What is a Landslide?
Landslides and slope instability cause problems in many parts of the world. A landslide is primarily a combination of several geological processes that include earth movements like extensive slope failure, rocks falling, and debris flow. These ground movements may take place in a coastal, offshore, or far onshore environment. Gravity, in addition to other factors that affect ground stability, is the main force that causes landslides.
Normally, the ground characteristics develop particular sub-surface soil conditions that cause the slope failure. However, the real landslide needs a trigger before it can be initiated. It is normally feasible to recognize the main landslide origin and the landslide triggers. The trigger may be a process concerning the weather, or a stimulation that produces instant slope reaction. Usually, such movement is generated due to an alteration in slope stresses.
Geological formations with this potential are most common in mountainous areas. Slope stabilization is necessary to reduce the probability of landslides.
Landslide Triggers – What Causes a Landslide?
Landslides may be caused due to several reasons that may be active individually or in combination. Some of the major causes are:
The most prominent landslide trigger is prolonged and heavy intensity rainfall. The trigger may also occur even if the rainfall intensity is moderate, but the rain duration and pore pressure are high. A universal landslide survey held in 2003 revealed that 90% of the landslides that occurred were activated by a heavy rainfall. Primarily, this is due to the enhancement of the pore water pressures in the soil.
Melting of Snow
In several cold mountain places, snowmelt is frequent and may be a vital process for the occurrence of landslides. This is particularly important when a rapid increase in the temperature initiates fast snow melting. The water produced infiltrates into the earth that has impermeable layers under the surface. The pore water pressures are increased rapidly, causing the initiation of the landslide process. This consequence is particularly significant when the warm weather is supplemented by precipitation that adds to the groundwater and increases the melt rate.
Rivers can damage the slopes, particularly during the floods that trigger a landslide. The slope disturbance increases the slope gradient, decreasing the stability. In some countries, this process is frequently observed after glacial floods that cause toe erosion along the water channel. As the flood waves pass, extensive land sliding normally occurs. This volatility continues for an extended period, especially during the succeeding periods of intense rainfall and floods.
Change in Water Level
Fast changes in the ground water level along a slope may trigger landslides. This is especially true when a slope is adjoining a river or another water body. As the water level close to the slope decreases swiftly, the ground water cannot dissipate as fast, causing a high water table. The slope is subjected to high shear stresses that lead to a potential instability. This is one of the most important methods by which a river bank fails.
Earthquake waves through the rocks and earth create accelerations that alter the gravitational forces on the slope. The vertical accelerations consecutively change the load on the slope; the horizontal accelerations cause a shearing force because of the landslide inertia. This process is complex, and can be enough to cause a slope failure. These processes are significant in the hilly areas where the seismic waves cause an increase in the ground accelerations. This development is called topographic amplification. The greatest acceleration generally occurs at the slope crest or beside the ridge line.
The movement of the earthquake waves through the ground can produce liquefaction. During this process, shaking causes the pore space reduction. The high density raises the pore pressure in the ground. The granular material is changed into flow slides that are rapid and can be extremely damaging. You can find detailed information about ill effects of soil liquefaction in Soil Liquefaction and Remedial Measures, also here at Bright Hub.
Landslide Warning Techniques
The material destruction and life loss caused by the landslides concerns many people. For this reason, sensors have been developed that are used for the landslide warning and detection. They work by monitoring slope, displacement, rainfall, and flow. A careful analysis of this data can assist in the determination of the weak spots and the ground water pressures. Data regarding the rate of slide movement is available that can forecast the state of the landslide process. The sensors can provide early information regarding harsh weather and slope conditions that are unsafe.
Geotechnical analysis, including the study of slope stability, seismic behavior, and assessment of the site, can also be useful to provide an early warning of the potential for a landslide.
- CDC Emergency Preparedness and Response Site, Landslide (Mudslide) Safety, www.bt.cdc.gov
- Author's experience and expertise
- SAARC Disaster Management Centre, Landslides, Saarc-sdmc.nic.in