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Article |
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Landslide Types and Processes |
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Types
of Landslides
Landslide
Causes
Landslide
Mitigation—How to Reduce the Effects of Landslides
Related
Links
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United States landslide incidence and
susceptibility map.
National Atlas of the United States® |
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La Conchita, coastal area of southern
California. This landslide and earthflow occurred in the
spring of 1995.
Photo by R.L. Schuster, U.S. Geological
SurveyLandslides in the United States occur in all 50 States.
The primary regions of landslide occurrence and potential
are the coastal and mountainous areas of California, Oregon,
and Washington, the States comprising the intermountain west,
and the mountainous and hilly regions of the Eastern United
States. Alaska and Hawaii also experience all types of landslides.
Landslides in the United States cause approximately $3.5
billion (year 2001 dollars) in damage, and kill between 25
and 50 people annually. Casualties in the United States are
primarily caused by rockfalls, rock slides, and debris flows.
Worldwide, landslides occur and cause thousands of casualties
and billions in monetary losses annually.
Here are some basic scientific facts about landslides-the different
types of landslides, how they are initiated, and some basic
information about how they can begin to be managed as a hazard.
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Types of Landslides |
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The term "landslide" describes a wide variety of processes
that result in the downward and outward movement of slope-forming
materials including rock, soil, artificial fill, or a combination
of these. The materials may move by falling, toppling, sliding,
spreading, or flowing. Figure 1 shows a graphic illustration of
a landslide, with the commonly accepted terminology describing
its features.
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Figure 1. An idealized
slump-earth flow showing commonly used nomenclature for labeling
the parts of a landslide. |
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The various types
of landslides can be differentiated by the kinds of material involved
and the mode of movement. A classification system based on these
parameters is shown in figure 2. Other classification systems incorporate
additional variables, such as the rate of movement and the water,
air, or ice content of the landslide material. |
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Type
of Movement |
Type of Material |
Bedrock |
Engineering Soils |
Predominantly
Coarse |
Predominantly
Fine |
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FALLS |
Rock fall |
Debris fall |
Earth fall |
TOPPLES |
Rock topple |
Debris slide |
Earth slide |
SLIDES |
ROTATIONAL |
Rock slide |
Debris slide |
Earth slide |
TRANSLATIONAL |
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LATERAL SPREADS |
Rock spread |
Debris spread |
Earth spread |
FLOWS |
Rock flow
(deep creep) |
Debris flow |
Earth flow |
(soil creep) |
COMPLEX |
Combination of two or more principal
types of movement |
Figure 2. Types of landslides.
Abbreviated version of Varnes' classification of slope movements
(Varnes, 1978). |
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Although landslides are primarily associated with mountainous
regions, they can also occur in areas of generally low relief.
In low-relief areas, landslides occur as cut-and-fill failures
(roadway and building excavations), river bluff failures, lateral
spreading landslides, collapse of mine-waste piles (especially
coal), and a wide variety of slope failures associated with quarries
and open-pit mines. The most common types of landslides are described
as follows and are illustrated in figure 3, figure 4, and figure
5.
Slides: Although many types of mass movements
are included in the general term "landslide," the more
restrictive use of the term refers only to mass movements, where
there is a distinct zone of weakness that separates the slide material
from more stable underlying material. The two major types of slides
are rotational slides and translational slides. Rotational slide:
This is a slide in which the surface of rupture is curved concavely
upward and the slide movement is roughly rotational about an axis
that is parallel to the ground surface and transverse across the
slide (fig. 3A). Translational slide: In this type of slide, the
landslide mass moves along a roughly planar surface with little
rotation or backward tilting (fig. 3B). A block slide is a translational
slide in which the moving mass consists of a single unit or a few
closely related units that move downslope as a relatively coherent
mass (fig. 3C).
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Figure 3. These schematics illustrate the major types of landslide
movement that are described within this article. |
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Falls: Falls are abrupt movements of masses of
geologic materials, such as rocks and boulders, that become detached
from steep slopes or cliffs (fig. 4D). Separation occurs along
discontinuities such as fractures, joints, and bedding planes,
and movement occurs by free-fall, bouncing, and rolling. Falls
are strongly influenced by gravity, mechanical weathering, and
the presence of interstitial water.
Topples: Toppling failures are distinguished
by the forward rotation of a unit or units about some pivotal point,
below or low in the unit, under the actions of gravity and forces
exerted by adjacent units or by fluids in cracks (fig. 4E).
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Figure 4. These schematics
illustrate the major types of landslide movement that are described
in this article. |
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Flows: There are five basic categories of flows
that differ from one another in fundamental ways.
- a. Debris flow: A debris flow is a form of rapid mass movement
in which a combination of loose soil, rock, organic matter, air,
and water mobilize as a slurry that flows downslope (fig. 4F).
Debris flows include <50% fines. 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.
Debris-flow source areas are often associated with steep gullies,
and debris-flow deposits are usually indicated by the presence
of debris fans at the mouths of gullies. Fires that denude slopes
of vegetation intensify the susceptibility of slopes to debris
flows.
- b. Debris avalanche: This is a variety of very rapid to extremely
rapid debris flow (fig. 5G).
- c. Earthflow: Earthflows have a characteristic "hourglass" shape
(fig. 5H). The slope material liquefies and runs out, forming
a bowl or depression at the head. The flow itself is elongate
and usually occurs in fine-grained materials or clay-bearing
rocks on moderate slopes and under saturated conditions. However,
dry flows of granular material are also possible.
- d. Mudflow: A mudflow is an earthflow consisting of material
that is wet enough to flow rapidly and that contains at least
50 percent sand-, silt-, and clay-sized particles. In some instances,
for example in many newspaper reports, mudflows and debris flows
are commonly referred to as "mudslides."
- e. Creep: Creep is the imperceptibly slow, steady, downward
movement of slope-forming soil or rock. Movement is caused by
shear stress sufficient to produce permanent deformation, but
too small to produce shear failure. There are generally three
types of creep: (1) seasonal, where movement is within the
depth of soil affected by seasonal changes in soil moisture and
soil temperature; (2) continuous, where shear stress continuously
exceeds the strength of the material; and (3) progressive, where
slopes are reaching the point of failure as other types of mass
movements. Creep is indicated by curved tree trunks, bent fences
or retaining walls, tilted poles or fences, and small soil ripples
or ridges (fig. 5I).
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Figure 5. These schematics
illustrate the major types of landslide movement that are described
in this article. |
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Lateral spreads: Lateral
spreads are distinctive because they usually occur on very gentle
slopes or flat terrain (fig. 5J). The dominant mode of movement
is lateral extension accompanied by shear or tensile fractures.
The failure is caused by liquefaction, the process whereby saturated,
loose, cohesionless sediments (usually sands and silts) are transformed
from a solid into a liquefied state. Failure is usually triggered
by rapid ground motion, such as that experienced during an earthquake,
but can also be artificially induced. When coherent material, either
bedrock or soil, rests on materials that liquefy, the upper units
may undergo fracturing and extension and may then subside, translate,
rotate, disintegrate, or liquefy and flow. Lateral spreading in
fine-grained materials on shallow slopes is usually progressive.
The failure starts suddenly in a small area and spreads rapidly.
Often the initial failure is a slump, but in some materials movement
occurs for no apparent reason. Combination of two or more of the
above types is known as a complex landslide. |
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Landslide Causes |
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Geological Causes |
Weak or sensitive materials |
Weathered materials |
Sheared, jointed, or fissured materials |
Adversely oriented discontinuity (bedding, schistosity, fault, unconformity,
contact, and so forth) |
Contrast in permeability and/or stiffness of materials |
Morphological Causes |
Tectonic or volcanic uplift |
Glacial rebound |
Fluvial, wave, or glacial erosion of slope toe or lateral margins |
Subterranean erosion (solution, piping) |
Deposition loading slope or its crest |
Vegetation removal (by fire, drought) |
Thawing |
Freeze-and-thaw weathering |
Shrink-and-swell weathering |
Human Causes |
Excavation of slope or its toe |
Loading of slope or its crest |
Drawdown (of reservoirs) |
Deforestation |
Irrigation |
Mining |
Artificial vibration |
Water leakage from utilities |
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Although there are multiple types of causes of landslides, the
three that cause most of the damaging landslides around the world
are these:
Landslides and Water
Slope saturation by water is a primary cause of landslides. This
effect can occur in the form of intense rainfall, snowmelt, changes
in ground-water levels, and water-level changes along coastlines,
earth dams, and the banks of lakes, reservoirs, canals, and rivers.
Landsliding and flooding are closely allied because both are related
to precipitation, runoff, and the saturation of ground by water.
In addition, debris flows and mudflows usually occur in small,
steep stream channels and often are mistaken for floods; in fact,
these two events often occur simultaneously in the same area.
Landslides can cause flooding by forming landslide dams that block
valleys and stream channels, allowing large amounts of water to
back up. This causes backwater flooding and, if the dam fails,
subsequent downstream flooding. Also, 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 can also cause overtopping of reservoirs and/or
reduced capacity of reservoirs to store water.
Landslides and Seismic Activity
Many mountainous areas that are vulnerable to landslides have
also experienced at least moderate rates of earthquake occurrence
in recorded times. The occurrence of earthquakes in steep landslide-prone
areas greatly increases the likelihood that landslides will occur,
due to ground shaking alone or shaking-caused dilation of soil
materials, which allows rapid infiltration of water. The 1964
Great Alaska Earthquake caused widespread landsliding and other
ground failure, which caused most of the monetary loss due to
the earthquake. Other areas of the United States, such as California
and the Puget Sound region in Washington, have experienced slides,
lateral spreading, and other types of ground failure due to moderate
to large earthquakes. Widespread rockfalls also are caused by
loosening of rocks as a result of ground shaking. Worldwide,
landslides caused by earthquakes kill people and damage structures
at higher rates than in the United States.
Landslides and Volcanic Activity
Landslides due to volcanic activity are some of the most devastating
types. Volcanic lava may melt snow at a rapid rate, causing a deluge
of rock, soil, ash, and water that accelerates rapidly on the steep
slopes of volcanoes, devastating anything in its path. These volcanic
debris flows (also known as lahars) reach great distances, once
they leave the flanks of the volcano, and can damage structures
in flat areas surrounding the volcanoes. The 1980 eruption of
Mount St. Helens, in Washington triggered a massive landslide
on the north flank of the volcano, the largest landslide in recorded
times.
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Landslide Mitigation—How to Reduce the Effects
of Landslides |
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Vulnerability to landslide hazards is a function of location,
type of human activity, use, and frequency of landslide events.
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 reduce landslide effects through land-use policies
and regulations. Individuals can reduce their exposure to hazards
by educating themselves on the past hazard history of a site and
by making inquiries to planning and engineering departments of
local governments. They can also obtain the professional services
of an engineering geologist, a geotechnical engineer, or a civil
engineer, who can properly evaluate the hazard potential of a site,
built or unbuilt.
The hazard from landslides can be reduced by avoiding construction
on steep slopes and existing landslides, or by stabilizing the
slopes. Stability increases when ground water is prevented from
rising in the landslide mass by (1) covering the landslide
with an impermeable membrane, (2) directing surface water away
from the landslide, (3) draining ground water away from the landslide,
and (4) minimizing surface irrigation. Slope stability is
also increased when a retaining structure and/or the weight of
a soil/rock berm are placed at the toe of the landslide or when
mass is removed from the top of the slope.
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Related Links |
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Adapted from July
2004, Landslide
Types and Processes, U.S. Geological Survey Fact
Sheet 2004-3072, Version 1.0 |
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