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14   The Landslide Handbook­—A Guide to Understanding Landslides

Spreads

An extension of a cohesive soil or rock mass combined with the general sub-

sidence of the fractured mass of cohesive material into softer underlying material.

Spreads may result from liquefaction or flow (and extrusion) of the softer under-

lying material. Types of spreads include block spreads, liquefaction spreads, and

lateral spreads.

Lateral Spreads

Lateral spreads usually occur on very gentle slopes or essentially flat terrain,

especially where a stronger upper layer of rock or soil undergoes extension and

moves above an underlying softer, weaker layer. Such failures commonly are accom-

panied by some general subsidence into the weaker underlying unit. In rock spreads,

solid ground extends and fractures, pulling away slowly from stable ground and

moving over the weaker layer without necessarily forming a recognizable surface of

rupture. The softer, weaker unit may, under certain conditions, squeeze upward into

fractures that divide the extending layer into blocks. In earth spreads, the upper stable

layer extends along a weaker underlying unit that has flowed following liquefaction

or plastic deformation. If the weaker unit is relatively thick, the overriding fractured

blocks may subside into it, translate, rotate, disintegrate, liquefy, or even flow.

Occurrence

Worldwide and known to occur where there are liquefiable soils.

Common, but not restricted, to areas of seismic activity.

Relative size/range

The area affected may start small in size and have a few cracks that may

spread quickly, affecting areas of hundreds of meters in width.

Velocity of travel

May be slow to moderate and sometimes rapid after certain triggering

mechanisms, such as an earthquake. Ground may then slowly spread

over time from a few millimeters per day to tens of square meters

per day.

Triggering mechanism

Triggers that destabilize the weak layer include:

• Liquefaction of lower weak layer by earthquake shaking

• Natural or anthropogenic overloading of the ground above an unstable slope

• Saturation of underlying weaker layer due to precipitation, snowmelt, and

(or) ground-water changes

• Liquefaction of underlying sensitive marine clay following an erosional

disturbance at base of a riverbank/slope

• Plastic deformation of unstable material at depth (for example, salt)

Effects (direct/indirect)

Can cause extensive property damage to buildings, roads, railroads, and

lifelines. Can spread slowly or quickly, depending on the extent of water

saturation of the various soil layers. Lateral spreads may be a precursor

to earthflows. Part B. Basic Landslide Types   15

Mitigation measures

Liquefaction-potential maps exist for some places but are not widely

available. Areas with potentially liquefiable soils can be avoided as

construction sites, particularly in regions that are known to experience

frequent earthquakes. If high ground-water levels are involved, sites can

be drained or other water-diversion efforts can be added.

Predictability

High probability of recurring in areas that have experienced previous

problems. Most prevalent in areas that have an extreme earthquake

hazard as well as liquefiable soils. Lateral spreads are also associated

with susceptible marine clays and are a common problem throughout the For further reading:

St. Lawrence Lowlands of eastern Canada. Figures 11 and 12 show a References 9, 39, 43, and 45

schematic and an image of a lateral spread. Firm clay

Soft clay with

water-bearing silt

and sand layers

Bedrock

Schematic of a lateral spread. A liquefiable layer underlies the surface layer.

Figure 11.

(Schematic modified from Reference 9.)

Photograph of lateral spread damage to a roadway as a result of the 1989 Loma

Figure 12.

Prieta, California, USA, earthquake. (Photograph by Steve Ellen, U.S. Geological Survey.)

16   The Landslide Handbook­—A Guide to Understanding Landslides

Flows

A flow is a spatially continuous movement in which the surfaces of shear are

short-lived, closely spaced, and usually not preserved. The component velocities in

the displacing mass of a flow resemble those in a viscous liquid. Often, there is a

gradation of change from slides to flows, depending on the water content, mobility,

and evolution of the movement.

Debris Flows

A form of rapid mass movement in which loose soil, rock and sometimes organic

matter combine with water to form a slurry that flows downslope. They have been

informally and inappropriately called “mudslides” due to the large quantity of fine

material that may be present in the flow. Occasionally, as a rotational or translational

slide gains velocity and the internal mass loses cohesion or gains water, it may evolve

into a debris flow. Dry flows can sometimes occur in cohesionless sand (sand flows).

Debris flows can be deadly as they can be extremely rapid and may occur without any

warning.

Occurrence

Debris flows occur around the world and are prevalent in steep gullies

and canyons; they can be intensified when occurring on slopes or in

gullies that have been denuded of vegetation due to wildfires or forest

logging. They are common in volcanic areas with weak soil.

Relative size/range

These types of flows can be thin and watery or thick with sediment and

debris and are usually confined to the dimensions of the steep gullies

that facilitate their downward movement. Generally the movement is

relatively shallow and the runout is both long and narrow, sometimes

extending for kilometers in steep terrain. The debris and mud usually

terminate at the base of the slopes and create fanlike, triangular deposits

called debris fans, which may also be unstable.

Velocity of travel

Can be rapid to extremely rapid (35 miles per hour or 56 km per hour)

depending on consistency and slope angle.

Triggering mechanisms

Debris flows are commonly caused by intense surface-water flow, due to

heavy precipitation or rapid snowmelt, that erodes and mobilizes loose

soil or rock on steep slopes. Debris flows also commonly mobilize from

other types of landslides that occur on steep slopes, are nearly saturated,

and consist of a large proportion of silt- and sand-sized material.

Effects (direct/indirect)

Debris flows can be lethal because of their rapid onset, high speed of

movement, and the fact that they can incorporate large boulders and

other pieces of debris. They can move objects as large as houses in

their downslope flow or can fill structures with a rapid accumulation

of sediment and organic matter. They can affect the quality of water by

depositing large amounts of silt and debris. Part B. Basic Landslide Types   17

Mitigation measures

Flows usually cannot be prevented; thus, homes should not be built in

steep-walled gullies that have a history of debris flows or are otherwise

susceptible due to wildfires, soil type, or other related factors. New flows For further reading:

can be directed away from structures by means of deflection, debris-flow References 9, 39, 43, and 45

basins can be built to contain flow, and warning systems can be put in

place in areas where it is known at what rainfall thresholds debris flows

are triggered. Evacuation, avoidance, and (or) relocation are the best

methods to prevent injury and life loss.

Predictability

Maps of potential debris-flow hazards exist for some areas. Debris flows

can be frequent in any area of steep slopes and heavy rainfall, either sea-

sonally or intermittently, and especially in areas that have been recently

burned or the vegetation removed by other means. Figures 13 and 14

show a schematic and an image of a debris flow.

Schematic of a debris flow. (Schematic modified from Reference 9.)

Figure 13. Debris-flow damage to the

Figure 14.

city of Caraballeda, located at the base of

the Cordillera de la Costan, on the north

coast of Venezuela. In December 1999, this

area was hit by Venezuela’s worst natural

disaster of the 20th century; several days

of torrential rain triggered flows of mud,

boulders, water, and trees that killed as

many as 30,000 people. (Photograph by

L.M. Smith, Waterways Experiment Station,

U.S. Army Corps of Engineers.)

18   The Landslide Handbook­—A Guide to Understanding Landslides

Lahars (Volcanic Debris Flows)

The word “lahar” is an Indonesian term. Lahars are also known as volcanic

mudflows. These are flows that originate on the slopes of volcanoes and are a type

of debris flow. A lahar mobilizes the loose accumulations of tephra (the airborne

solids erupted from the volcano) and related debris.

Occurrence

Found in nearly all volcanic areas of the world.

Relative size/range

Lahars can be hundreds of square kilometers or miles in area and can

become larger as they gain speed and accumulate debris as they travel

downslope; or, they can be small in volume and affect limited areas of

the volcano and then dissipate downslope.

Velocity of travel

Lahars can be very rapid (more than 35 miles per hour or 50 kilometers

per hour) especially if they mix with a source of water such as melting

snowfields or glaciers. If they are viscous and thick with debris and less

water, the movement will be slow to moderately slow.

Triggering mechanism

Water is the primary triggering mechanism, and it can originate from

crater lakes, condensation of erupted steam on volcano particles, or the

melting of snow and ice at the top of high volcanoes. Some of the largest

and most deadly lahars have originated from eruptions or volcanic vent-

ing which suddenly melts surrounding snow and ice and causes rapid

liquefaction and flow down steep volcanic slopes at catastrophic speeds.

Effects (direct/indirect)

Effects can be extremely large and devastating, especially when trig-

gered by a volcanic eruption and consequent rapid melting of any

snow and ice—the flow can bury human settlements located on the

volcano slopes. Some large flows can also dam rivers, causing flooding

upstream. Subsequent breaching of these weakly cemented dams can

cause catastrophic flooding downstream. This type of landslide often

results in large numbers of human casualties.

Mitigation measures

No corrective measures are known that can be taken to prevent damage

from lahars except for avoidance by not building or locating in their paths

or on the slopes of volcanoes. Warning systems and subsequent evacua-

tion work in some instances may save lives. However, warning systems

require active monitoring, and a reliable evacuation method is essential.

Part B. Basic Landslide Types   19

Predictability

Susceptibility maps based on past occurrences of lahars can be con-

structed, as well as runout estimations of potential flows. Such maps are For further reading:

not readily available for most hazardous areas. Figures 15 and 16 show a References 9, 39, 43, and 45

schematic and an image of a lahar.

Schematic of a lahar. (Graphic by U.S. Geological Survey.)

Figure 15. Photograph of a lahar caused by the 1982 eruption of Mount St. Helens in

Figure 16.

Washington, USA. (Photograph by Tom Casadevall, U.S. Geological Survey.)

20   The Landslide Handbook­—A Guide to Understanding Landslides

Debris Avalanche

Debris avalanches are essentially large, extremely rapid, often open-slope flows

formed when an unstable slope collapses and the resulting fragmented debris is rap-

idly transported away from the slope. In some cases, snow and ice will contribute to

the movement if sufficient water is present, and the flow may become a debris flow

and (or) a lahar.

Occurrence

Occur worldwide in steep terrain environments. Also common on very

steep volcanoes where they may follow drainage courses.

Relative size/range

Some large avalanches have been known to transport material blocks as

large as 3 kilometers in size, several kilometers from their source.

Velocity of travel

Rapid to extremely rapid; such debris avalanches can travel close to

100 meters/sec.

Triggering mechanism

In general, the two types of debris avalanches are those that are “cold”

and those that are “hot.” A cold debris avalanche usually results from a

slope becoming unstable, such as during collapse of weathered slopes in

steep terrain or through the disintegration of bedrock during a slide-type

landslide as it moves downslope at high velocity. At that point, the mass

can then transform into a debris avalanche. A hot debris avalanche is one

that results from volcanic activity including volcanic earthquakes or the

injection of magma, which causes slope instability.

Effects (direct/indirect)

Debris avalanches may travel several kilometers before stopping, or they

may transform into more water-rich lahars or debris flows that travel

many tens of kilometers farther downstream. Such failures may inun-

date towns and villages and impair stream quality. They move very fast

and thus may prove deadly because there is little chance for warning

and response.

Corrective measures/mitigation

Avoidance of construction in valleys on volcanoes or steep mountain

slopes and real-time warning systems may lessen damages. However,

warning systems may prove difficult due to the speed at which debris

avalanches occur—there may not be enough time after the initiation of

the event for people to evacuate. Debris avalanches cannot be stopped

or prevented by engineering means because the associated triggering

mechanisms are not preventable. Part B. Basic Landslide Types   21

Predictability

If evidence of prior debris avalanches exists in an area, and if such

evidence can be dated, a probabilistic recurrence period might be

established. During volcanic eruptions, chances are greater for a debris For further reading:

avalanche to occur, so appropriate cautionary actions could be adopted. References 9, 39, 43, and 45

Figures 17 and 18 show a schematic and an image of a debris avalanche.

Schematic of a debris avalanche. (Schematic modified from Reference 9.)

Figure 17. A debris avalanche that buried the village of Guinsaugon, Southern Leyte,

Figure 18.

Philippines, in February 2006. (Photograph by University of Tokyo Geotechnical Team.)

Please see figure 30 for an image of another debris avalanche.

22   The Landslide Handbook­—A Guide to Understanding Landslides

Earthflow

Earthflows can occur on gentle to moderate slopes, generally in fine-grained

soil, commonly clay or silt, but also in very weathered, clay-bearing bedrock. The

mass in an earthflow moves as a plastic or viscous flow with strong internal defor-

mation. Susceptible marine clay (quick clay) when disturbed is very vulnerable and

may lose all shear strength with a change in its natural moisture content and sud-

denly liquefy, potentially destroying large areas and flowing for several kilometers.

Size commonly increases through headscarp retrogression. Slides or lateral spreads

may also evolve downslope into earthflows. Earthflows can range from very slow

(creep) to rapid and catastrophic. Very slow flows and specialized forms of earthflow

restricted to northern permafrost environments are discussed elsewhere.

Occurrence

Earthflows occur worldwide in regions underlain by fine-grained soil

or very weathered bedrock. Catastrophic rapid earthflows are common

in the susceptible marine clays of the St. Lawrence Lowlands of North

America, coastal Alaska and British Columbia, and in Scandinavia.

Relative (size/range)

Flows can range from small events of 100 square meters in size to large

events encompassing several square kilometers in area. Earthflows in

susceptible marine clays may runout for several kilometers. Depth of the

failure ranges from shallow to many tens of meters.

Velocity of travel

Slow to very rapid.

Triggering mechanisms

Triggers include saturation of soil due to prolonged or intense rainfall

or snowmelt, sudden lowering of adjacent water surfaces causing rapid

drawdown of the ground-water table, stream erosion at the bottom of

a slope, excavation and construction activities, excessive loading on a

slope, earthquakes, or human-induced vibration.

Effects (direct/indirect)

Rapid, retrogressive earthflows in susceptible marine clay may devastate

large areas of flat land lying above the slope and also may runout for

considerable distances, potentially resulting in human fatalities, destruc-

tion of buildings and linear infrastructure, and damming of rivers with

resultant flooding upstream and water siltation problems downstream.

Slower earthflows may damage properties and sever linear infrastructure.

Corrective measures/mitigation

Improved drainage is an important corrective measure, as is grading of

slopes and protecting the base of the slope from erosion or excavation.

Shear strength of clay can be measured, and potential pressure can be

monitored in suspect slopes. However, the best mitigation is to avoid

development activities near such slopes. Part B. Basic Landslide Types   23

Predictability

Evidence of past earthflows is the best indication of vulnerability. Dis-

tribution of clay likely to liquefy can in some cases be mapped and has For further reading:

been mapped in many parts of eastern North America. Cracks opening References 9, 39, 43, and 45

near the top of the slope may indicate potential failure. Figures 19 and

20 show a schematic and an image of an earthflow.

Original

position

Earthflow

Schematic of an earthflow. (Schematic from Geological Survey of Canada.)

Figure 19. The 1993 Lemieux landslide—a rapid earthflow in sensitive marine clay

Figure 20.

near Ottawa, Canada. The headscarp retrogressed 680 meters into level ground above

the riverbank. About 2.8 million tons of clay and silt liquefied and flowed into the South

Nation River valley, damming the river. (Photograph by G.R. Brooks, Geological Survey of

Canada.)

24   The Landslide Handbook­—A Guide to Understanding Landslides

Slow Earthflow (Creep)

Creep is the informal name for a slow earthflow and consists of the impercep-

tibly slow, steady downward movement of slope-forming soil or rock. Movement

is caused by internal shear stress sufficient to cause deformation but insufficient to

cause failure. Generally, the three types of creep are: (1) seasonal, where movement

is within the depth of soil affected by seasonal changes in soil moisture and tem-

perature; (2) continuous, where shear stress continuously exceeds the strength of the

material; and (3) progressive, where slopes are reaching the point of failure for other

types of mass movements.

Occurrence

Creep is widespread around the world and is probably the most common

type of landslide, often preceding more rapid and damaging types of

landslides. Solifluction, a specialized form of creep common to perma-

frost environments, occurs in the upper layer of ice-rich, fine-grained

soils during the annual thaw of this layer.

Relative size/range

Creep can be very regional in nature (tens of square kilometers) or

simply confined to small areas. It is difficult to discern the boundaries of

creep since the event itself is so slow and surface features representing

perceptible deformation may be lacking.

Velocity of travel

Very slow to extremely slow. Usually less than 1 meter (0.3 foot)

per decade.

Triggering mechanism

For seasonal creep, rainfall and snowmelt are typical triggers, whereas

for other types of creep there could be numerous causes, such as chemi-

cal or physical weathering, leaking pipes, poor drainage, destabilizing

types of construction, and so on.

Effects

Because it is hard to detect in some places because of the slowness of

movement, creep is sometimes not recognized when assessing the suit-

ability of a building site. Creep can slowly pull apart pipelines, build-

ings, highways, fences, and so forth, and can lead to more drastic ground

failures that are more destructive and faster moving.

Corrective measures/mitigation

The most common mitigation for creep is to ensure proper drainage

of water, especially for the seasonal type of creep. Slope modification

such as flattening or removing all or part of the landslide mass, can be

attempted, as well as the construction of retaining walls.

Part B. Basic Landslide Types   25

Predictability For further reading:

References 9, 39, 43, and 45

Indicated by curved tree trunks, bent fences and (or) retaining walls,

tilted poles or fences, and small soil ripples or ridges on the surface.

Rates of creep can be measured by inclinometers installed in boreholes

or by detailed surface measurements. Figures 21 and 22 show a sche-

matic and an image of creep. Curved tree trunks Tilted pole

Soil ripples

Fence out of alignment

Schematic of a slow earthflow, often called creep. (Schematic modified from

Figure 21.

Reference 9.)

This photograph shows the effects of creep, in an area near East Sussex,

Figure 22.

United Kingdom, called the Chalk Grasslands. Steep slopes of thin soil over marine

chalk deposits, develop a ribbed pattern of grass-covered horizontal steps that are

0.3 to 0.6 meter (1 to 2 feet) high. Although subsequently made more distinct by cattle

and sheep walking along them, these terraces (commonly known as sheep tracks)

were formed by the gradual, creeping movement of soil downhill. (Photograph by

Ian Alexander.)

26   The Landslide Handbook­—A Guide to Understanding Landslides

Flows in Permafrost

Failures in permafrost conditions involve the movement of fine-grained, previ-

ously ice-rich soil and can occur on gentle slopes. Seasonal thaw of the upper meter

of frozen ground melts ground ice and results in oversaturation of the soil, which in

turn loses shear strength and initiates flows. Solifluction, a form of cold environment

creep, involves very slow deformation of the surface and forms shallow lobes elon-

gated downslope. Active layer detachments, also known as skinflows, involve rapid

flow of a shallow layer of saturated soil and vegetation, forming long, narrow flows

moving on the surface but over the underlying permanently frozen soil. This type of

movement may expose buried ice lenses, which when thawed may develop into ret-

rogressive thaw flows or possibly debris flows. Retrogressive thaw flows are larger

features with a bimodal shape of a steep headwall and low-angle tongue of saturated

soil. This type of feature will continue to expand through headscarp retrogression

until displaced vegetation buries and insulates the ice-rich scarp.

Occurrence

Flows are common in ice-rich permafrost soils in northern latitudes and

high altitudes (cold environments).

Relative size/range

Flows are generally small but can increase in size through headscarp

retrogression. They may evolve into a larger debris flow.

Velocity of travel

Very slow (solifluction); slow (retrogressive thaw flow); rapid (active

layer detachment).

Triggering mechanisms

Above-average summer temperatures, frost wedges, wildfire, and

anthropogenic disturbances to insulating peat layer. Such landslides are

particularly likely in warming climates.

Effects (direct/indirect)

Damage to pipelines and roads and other structures can be severe.

Corrective measures/mitigation

Infrastructure designs that have minimal effect on the surface peat layer

or temperature of the active layer and avoidance, when possible, of ice-

rich soils when planning roads and other infrastructure, can reduce risk.

Ice content of the upper soil can be readily tested.

Predictability

If ice-rich soil thaws, it will flow. In some areas, ice content has been

mapped; in other areas, ice content can be estimated on the basis of

For further reading: specific mapped units shown on surficial geology maps. Figures 23 and

References 2, 9, 39, 43, and 45 24 show a schematic and an image of permafrost-related flow.

Part B. Basic Landslide Types   27

A. Original slope

Active layer

(seasonally thawed) Permanently frozen

icy sediment

Segregated ice bodies

B. Retrogressive thaw flow in progress

Original slope sion

Ice s

Tongue of re

g

flowing mud tro

re

e

ibl

ss

o

P

Retrogression

Schematic of a retrogressive thaw flow slide. (Schematic by Jan Aylsworth,

Figure 23.

Geological Survey of Canada.)

A note about complex landslides:

These are landslides that feature components of two or more of the basic types

of landslides and can occur either simultaneously or at different times during the

onset of slope failure.

28   The Landslide Handbook­—A Guide to Understanding Landslides

Photograph of a retrogressive thaw flow in the Northwest Territories, Canada. Wildfire has likely contributed to the size of

Figure 24.

the flow by means of damage to an insulating moss layer, resulting in the thickening of the active layer, which is thawing permafrost.

(Photograph by Marten Geertsema, Ministry of Forests, British Columbia, Canada.) Part C. Where Do Landslides Occur?   29

Part C. Where Do Landslides Occur?

A surprising fact to many people is that landslides can occur virtually anywhere

in the world. The traditional viewpoint that landslides are restricted to extremely For further reading:

steep slopes and inhospitable terrain does not accurately reflect the real nature of the Reference 2

problem. Most countries in the world have been affected in some manner by land-

slides. The reason for such wide geographic coverage has much to do with the many

different triggering mechanisms for landslides.

Excessive precipitation, earthquakes, volcanoes, forest fires and other mecha-

nisms, and more recently, certain dangerous human activities are just some of the

key causes that can trigger landslides. See “Part What Causes Landslides?” for

D.

more information on triggering mechanisms. Figure 25 shows an example of lateral

spreading, a type of ground failure often associated with earthquakes.

Similarly, landslides are known to occur both on land and under water; they can

occur in bedrock or on soils; cultivated land, barren slopes and natural forests are all

subject to landslides. Both extremely dry areas and very humid areas can be affected

by slope failures, and most important, steep slopes are not a necessary prerequisite

for landslides to occur. In some cases, gentle slopes as shallow as 1–2 degrees have

been observed to fail.

Bearing in mind that landslides can happen virtually anywhere around the

world, we do, however, recognize certain patterns in their occurrence. At the

national scale in countries such as Canada and the United States, the association of

hilly terrain such as the Rocky Mountains with certain types of landslides is clear.

Other geographic trends in landslide distribution can be linked to natural patterns of

climate and weather, wildfires, and stream/river courses or human patterns involving

the clearing of vegetation, modification to slopes, and other urban and rural prac-

tices. In each of these cases, it is important to recognize that landslide types vary in

relation to the local and regional conditions.

Debris torrents require channels and ravines to occur, whereas rockfalls will

happen only where steep, exposed faces of bedrock or boulder-rich deposits are

present. Geology itself figures prominently in the occurrence of many landslides.

The correlation of seismic and volcanic activity to landslides is of paramount impor- For further reading:

tance, so specialists often approach the evaluation of hazards from a multi-hazard References 8, 16, 19, 25, 30, and 45

perspective, which by definition takes into account most of the factors discussed

previously. Lateral spreading damage.

Figure 25.

Photograph shows the Puget Sound

area in Washington, USA, after the 2001

Nisqually earthquake. (Photograph

courtesy of the Seattle Times.)

30   The Landslide Handbook­—A Guide to Understanding Landslides

Part D. What Causes Landslides?

There are two primary categories of causes of landslides: natural and human-

caused. Sometimes, landslides are caused, or made worse, by a combination of the

two factors.

Natural Occurrences

This category has three major triggering mechanisms that can occur either singly

or in combination —(1) water, (2) seismic activity, and (3) volcanic activity. Effects

of all of these causes vary widely and depend on factors such as steepness of slope,

morphology or shape of terrain, soil type, underlying geology, and whether there are

people or structures on the affected areas. Effects of landslides will be discussed in

more detail in Part E.

Landslides and Water

Slope saturation by water is a primary cause of landslides. Saturation can occur

in the form of intense rainfall, snowmelt, changes in ground-water levels, and surface-

water level changes along coastlines, earth dams, and in the banks of lakes, reser-

voirs, canals, and rivers. Landslides and flooding are closely associated because both

are related to precipitation, runoff, and the saturation of ground by water. Flooding

may cause landslides by undercutting banks of streams and rivers and by saturation

of slopes by surface water (overland flow). In addition, debris flows and mudflows

usually occur in small, steep stream channels and commonly are mistaken for floods;

in fact, these two events often occur simultaneously in the same area. Conversely,

landslides also can cause flooding when sliding rock and debris block stream chan-

nels and other waterways, allowing large volumes of water to back up behind such

dams. This causes backwater flooding and, if the dam fails, subsequent downstream

flooding. Moreover, solid landslide debris can “bulk” or add volume and density to

otherwise normal streamflow or cause channel blockages and diversions, creating

flood conditions or localized erosion. Landslides also can cause tsunamis (seiches),

overtopping of reservoirs, and (or) reduced capacity of reservoirs to store water. Steep

wildfire-burned slopes often are landslide-prone due to a combination of the burning

and resultant denudation of vegetation on slopes, a change in soil chemistry due to

burning, and a subsequent saturation of slopes by water from various sources, such

as rainfall. Debris flows are the most common type of landslide on burned slopes

(for a description and images of a debris flow, see “Part Basic Landslide Types”

B.

in Section I). Wildfires, of course, may be the result of natural or human causes.

Figure 26 shows a devastating landslide caused by rainfall, and possibly made worse

by a leaking water pipe, which added even more water to the soil.

Part D. What Causes Landslides?   31

The Mameyes, Puerto Rico, landslide, 1985. This landslide destroyed 120 houses and killed at least 129 people. The

Figure 26.

catastrophic slide was triggered by a tropical storm that produced extremely heavy rainfall. Contributing factors could also have

included sewage saturating the ground in the densely populated area, and a leaking water pipe at the top of the landslide. (Photograph

by Randall Jibson, U.S. Geological Survey.)

32   The Landslide Handbook­—A Guide to Understanding Landslides

Landslides and Seismic Activity

Many mountainous areas that are vulnerable to landslides have also experienced

at least moderate rates of earthquake activity in recorded times. Earthquakes in steep

landslide-prone areas greatly increase the likelihood that landslides will occur, due

to ground shaking alone, liquefaction of susceptible sediments, or shaking-caused

dilation of soil materials, which allows rapid infiltration of water. For instance, the

1964 Great Alaska earthquake in the United States caused widespread landsliding

and other ground failure, which led to most of the monetary loss attributed to the

earthquake. Other areas in North America, such as the State of California, the Puget

Sound region in Washington, and the St. Lawrence lowlands of eastern Canada, have

experienced landslides, lateral spreading, and other types of ground failure classified

as landslides, due to moderate to large earthquakes. Rockfalls and rock topples can

also be caused by loosening of rocks or rocky formations as a result of earthquake

ground shaking. Figure 27 shows damage from a landslide that was triggered by

an earthquake. There is also a great danger of landslide dams forming in streams

and rivers below steep slopes, a result of rock and earth being shaken down by the

earthquake. These landslide dams often completely or partially block the flow of

water, causing water to back up behind the landslide dam, flooding areas upriver. As

Figures 32, 42, C53, C54, and C55 these dams are often unstable, they may erode either quickly or over a period of time

show examples of large landslide and fail catastrophically, unleashing the backed up water as a rapid deluge below the

dams that still exist dam. This deluge is capable of causing a great deal of damage downriver.

Earthquake-induced landslide damage to a house built on artificial fill, after

Figure 27.

the 2004 Niigata Prefecture earthquake in Japan. (Photograph by Professor Kamai,

Kyoto University, Japan.) Part D. What Causes Landslides?   33

Landslides and Volcanic Activity

Landslides due to volcanic activity represent some of the most devastating

types of failures. Volcanic lava may melt snow rapidly, which can form a deluge

of rock, soil, ash, and water that accelerates rapidly on the steep slopes of volca-

noes, devastating anything in its path. These volcanic debris flows (also known as

lahars, an Indonesian term) can reach great distances after they leave the flanks

of the volcano and can damage structures in flat areas surrounding the volcanoes.

Volcanic edifices are young, unconsolidated, and geologically weak structures that

in many cases can collapse and cause rockslides, landslides, and debris avalanches.

Many islands of volcanic origin experience periodic failure of their perimeter areas

(due to the weak volcanic surface deposits), and masses of soil and rock slide into

the ocean or other water bodies, such as inlets. Such collapses may create massive

sub-marine landslides that may also rapidly displace water, subsequently creating

deadly tsunamis that can travel and do damage at great distances, as well as locally.

Figure 28 shows a collapse of the side of a volcano and the resulting devastation.

The side of Casita Volcano in Nicaragua, Central America, collapsed on

Figure 28.

October 30, 1998, the day of peak rainfall as Hurricane Mitch moved across Central

America. This lahar killed more than 2,000 people as it swept over the towns of

El Porvenir and Rolando Rodriguez. (Photograph by K.M. Smith, U.S. Geological Survey.)

Human Activities

Populations expanding onto new land and creating neighborhoods, towns, and

cities is the primary means by which humans contribute to the occurrence of land-

slides. Disturbing or changing drainage patterns, destabilizing slopes, and removing

vegetation are common human-induced factors that may initiate landslides. Other

examples include oversteepening of slopes by undercutting the bottom and loading

the top of a slope to exceed the bearing strength of the soil or other component mate-

rial. However, landslides may also occur in once-stable areas due to other human

activities such as irrigation, lawn watering, draining of reservoirs (or creating them),

leaking pipes, and improper excavating or grading on slopes. New construction on

landslide-prone land can be improved through proper engineering (for example,

grading, excavating) by first identifying the site’s susceptibility to slope failures and

by creating appropriate landslide zoning. For further reading:

See Appendix A for an expanded, detailed list of causes/triggering mechanisms References 16, 19, 32, 38, 39, 43,

of landslides. and 45

34   The Landslide Handbook­—A Guide to Understanding Landslides

Part E. What are the Effects and Consequences of

Landslides?

Landslide effects occur in two basic environments: the built environment

and the natural environment. Sometimes there is intersection between the two; for

example agricultural lands and forest lands that are logged.

Effects of Landslides on the Built Environment

Landslides affect manmade structures whether they are directly on or near a

landslide. Residential dwellings built on unstable slopes may experience partial

damage to complete destruction as landslides destabilize or destroy foundations,

walls, surrounding property, and above-ground and underground utilities. Landslides

can affect residential areas either on a large regional basis (in which many dwell-

In many areas of the world that

Note: ings are affected) or on an individual site basis (where only one structure or part of

provide private disaster insurance, a structure is affected). Also, landslide damage to one individual property’s lifelines

damage from landslides is not covered (such as trunk sewer, water, or electrical lines and common-use roads) can affect the

in these insurance policies, and the lifelines and access routes of other surrounding properties. Commercial structures

costs of damages must be borne by the are affected by landslides in much the same way residential structures are affected.

individual homeowner. In such a case, consequences may be great if the commercial structure is a common-

use structure, such as a food market, which may experience an interruption in busi-

ness due to landslide damage to the actual structure and (or) damage to its access

roadways.

Fast-moving landslides such as debris flows are the most destructive type of

landslide to structures, as they often occur without precursors or warnings, move too

quickly for any mitigation measures to be enacted, and due to velocity and mate-

rial are often very powerful and destructive. Fast-moving landslides can completely

destroy a structure, whereas a slower moving landslide may only slightly damage

it, and its slow pace may allow mitigation measures to be enacted. However, left

unchecked, even slow landslides can completely destroy structures over time. Debris

avalanches and lahars in steep areas can quickly destroy or damage the structures

and lifelines of cities, towns, and (or) neighborhoods due to the fact that they are an

extremely fast-moving, powerful force. The nature of landslide movement and the

fact that they may continue moving after days, weeks, or months preclude rebuilding

on the affected area, unless mitigative measures are taken; even then, such efforts are

not always a guarantee of stability.

One of the greatest potential consequences from landslides is to the transporta-

tion industry, and this commonly affects large numbers of people around the world.

Cut and fill failures along roadways and railways, as well as collapse of roads from

underlying weak and slide-prone soils and fill, are common problems. Rockfalls may

injure or kill motorists and pedestrians and damage structures. All types of landslides

can lead to temporary or long-term closing of crucial routes for commerce, tour-

ism, and emergency activities due to road or rail blockage by dirt, debris, and (or)

rocks (fig. 29). Even slow creep can affect linear infrastructure, creating maintenance

problems. Figure 29 shows a landslide blocking a major highway. Blockages of

highways by landslides occur very commonly around the world, and many can sim-

ply be bulldozed or shoveled away. Others, such as the one shown in figure 29, will

require major excavation and at least temporary diversion of traffic or even closure of

the road.

As world populations continue to expand, they are increasingly vulnerable

to landslide hazards. People tend to move on to new lands that might have been

deemed too hazardous in the past but are now the only areas that remain for a

growing population. Poor or nonexistent land-use policies allow building and other

construction to take place on land that might better be left to agriculture, open-space

parks, or uses other than for dwellings or other buildings and structures. Communi-

ties often are not prepared to regulate unsafe building practices and may not have the

legitimate political means or the expertise to do so.

Part E. What are the Effects and Consequences of Landslides?   35

A landslide on the Pan American Highway in El Salvador, Central America, near the town of San Vicente, in 2001.

Figure 29.

(Photograph by Ed Harp, U.S. Geological Survey.)

36   The Landslide Handbook­—A Guide to Understanding Landslides

Effects of Landslides on the Natural Environment

Landslides have effects on the natural environment:

• The morphology of the Earth’s surface—mountain and valley systems, both

on the continents and beneath the oceans; mountain and valley morphologies

are most significantly affected by downslope movement of large landslide

masses;

• The forests and grasslands that cover much of the continents; and

• The native wildlife that exists on the Earth’s surface and in its rivers, lakes,

and seas.

Figures 30, 31, and 32 show the very large areal extent of some landslides and

how they may change the face of the terrain, affecting rivers, farmland, and forests.

Forest, grasslands, and wildlife often are negatively affected by landslides,

with forest and fish habitats being most easily damaged, temporarily or even rarely,

destroyed. However, because landslides are relatively local events, flora and fauna

can recover with time. In addition, recent ecological studies have shown that, under

certain conditions, in the medium-to-long term, landslides can actually benefit fish

and wildlife habitats, either directly or by improving the habitat for organisms that

the fish and wildlife rely on for food.

The following list identifies some examples of landslides that commonly occur

in the natural environment:

Submarine landslide

• is a general term used to describe the downslope mass

movement of geologic materials from shallower to deeper regions of the

ocean. Such events may produce major effects to the depth of shorelines,

ultimately affecting boat dockings and navigation. These types of landslides

can occur in rivers, lakes, and oceans. Large submarine landslides triggered

by earthquakes have caused deadly tsunamis, such as the 1929 Grand Banks

(off the coast of Newfoundland, Canada) tsunamis.

Coastal cliff retreat cliff erosion,

• , or is another common effect of landslides

on the natural environment. Rock-and-soil falls, slides, and avalanches are

the common types of landslides affecting coastal areas; however, topples

and flows also are known to occur. Falling rocks from eroding cliffs can be

especially dangerous to anyone occupying areas at the base of cliffs, or on

the beaches near the cliffs. Large amounts of landslide material can also be

destructive to aquatic life, such as fish and kelp, and the rapid deposition of

sediments in water bodies often changes the water quality around vulnerable

shorelines.

Landslide dams

• can naturally occur when a large landslide blocks the flow

of a river, causing a lake to form behind the blockage. Most of these dams

are short-lived as the water will eventually erode the dam. If the landslide

dam is not destroyed by natural erosional processes or modified by humans,

it creates a new landform—a lake. Lakes created by landslide dams can last

a long time, or they may suddenly be released and cause massive flooding

For further reading: downstream. There are many ways that people can lessen the potential dan-

References 4, 11, 14, 16, 19, 31, 35, gers of landslide dams, and some of these methods are discussed in the safety

36, 39, and 43 and mitigation sections of this volume. Figure 32 shows the Slumgullion

landslide one of the largest landslides in the world—the landslide dam it has

formed is so large and wide, that it has lasted 700 years. Figures C53, C54,

and C55 (in Appendix C) also show aspects of another large landslide dam.

See Appendix C for more information on mitigating the effects of landslide dams.

Part E. What are the Effects and Consequences of Landslides?   37

The active volcano, Mount Shasta in California, USA. Note the landforms in the foreground, caused by a debris avalanche

Figure 30.

that occurred about 300,000 years ago. The debris avalanche traveled great distances from the volcano and produced lasting landform

effects that can still be seen today. (Photograph by R. Crandall, U.S. Geological Survey.)

38   The Landslide Handbook­—A Guide to Understanding Landslides

View looking downstream at the confluence of the Río Malo (flowing from lower left) and the Río Coca, northeastern

Figure 31.

Ecuador, in South America. Both river channels have been filled with sediment left behind by debris flows triggered by the 1987

Reventador earthquakes. Slopes in the area had been saturated by heavy rains in recent days before the earthquake. Debris/earth

slides, debris avalanches, debris/mudflows, and resulting floods destroyed about 40 kilometers of the Trans-Ecuadorian oil pipeline and

the only highway from Quito. (Photograph by R.L. Schuster, U.S. Geological Survey; information from Reference 32.)

Part E. What are the Effects and Consequences of Landslides?   39

The Slumgullion landslide, Colorado, USA. This landslide (formally referred to also as an earthflow) dammed the Lake Fork

Figure 32.

of the Gunnison River, which flooded the valley and formed Lake Cristobal. (Photograph by Jeff Coe, U.S. Geological Survey.)

40   The Landslide Handbook­—A Guide to Understanding Landslides

Part F. Interrelationship of Landslides with Other

Natural Hazards—The Multiple Hazard Effect

Natural hazards such as floods, earthquakes, volcanic eruptions, and landslides

can occur simultaneously, or one or more of these hazards can trigger one or more of

the others. Landslides are often the result of earthquakes, floods, and volcanic activ-

ity and may in turn cause subsequent hazards; for example, an earthquake-induced

landslide can cause a deadly tsunami if sufficient landslide material slides into a

body of water to displace a large volume of water. Another example would be a

volcanic eruption-induced or earthquake-induced landslide that blocks a river, caus-

ing water to back up behind the mass and flood the upstream area. Should the dam

fail, the impounded water will be suddenly unleashed to cause flooding downstream.

This flooding can then add to riverbank and coastal erosion and destabilization

through rapid saturation of slopes and undercutting of cliffs and banks. It is there-

fore imperative, when evaluating an area’s vulnerability to landslides, to examine

all other possible natural hazards. At this time, few maps exist that show multiple-

hazard susceptibilities; for most areas, if hazards are mapped at all, only a single

hazard is mapped.

Figures 33–35 show multi-hazard events involving landslides.

For further reading:

References 17, 20, 35, 39, 43, and 45

Part F. Interrelationship of Landslides with Other Natural Hazards—The Multiple Hazard Effect   41

An example of a multi-hazard event. Photograph shows an aerial view of Lituya Bay, Alaska, USA. On July 9, 1958, an

Figure 33.

earthquake occurred which caused a landslide to enter the bay. The landslide in turn, caused a tsunami wave that had a run-up of

174 meters on the opposite shore, and a 30-meter wave passed beyond Lituya Bay. It is the largest landslide-generated wave ever

documented. Note the extent of the nonforested areas of land lining the shore of the bay, which marks the approximate reach of the

tsunami wave. (Photograph by D.J. Miller, U.S. Geological Survey.)

42   The Landslide Handbook­—A Guide to Understanding Landslides

The 1999 multi-hazard event in Tanaguarena, in coastal Venezuela, South

Figure 34.

America. The floods and landslides were triggered by heavy rains. (Photograph by

Matthew Larsen, U.S. Geological Survey.) Rock

avalanche

deposit

Ridge

Debris

flow

deposit

This is a photograph showing the aftereffects of a multi-hazard event. It is

Figure 35.

an aerial view showing part of the Andes Mountains and Nevado Huascaran, the highest

peak in Peru, South America. A massive avalanche of ice and rock debris, triggered by

the May 31, 1970, earthquake, buried the towns of Yungay and Ranrahirca, killing more

than 20,000 people, about 40 percent of the total death toll of 67,000. The avalanche

started with a sliding mass of glacial ice and rock about 1,000 meters (3,000 feet) wide

and 1.6 kilometers (one mile) long that swept downslope about 5.4 kilometers (3.3 miles) to

Yungay at average speed of more than 160 kilometers per hour. The ice picked up morainal

material of water, mud, and rocks. (Photograph by Servicio Aerofotográfico National,

graphics by George Plafker, U.S. Geological Survey.) Photograph and information from the

U.S. Geological Survey Photographic Archives: http://libraryphoto.cr.usgs.gov/

Section II.

Evaluating and Communicating Landslide Hazard

Worldwide, information about landslides varies in its quantity and complexity

and ranges in quality from detailed inventories of past landslides and resultant

susceptibility and hazard maps to no information at all. People in some regions of

the world have collective memories of past landslide events and know where slopes

are unstable and (or) dangerous, and as a result, intuitively know where it is advan-

tageous to build or not build. However, many areas are not readily obvious as to

potential landslide hazard, and ground failure does not occur on any kind of regular

basis. Also, some triggering mechanisms occur sporadically and have a gradual

and cumulative effect and are not readily obvious.

44   The Landslide Handbook­—A Guide to Understanding Landslides

Part A. Evaluating Landslide Hazards

There are many different means of assessing landslide hazard for an area; it

is always advisable to consult with an expert for the most accurate assessment,

although this is not always possible. Two types of landslide hazard evaluation, direct

observation and the use of technological tools, are discussed here.

Observation and (or) inspection by local experts and (or)

municipal officials, and property owners.

For further reading: The following simple guide may assist individuals in observing and

References 1, 3, 4, 19, 20, 21, 25, 26, assessing a potential landslide hazard. It is important to note that some

36, 39, 42, and 44 of these features can also be due to causes other than landslides, such

as swelling soils.

Features that might indicate landslide movement:

• Springs, seeps, and wet or saturated ground in previously dry areas on or

below slopes.

• Ground cracks—cracks in snow, ice, soil, or rock on or at the head of slopes.

• Sidewalks or slabs pulling away from structures if near a slope; soil pulling

away from foundations.

• Offset fence lines, which were once straight or configured differently (see

photograph in figure 22).

• Unusual bulges or elevation changes in the ground, pavements, paths,

or sidewalks.

• Tilting telephone poles, trees, retaining walls, fences.

• Excessive tilting or cracking of concrete floors and foundations.

• Broken water lines and other underground utilities.

• Rapid increase or decrease in stream-water levels, possibly accompanied by

increased turbidity (soil content clouding the water).

• Sticking doors and windows and visible open spaces, indicating walls and

frames are shifting and deforming.

• Creaking, snapping, or popping noises from a house, building, or grove of

trees (for example, roots snapping or breaking).

• Sunken or down-dropped roads or paths.

It is important that governing bodies provide a means of

Note to Managers:

keeping records, preferably in written format, about the occurrences of land-

slides, with photographs and (or) diagrams where possible. For areas of the world

that do not already have laws or regulations that require disclosure of landslide

hazards to property owners, it is important for villages, municipalities, or cities

to establish some source of authority for hazard information. This need not be

sophisticated or expensive but will provide a means to sustain landslide knowl-

edge through time. Although some information may be politically sensitive, such

as landowner rights, it is important that landslide information be made available,

in some manner, to the general population.

Part A. Evaluating Landslide Hazards   45

Ground cracks. (Phtograph courtesy of Utah Geological Survey.)

Figure 36. Sidewalk pulling away from house. (Photograph courtesy of Utah

Figure 37.

Geological Survey.)

Cracking of the foundation of a structure. (Photograph courtesy of Utah

Figure 38.

Geological Survey.)

46   The Landslide Handbook­—A Guide to Understanding Landslides

Technological Tools for Evaluation of Landslides—

Mapping, Remote Sensing, and Monitoring

the past is the key to the

One of the guiding principles of geology is that

future. In evaluating landslide hazards this means that future slope failures could

occur as a result of the same geologic, geomorphic, and hydrologic situations that

led to past and present failures. Based on this assumption, it is possible to estimate

the types, frequency of occurrence, extent, and consequences of slope failures that

may occur in the future. However, the absence of past events in a specific area does

not preclude future failures. Human-induced conditions, such as changes in the natu-

ral topography or hydrologic conditions, can create or increase an area’s susceptibil-

ity to slope failure.

In order to predict landslide hazards in an area, the conditions and processes

that promote instability must be identified and their relative contributions to slope

failure estimated, if possible. Useful conclusions concerning increased probability

of landsliding can be drawn by combining geological analyses with knowledge of

short- and long-term meteorological conditions. Current technology enables persons

monitoring earth movements to define those areas most susceptible to landsliding

and to issue warnings and “alerts” covering time spans of hours to days when

meteorological conditions or thresholds known to increase or initiate certain types of

landslides are met.

Map Analysis

Map analysis is usually one of the first steps in a landslide investigation. Neces-

sary maps include bedrock and surficial geology, topography, soils, and if available,

geomorphology maps. Using knowledge of geologic materials and processes, a

trained person can obtain a general idea of landslide susceptibility from such maps.

Appendix B at the end of this report contains a section on the various types of maps

used in landslide analysis.

Aerial Reconnaissance

Analysis of aerial photography is a quick and valuable technique for identify-

ing landslides, because it provides a three-dimensional overview of the terrain and

indicates human activities as well as much geologic information to a trained person.

In addition, the availability of many types of aerial imagery (satellite, infrared, radar,

and so forth) makes aerial reconnaissance very versatile although cost-prohibitive in

some cases.

Field Reconnaissance

Many of the more subtle signs of slope movement cannot be identified on maps

or photographs. Indeed, if an area is heavily forested or has been urbanized, even

major features may not be evident. Furthermore, landslide features change over time

on an active slide. Thus, field reconnaissance is always mandatory to verify or detect

landslide features, and to critically evaluate the potential instability of vulnerable

slopes. It identifies areas with past landslides (which could indicate future likelihood

of landslides) by using field mapping and laboratory testing of terrain through the

sampling of soil and rock. Mapping and laboratory testing for example, may identify

vulnerable clays or other susceptible soils and show where they exist and their size

and extent. Part A. Evaluating Landslide Hazards   47

Drilling

At most sites, drilling is necessary to determine the type of earth materi-

als involved in the slide, the depth to the slip surface, and thus the thickness and

geometry of the landslide mass, the water-table level, and the degree of disruption

of the landslide materials. It also can provide suitable samples for age-dating and

testing the engineering properties of landslide materials. Finally, drilling is needed

for installation of some monitoring instruments and hydrologic observation wells.

Note that drilling for information on stratigraphy, geology, water-table levels and for

installation of instruments, for example, is also done for areas that have never had a

landslide but for which the possibility exists.

Instrumentation

Sophisticated methods such as electronic distance measurement (EDM), instru-

ments such as inclinometers, extensometers, strain meters, and piezometers (see

Glossary for definitions of these instruments), and simple techniques, such as estab-

lishing control points by using stakes can all be used to determine the mechanics of

landslide movement and to monitor and warn against impending slope failure.

Geophysical Studies

Geophysical techniques (measurement of soil’s electrical conductivity/

resistivity, or measurement of induced seismic behavior) can be used to determine

some subsurface characteristics such as the depth to bedrock, stratigraphic layers,

zones of saturation, and sometimes the ground-water table. It can also be used to

determine texture, porosity, and degree of consolidation of subsurface materials and

the geometry of the units involved. In most instances, such surface survey meth-

ods can best be used to supplement drilling information, spatially extending and

interpolating data between boreholes. They can also offer an alternative if drilling

is impossible. Downhole geophysical methods (nuclear, electrical, thermal, seis-

mic) also can be applied to derive detailed measurements in a borehole. Monitor-

ing of natural acoustic emissions from moving soil or rock has also been used in

landslide studies.

Acoustic Imagery and Profiles

Profiles of lakebeds, river bottoms, and the sea floor can be obtained using

acoustic techniques such as side-scan sonar and subbottom seismic profiling.

Surveying of controlled grids, with accurate navigation, can yield three-dimen-

sional perspectives of subaqueous geologic phenomena. Modern, high-resolution

techniques are used routinely in offshore shelf areas to map geologic hazards for

offshore engineering.

Computerized Landslide Terrain Analysis

In recent years, computer modeling of landslides has been used to determine

the volume of landslide masses and changes in surface expression and cross section

over time. This information is useful in calculating the potential for stream blockage,

cost of landslide removal (based on volume), and type and mechanism of move-

ment. Very promising methods are being developed that use digital elevation models For further reading:

(DEMs) to evaluate areas quickly for their susceptibility to landslide/debris-flow References 4, 15, 18, 24, 25, 39,

events. Computers also are being used to perform complex stability analyses. Soft- and 46

ware programs for these studies are readily available for personal computers.

See Appendix B for more information and images of map types.

48   The Landslide Handbook­—A Guide to Understanding Landslides

Part B. Communicating Landslide Hazard

The successful translation of natural hazard information into a form useful for

nontechnical users conveys the following three elements in one form or another:

• Likelihood of the occurrence of an event of a size and in a location that

would cause casualties, damage, or disruption to an existing standard

of safety.

• Expected location and extent of the effects of the event on the ground,

structures or socioeconomic activity.

• Estimated severity of the effects on the ground, structures, or socioeconomic

activity.

These elements are needed because engineers, planners, and decisionmakers

usually will not be concerned with a potential hazard if its likelihood is rare, its loca-

tion is unknown, or its severity is slight.

Unfortunately, these three pieces of information can come in different forms

with many different names, some quantitative and precise, others qualitative and

general. For a product to qualify as useful hazard information, the nontechnical user

must be able to perceive likelihood, location, and severity of the hazard so that they

become aware of the danger, can convey the risk to others, and can use the translated

information directly to reduce a threat.

Safety Information

Safety is no doubt the first order of business for managers and municipal

officials. People living in areas prone to fast-moving, deadly debris flows need

information on the likelihood of the hazard; for example, when it is most dangerous

to be in the path of potential debris flows (such as during heavy rainstorms) and at

what point to evacuate and (or) cease walking or driving in a hazardous area. Safety

information about slow-moving landslides is equally important as these types of

landslides can damage and (or) break electrical and gas lines, creating an additional

hazard of fire, electrocution, and gas fumes.

Building and Construction Information

This information is also valuable to communities, so that some of the causes

of landslides might be avoided. This information is discussed in Section III,

“Mitigation Concepts and Approaches.”

Safety, education, and building information can be made available to com-

munity residents in various ways. A list of building codes, building inspection

processes, and potential areas where destabilization may cause landslide problems

can be made available through the following:

Part B. Communicating Landslide Hazard   49

Suggested Local Government Outreach for Landslide Hazard

• Newspaper bulletins/advertisements.

• Public-service brochures distributed door-to-door or displayed in public places.

• Community meeting discussions.

• Posters in public buildings and (or) marketplaces with as much visual information as possible.

• Media announcements through radio, television, loudspeakers, or other means.

• Public lectures by experts or other officials.

• Signs posted in immediate areas of hazards, informing people of the kind of hazard and warning them to

be cautious. An example of this would be a sign warning of rockfall hazards along well-used footpaths.

• In areas where literacy rates are low, oral communications with graphics, photographs, and illustrations

of hazards can be extremely effective. Pictures may take the place of a thousand words!

• Telephone book—In areas where phone service is widespread or accessible, municipal listings for

engineering, emergency planning, and police or fire departments.

• Where possible a municipal Internet Web site is a useful source of safety information and contact phone

numbers and emails for emergency personnel and engineering and (or) planning departments.

• Determine local landslide hazard problems through a working committee and (or) secure professional

advice. State/Provincial or Federal geological surveys, university geology or engineering departments,

and private geotechnical companies are sources of advice. Provide for a mapping program where

possible, either within local government or contracted with professionals.

• Conduct public education and information programs through community meetings, city council, or

other councils.

• Adopt and enforce appropriate land-use policies—discuss with landowners, developers, buyers, and

sellers. One option is to require disclosure of geological hazards during property sales to ensure that the

new buyer is aware of any problems.

• Monitor changes in unstable slopes and take appropriate actions (see “Mitigation” section of this

handbook).

• Construct street and drainage projects that meet local safety needs and ordinances.

• Pursue public grant programs, government programs for infrastructure, and public works

improvement projects.

• Be informed about insurance programs available and liability issues and know where local government

responsibility lies for public safety and well-being.

• Have an emergency response plan for the community. Consult with neighboring towns and (or)

communities that have plans and have used them in an emergency. Evaluate their effectiveness for your

own situation.

50   The Landslide Handbook­—A Guide to Understanding Landslides

Examples of Hazard Warning Signs

The following figures (figs. 39, 40, and 41) show examples of some simple

warning signs that can be placed in hazardous areas. This information can also be

For further reading: used in emergency management policies for municipalities and (or) communities.

References 1, 6, 19, 21, 23, 24, 26, Please see Appendix B for samples of basic safety information for debris flow

36, and 41 and other landslide hazards that are suitable for posting and distributing in public

places. Example of a

Figure 39.

rockfall hazard sign. A notice for cliff

Figure 40.

hazards, city of Wanneroo,

Australia.

Sign along a highway in Virginia, USA.

Figure 41.

Section III.

Mitigation Concepts and Approaches

Vulnerability to landslide hazards is a function of a site’s location (topog-

raphy, geology, drainage), type of activity, and frequency of past landslides. The

effects of landslides on people and structures can be lessened by total avoidance

of landslide hazard areas or by restricting, prohibiting, or imposing conditions

on hazard-zone activity. Local governments can accomplish this through land-

use policies and regulations. Individuals can reduce their exposure to hazards by

educating themselves on the past hazard history of a desired site and by making

inquiries to planning and engineering departments of local governments. They

could also hire the professional services of a geotechnical engineer, a civil engi-

neer, or an engineering geologist who can properly evaluate the hazard potential of

a site, built or unbuilt.

52   The Landslide Handbook­—A Guide to Understanding Landslides

Part A. Overview of Mitigation Methods for Various

Types of Landslide Hazards

Seeking the advice of professionals is always advised where possible, but

managers and homeowners should be educated about mitigation in order to make

informed decisions concerning construction and land use. A few of these measures

are discussed in this section. More detailed information on landslide mitigation is

available in Appendix C and in Turner and Schuster (1996) ( Reference 39).

The simplest means of dealing with landslide hazards is to avoid construction

on steep slopes and existing landslides; however, this is not always practical. Regu-

lating land use and development to ensure that construction does not reduce slope

stability is another approach. Avoidance and regulation rely on landslide maps and

the underlying definitions of landslide areas to reduce hazard (Appendix B). In cases

where landslides affect existing structures or cannot be avoided, physical controls

can be used. In some cases, monitoring and warning systems (Appendix B) allow

residents to evacuate temporarily during times when the probability of landslide

activity is high.

Soil Slope Stabilization

Stability increases when ground water is prevented from rising in the slide

mass by

• directing surface water away from the landslide,

• draining ground water away from the landslide to reduce the potential for a

rise in ground-water level,

• covering the landslide with an impermeable membrane, and (or)

• minimizing surface irrigation. Slope stability is also increased when weight

or retaining structures are placed at the toe of the landslide or when mass

(weight) is removed from the head of the slope.

Planting or encouraging natural growth of vegetation can also be an effective means

of slope stabilization—this is further discussed in the section on biotechnical mitiga-

tion methods and Appendix C.

An example of one means of slope stabilization is the use of retaining walls.

Retaining walls are structures built to support a soil mass permanently. They also are

used whenever space requirements make it impractical to slope the side of an exca-

vation, or to prevent sloughing of loose hillslope soils onto roads or property. Retain-

ing walls are also used to prevent or minimize toe erosion by river scour or to retard

creep. They cannot, however, be used to stop landslides from occurring. Several

basic types of wall are timber crib, steel bin, pile, cantilever, sheet pile, plastic mesh,

and reinforced earth. Each of these types has advantages in certain situations, but

cost is usually what determines which is type is adopted. More information about

retaining walls is given in Appendix C.

See Appendix C for more information on stabilization methods.

Part A. Overview of Mitigation Methods for Various Types of Landslide Hazards   53

Rockfall Hazard Mitigation

Rockfall is common in areas of the world with steep rock slopes and cliffs.

Commonly, these are mountainous or plateau areas, whether in coastal areas or

among isolated rock formations. Rockfall causes extraordinary amounts of monetary

damage and death, the former mostly by impeded transportation and commerce due

to blocked highways and waterways and the latter as direct casualties from falling

rocks. Diverting paths and highways around rockfall areas is sometimes imple-

mented but is not always practical. Many communities post danger signs around

areas of high rockfall hazard. Some methods of rockfall hazard mitigation include

catch ditches, benches, scaling and trimming, cable and mesh, shotcrete, anchors,

bolts, dowels, and controlled blasting.

See Appendix C for more information on mitigation means for preventing and

diverting rockfall.

Debris-Flow Hazard Mitigation

Due to the speed and intensity of most debris flows, they are very hard to stop

once they have started. However, methods are available to contain and deflect debris

flows primarily through the use of retaining walls and debris-flow basins. Other mit-

igation methods include modifying slopes (preventing them from being vulnerable

to debris-flow initiation through the use of erosion control), revegetation, and the

prevention of wildfires, which are known to intensify debris flows on steep slopes.

See Appendix C for more information on methods for debris-flow hazard

mitigation.

Landslide Dam Mitigation

Many problems arise when landslides dam waterways. Dams caused by

landslides are a common problem in many areas of the world. Landslides can occur

on the valley walls of streams and rivers. If enough displaced material (rock, soil,

and (or) debris) fills the waterway, the landslide will act as a natural dam, block-

ing the flow of the river and creating flooding upstream. As these natural dams

are frequently composed of loose, unconsolidated material, they commonly are

inherently weak and are soon overtopped and fail due to erosion. When breach-

ing happens quickly, the backed-up water rushes down the waterway, potentially

causing catastrophic downstream flooding. An example of a landslide dam is the

600-meter-high Usoi landslide dam in Tajikistan, one of the largest landslide dams in

the world. A large earthquake-induced landslide dammed the Murghab River, creat-

ing Lake Sarez. The dam poses a hazard for people living downstream. Also, future

seismic action may cause more landslides to slide into the dammed lake, causing a

seiche (a tsunami-like wave in a closed water basin), which may weaken and (or)

overtop the landslide dam. Figure 42 shows a landslide dam caused by the sliding of

saturated slopes, and figure 43 shows a landslide dam caused by an earthquake.

See Appendix C for more details on mitigation methods for landslide dams.

54   The Landslide Handbook­—A Guide to Understanding Landslides

The Thistle landslide in Utah, USA. This 1983 landslide dammed the Spanish Fork River, backing up water that flooded the

Figure 42.

town of Thistle. Many landslide dams are much smaller than the one shown here and potentially can be overtopped by backed-up

water, or eroded through. Some are much larger, and roads and railroad lines that are blocked or damaged must be diverted around

the landslide mass. The concrete tunnel at the lower part of the bottom photograph shows where the rail line was rerouted around the

Thistle slide and excavated through an adjacent mountain.

Part A. Overview of Mitigation Methods for Various Types of Landslide Hazards   55

The great earthquake that

Figure 43.

struck China on May 12, 2008, caused

extensive damage in the mountainous

terrain of Beichuan County. In many

cases, landslides in steep valleys formed

landslide dams, creating new lakes

in a period of hours. This pair of high-

resolution, photo-like images from Taiwan’s

Formosat-2 satellite on May 14, 2006 (top),

and May 14, 2008 (bottom), before and after

the earthquake, show the large landslide

that blocked the Jiangjian River, forming a

dangerous landslide-dammed lake.

Methods of Biotechnical Landslide Mitigation

This type of slope protection is used to reduce the adverse environmental con-

sequences of landslide-mitigation measures. When used for landslide remediation or

mitigation, conventional earth-retaining structures made of steel or concrete usually

are not visually pleasing or environmentally friendly. These traditional “hard” reme-

dial measures are increasingly being supplanted by vegetated composite soil/structure

bodies that are environmentally more friendly; that is, a process that has come to be

known as biotechnical slope protection. Common biotechnical systems include nets

of various materials anchored by soil nails that hold in place soil seeded with grass.

Research has been done on using plants to stabilize soil to prevent excessive erosion

and also to mitigate the effect of landslides. One of the most promising types of plants

is Vetiver, a type of grass that works very well to stabilize slopes against erosion in

many different environments. See Appendix C for more information on Vetiver grass

uses and its geographical suitability.

See Appendix C for more information on mitigation techniques.

56   The Landslide Handbook­—A Guide to Understanding Landslides

Part B. Simple Mitigation Techniques for Home and

Businesses, Managers, and Citizens

There are simple and low-technology means for homeowners and others to

implement methods and techniques that are effective and lessen the effects of

landslides. First, it is always best to consult a professional, such as a geotechnical

engineer or a civil engineer, as they have had the training and experience to solve

instability problems; a local company or professional may be the best, as they may

be familiar with the geology, soil types, and geography of the area in question. This

is not always the case, but it is a basis for making inquiries. When there are local

jurisdictions such as county and (or) city municipal offices, individuals within these

institutions may be professional geologists, planners, and (or) building experts who

can answer questions, provide maps, and explain building regulations and inspec-

tion procedures. Access to these types of officials varies widely around the world,

and local situations may be handled differently. When consulting a professional is

not possible, some steps can be taken in the meantime, as detailed in Appendixes C

and D.

For further reading: See Appendixes C and D for detailed information on mitigation techniques for

References 4, 8, 11, 19, 20, 28, 30, property owners, citizens, and managers.

31, 32, and 37 Part C. List of Works Consulted/Cited/Quoted and for Further Reading   57

Part C. List of Works Consulted/Cited/Quoted and for Further Reading

1. A 7. Case, William F., 2003, Debris-flow 16. Highland, Lynn, 2004, Landslide types

dvisory Committee on the Inter- hazards: Utah Geological Survey, and processes: U.S. Geological Survey

national Decade for Natural Hazard Public Information Series 70. Online: Fact Sheet FS–2004–3072. Online:

Reduction,Commission on Engineering http://geology.utah.gov/online/pi-70/ http://pubs.usgs.gov/fs/2004/3072/

and Technical Systems, 1987, Confront- debrisflow.htm

ing natural disasters, an International 17. Jackson, Julia A., ed., 1997, Glossary of

Decade for Natural Hazard Reduc- 8. Case, William F., 2000, Rock-fall geology, fourth edition: American Geo-

tion, National Research Council: U.S. hazards: Utah Geological Survey, Public physical Institute, Alexandria, Virginia,

National Academy of Sciences, Wash- Information Series 69. Online: USA, 769 p.

http://

ington, D.C. geology.utah.gov/online/pdf/pi-69.pdf 18. Jibson, Randall W., Harp, Edwin L., and

2. Aylsworth, J.M., Duk-Rodkin, A., 9. Cruden, D.M, and Varnes, D.J., 1996, Michael, John A., 1998, A method for

Robertson, T., and Traynor, J.A., 2000, Landslide types and processes, producing digital probabilistic seismic

in

Landslides, in the physical environment Turner, A. Keith, and Schuster, Robert landslide hazard maps—An example

of the Mackenzie Valley, Northwest L. eds. Landslides—Investigation and from the Los Angeles, California, area:

Territories—A baseline for the assess- mitigation: Transportation Research U.S. Geological Survey Open-File

ment of environmental change, Dyke, Board, Special report no. 247, National Report 98–113, 17 p. Online: http://

L.D., and Brooks, G.R., eds.: Geologi- Research Council, National Academy pubs.usgs.gov/of/1998/ofr-98-113/

cal Survey of Canada, Bulletin no. 547, Press, Washington, D.C., p. 36–75.

p. 41–48. 19. Jochim, Candice, Rogers, William

10. Cruden, D.M., 1993, The multilingual P., Truby, John O., Wold, Jr., Robert

3. Barrows, Alan, and Smith, Ted, landslide glossary: Richmond., British L., Weber, George, and Brown, Sally

2004, Hazards from “mudslides,” Columbia, Canada, Bitech Publishers,for P., 1988, Colorado landslide hazard

debris avalanches and debris flows the UNESCO Working Party on World mitigation plan: Department of Natural

in hillside and wildfire areas: Cali- Landslide Inventory, 1993. Resources, Colorado Geological Survey,

fornia Geological Survey Note 33. Denver, Colo., USA.

Online: http://www.consrv.ca.gov/cgs/ 11. Chatwin, S.C., Howes, D.E., Schwab,

information/publications/cgs_notes/ J.W., and Swanston, D.N., 1994, A 20. Los Angeles County Department of

note_33/index.htm guide for management of landslide- Public Works, Board of Supervisors,

prone terrain in the Pacific Northwest, 1993, Homeowner’s guide for flood,

4. Blake, T.F., Holingsworth, R.A., and second edition: Ministry of Forests, debris, and erosion control: Alhambra,

J.P. Stewart, eds., 2002, Recommended 31 Bastion Square, Victoria, British California, in English and Spanish.

procedures for implementation of Columbia V8W3E7, 220 p. Online: Online: http://dpw.lacounty.gov/wmd/

guidelines for analyzing and mitigat- http://www.for.gov.bc.ca/hfd/pubs/docs/ Homeowners/index.cfm

ing landslide hazards in California: Lmh/Lmh18.htm

Department of Mining and Geology 21. McInnes, Robin, 2000, Managing

special publication 117 American 12. Creath, W.B., 1996. Homebuyers’ guide ground instability in urban areas, a

Society of Civil Engineers (ASCE), to geologic hazards—An AIPG issues guide to best practice, Centre for the

Los Angeles Section Geotechnical and answers publication: Department Coastal Environment, Isle of Wight

Group, published by Southern California of Natural Resources, Denver, Colo- Council: United Kingdom, Cross

Earthquake Center (SCEC). Online: rado Geological Survey, Miscellaneous Publishing, Walpen Manor, Chale,

http://www.scec.org/resources/catalog/ Publication (MI) no. 58, 30 p. Isle of Wight.

LandslideProceduresJune02.pdf 13. Fleming, Robert W., and Taylor, Fred 22. National Research Council, 1993,

5. California Department of Conserva- A., 1980, Estimating the costs of land- Vetiver grass—A thin green line against

tion, Division of Mines and Geology, slide damage in the United States: U.S. erosion: National Academy Press,

1997, Factors affecting landslides in Geological Survey Circular 832, 21p. Washington, D.C. Online:

forested terrain, Note 50. Online: http:// http://www.vetiver.org

www.consrv.ca.gov/cgs/information/ 14. Gray, D.H., and Sotir, R.B., 1996, Bio-

publications/cgs_notes/note_50/ technical and soil bioengineering slope 23. Nichols, Donald R., and Catherine C.

Documents/note50.pdf stabilization—A practical guide for Campbell, eds., 1971, Environmental

erosion control: New York, John Wiley, planning and geology: U.S. Department

6. Case, William F., (no date) Landslides— 378 p. of Housing and Urban Development,

What they are, why they occur: Utah U.S. Department of the Interior, U.S.

Geological Survey, Utah Department of 15. Haugerud, Ralph A., Harding, David J., Government Printing Office.

Natural Resources, Public Information Johnson, Samuel Y., Harless, Jerry L.,

Series 74. Online: http://geology.utah. Weaver, Craig S., and Brian L. Sherrod,

gov/online/pdf/pi-74.pdf 2003, High-resolution LiDAR topog-

raphy of the Puget Lowland, Washing-

ton—A Bonanza for earth science: GSA

Today, Geological Society of America,

p. 4–10.

58   The Landslide Handbook­—A Guide to Understanding Landslides

24. Norheim, Robert A., Queija, Vivian R., 33. Schwab, J.C., Gori, P.L., and Jeer, S., 41. U.S. Geological Survey, 2005,

and Haugerud, Ralph A., 2002, Com- eds., 2005, Landslide hazards and Monitoring ground deformation

parison of LiDAR and InSAR DEMs planning: American Planning Associa- from space: U.S. Geological Survey

with dense ground control: Proceed- tion Planning Advisory Service Report Fact Sheet FS–2005–3025. Online:

ings, Environmental Systems Research no. 533/534. http://volcanoes.usgs.gov/insar/public_

Institute 2002 User Conference. Online: files/InSAR_Fact_Sheet/2005-3025.pdf

34. Shelton, David C., and Prouty, Dick,

http://gis.esri.com/library/userconf/ 1979, Nature’s building codes, geology 42. Utah Geological Survey, 2003, Home

proc02/pap0442/p0442.htm and construction in Colorado: Depart- owner’s guide to recognizing and reduc-

25. Nuhfer, Edward B., Proctor, Richard J., ment of Natural Resources, Colorado ing landslide damage on their property:

and Moser, Paul H., 1993. The citizen’s Geological Survey Bulletin 48, 72 p. Public Information Series no. 58.

guide to geologic hazards: The Ameri- Online: http://geology.utah.gov/online/

35. Soeters R., and van Westen, C.J., 1996,

can Institute of Professional Geologists, pi-58/index.htm

Slope instability recognition, analysis,

134 p. and zonation, in Turner, A.K.,and Schus- 43. Varnes, D.J., 1978, Slope movement

26. Olshansky, Robert B., 1996, Planning ter, R.L. eds., Landslides—Investigation types and processes, Schuster, R.L.,

in

for hillside development: American and mitigation: Transportation Research and Krizek, R.J., eds., Landslides—

Planning Association (APA), Planning Board Special Report 247, National Analysis and control: Transportation

Advisory Service Report no. 466, 50 p. Research Council, Washington, D.C., Research Board Special Report 176,

p. 129–177. National Research Council, Washington,

27. Pelletier, B.R., ed., 2000, Environmen- D.C., p. 11–23.

tal atlas of the Beaufort coastlands, 36. Solomon, Barry J., 2001, Using geologic

supplement to the Marine Science Atlas hazards information to reduce risks 44. Weber, G., Von Schulez, W., and Czer-

of the Beaufort Sea: Geological Survey and Losses—A guide for local gov- niak, R., 1983, Flood hazard manage-

of Canada, Natural Resources Canada. ernments: Utah Geological Survey, ment plan for the Sheridan watershed

Online: http://gsc.nrcan.gc.ca/beaufort/ Public Information Series 75. Online: area: Sheridan, Wyoming, Geographic

index_e.php Applications and Research Group,

http://geology.utah.gov/online/pdf/ Boulder, Colorado.

pi-75.pdf

28. Reid, Mark, and Ellis, William L.,

1999, Real-time monitoring of active 37. Swanston, D., ed., 1985, Proceedings of 45. Wieczorek, Gerald F., 1996, Landslide

landslides: U.S. Geological Survey a workshop on slope stability—Prob- triggering mechanisms, Turner,

in

Fact Sheet FS–091–99. Online: lems and solutions in forest manage- A. Keith, and Schuster, Robert L.,

http://pubs.usgs.gov/fs/fs-091-99/ ment: USDA Forest Service General eds., Landslides—Investigation and

Technical Report PNW–180, Pacific mitigation: Transportation Research

29. Rickenmann, Dieter, and Cheng-lung Northwest Forest and Range Experimen- Board, Special report no. 247, National

Chen, eds., 2003, Debris-flow hazards tal Station, Portland, Oregon, 122 p. Research Council, National Academy

Mitigation—Mechanics, prediction, Press, Washington, D.C., p. 76–90.

and assessment: Millpress, Rotterdam, 38. Swanston, D.N., 1983, Assessment of

The Netherlands. mass erosion risk from forest operations 46. Wold, Robert L., and Jochim, Candace

in steep terrain: International Associa- L.,1989, Landslide loss reduction—A

30. Schuster, Robert L., and Highland, tion of Forestry Research Organizations guide for state and local government

Lynn M., 2004, Impact of landslides and Congress, Division 3, Forest Operations planning: Special Publication 33,

innovative landslide-mitigation measures and Techniques, Munich, Germany, Department of Natural Resources,

on the natural environment: International 1982, Proceedings. Colorado Geological Survey, Denver,

Conference on Slope Engineering, Hong Colo., 50 p.

Kong, China, December 8–10, 2003, 39. Turner, A. Keith, and Schuster, Robert

keynote address, Proceedings 29. L., 1996, Landslides— Investigation and 47. Yoon, P.K., 1994, Important biological

mitigation: National Research Council, considerations in use of Vetiver grass

31. Schuster, R.L., 2004, Risk-reduction Transportation Research Board Special hedgerows (VGHR) for slope protec-

measures for landslide dams, Security

in Report 247, National Academy Press, tion and stabilisation, Vegetation and

in

of natural and artificial rockslide dams: Washington, D.C., 673 p. slopes— Stabilisation, protection and

Extended Abstracts Volume, NATO ecology: Proceedings, International

Advanced research Workshop on 40. United States Agency for International Conference Institution of Civil Engi-

Landslide Dams, Bishkek, Kyrgyzstan, Development, Bureau for Humanitarian neers, University Museum, Oxford,

June 8–13, p.170–176 [theme keynote Response, Office of Foreign Disaster September 29–30, 1994, Thomas

paper]. Assistance, 1998, Field operations Telford, London, p. 212–221.

guide for disaster assessment and

32. Schuster, Robert L., and Highland, response: U.S. Government Printing

Lynn M., 2001, Socioeconomic Office. Online: http://www.usaid.gov/

effects of landslides in the west- our_work/humanitarian_assistance/

ern hemisphere: U.S. Geological disaster_assistance/resources/pdf/

Survey Open-File Report 2001–0276. fog_v3.pdf

Online: http://pubs.usgs.gov/of/2001/

ofr-01-0276/

Appendix A.

Basic Information about Landslides

60   The Landslide Handbook­—A Guide to Understanding Landslides

Part 1. Glossary of Landslide Terms

F ull references citations for glossary are at the end of the list.

alluvial fan Digital Terrain Model (DTM) Geographic Information System (GIS)

An outspread, gently sloping The term used A

by United States Department of Defense and computer program and associated data bases

mass of alluvium deposited by a stream, other organizations to describe digital eleva- that permit cartographic information (includ-

especially in an arid or semiarid region tion data. (Reference 3) ing geologic information) to be queried

where a stream issues from a narrow canyon by the geographic coordinates of features.

onto a plain or valley floor. Viewed from drawdown Lowering of water levels in riv- Usually the data are organized in “layers”

above, it has the shape of an open fan, the ers, lakes, wells, or underground aquifers due representing different geographic entities

apex being at the valley mouth. (Reference 3) to withdrawal of water. Drawdown may leave such as hydrology, culture, topography, and

unsupported banks or poorly packed earth

bedding surface/plane In sedimentary or so forth. A geographic information system,

that can cause landslides. (Reference 3)

stratified rocks, the division planes that sepa- or GIS, permits information from different

rate each successive layer or bed from the electronic distance meter (EDM) A device layers to be easily integrated and analyzed.

one above or below. It is commonly marked that emits ultrasonic waves that bounce off (Reference 3)

by a visible change in lithology or color. solid objects and return to the meter. The geologic hazard A geologic condition,

(Reference 3) meter’s microprocessor then converts the either natural or manmade, that poses

elapsed time into a distance measurement.

bedrock The solid rock underlying gravel, a potential danger to life and property.

Sound waves spread 1 foot wide for every

sand, clay, and so forth; any solid rock Examples: earthquake, landslides, flooding,

10 feet measured. There are various types

exposed at the surface of the earth or overlain faulting, beach erosion, land subsidence,

available.

by unconsolidated superficial material. pollution, waste disposal, and foundation and

(Reference 3) epicenter The point on the Earth’s surface footing failures. (Reference 3)

directly above the focus of an earthquake.

borehole A circular hole drilled into the geologic map A map on which is recorded

(Reference 3)

earth, often to a great depth, as a prospec- the distribution, nature, and age relationships

tive oil, gas, or water well or for exploratory expansive soils Types of soil that shrink of rock units and the occurrence of structural

purposes. (Reference 3) or swell as the moisture content decreases or features. (Reference 3)

increases. Structures built on these soils may

check dams Check dams are small sedi- geomorphology The science that treats the

shift, crack, and break as soils shrink and

ment storage dams built in the channels of general configuration of the Earth’s surface;

subside or expand. Also known as swelling

steep gullies to stabilize the channel bed. A specifically, the study of the classification,

soils. (Reference 5)

common use is to control channelized debris- description, nature, origin, and develop-

flow frequency and volume. Check dams are extensometer An instrument for measur- ment of landforms and their relationships to

expensive to construct and are therefore usu- ing small deformations, as in tests of stress. underlying structures, and the history of geo-

ally only built where important installations (Reference 3) logic changes as recorded by these surface

or natural habitat (such as a camp or unique factor of safety features. (Reference 3)

The factor of safety, also

spawning area) lies downslope. (Reference 2) known as Safety Factor, is used to provide geophysical studies The science of the

colluvium A general term applied to loose a design margin over the theoretical design Earth, by quantitative physical methods,

and incoherent deposits, usually at the foot of capacity to allow for uncertainty in the with respect to its structure, composition,

a slope or cliff and brought there chiefly by design process. The uncertainty could be any and development. It includes the sciences of

gravity. (Reference 2) one of a number of the components of the dynamical geology and physical geography

design process including calculations and

(sometimes called catch

debris basin and makes use of geodesy, geology, seismol-

material strengths for example. Commonly,

basins) A large excavated basin into which ogy, meteorology, oceanography, magnetism,

a factor of safety of less than 1, for instance,

a debris flow runs or is directed and where it and other Earth sciences in collecting and

on an engineered slope indicates potential

quickly dissipates its energy and deposits its interpreting Earth data. (Reference 3)

failure, where a factor of safety of greater

load. Abandoned gravel pits or rock quarries in

hydraulic Of or pertaining to fluids

than 1, indicates stability. (Reference 6)

are often used as debris basins. (Reference 3). motion; conveying, or acting, by water;

geodesic/geodetic measurements The

delta-front landsliding Delta fronts are operated or moved by means of water, as

investigation of any scientific questions con-

where deposition in deltas is most active— hydraulic mining. (Reference 3)

nected with the shape and dimensions of the

underwater landsliding along coastal and hydrology The science that relates to the

Earth. (Reference 3)

delta regions due to rapid sedimentation of water of the Earth. (Reference 3)

loosely consolidated clay, which is low in fracture Brittle deformation due to a inclinometer

strength and high in pore-water pressures. Instrument for measuring

momentary loss of cohesion or loss of resis- inclination to the horizontal. (Reference 3)

tance to differential stress and a release of

Digital Elevation Model (DEM) A digital stored elastic energy. Both joints and faults landslide dam

elevation model (DEM) is a digital file con- An earthen dam created

are fractures. (Reference 3)

sisting of terrain elevations for ground posi- when a landslide blocks a stream or river.

tions at regularly spaced horizontal intervals. (Reference 3)

(A commercial definition – new technology) Part 1. Glossary of Landslide Terms   61

lahar mudslide sag pond

Landslide, debris flow or mudflow, of An imprecise but popular term A small body of water occupying

an enclosed depression or sag formed where

pyroclastic material on the flank of a volcano; coined in California, USA, frequently used

deposit produced by such a debris flow. active or recent fault or landslide movement

by the general public and the news media

Lahars are described as wet if they are mixed has impounded drainage. (Reference 3)

to describe a wide scope of events, ranging

with water derived from heavy rains, escaping from debris-laden floods to landslides. Not seepage Concentrated subsurface drain-

from a crater lake, or produced by melting technically correct. Please see “mudflow,” age indicated by springs, sag ponds, or moist

snow. Dry lahars may result from tremors of next Glossary entry. (Reference 5) areas on open slopes, and seepage sites along

a cone or by accumulating material becom- road cuts. The locations of these areas of con-

mudflow A general term for a mass-move-

ing unstable on a steep slope. If the material centrated subsurface flow should be noted on

ment landform and process characterized by

retains much heat, it is termed a hot lahar. maps and profiles as potential sites of active,

a flowing mass of predominately fine-grained

(Reference 3) unstable ground. (Reference 2)

earth material possessing a high degree of

liquefaction The transformation of satu- fluidity during movement. The water content sea cliff retreat A cliff formed by wave

rated, loosely packed, coarse-grained soils may range up to 60 percent. (Reference 3) action, causing the coastal cliff to erode and

from a solid to a liquid state. The soil grains recede toward land. (Reference 3)

perched ground water Unconfined ground

temporarily lose contact with each other, and water separated from an underlying main shear A deformation resulting from stresses

the particle weight is transferred to the pore body of ground water by an unsaturated that cause contiguous parts of a body to slide

water. (Reference 4) zone. (Reference 3) relative to each other in a direction parallel to

landslide inventory maps Inventories their plane of contact. (Reference 3)

piezometer An instrument for measuring

identify areas that appear to have failed by pressure head in a conduit, tank, or soil—it is slurry A highly fluid mixture of water and

landslide processes, including debris flows a small diameter water well used to mea- finely divided material; for example, pulver-

and cut-and-fill failures. (Reference 4) sure the hydraulic head of ground water in ized coal and water for movement by pipeline

landslide susceptibility map This map goes aquifers. (Reference 3) or of cement and water for use in grouting.

beyond an inventory map and depicts areas (Reference 3)

pore-water pressure A measure of the

that have the potential for landsliding. These pressure produced by the head of water in soil mechanics The application of the

areas are determined by correlating some of a saturated soil and transferred to the base principles of mechanics and hydraulics to

the principal factors that contribute to land- of the soil through the pore water. This is engineering problems dealing with the behav-

sliding, such as steep slopes, weak geologic quantifiable in the field by the measure- ior and nature of soils, sediments, and other

units that lose strength when saturated, and ment of free water-surface level in the soil unconsolidated accumulations; the study of

poorly drained rock or soil, with the past or by direct measurement of the pressure by the physical properties and utilization of soils,

distribution of landslides. (Reference 5) means of piezometers. Pore-water pressure especially in relation to highway and founda-

landslide hazard map Hazard maps show is a key factor in failure of a steep slope tion engineering. (Reference 3)

the areal extent of threatening processes: soil and operates primarily by reducing the strainmeter A seismometer that is designed

where landslide processes have occurred in weight component of soil shear strength. to detect deformation of the ground by mea-

the past, where they occur now, and the likeli- (Reference 2) suring relative displacement of two points.

hood in various areas that a landslide will pore water, or interstitial water Subsurface (Reference 3)

occur in the future. (Reference 5) water in an interstice, or pore. (Reference 3) stress In a solid, the force per unit area,

landslide risk map Landslide hazards and quick clay A clay that loses nearly all its acting on any surface within it, and variously

the probability that they will occur, expressed shear strength after being disturbed; a clay expressed as pounds or tons per square inch,

in statistical recurrence rates; risk maps may that shows no appreciable gain in strength or dynes or kilograms per square centimeter;

show cost/benefit relationships, loss potential after remolding. (Reference 3) also, by extension, the external pressure that

and other potential socioeconomic effects on creates the internal force. (Reference 3)

an area and (or) community. reconnaissance geology/mapping A (German language term for “fall

sturzstroms

general, exploratory examination or survey

lithology The physical character of a rock, stream”)

of the main features of a region, usually A huge mass of rapidly mov-

generally as determined at the microscopic preliminary to a more detailed survey. It ing rock debris and dust, derived from the

level, or with the aid of a low-power magni- may be made in the field or office, depend- collapse of a cliff or mountainside, flowing

fier; the microscopic study and description of ing on the extent of information available. down steep slopes and across low ground,

rocks. (Reference 3) (Reference 2) often for several kilometers at speeds of

loess A widespread, homogenous, com- more than 100 km/hr. Sturzstroms are the

relief The difference in elevation between

monly nonstratified, porous, friable, slightly most catastrophic of all forms of mass

the high and low points of a land surface.

coherent, usually highly calcareous, fine- movement. (Reference 3)

(Reference 3)

grained blanket deposit (generally less than subaqueous (submarine) landslide Condi-

30 m thick) consisting predominantly of silt, risk The probability of occurrence or tions and processes, or features and deposits,

with subordinate grain sizes ranging from expected degree of loss, as a result of expo- that exist or are situated in or under water.

amounts of clay to fine sand. (Reference 3) sure to a hazard. (Reference 4) Generally used to specify a process that

mitigation Activities that reduce or rock mechanics The theoretical and occurs either on land (the slide extending

eliminate the probability of occurrence of a applied science of the mechanical behavior underwater) or that begins under water; for

disaster and (or) activities that dissipate or of rocks, representing a “branch of mechan- example, slumping, gravitational slides.

lessen the effects of emergencies or disasters ics concerned with the response of rock to (Reference 3)

when they actually occur. (Reference 5) the force fields of its physical environment.”

(Reference 3)

62   The Landslide Handbook­—A Guide to Understanding Landslides References for Glossary:

subsidence weathering, differential

Sinking or downward settling When weather-

of the Earth’s surface, not restricted in rate, ing across a rock face or exposure occurs at

magnitude, or area involved. Subsidence may different rates mainly due to variations in the 1. C

reath, W.B., 1996, Homebuyers’ guide

be caused by natural geologic processes, composition and resistance of the rock. This to geologic hazards: An AIPG issues

such as solution, compaction, or withdrawal results in an uneven surface with the more and answers publication: Department of

of fluid lava from beneath a solid crust or resistant material protruding. (Reference 4) Natural Resources, Colorado Geological

by human activity such as subsurface min- Survey, Miscellaneous Publication (MI)

weathering, mechanical The physical

ing or the pumping of oil or ground water. no. 58, 30 p.

(Reference 3) processes by which rocks exposed to the 2. Chatwin, S.C., Howes, D.E., Schwab,

weather change in character, decay, and

surficial geology Geology of surficial J.W., and Swanston, D.N., 1994, A

crumble into soil. Processes include tem-

deposits, including soils; the term is some- guide for management of landslide-

perature change (expansion and shrinkage),

times applied to the study of bedrock at or prone terrain in the Pacific Northwest,

near the Earth’s surface. (Reference 3) freeze-thaw cycle, and the burrowing activity 2d edition: Research Branch, Ministry

of animals. (Reference 4)

swelling soils These are soils or soft of Forests, Province of British Colum-

bedrock that increases in volume as they get zonation A term used generally, even bia, Victoria, British Columbia, Crown

wet and shrink as they dry out. They are also Publications.

vaguely, for a region of latitudinal charac-

commonly known as bentonite, expansive, or ter more or less set off from surrounding 3. Jackson, Julia A., ed., 1997, Glossary of

montmorillinitic soils. (Reference 1) regions by some distinctive characteristic, for geology, fourth edition: Prepared by the

tensile stress A normal stress that tends to instance, the Earth’s torrid zone, two temper- American Geological Institute, Alexan-

pull apart the material on the opposite sides dria, Virginia, USA, Doubleday.

ate zones, and two frigid zones. For hazards,

of the plane on which it acts. (Reference 3) zones are geographic regions or designations 4. Jochim, Candice L., Rogers, William P.,

weathering The destructive process by that are differentiated through a variety of Truby, John O., Wold, Robert L., Jr.,

which earth and rock materials exposed to different criteria, for example, residential Weber, George, and Brown, Sally P.,

the atmosphere undergo physical disintegra- zones, zones of low hazard, zones of high 1988, Colorado landslide hazard

tion and chemical decomposition resulting mitigation plan: Department of Natural

hazard. (Reference 3)

in changes in color, texture, composition, or Resources, Colorado Geological Survey,

form. Processes may be physical, chemical, Bulletin 48.

or biological. (Reference 4) 5. Shelton, David C., and Prouty, Dick,

1979, Nature’s building codes, geology

and construction in Colorado: Depart-

ment of Natural Resources, Colorado

Geological Survey Special Publication

No. 48, 72 p.

6. Turner, A. Keith, and Schuster,

Robert L., 1996, Landslides—

Investigation and mitigation: National

Research Council, Transportation

Research Board, Special Report 247,

National Academy Press, Washington,

D.C., 673 p.

Part 2. Parts of a Landslide—Description of Features/Glossary   63

Part 2. Parts of a Landslide—Description of Features/Glossary

Crown cracks

Original ground Crown

surface Ma in

Minor scarp s car p

He ad

Transverse cracks k

an

fl

ht

g

Ri

Transverse ridges

Radial

cracks Surface of rupture

Toe Main body

Toe of surface of rupture

Foot

Surface of separation

Parts of a landslide. (Modified from Varnes, 1978, reference 43).

Figure A1.

accumulation main body top

The volume of the displaced The part of the displaced mate- The highest point of contact between

rial of the landslide that overlies the surface the displaced material and the main scarp.

material, which lies above the original of rupture between the main scarp and the

ground surface. toe of surface of rupture The intersection

toe of the surface of rupture. (usually buried) between the lower part of

crown The practically undisplaced material main scarp A steep surface on the undis- the surface of rupture of a landslide and the

still in place and adjacent to the highest parts turbed ground at the upper edge of the land- original ground surface.

of the main scarp. slide, caused by movement of the displaced zone of accumulation The area of the land-

depletion The volume bounded by the material away from the undisturbed ground. slide within which the displaced material lies

main scarp, the depleted mass and the origi- It is the visible part of the surface of rupture. above the original ground surface.

nal ground surface. minor scarp A steep surface on the dis- zone of depletion The area of the landslide

depleted mass The volume of the displaced placed material of the landslide produced by within which the displaced material lies

material, which overlies the rupture surface differential movements within the displaced below the original ground surface.

but underlies the original ground surface. material.

displaced material Material displaced original ground surface The surface of the Sources of information on

from its original position on the slope by slope that existed before the landslide took nomenclature:

movement in the landslide. It forms both the place.

depleted mass and the accumulation. surface of separation The part of the

flank 1. Cruden, D.M., 1993, The multilingual

The undisplaced material adjacent original ground surface overlain by the foot landslide glossary: Richmond, British

of the landslide.

to the sides of the rupture surface. Compass Columbia, Bitech Publishers, for the

directions are preferable in describing the surface of rupture The surface that forms IUGS Working Party on World

flanks, but if left and right are used, they (or which has formed) the lower boundary Landslide Inventory in 1993.

refer to the flanks as viewed from the crown. of the displaced material below the original 2. Varnes, D.J., 1978, Slope movement

ground surface.

foot The portion of the landslide that has types and processes, Schuster, R.L.,

in

moved beyond the toe of the surface of rup- tip The point of the toe farthest from the and Krizek, R. J., eds., Landslides—

ture and overlies the original ground surface. top of the landslide. Analysis and control: Transportation

head The upper parts of the landslide along toe The lower, usually curved margin of Research Board Special Report 176,

the contact between the displaced material the displaced material of a landslide, it is the National Research Council, Washington,

and the main scarp. most distant from the main scarp. D.C., p. 11–23.

64   The Landslide Handbook­—A Guide to Understanding Landslides

Part 3. Landslide Causes and Triggering Mechanisms

Natural Causes

Physical Causes—Triggers Morphological causes

Geological causes

• Intense rainfall

• Rapid snowmelt • Tectonic or volcanic uplift

• Weak materials, such as some

• Prolonged intense precipitation volcanic slopes or unconsolidated • Glacial rebound

marine sediments, for example

• Rapid drawdown (of floods and • Glacial meltwater outburst

tides) or filling • Susceptible materials • Fluvial erosion of slope toe

• Earthquake • Weathered materials • Wave erosion of slope toe

• Volcanic eruption • Sheared materials • Glacial erosion of slope toe

• Thawing • Jointed or fissured materials • Erosion of lateral margins

• Freeze-and-thaw weathering • Adversely oriented mass disconti- • Subterranean erosion (solution,

nuity (bedding, schistosity, and so

• Shrink-and-swell weathering piping)

forth)

• Flooding • Deposition loading slope or its crest

• Adversely oriented structural • Vegetation removal (by forest fire,

discontinuity (fault, unconformity, drought)

contact, and so forth)

• Contrast in permeability

For further reading: • Contrast in stiffness (stiff, dense

References 9, 3, and 45 material over plastic materials)

Human Causes

• Excavation of slope or its toe

• Use of unstable earth fills, for construction

• Loading of slope or its crest, such as placing earth fill at the top of a slope

• Drawdown and filling (of reservoirs)

• Deforestation—cutting down trees/logging and (or) clearing land for crops;

unstable logging roads

• Irrigation and (or) lawn watering

• Mining/mine waste containment

• Artificial vibration such as pile driving, explosions, or other strong ground

vibrations

• Water leakage from utilities, such as water or sewer lines

• Diversion (planned or unplanned) of a river current or longshore current by

construction of piers, dikes, weirs, and so forth

Appendix B.

Introduction to Landslide Evaluation Tools—

Mapping, Remote Sensing, and Monitoring

of Landslides

66   The Landslide Handbook­—A Guide to Understanding Landslides

Part 1. Mapping

Maps are a useful and convenient tool for presenting information on landslide

hazards. They can present many kinds and combinations of information at different

levels of detail. Hazard maps used in conjunction with land-use maps are a valu-

able planning tool. Commonly, there is a three-stage approach to landslide hazard

mapping. The first stage is regional or reconnaissance mapping, which synthesizes

available data and identifies general problem areas. This regional scale (sometimes

called “small-scale”) mapping is usually performed by a Provincial, State, or Federal

geological survey. The next stage is community-level mapping, a more detailed

surface and subsurface mapping program in complex problem areas. Finally, detailed

site-specific large-scale maps are prepared. If resources are limited, it may be more

prudent to bypass regional mapping and concentrate on a few known areas of con-

cern. We discuss three types of general mapping; (1) Regional, (2) Community level,

and (3) Site specific.

Regional mapping

Regional or reconnaissance mapping supplies basic data for regional planning

by providing baseline information for conducting more detailed studies at the com-

munity and site-specific levels and for setting priorities for future mapping.

Such maps are usually simple inventory or susceptibility maps and are directed

primarily toward the identification and delineation of regional landslide problem

areas and the conditions under which they occur. They concentrate on those geo-

logic units or environments in which additional movements are most likely. The

geographical extent of regional maps can vary from a map of a State or Province to

a national map, which delineates an entire country. Such mapping relies heavily on

photogeology (the geologic interpretation of aerial photography), reconnaissance

field mapping, and the collection and synthesis of all available pertinent geologic

data. Map scales at this level are typically at scales ranging from 1:10,000 down to

1:4,000,000 or even smaller.

Community-level mapping

This type of mapping identifies both the three-dimensional potential of land

sliding and considers its causes. Guidance concerning land use, zoning, and build-

ing, and recommendations for future site-specific investigations also are made at this

stage. Investigations should include subsurface exploratory work in order to produce

a map with cross sections. Map scales at this level typically vary from 1:1,000 to

1:10,000.

Site-specific mapping

Site-specific mapping is concerned with the identification, analysis, and solu-

tion of actual site-specific problems, often presented in the size of a residential lot. It

is usually undertaken by private consultants for landowners who propose site devel-

opment and typically involves a detailed drilling program with downhole logging,

sampling, and laboratory analysis in order to procure the necessary information

for design and construction. Map scales vary but usually are about 1:600 or 25 mm

(1 inch) equal to 16 m (50 feet). Part 1. Mapping   67

Three Important Criteria for Landslide Maps

The three types of landslide maps most useful to planners and the general public

are (1) landslide inventories, (2) landslide susceptibility maps, and (3) landslide

hazard maps.

Landslide inventory maps

Inventories denote areas that are identified as having failed by landslide processes

(fig. B1). The level of detail of these maps ranges from simple reconnaissance inven-

tories that only delineate broad areas where landsliding appears to have occurred to

complex inventories that depict and classify each landslide and show scarps, zones of

depletion and accumulation, active and inactive slides, geological age, rate of move-

ment, and (or) other pertinent data on depth and kind of materials involved in sliding.

Simple inventories give an overview of the areal extent of landslide occur-

rence and highlight areas where more detailed studies should be conducted. Detailed

inventories provide a better understanding of the different landslide processes operat-

ing in an area and can be used to regulate or prevent development in landslide-prone

areas and to aid the design of remedial measures. They also provide a good basis for

the preparation of derivative maps, such as those indicating slope stability, for rating

landslide hazard, and to identify land use. One way is to use aerial photography with

selective field checking to detect landslide areas and then to present the information

in map form using a coded format. The maps show any or all of the following: state

of activity, certainty of identification, dominant types of slope movement, estimated

thickness of landslide material, type of material, and dates or periods of activity.

In the United States, regional maps are most often prepared at a scale of

1:24,000 (1:50,000 in Canada) because high-quality U.S. Geological Survey

topographic base maps at this scale are widely available and aerial photographs are

commonly of a comparable scale. Other scales commonly used in the United States,

×

for example, include 1:50,000 (county series), 1:100,000 (30 60 minute series),

×

and 1:250,000 (1 2 degree series).

EXPLANATION

Study area

Drainages

Landslides

Elevation, in meters

149−251

252−354

355−456

457−559

560−661

662−764

765−866

867−969

970−1,071

1,072−1,174

Example of a landslide inventory map showing the locations of past

Figure B1.

landslides and including topographical information consisting of elevation (measured in

meters) and drainage courses (map from U.S. Geological Survey).

68   The Landslide Handbook­—A Guide to Understanding Landslides

Landslide susceptibility maps

A landslide susceptibility map goes beyond an inventory map and depicts areas

that have the potential for landsliding (fig. B2). These areas are determined by cor-

relating some of the principal factors that contribute to landsliding (such as steep

slopes, weak geologic units that lose strength when saturated or disturbed, and poorly

drained rock or soil) with the past distribution of landslides. These maps indicate only

the relative stability of slopes; they do not make absolute predictions.

Landslide susceptibility maps can be considered derivatives of landslide inven-

tory maps because an inventory is essential for preparing a susceptibility map. For

example, overlaying a geologic map with an inventory map that shows existing land-

slides can identify specific landslide-prone geologic units. This information can then

be extrapolated to predict other areas of potential landsliding. More complex maps

may include additional information such as slope angle and drainage.

An example of a landslide susceptibility map. This map shows an area in

Figure B2.

Canada, the Mackenzie River Valley, Northwest Territories. Graphic by Réjean Couture,

Geological Survey of Canada. Part 1. Mapping   69

Landslide hazard maps

Hazard maps show the areal extent of threatening processes (fig. B3): where

landslide processes have occurred in the past, recent occurrences, and most important,

the likelihood in various areas that a landslide will occur in the future. For a given

area, hazard maps contain detailed information on the types of landslides, extent of

slope subject to failure, and probable maximum extent of ground movement. These

maps can be used to predict the relative degree of hazard in a landslide area. Areas

may be ranked in a hierarchy such as low, moderate, and high hazard areas. For mapping references and further

reading: 4, 12, 18, 19, 21, 25, 29, 33,

34, 35, 41 and 46.

Relative hazard

High (more than 75 shallow landslides/km )

2

)

Medium (20−75 shallow landslides/km

2

Low (fewer than 20 shallow landslides/km )

2 ug

P et So

Failure locations from landslide database un d

Portion of shallow landslide hazard map showing part of the Magnolia area

Figure B3.

of the city of Seattle, Washington, USA. (km is notation for square kilometers.)

2

70   The Landslide Handbook­—A Guide to Understanding Landslides

Part 2. Remote Sensing and Other Tools that Show

Features of Landslide Activity

Maps and other forms of information are sometimes overlaid on each other

using a GIS (Geographical Information System) so that the different types of infor-

mation can be viewed all at once. In the absence of a GIS computerized system,

transparencies can be made of each map and then can be overlaid together. It is

important that the maps and data be at the same scale. The following list describes

many types of information that might be useful in constructing layers for GIS analy-

sis of landslide potential.

Topographic Map

• Indicates slope gradient, terrain configuration, drainage

pattern.

Terrain Map

• Identifies material, depth, geological processes, terrain config-

uration, surface and subsurface drainage, slope gradient (also called surficial

geology or Quaternary geology maps).

Bedrock Map

• Identifies bedrock type, surface and subsurface structure,

surficial cover (overburden), and age of rock over a topographic map base.

Engineering Soil Map

• Identifies surficial material type, drainage, limited

engineering characteristics, soils characteristics, vegetation cover.

Forest Cover Map

• Identifies surface vegetation, topographic features, sur-

face drainage pattern, and in some cases, soil drainage character.

Research Studies

• May provide information on all of the above, plus quanti-

tative data on controlling factors and possibly local stability risk assessment.

Aerial Photography Remote Sensing

• (Examples shown in figs. B4 through

B7.) Identification can be made of vegetation cover, topography, drainage

pattern, soil drainage character, bedrock geology, surficial geology, landslide

type, and relationship to other factors. Careful study of a given area of terrain

with the aid of oblique aerial photographs and vertical stereo pairs can yield

significant information on landslide type and frequency and the effects of

management practices. A review of recent and past aerial photographs of the

area should be undertaken whenever possible, as older slides may not be evi-

dent on more recent photographs. Features discernible on aerial photographs

can help users identify landslide type and develop a reasonable assessment of

overburden characteristics. These, in turn, provide a means for estimating the

landslide hazard at a site.

InSAR Imaging

• InSAR is an acronym for Interferometric Synthetic Aper-

ture Radar. Both InSAR and LIDAR (description follows) use active sensors

emitting a pulse of energy (from a satellite) and recording its return, from

the ground, at the sensor. Most InSAR equipment is able to penetrate fog and

rain and can be used in areas difficult to access by foot. By bouncing signals

from a radar satellite off the ground, digital elevation model (DEM) maps

can be produced that will show the ground terrain. Two images of the same

place are taken at different times then merged, forming a map called an inter-

ferogram. The merging of the two images shows the ground displacement (if

any) that would indicate any movement that has occurred between the time

the two images were taken. In this way, one can determine if a hillside, for

example, has moved.

Ordinary radar on a typical Earth-orbiting satellite has a very poor

ground resolution of about 3 to 4 miles because of the restricted size of the

antenna on the satellite. Synthetic Aperture Radar (SAR) takes advantage of

the motion of the spacecraft along its orbital track to mathematically recon-

struct (synthesize) an operationally larger antenna and yield high-spatial-

resolution imaging capability on the order of hundreds of feet.

Part 2. Remote Sensing and Other Tools that Show Features of Landslide Activity   71

An example of an aerial photograph of the La Conchita landslide in California,

Figure B4.

USA, taken in 2005. Blue line delineates an older landslide, yellow a more recent

landslide. (Photograph courtesy of AirPhoto USA and County of Ventura, California, and

Randy Jibson, U.S. Geological Survey.)

Schematic showing satellite passes over an area of the Earth’s surface

Figure B5.

(graphic modified from Reference 41.)

72   The Landslide Handbook­—A Guide to Understanding Landslides

Interferogram from InSAR imaging process showing the area of uplift

Figure B6.

(1997–2001) at the Three Sisters volcanoes (red triangles) in the Cascade Range in

central Oregon, USA (circles show locations of earthquakes). (Photograph modified from

Reference 41.)

Part 2. Remote Sensing and Other Tools that Show Features of Landslide Activity   73

LiDAR Imaging

• LiDAR is an acronym for Light Detection and Ranging, For remote sensing and other map

also known as ALSM or Airborne Laser Swath Mapping. Using a narrow references and further reading: 4, 12, 14,

laser beam to probe through dense ground cover, such as trees, LiDAR can 15, 18, 19, 21, 24, 35, 39, and 41

produce accurate terrain maps even where forest cover gets in the way of

traditional photography. The technique produces a very accurate Digital

Elevation Model map (DEM) (fig. B7). Accurate bare-earth DEMs can be

produced when the imagery is acquired during the leaf-off season in areas

covered by deciduous forests.

Essential elements of a LiDAR mapping system are a scanning laser

rangefinder mounted in an aircraft, differential Global Positioning System

(GPS) to locate the aircraft, and an internal measurement unit (IMU) to

measure aircraft orientation. LiDAR is a useful topographic mapping tool for

three reasons. First is accuracy, the second is productivity; measurements are

made at rates of 10,000 to 80,000 laser pulses per second. Third, LiDAR is

monoscopic and provides its own illumination. These characteristics over-

come the major liabilities of photogrammetry in forested terrain. The maps

produced by LiDAR are very clear and detailed and in many cases reveal

evidence of past landslides that are virtually invisible by other means due to

heavy vegetation cover. LiDAR is expensive and highly technical and is used

mainly by government agencies, universities, and some private entities. One

drawback is that many LiDAR systems use a near-infrared laser that does not

penetrate fog or rain.

An oblique LiDAR image of La Conchita, California, USA, landslide, taken

Figure B7.

in 2005. Outline of 1995 (blue) and 2005 (yellow) landslides are shown; arrows show

examples of other landslides in the area; red line outlines main scarp of an ancient

landslide that involved the entire bluff. (Photograph by Airborne 1, El Segundo, California,

USA, and Randy Jibson, U.S. Geological Survey.)

74   The Landslide Handbook­—A Guide to Understanding Landslides

Part 3. Real-Time Monitoring of Landslides and

Landslide Instrumentation

The immediate detection of landslide activity that is provided by real-time

systems can be crucial in saving human lives and protecting property. Traditional

field observations, even if taken regularly, cannot detect changes at the moment

they occur. Moreover, active landslides can be hazardous to work on, and large

movements often occur during storms when visibility is poor. The continuous data

provided by remote real-time monitoring permits a better understanding of dynamic

For more information: References 4, landslide behavior that, in turn, enables engineers to create more effective designs to

21, 25, 28, 35, 38, 39, and 46 prevent or halt landslides. Landslide monitoring can be expensive, and most moni-

toring systems require installation by experts. The advantage is that systems that

detect landslide movement can be coordinated with warning systems.

Measuring landslide

Figure B8. Testing a solar-powered

Figure B9.

movement using an extensometer, an radiotelemetry system for remote

instrument that can detect movement of transmission of real-time landslide data.

the ground surface between stable ground (Photograph by Mark Reid, U.S. Geological

and sliding ground. (Photograph by Richard Survey.)

LaHusen, U.S. Geological Survey.) Slope

movement

sensor

Monitoring Instruments nt

eme

ov

m

of

n

ctio

Dire

Rain Shallow

gage pore pressure

Geophone

Battery Deep

pore pressure

Example of a network for measurement and transmission of real-time

Figure B10.

landslide data. (Schematic from U.S. Geological Survey.)

Appendix C.

Introduction to Landslide Stabilization and Mitigation

Much of the material that follows on slope stabilization meth-

Note:

ods has been reproduced directly from “A Guide for Management of

Landslide-Prone Terrain in the Pacific Northwest,” published by the

Research Branch of the Ministry of Forests, British Columbia, Canada.

However, this volume contains a much more comprehensive overview

of mitigation, and it is highly recommended by the authors for those

desiring more detailed information on mitigation measures. Please see

reference 11, Chatwin and others, for full publication citation.

76   The Landslide Handbook­—A Guide to Understanding Landslides

Part 1. Earth Slope Stabilization/Mitigation

Some of the stabilization techniques that are currently available in North

America are illustrated in this discussion. We highlight simple methods that can be

used safely in the absence of detailed soil or bedrock analysis or in low-risk situa-

tions. Some stabilization methods are very expensive and require significant time to

implement. This is an overview of stabilization methods; many other methods are

in use around the world. Professional advice is essential before, during, and after

implementation (where possible), as is further literature consultation.

The stability of any slope will be improved if certain actions are carried out.

To be effective, first one must identify the most important controlling process that

is affecting the stability of the slope; second, one must determine the appropriate

technique to be sufficiently applied to reduce the influence of that process. The miti-

gative prescription must be designed to fit the condition of the specific slope under

study. For example, installation of drainage pipes into a slope that has very little

ground water is pointless. Slope stabilization efforts take place during construction

or when stability problems develop unexpectedly following construction. Most slope

engineering techniques require a detailed analysis of soil properties and a sound

knowledge of the underlying soil and rock mechanics.

In any high-risk situation, where a landslide may endanger lives or

adversely affect property, a professional landslide expert such as a

geotechnical or civil engineer should always be consulted before any

stabilizing work is undertaken.

The following sections provide a general introduction to techniques that can be

used to increase slope stability.

Excavation

Figures C1, C2, and C3 provide a cross-sectional view, in schematic form, of

general principles for slope excavation, showing the effects and consequences of

where on a slope the excavation takes place. These graphics are general in nature,

and a geotechnical engineer or other professional should always be a consulted if

possible.

Removal of soil from the head of a slide

This method reduces the driving force and thereby improves stability. This

method is suitable only for cuts into deep soil where rotational landslides (see “Basic

Landslide Types” in Section I) may occur. It is ineffective on translational failures on

long, uniform or planar slopes or on flow-type landslides.

Reducing the height of the slope

Reducing the height of a cut bank reduces the driving force on the failure plane

by reducing the weight of the soil mass and commonly involves the creation of an

access road above the main road and the forming of a lower slope by excavation.

Also, it is possible to excavate deeply and lower the main road surface if the right-

of-way crosses the upper part of a landslide. This method is only moderately effi-

cient in increasing stability, and a complete solution may involve additional modi-

fication of the land. According to Chatwin (Reference 11), it usually increases the

Factor of Safety by only 10 or 15 percent. (“Factor of Safety” in its simple definition

is the ratio of the maximum strength of a piece of material or a part to the probable

maximum load to be applied to it.) Part 1. Earth Slope Stabilization/Mitigation   77

Removing material from head reduces

driving forces and increases stability

Ground surface

Slip surface

Ground surface Removing material from toe reduces

resisting forces and reduces stability

Slip surface

Illustration of the differences in stability resulting in

Figure C1.

excavation at the head and toe surfaces of a slope. (Graphic by Rex

Baum, U.S. Geological Survey.)

Load applied at head increases

driving forces and decreases stability

Ground surface

Slip surface

Ground surface Load applied at toe increases

resisting forces and increases stability

Slip surface

Illustration of the difference in stability of loading either

Figure C2.

the head or the toe of a slope. (Graphic by Rex Baum, U.S. Geological

Survey.) Falling water table/pore pressure increases

resisting forces and increases stability

Ground surface

Slip surface

Ground surface Rising water table/pore pressure reduces

resisting forces and reduces stability

Slip surface

Illustration of the importance of water in the stability of

Figure C3.

a slope. (Graphic by Rex Baum, U.S. Geological Survey.)

78   The Landslide Handbook­—A Guide to Understanding Landslides

Backfilling with lightweight material

A technique related to height reduction is to excavate the upper soil and replace

it with a lightweight backfill material such as woodchips or logging slash. Then,

covered with a thin layer of coarse aggregate, the backfilled material can form a

foundation for limited-use traffic (fig. C4). vel

Gra

Underdrain ing

p

Wood ap

f ib C

er Till

Schematic and photograph of a lightweight backfill. There has been an

Figure C4.

increased growth in the use of recycled tire shreds in civil engineering applications.

Highway applications include using shredded tires as lightweight fill over weak soils

in bridge embankments and retaining wall reinforcements or, in very cold climates, as

insulation of the road base to resist frost heaves and as a high-permeability medium

for edge drains. (Graphic from reference 11, photograph from U.S. Department of

Transportation, Federal Highway Administration.)

Part 1. Earth Slope Stabilization/Mitigation   79

Benches

Benches are a series of “steps” cut into a deep soil or rock face for the purpose

of reducing the driving forces. They are mainly effective in reducing the incidence

of shallow failures but generally are not very efficient in improving the overall slope

stability for which other methods are recommended. Benches are useful in providing

protection structures beneath rockfall-prone cliffs, for controlling surface drainage,

or for providing a work area for installing drainpipe or other structures.

Please see figure C12 for a photo of benches cut into a slope.

Flattening or reducing slope angle, or other slope modification

This reduces the weight of material and reduces the possibility of stream/river

undercutting or construction loading.

When not to excavate a slide mass

In some situations, removing the entire slide mass is an effective and economic

solution. Generally, however, it is only practical on small slumps or small rotational

failures. Large-scale excavation of larger landslide areas is usually not recommended

for several reasons:

• Excavation is not always effective­—for large planar failures, excavation may

not cause movement to stop and may allow the landslide to expand.

trigger a larger landslide

• Excavation may by removing the support provided

by the toe of the landslide.

destabilize

• Excavation may actually the ground farther upslope by undercut-

ting, which weakens the slope.

In deeper soils

• , especially soft clays, where there are two potential failure

surfaces, one deep and one shallow, excavating down to the first failure sur-

face might trigger a sudden slippage on the deeper failure surface. A stability

analysis using soil strength data is advised and most always necessary for

any major excavation project in deep clay soils.


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AUTORE

Atreyu

PUBBLICATO

+1 anno fa


DETTAGLI
Corso di laurea: Corso di laurea in scienze e tecnologie per l'ambiente (Facoltà di Agraria, di Scienze Matematiche, Fisiche e Naturali)
SSD:
Università: Pisa - Unipi
A.A.: 2009-2010

I contenuti di questa pagina costituiscono rielaborazioni personali del Publisher Atreyu di informazioni apprese con la frequenza delle lezioni di Fondamenti di scienze della terra e studio autonomo di eventuali libri di riferimento in preparazione dell'esame finale o della tesi. Non devono intendersi come materiale ufficiale dell'università Pisa - Unipi o del prof Santacroce Roberto.

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