What is Landslide?
A landslide is the movement of rock, debris or earth down a slope. They result from the failure of the materials which make up the hill slope and are driven by the force of gravity. Landslides are known also as landslips, slumps or slope failure.
Landslides are being increasingly viewed as natural hazard. The concern regarding the lanslide problem is understood in the light of the fact that a majority of the landslides are triggered by natural causes including substantial rain falls, cloud bursts, earthquakes, etc and as such these are difficult to predict.
However, the landslide problem has increased in magnitude due to man-made activities as well. Large scale construction works involving dams, hydroelectric projects, mining activities, housing projects, extensive expansion of road network, as well as deforestation resulting from the exploitation of the forests, have all taken their toll of the fragile eco-systems of hill ranges. At the same time, increasing needs for defence of the country, development of hilly areas, providing uninterrupted communication systems to the isolated and far-flung areas, have all created a very high demand potential for developing and keeping the road communication network in hills always open. This standard has been formulated with a view to understand the landslide phenomenon, their evaluation and control methods for effective correction measures.
- The five principal types of mass movements such as falls, topples, slides, lateral spreads, flows and sixth as complex which is a combination of two or more principal types of movements and the recommended control measures are summarized .
THE BASIC TYPES OF LANDSLIDE MOVEMENT ARE: FALL, TOPPLE, FLOW, SLIDE & SPREAD
- Falls are commonly triggered by earthquakes or erosion processes.
- Topple is characterised by the tilting of rock without collapse, or by the forward rotation of rocks about a pivot point.
- Flow is the most destructive and turbulent form of landslide.
- Slide is one of the most common forms of failure and can be subdivided into translational and rotational slides.
- Spread phenomenon is characterised by the gradual lateral displacement of large volumes of distributed material over very gentle or flat terrain.
RECOMMENDED REMEDIAL MEASURES/METHODS
Landslide preventive techniques are divided in two groups:
- Direct methods, and
- Indirect methods. Direct methods are further subdivided into:
i. restraining structure, for example; retaining walls, anchored walls, restraining piles etc;
ii. easing of pressure by excavation;
iii. reconstruction of slope using reinforced earth; and
iv. rock reinforcement.
Indirect methods involve erosion control measures, improvement in surface and sub-surface drainage.
Erosion Control Measures
Plantation of grasses and shrubs to restore the vegetative cover on denuded slope help in arresting the surface erosion. Some of the techniques of establishing a vegetative cover on hill slopes are:
- Asphalt mulch treatment
For this treatment, the proposed slope area is prepared into vast seed beds by sounding off the tops, regrading or reshaping and finally raking the top soil about 20 mm thick. Seeds and the root slips of locally available grasses are dibbled 150 to 200 mm apart, root to root and row to row. An asphalt emulsion (mulch) of a suitable grade is then spread by a sprayer. The optimum rate of application of the emulsion is 0.9 litre/m2 which is about 1.0 mm thick film. The asphaltic film gradually disintegrates and its place is taken by a carpet of green vegetation.
- Slope treatment by jute/coir netting
The slopes are initially demarcated, graded and uniformally levelled. Seeding at the rate of 5 kg per acre or dibbling of the root slips of locally available grasses 150 mm apart row to row and plant to plant is done. The rolls of the coir/jute netting are then spread out on the slope prepared as above. The edges of the netting are firmly anchored in the ground using 150 mm iron nails. Due to the 'check dam' action, erosion of soil is prevented during rains and the danger of seeds and nutrients being washed away along with top soil is stopped. Thus, vegetation takes roots quickly and grows to cover the entire slope.
- Bally benching
This technique is used for control of surface erosion on slide areas and in preventing the deepening of gullies/chutes, caused by the eroding action of flowing water. Wooden ballies (posts) of 120 to 150 mm dia and 2.0 to 2.7 m long are vertically driven in rows into the slope. The spacing of ballies range from 0.60 to 1.20 m centre to centre. The ballies are embedded into the slope by about 1.0 to 1.50 m and protrude out by about 1.0 to 1.20 m. The vertical posts are tied with three tiers of horizontal runners about 80 to 100 mm dia from uphill side with the help of 6 mm dia and about 200 to 250 mm long nails or braced with galvanised wires of about 4.00 mm diameter. Finally the uphill side of gullies/chutes is backfilled with boulders to avoid erosion.
Catch water or interceptor drains, side drains and cross-drains constitute some of the important types of drains used in a system of surface drainage.
- Catch water or interceptor drains:
In order to intercept and divert the water from the hill slope, catch water drains shall be located very carefully, after the topography of the ground is studied in detail. Catch water drains shall be lined and properly maintained and shall be given a gradient of 1 in 50 to 1 in 33 to avoid high water velocity and possible wash out. A number of inter-connecting lined catch water drains may need to be reconstructed on the slope to collect the surface run-off if the area of slide is large. Water from the catch water drains shall be diverted into a chute or a natural hill-side drain or diverted by sloping drains and lead into culverts at a lower level finally to be lead through chutes into the nearest natural watercourse.
- Road side drains
Road side drains are provided on the road side at the foot of the hill slope to drain out water from the road surface and the water from the portion of the hill slope below the catch water drains. Road side drains are constructed of dry rubble stone masonry with semi-circular saucer, rectangular, trapezoidal, angle drain and kerb and channel drain in sections. Angle or kerb and channel drains (see Fig. 3) are suitable where road width available is restricted and in emergencies, it serves as an extra width and not easily damaged.
The slope of the bed shall be 1:20 to 1:25 to allow water to flow at self-cleaning velocity. If the grades are rather steep, the side drains shall be stepped to break the velocity of water or provided with small dry rubble stone masonry check walls to provide falls to minimize bed scour. A shoulder of 0.3 m width may be provided between the edge of the drain and the hill slope. Generally, lined side drains shall be constructed. However, unlined side drains are sometimes provided on hard/stiffer strata.
- Cross drains:
Cross drainage shall be provided at intervals of 4 to 6 per km depending upon the nature of the terrain to prevent the road side drains from being overloaded and flooding the road surface. These shall be provided at every point of natural nallah and water crossing. The cross drainage structures, are culverts, scuppers, causeways and minor or major bridges.
Control of surface water consists of two main parts
- The collection of run-off at the uphill boundary of any unstable area,
- Maximising run-off from the unstable area and controlling and collecting the run-off.