Mass movement and its mechanism

Mass movement (mass wasting) is the downslope movement of rock, debris or soil under the influence of gravity, with or without water as a helping agent. Landslides, rock falls, creep, debris flows and subsidence are all forms of it.

Mechanism

A slope fails when the driving (shear) forces exceed the resisting forces (shear strength). For a mass of weight $W$ on a slope of angle $\beta$:

$$\text{Factor of safety } F = \frac{\text{Resisting force}}{\text{Driving force}} = \frac{c A + W\cos\beta\tan\phi}{W\sin\beta}$$

Failure begins when $F<1$.

• Increase of driving force: extra load (buildings, fill, rainwater), steepening by river cutting or road excavation, removal of toe support, earthquake shaking, vibration.
• Decrease of resisting force: rise in pore water pressure (rain, snow melt, reservoir drawdown), weathering, loss of cohesion, removal of vegetation, clay softening, joint opening.

Steps

1. The slope is made unstable by preparatory factors (weak rock, steep slope, adverse structure).
2. A trigger (heavy rain, earthquake, cutting) acts.
3. Tension cracks form at the crown.
4. A slip surface develops and the mass moves downward by sliding, falling, toppling or flowing.
5. The material comes to rest on a gentler slope and forms a deposit.