Causes of slope failure

Slope failure occurs when the shear stress along a surface exceeds the shear strength of the soil. The causes can be grouped into those that increase the shear stress and those that decrease the shear strength.

A. Causes that increase shear stress

1. Removal of lateral support by erosion, undercutting or excavation at the toe.
2. Extra load on the top (surcharge): buildings, stored materials, vehicles, or fill placed on the slope.
3. Increase in slope steepness or height by cutting or filling.
4. Increase in weight of soil from rain or seepage water (higher unit weight).
5. Earthquakes and vibrations from blasting, traffic or machinery, which add dynamic forces.
6. Tension cracks filled with water, which cause hydrostatic thrust on the potential slip surface.
7. Transient loads such as wind or water pressure acting on the slope.

B. Causes that decrease shear strength

1. Increase in pore water pressure from rainfall, rising water table or seepage, which lowers effective stress.
2. Rapid drawdown of water against a slope (e.g. reservoir bank), leaving excess pore pressure inside the soil.
3. Weathering and chemical changes that weaken the soil or rock and reduce cohesion.
4. Swelling of clay and loss of cementation after wetting.
5. Progressive failure and strain softening, reducing strength from peak to residual.
6. Decay of roots and removal of vegetation, which reduces cohesion near the surface.
7. Liquefaction or loss of strength in loose saturated sand due to shaking.
8. Freeze-thaw and cracking that open fissures and weaken the mass.