Mass movement is defined as the downhill transfer of regolith (weathered material such as earth/rocks).
Causes: Shear strength and shear stress
Shear strength is the internal resistance of a body to movement. Shear stress is the force acting on a body that causes movement downslope. Slope failure is caused by a disruption to the equilibrium of these two.
- Factors contributing to increased shear stress
- Removal of support through undercutting or slope steepening. This can be caused by erosion, weathering or faulting (caused by previous mass movement).
- Loading of the slope through weight of water, vegetation or sediment.
- Lateral pressure cased by freeze-thaw or root expansion.
- Short-term stresses such as earthquakes (destabilisation), trees moved by strong wind.
- Factors contributing to reduced shear strength
- Weathering effects: disintegration of rocks, hydration, solution - reducing cohesion.
- Changes in soil and ground-water pressure; saturation, weakening.
- Changes of structure, such as creation of fissures in clay, remoulding.
Water is vital in many slope failures. It can weaken a slope by increasing shear stress and decreasing shear strength. Water also reduces cohesion of particles by saturation. It decreases frictional strength also.
Types of mass movement
The types of processes can be classified in a number of ways, such as the type of movement, the rate of movement, and the type of material.