Posted by: drazizul | December 20, 2009

Lecture 7: Landslide

Landslide in Ymaguchi, Japan 2009

A landslide (or landslip) is a geological phenomenon which includes a wide range of ground movement, such as rock falls, deep failure of slopes and shallow debris flows, which can occur in offshore, coastal and onshore environments.
Although the action of gravity is the primary driving force for a landslide to occur, there are other contributing factors affecting the original slope stability.
Typically, pre-conditional factors build up specific sub-surface conditions that make the area/slope prone to failure, whereas the actual landslide often requires a trigger before being released.

Damage caused by landslide
block roads;
damage and destroy homes;
locally disrupt water mains, sewers, and power lines;
damage oil- and gas-production facilities.
Kill many people

Concepts

Earth materials are subject to movement under the force of gravity:
– these are generally termed landslides, slope failure or mass wasting
when movement is strictly vertical it often called
subsidence
when dominated by water (e.g like a slurry) they will be termed debris flows .

Landslides occur and can cause damage. Severe storms, earthquakes, volcanic activity, coastal wave attack, and wildfires can cause widespread slope instability. Landslide danger may be high even as emergency personnel are providing rescue and recovery services.
To address landslide hazards, several questions must be considered:
Where and when will landslides occur?
How big will the landslides be?
How fast and how far will they move?
What areas will the landslides affect or damage?
How frequently do landslides occur in a given area?
Answers to these questions are needed to make accurate landslide hazard maps and forecasts of landslide occurrence, and to provide information on how to avoid or mitigate landslide impacts.

Landsliding is controlled by the ratio of resisting to driving forces

Factor of safety (FS) for slope stability is equal to this
ratio
FS greater than 1 means slope considered stable
Driving forces are increased by:
steep slopes (gravitational driving force oriented more
parallel to slip planes)
increasing weight on a slope
Resisting forces are weakened

Causes of landslides

Landslides are caused when the stability of a slope changes from a stable to an unstable condition. A change in the stability of a slope can be caused by a number of factors, acting together or alone. Natural causes of landslides include:
groundwater (porewater) pressure acting to destabilize the slope
Loss or absence of vertical vegetative structure, soil nutrients, and soil structure (e.g. after a wildfire)
erosion of the toe of a slope by rivers or ocean waves
weakening of a slope through saturation by snowmelt, glaciers melting, or heavy rains
earthquakes adding loads to barely-stable slopes
earthquake-caused liquefaction destabilizing slopes (see Hope Slide)
volcanic eruptions

Man made cause
landslides are aggravated by human activities, Human causes include: deforestation, cultivation and construction, which destabilize the already fragile slopes
vibrations from machinery or traffic
blasting
earthwork which alters the shape of a slope, or which imposes new loads on an existing slope
in shallow soils, the removal of deep-rooted vegetation that binds colluvium to bedrock
Construction, agricultural or forestry activities (logging) which change the amount of water which infiltrates the soil.

Timber Harvesting: clear-cutting reduces transpiration,
increasing soil moisture content. Landslides often occur
in forested areas of the Pacic Northwest in spring after a
clear-cut is completed the prior fall
Urbanization
{ poor slope design (e.g. use slump as part of house pad,
{ excess irrigation
{ overloading slope via construction
{ cut-o toe of pre-historic landslide

Human causes
Excavation
Loading
Drawdown
Land use change
Water management
Mining
Quarrying
Vibration
Water leakage
Deforestation

Geological causes
Weak materials
Sensitive materials
Weathered materials
Sheared materials
Jointed or fissured materials
Adversely orientated discontinuities
Permeability contrasts
Material contrasts
Rainfall and snow fall

Morphological causes
Slope angle
Uplift
Rebound
Fluvial erosion
Wave erosion
Glacial erosion
Erosion of lateral margins
Subterranean erosion
Slope loading
Vegetation change

Physical causes
Intense rainfall
Rapid snow melt
Prolonged precipitation
Rapid drawdown
Earthquake
Volcanic eruption
Thawing
Freeze-thaw
Shrink-swell
Ground water changes
Soil pore water pressure
Surface runoff
Seismic activity

Triggers of landslides

Water
Rainfall
In the majority of cases the main trigger of landslides is heavy or prolonged rainfall. Generally this takes the form of either an exceptional short lived event, such as the passage of a tropical cyclone or even the rainfall associated with a particularly intense thunderstorm or of a long duration rainfall event with lower intensity, such as the cumulative effect of monsoon rainfall in South Asia.
In the former case it is usually necessary to have very high rainfall intensities, whereas in the latter the intensity of rainfall may be only moderate – it is the duration and existing pore water pressure conditions that are important. The importance of rainfall as a trigger for landslides cannot be under-estimated.
A global survey of landslide occurrence in the 12 months to the end of September 2003 revealed that there were 210 damaging landslide events worldwide. Of these, over 90% were triggered by heavy rainfall. One rainfall event for example in Sri Lanka in May 2003 triggered hundreds of landslides, killing 266 people and rendering over 300,000 people temporarily homeless.

Snowmelt

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.

Water-level change
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 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 is declining (i.e. on the falling limb of the hydrograph) as shown in the following figures.

Rivers
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.
For example, in Nepal this process is often seen after a glacial lake outburst flood, when toe erosion occurs along the channel. Immediately after the passage of flood waves extensive landsliding often occurs.
This instability can continue to occur for a long time afterwards, especially during subsequent periods of heavy rain and flood events.

Disaster Minimization
Simplest: identify hazard areas in advance and don’t allow building there

Consumer: before you buy on a slope, look for signs of sliding!

Zoning: establish grading codes, require engineering geology study before construction

a) Surface Drainage Control Works
The surface drainage control works are implemented to control the movement of landslides accompanied by infiltration of rain water and spring flows. The surface drainage control works include two major works: drainage collection works and drainage channel works. The drainage collection works are designed to collect surface flow by installing corrugated half pipes or lined U-ditches along the slopes, and then connected to the drainage channel. The drainage channel works are designed to remove the collected water out of the landslide zone as quickly as possible, and are constructed from the same materials as the drainage collection works. The surface drainage control works are often combined with the subsurface control works

The purpose of the subsurface drainage control works is to remove the ground water within the landslide mass and to prevent the inflow of ground water into the landslide mass from outside sources. The subsurface drainage control works include shallow and deep subsurface drainage control works.

Intercept Under Drains and Interceptor Trench Drains These systems are most useful to remove shallow ground water from up to 3m from the ground surface. The interceptor under drains contain impervious sheets at the bottom of the trench, and the gravels are wrapped with filter fabric and the drains are connected at groundsils and catch basin. Structurally, the interceptor trench drain is a combination of the interceptor under drain and surface drainage control, and are commonly used .

Horizontal Gravity Drains In order to remove the shallow groundwater within about 3m from ground surface, 30 to 50 m-long horizontal gravity drains are drilled. The pipes could be either perforated P.V.C. (polyvinyl chloride) or steel construction, and should be drilled at an angle of 10 to 15 degrees from the horizontal line .

Drainage Wells Drainage wells of up to 25m deep and at least 3.5m in diameter are excaveted within areas of concentrated ground water. A series of radially-positioned horizontal gravity drains with multi-levels are drilled to collect the ground water into the drainage wells where the water can be removed through drainage tunnels. They are constructed of either steel or reinfored concrete segment, and concrete is used at the well bottoms and the upper portion of the well .

The primary purpose of the drainage tunnels (which are constructed below the slide plane) is to remove collected water out of the landslide mass by interconnecting the drainage wells. Instead of excavating the drainage wells from the ground surface, they can be constructed upward from the drainage tunnels. The series of gravity drains drilled from the tunnel tends to increase the effectiveness of the drain system. This is the most effective and reliable drainage work where numerous ground water veins exist within the landslide mass Furthermore, this work is effective to maintain existing facilities. Generally, the diameter of the tunnel is between 1.8 and 2.5m, and the drainage channel is constructed along the invert.
c) Soil Removal Works
This is one of the methods where the most reliable results can be expected, and generally applies to small to medium sized landslides. Except for special cases, the soil removal is focused on the head portion the slide (Fig.46).
d) Buttress Fill Works
The buttress fill is placed at the lower portions of the landslide in order to counterweight the landslide mass. It is most effective if the soils generated by the soil removal works are used (Fig.47).
e) River Structures
Degradation and channel bank erosion reduce earth stability and often tends to induce slide activity. In such cases, check dams, groundsils and bank protection can be constructed to prevent further erosion.

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Responses

  1. Dear DR. Aziz.
    Konnichiwa
    It was for a long time not to meet you. Ogenki desuka?
    Wow, I surprised when know that you are now are still stay in Saga and You are a teacher of my former lovely campus.
    I hopefully can keep in touch with you.
    Ja mata ne. Soiginta.


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