When a mass of earth or rock slides down the slope along a definite zone or surface, the movement is called Landslide. The primary factor behind this movement is gravity and is facilitated by moisture, which acts as a lubricating agent. The landslide starts with slow movement along a slip surface, followed by a more rapid movement of the separated portion of the earth mass. The slip surface is usually bounded by a crack, which distinguishes a slide from creep in which a continuous crack is often absent The chief types of landslides are (i) Slump, (ii) rock slides, and (iii) rock falls.
(1) Slump type Landslide:
In a nearly homogeneous cohesive material, such as clays and some soils, a slope fails primarily by shear and the slip surface is approximately cylindrical or spoon-shaped. The movement of the mass starts by cracking along a shearing surface and then the separated mass slides down rapidly. Such a slide is called a ”slump” or ”shear slide”. Slump is often accompanied by bulges at the toe.
(2). Rock Sliding:
When detached blocks of bedrock move down the hill, it is called a ”rock slide”. In a rockslide, the movement takes place on bedding planes, joints, or any other planes of weakness in the country rocks.
(3). Rock Fall:
From steep rock slopes, blocks of rock of varying sizes, which are loosened by weathering, suddenly fall in the downward direction under the influence of gravity. This phenomenon is called ”rock fall”. The rockfalls supply ”talus” which are commonly found at the foot of the cliff in the higher mountain region.
Causes of Landsliding:
The factors which promote landslides are (i) water, (ii) slopes, (iii) nature of rocks, (iv) structure of rocks, and (v) disturbance of equilibrium.
(1). Water: The essential conditions which cause landslides, are lack of support in front and lubrication behind. Thus water acts as a lubricant between the rock strata thereby making them slip down. Water acts in three ways to facilitate landsliding.
- Water diminishes the strength of rocks and thus helps in their movement.
- The water that seeps into the rock or soil, not only produces lubrication but also exerts an additional force on the grains tending to displace them along the direction of water movement.
- It adds weight to the material. Hence many landslides occur after rains.
- It exerts an expansive force on freezing.
(2). Slope: It has been observed that the majority of the earth or rock failures are confined to slopes. This indicates that the slopes are directly responsible for landslides. As a rule, the steeper the slope, the greater is the instability of such a mass.
(3). Nature of the Rocks: Unconsolidated sediments, such as clay, sand, gravel, etc. can not stand permanently along slopes greater than their angle of repose (about 35°) and are likely to be affected by landslides. Where weak and slippery rocks like shale, volcanic tuff, phyllites or mica-schist are present, they are not likely to remain stable on steep slopes for a considerable period of time.
(4). Structure of Rocks: Joints, fractures, shear zones, and bedding planes usually become the slip surface in case of rock slides. Landslides are particularly common on hill slopes where the dip of rock beds is also in the same direction.
Prevention of Landslides:
Landsliding is a terrible and disastrous natural phenomenon, which can cause a threat to the lives and infrastructures. Man, so far, has made various efforts in order to prevent landslides, and reduce their effect. The methods, which are commonly used for the prevention of landslides are as follows.
- Slides in the impervious material are prevented by reducing slopes. Efforts should also be made not to allow the additional water to enter into the material.
- In order to check the surface water to enter into the unstable ground, it is diverted and made to run off as rapidly as possible away from that area.
- In porous materials, the landslides may be prevented by increasing the internal friction of the mass by lowering the water content. Their water may be removed b drain pipes, by drainage through tunnels or by pumping from wells.
- In situations where slides may cause loss of life and property, the loose rock material is prevented from by constructing retaining walls, concrete piers, or by use of piling.
- At some places, the unstable and unconsolidated materials may be consolidated by cement grouting, chemical means, and artificial freezing.
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