Why do cliffs slump?

Answer 1

There are a number of potential causes for the collapse of a cliff.

In general it happens because some event occurs which acts to reduce the strength of the rockmass.

The term rockmass is used to describe the solid rock and all the bedding, joint and any other fracture planes which are spread throughout the rock and act to reduce it's strength.

Under normal conditions, the rockmass is strong enough to support itself (if it wasn't it would have already collapsed until achieving a stable state!). However if there is a change in the conditions then this can lead to failure.

So what conditions can change that may cause failure?

Pore water pressure: during periods of heavy rainfall, water infiltrates the ground and can act to increase the groundwater table. It will also flow along any fracture zones or other discontinuities in the rockmass which are permeable. Normally the self weight of the rockmass (known as the in-situstress) acts to force these discontinuities together and the roughness of their surfaces and the resultant friction acts to stop movement. However the water is relatively incompressible and so forms a small barrier between the two surfaces which acts to reduce the strength of the interface by reducing friction and the shear strength. If the alignment of the interface in question is "bad" i.e. one along which a slip or failure would result in a block of rock falling from the slope or cliff face and the pore water pressure becomes high enough to negate the frictional resistance to sliding, then a rock failure will occur.

Fractures and discontinuities also commonly have a clay infilling. The pore water can cause these clay fills to swell (and soften) and then shrink again when they dry out. These shrink swell cycles can act to reduce the friction on the interface between two rock blocks and again increase the rick of failure and a rock slide occurring.

Weathering can also act to increase the dimensions of fractures (e.g. freeze thaw weathering and frost heave) which will reduce their strength and can act to reduce stability and may contribute towards a rockfall.

Changes in the stress state: Construction or the application of a large load on the top of the slope or cliff will increase the stress within the rock in the cliff face and this may exceed the strength of the discontinuities in certain sections leading to rockfalls.

A change in the slope or cliff geometry (e.g. cutting a steeper slope into a rock face when building a road) may also lead to instability and a rockfall.

This may also be caused by undercutting in coastal cliff faces due to wave action creating a wave cut notch leading to a loss of support to the overlying rockmass causing it to collapse.

Dynamic loading: The seismic waves / vibrations caused by earthquakes, explosions (from quarrying or during the blasting used during the construction of road cuttings through rock) can act to cause rock falls.

Answer 2

because the rock is impermeable they get all soft so it collapses x :]