LANDSLIDES
11.1 Introduction
Landslides are geological phenomena causing flow of rock, earth, or debris flows on slopes due
to gravity in coastal, offshore or onshore environment. It also known as mud flows, debris flows,
earth failures, slope failures, etc., and can be triggered by rains, floods, earthquakes, and other
natural causes as well as human-made causes, such as grading, terrain cutting and filling,
excessive development, etc. The factors affecting landslides can be geophysical or human-made,
they can occur in developed areas, undeveloped areas, or any area where the terrain was altered
for roads, houses, utilities, and buildings. The down-slope movement of material is further
subdivide into two broad categories i.e. Slope Failures and Sediment Flows.
11.2 Slope Failure
Slope Failures are characteristics by a sudden failure of the slope resulting in transport of debris
down hill by sliding, rolling, falling, and slumping.
11.2.1 Slumps
It is a form of mass wasting that occurs when a coherent mass of loosely consolidated materials
or rock layers moves a short distance down a slope. Movement in slumps, involves the
movement of relatively intact masses of rock or sediment downslope along a curved concave
upward failure plane, characterized by sliding along a concave-upward or planar surface (figure
1). The upper surface of each slump block remains relatively undisturbed. Causes of slumping
include earthquake shocks, thorough wetting, freezing and thawing, undercutting, and loading of
a slope. Heavy rains or earthquakes usually trigger slumps.
Translational slumps occur when a detached landmass moves along a planar surface. Common
planar surfaces of failure include joints or bedding planes, especially where a permeable layer
overrides an impermeable surface (figure 2). Block slumps are a type of translational slump, in
which one or more related block units move downslope as a relatively coherent mass.
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Figure 1: Damage of electrial line and movement of surface due to Slump failure
Figure 2: Movement of rock mass or sediment downslope along a curved or concave upward.
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10.2.2 Rock Falls and Debris Falls
Rock fall refers to quantities of rock falling freely from a cliff face. A rockfall is a fragment of
rock (a block) detached by sliding, toppling, or falling, that falls along a vertical or sub-vertical
cliff, proceeds down a slope by bouncing and flying along ballistic trajectories or by rolling on
talus or debris slopes(figure 3 & 4). It is the natural downward motion of a detached block or
series of blocks with a small volume involving free falling, bouncing, rolling, and sliding. Rock
falls occur when a piece of rock on a steep slope becomes dislodged and falls down the slope.
Debris falls are similar, except they involve a mixture of soil, regolith, and rocks. A rock fall
may involve a single rock, or a mass of rocks, and the falling rocks can dislodge other rocks as
they collide with the cliff.
Figure 3: Rock material may breaks from an outcrop and falls due to gravity
Figure 4: Damage of rock and vechicals due to rock fall
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10.2.3 Rock and Debris Slides
Rock slides and debris slides result when rocks or debris slide down a pre-existing surface, such
as a bedding plane or joint surface. A rockslide is a type of landslide caused by rock failure in
which part of the plane of failure passes through intact rock (figure 5).
A debris slide is characterized by unconsolidated rock and soil that has moved downslope along
a relatively shallow failure plane. Debris slides are most likely to occur on slopes greater than 65
percent where unconsolidated alluvium overlie a shallow soil/bedrock. The shallow slide surface
is usually less than 5 m deep. The probability of sliding is low where bedrock is exposed, except,
where weak bedding planes and extensive bedrock joints and fractures exists parallel to the
slope.
Debris slides generally start with big rocks that start at the top of the slide and begin to break
apart as they slide towards the bottom. Debris avalanches are very fast and the entire mass seems
to liquefy as it slides down the slope. This is caused by a combination of saturated material and
steep slopes (figure 6).
Figure 5: Movement of slope along rock layer.
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Figure 6: Rock and Debris Slides occurs near roadway and damege or block the roadway
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Figure 7: Different types sudden slope failure
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10.3 Sediment Flows
In the sediment flow the material flows down hill mixed with water or air. They can be of two
types depending on the amount of water present.
Slurry Flows are sediment flows that contain between 20 to 40% water. As the water content
increases above 40%, slurry flows grade into streams. Major Types of slurry solifluction, debris
flow and mud flow.
Granular Flows are sediment flows that contain between 20 and 0% water. Granular flows are
possible with little or no water as well. Fluid-like behavior is given in these flows by mixing with
air. Each of these classes of sediment flows can be further subdivided on the basis of the velocity
at which flowage occurs. Major types of granuler flows are creep, earth flow again flow & debris
avalanche.
10.3.1 Solifluction
It produces distinctive lobes on hill slopes. These occur in areas where the soil remains saturated
with water for long period of time. Solifluction, also known as soil fluction, is a type of landslide
where waterlogged sediment moves slowly downslope, over impermeable material. It can occur
on slopes as shallow as 0.5 degrees at a rate of between 0.5 and 15 cm per year. It occurs in
periglacial environments where melting during the warm season leads to water saturation in the
thawed surface material (active layer), causing a form of downslope flow to occur. A Solifluction
lobe is a type of slope failure where sediments form a tongue-shaped feature due to differential
downhill flow rates (figure 8). Where the underlying ground is permanently frozen (permafrost),
the process is often called gelifluction.
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Figure 8: Rock and soil moves vertically back to the surface in Solifluction
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10.3.2 Debris Flows- these occur at higher velocities than solifluction, and often result from
heavy rains causing saturation of the soil and regolith with water. They sometimes start with
slumps and then flow down the hill forming lobes with an irregular surface consisting of ridges
and furrows (figure 9). They contain a mixture of rock fragments, soil, vegetation, water and, in
some cases, entrained air that flows downhill as a fluid (figure 10). Debris flows can be further
classified as mudflows and earthflows depending on the ratio of water to soil and rock debris.
Lahars are a special form of debris flow caused by volcanic eruptions. Some debris flows are
very fast. In areas of very steep slopes, they can reach speeds of over 160 km/hour. However,
many debris flows are very slow, creeping down slopes by slow internal movements at speeds of
just 30 to 60 centimeters per year.
Figure 9: Debris flow in hill slope area
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Figure 10: debris flow in the forest and city.
10.3.3 Mudflows are a highly fluid, high velocity mixture of sediment and water that has a
consistency of wet concrete. These usually result from heavy rains in areas where there is an
abundance of unconsolidated sediment that can be picked up by streams. Thus, after a heavy
rain, streams can turn into mudflows as they pick up more and more loose sediment. Mudflows
can travel for long distances over gently sloping stream beds. Because of their high velocity and
long distance of travel they are potentially very dangerous. Mudflows involve the downslope
movement of soil or unconsolidated, clay-rich sediment in a fluid motion. Mudflows occur when
the material within the sloped surface are saturated or nearly saturated with water. The slopes are
stable when dry, but become unstable when saturated with water. Mudflows occur on steep
slopes where vegetation is not sufficient to prevent rapid erosion but can occur on gentle slopes
if other conditions are met. Other factors are heavy precipitation in short periods and an easily
erodible source material. The speeds of mud flow at speeds as great as 100 km per hour and can
cause great damage to life and property (figure 11). Boulders as large as houses have been
moved by mudflows.
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Figure 11: Example shows the damage of house and other construciton due to mud flow
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10.3.4 Creeps are defined as very slow, usually continuous movement of regolith down slope.
Creep occurs on almost all slopes, but the rates vary. Evidence for creep is often seen in bent
trees, offsets in roads and fences, and inclined utility poles (figure 12). The combination of small
movements of soil or rock in different directions over time are directed by gravity gradually
downslope. The steeper the slope, the faster the creep. Creep makes trees and shrubs curve to
maintain their perpendicularity, and they can trigger landslides if they lose their root footing.
This happens at a rate that is not noticeable to the naked eye.
There are generally three types of creep: (a) seasonal, where movement is within the depth of
soil affected by seasonal changes in soil moisture and soil temperature; (b) continuous, where
shear stress continuously exceeds the strength of the material; and (c) progressive, where slopes
are reaching the point of failure as other types of mass movements.
Figure 12: Example of creap occurs in road slope and forest slope.
10.3.5 Earthflows are downslope, viscous flows of saturated, fine-grained materials, which
move at any speed from slow to fast (figure 13). Typically, they can move at speeds from 0.17 to
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20 km/h. Clay, fine sand and silt, and fine-grained, pyroclastic material are all susceptible to
earthflows. The velocity of the earthflow is dependent on water content in the flow itself.
Earthflows occur much more during periods of high precipitation, which saturates the ground
and adds water to the slope content. Water then increases the pore-water pressure and reduces the
shearing strength of the material.
Figure 13: example of Earthflows that damage the city
10.3.6 Debris Avalanches - These are very high velocity flows of large volume mixtures of rock
and regolith that result from complete collapse of a mountainous slope. A debris avalanche is a
type of slide characterized by the chaotic movement of rocks soil and debris mixed with water or
ice (or both) (figure 14). They move down the slope and then travel for considerable distances
along relatively gentle slopes. They are often triggered by earthquakes and volcanic eruptions.
Debris avalanches differ from debris slides because their movement is much more rapid. This is
usually a result of lower cohesion or higher water content and commonly steeper slopes.
Debris avalanches usually occur on large, steep volcanoes and are mainly caused by instability of
the volcano's slope. When a slope of a volcano is not stable it can easily collapse (possibly
riggered by volcanic earthquakes) causing debris to be transported away from the slope. The
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bigger the avalanche the bigger its speed and thus its danger. Debris avalanches and landslides
can produce numerous dangers. The mixture of debris from a landslide or avalanche with water
may produce harmful lahars. They also can dam rivers and cause flooding. Perhaps one of the
most important hazards that can be produced by avalanches or landslides is a tsunami.
Figure 14: Example of Debris avalanches in snow mountains
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Figure 15: Explains velocity wise classification of different types of landslide.
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Figure 16: Different types of load slide or Mass wasting Process.
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Table 1: Summarizes landslide based on the types of landslide involving failure (Varnes (1975).
Historical landslides (wiki )
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The Goldau on September 2, 1806
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The Cap Diamant Québec rockslide on September 19, 1889
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Frank Slide, Turtle Mountain, Alberta, Canada, on 29 April 1903
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Khait landslide, Khait, Tajikistan, Soviet Union, on July 10, 1949
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Monte Toc landslide (260 millions cubic metres) falling into the Vajont Dam basin in
Italy, causing a megatsunami and about 2000 casualties, on October 9, 1963
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Hope Slide landslide (46 million cubic metres) near Hope, British Columbia on January
9, 1965.
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The 1966 Aberfan disaster
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Tuve landslide in Gothenburg, Sweden on November 30, 1977.
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The 1979 Abbotsford landslip, Dunedin, New Zealand on August 8, 1979.
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Val Pola landslide during Valtellina disaster (1987) Italy
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Thredbo landslide, Australia on 30 July 1997, destroyed hostel.
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Vargas mudslides, due to heavy rains in Vargas State, Venezuela, on December, 1999,
causing tens of thousands of casualties.
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2007 Chittagong mudslide, in Chittagong, Bangladesh, on June 11, 2007.
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2008 Cairo landslide on September 6, 2008.
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The 2010 Uganda landslide caused over 100 deaths following heavy rain in Bududa
region.
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Zhouqu county mudslide in Gansu, China on August 8, 2010.
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Devil's Slide, an ongoing landslide in San Mateo County, California
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2011 Rio de Janeiro landslide in Rio de Janeiro, Brazil on January 11, 2011, causing 610
casualties so far.
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10.4 Major factors affecting landslides
Factors causing landslides to occur fall into two categories: 1) things increasing driving forces,
and 2) things reducing resisting forces.
Factors increasing driving forces:
1. Steepening the slope
2. Adding weight to (loading) the slope, especially the upper parts
3. Increasing the height of a slope (either by human or natural downcutting)
3. Seismic shaking
Factor reducing resisting forces:
1. Adding water to the slope --> causes increased pore pressure --> reduces frictional strength
2. Steepening the slope --> reduces normal stress, and thus reduces internal friction
3. Bedding, jointing, or foliation parallel to slope or dipping out of slope --> these discontinuities
are low-strength zones along which the rock can fail and slide out of the slope
4. Intrinsically weak materials (e.g., deeply weathered, sheared, unconsolidated, or clay-rich
materials)
5. Undercutting the slope --> reduces support
6. Removing vegetation, especially trees --> loss of root strength, also increased water in soil due
to reduced evaporation losses
7. Seismic shaking
Most of the factors causing landslides are similar to slope failure, however the two most
significant factors that are most critical for landslides are the rainfall and seismicity. The
following discussion explains their mechanism in the context of landslides.
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Majority of landslides are triggered by heavy rainfall. This is because the rainfall drives an
increase in pore water pressures within the soil. The Figure 17 illustrates the forces acting on an
unstable block on a slope. Movement is driven by shear stress, which is generated by the mass of
the block acting under gravity down the slope. Resistance to movement is the result of the
normal load. When the slope fills with water, the fluid pressure provides the block with
buoyancy, reducing the resistance to movement. In addition, in some cases fluid pressures can
act down the slope as a result of groundwater flow to provide a hydraulic push to the landslide
that further decreases the stability. Whilst the example given in Figures 17 and 18 is clearly an
artificial situation, the mechanics are essentially as per a real landslide.
Figure 17: Diagram illustrating the resistance to, and causes of, movement in a slope system
consisting of an unstable block
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Figure 18: Diagram illustrating the resistance to, and causes of, movement in a slope system
consisting of an unstable block
• Loss of suction forces in silty materials, leading to generally shallow failures (this may be an
important mechanism in residual soils in tropical areas following deforestation);
• Undercutting of the toe of the slope through river erosion.
In many cold mountain areas, snowmelt can be a key mechanism by which landslide initiation
can occur. This can be especially significant when sudden increases in temperature lead to rapid
melting of the snow pack. This water can then infiltrate into the ground, which may have
impermeable layers below the surface due to still-frozen soil or rock, leading to rapid increases
in pore water pressure, and resultant landslide activity. This effect can be especially serious when
the warmer weather is accompanied by precipitation, which both adds to the groundwater and
accelerates the rate of thawing.
Rapid changes in the groundwater level along a slope can also trigger landslides. This is often
the case where a slope is adjacent to a water body or a river. When the water level adjacent to the
slope falls rapidly, the groundwater level frequently cannot dissipate quickly enough, leaving an
artificially high water table. This subjects the slope to higher than normal shear stresses, leading
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to potential instability. This is probably the most important mechanism by which river bank
materials fail, being significant after a flood as the river level declines.
Figure 19: Groundwater conditions when the river level is stable
Figure 20: Groundwater conditions on the falling limb of the hydrograph.
In some cases, failures are triggered as a result of undercutting of the slope by a river, especially
during a flood. This undercutting serves both to increase the gradient of the slope, reducing
stability, and to remove toe weighting, which also decreases stability. Subsurface water, if
present in a slope or if it could develop during the life of a project, should be considered in slope
stability analyses. The presence of subsurface water in a slope can reduce effective stresses when
positive pore-water pressures develop, causing a reduction in shear resistance. Subsurface water
can also increase de-stabilizing forces in the slope via the additional weight associated with a
moist slide mass or via seepage forces.
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10.5 Seismicity
The second major factor in the triggering of landslides is seismicity. Landslides occur during
earthquakes as a result of two separate but interconnected processes: seismic shaking and pore
water pressure generation. The passage of the earthquake waves through the rock and soil
produces a complex set of accelerations that effectively act to change the gravitational load on
the slope. So, for example, vertical accelerations successively increase and decrease the normal
load acting on the slope. Similarly, horizontal accelerations induce a shearing force due to the
inertia of the landslide mass during the accelerations. These processes are complex, but can be
sufficient to induce failure of the slope. These processes can be much more serious in
mountainous areas in which the seismic waves interact with the terrain to produce increases in
the magnitude of the ground accelerations. This process is termed 'topographic amplification'.
The maximum acceleration is usually seen at the crest of the slope or along the ridge line,
meaning that it is a characteristic of seismically triggered landslides that they extend to the top of
the slope.
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