Landslide clearance is a main maintenance activity in the case of -roads in many hill ranges of India. The Himalayas have been observed to be a scene of most frequent landslide instances. The other hill ranges can be grouped in decreasing order of landslide proneness as North-Eastern hill ranges, Western ghats, Nilgiris, Eastern ghats and Vindyas. Quite often landslides result in loss of valuabe life and property apart from traffic hold ups. Control of landslides is an on-going process and requires in-depth study of a number of aspects. Soil mass beneath a slope has tendency to slide downwards or outwards under the influence of gravity. If the shear -strength of soil is adequate to counteract this tendency, the hill slope remains stable otherwise a slide occurs. Landslide prevention techniques basically adopt measures to decrease activating force and or increase resisting force..
Till recently methods of landslide correction in India relied to a great extent on individual experience- without taking proper recourse to appropriate geological investigations in the field. Detailed geological and laboratory investigations together with proper design of preventive and control measures are essential to have a stable slope. Because of highly hetrogenous and variable nature of rock formation as well as geological environment in which they are found, it is difficult to achieve a high degree of standardisation in regard to landslide control measures. However, a fairly correct measure can be determined from the broad spectrum of available measures with the help of experience and expertise and from the investigations and test results.
Investigations for working out landslide control techniques basically serve two main objectives:
- To identify landslide prone areas before the construction work is taken up so as to suggest preventive measures or alternative routes that are not susceptible or less susceptible to landslide hazard.
- In an existing slide site, it helps to identify factors responsible for the instability or movement and to determine the appropriate corrective measures needed to prevent or minimise recurring instability problems
Common methods adopted for landslide prevention can be divided into two types viz. Direct Method and Indirect Method. Direct Method consists of construction of retaining walls constructed in stone, RCC, CC, wire crates, wooden ballis etc., according to the strength required and depending upon situations. Apart from this, Timber, Metal or Concrete cribwalls and sausage walls are also used as restraining structures. Butteresses are often used as retaining device to prevent landslide on the hill side. Anchor walls, restraint piles, prestressed rock anchors, tied back walls, reinforced earth, geo-grids and soil nailing are also adopted for resisting slides. Other techniques consist of design of proper slopes in soil, use of proper material to increase shear strength. excavation or removal of unsuitable materials, benching of slopes, change of grades, growing vegetation on slopes and giving slope treatment using jute or coir netting, geo- fabrics etc. The Indirect Method of landslide prevention consists of methods for effecting proper surface and sub- surface drainage. This is achieved by construction of properly designed catch water drains, road slide drains, cross drains, horizontal drains in sub-surface, deep drainage drains etc.
After working out the appropriate measures for landslide prevention and constructing the same in the field, monitoring of their performance is very vital for a feed back and to take any advance action if there is an impending failure. Geo- technical instrumentation has a vast role to play in monitoring the stability of the slopes and also in gathering factual information on performance of structures located thereon. Generally, instruments are required to measure subsidence or settlement of slopes, lateral displacement, tilting of structures, measurement of pore water pressure etc. A number of modern instruments are available for this purpose.
The Indian Roads Congress is shortly bringing out a State-of-the-Art Report on 'Landslide Correction Techniques'. This exhaustive publication will be dealing with various aspects of landslide investigation requirements, correction techniques and monitoring procedures, It is hoped that this Report will be of immense use to the practicing highway engineers.
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