Landslide causes and mitigation

A landslide is the downslope movement of a mass of rock, debris or earth under gravity. It occurs when the shear stress exceeds the shear strength.

Causes

1. Natural

• Steep slope and high relief; weak rocks (phyllite, shale, Siwalik mudstone), weathered rock and thick colluvium.
• Adverse geological structure: joints, bedding or foliation dipping out of the slope, faults and shear zones.
• Heavy rainfall, snow melt, and high groundwater (pore pressure).
• Earthquake shaking.
• River undercutting and toe erosion.
• Weathering and loss of vegetation.

2. Human (anthropogenic)

• Unplanned road cutting and slope excavation.
• Blasting, quarrying and mining.
• Deforestation and overgrazing.
• Irrigation and leakage of water, poor drainage.
• Loading by buildings and fills; reservoir fluctuation.

Mitigation (preventive and control) measures

1. Avoidance: hazard mapping and zoning; avoid building on unstable slopes.
2. Drainage: surface drains, catch drains, sub-surface drains and horizontal drains to reduce water.
3. Slope geometry: reduce slope angle, benching, removal of loose material at the head; add weight at the toe.
4. Retaining structures: retaining and breast walls, gabions, crib walls at the toe.
5. Reinforcement: rock bolts, anchors, soil nailing, wire mesh, shotcrete.
6. Bio-engineering: grass, shrubs and trees to bind the soil.
7. River training: spurs and check dams to prevent toe erosion.
8. Monitoring and early warning: inclinometers, crack gauges, rain gauges.
9. Community awareness and land use control.