Landslides - Understanding

Let us first understand the movements that happen during landslides ((:
There are two nature of movements of landslides.
- Rotational Slide
- Slumping


Rotational Slide

Rotational slides move downward and outward on top of curved slip surfaces. Movement is more or less rotational about an axis parallel to the slope. This type of slide has an arcuate slide plane, along which sliding occurs and they produce slumps by their backward rotational movement. Rotational slides leave a series of concave slope forms. The rotational slide is characterized by sliding of land as a coherent large slump block.

Rotational slides are common on steep slopes and usually occur on weaker rocks. They are also commonly associated with permeable caprock overlying an impermeable substratum and characterized by over-steepened slopes such as marine cliffs or actively retreating escarpments. But sometime, rotational slides can take place on slope made up of one type of rock/soil.

Coherent blocks of material slip down across one or more converging slide planes so that back tilted slopes are found at the surface and may trap sediment or standing water. Rotational slides move only short distances, and their accurate movements tend to restore equilibrium soon because the driving mass decreases and resting mass increases.



Slumping

Slumps have rotational movements along a curved slide plane. The coherence of the mass is then lost. This movement also takes place as a flow, near the toe. It results in (a) an arcuate scar at the head where the material evacuated, (b) a linear tongue of mobile material and (c) a bulging toe dominated by flowage. Slumping results in sliding of masses of rock but not as coherent as a whole. The coherence of the mass is lost and often flow is observed at the toe of slumping. Usually, the resultant slope form is concave and sometimes the undulating nature of toe of the slope will depict a miniature series of concave and convex at the base of the slope.

There are several causes to this movement.
1) Heavy rainfall
Water increase pore pressures. This reduces the frictional forces between the soil particles which in turn results in destabilizing the slope. The presence of water also results in increase in weight, lubricating effect between rock layers and reduced friction between soil particles and between rock layers.

2) Geographical Conditions
The nature of rock structure (jointing planes and bedding planes) is an important factor. If permeable rocks such as Chalk, Sand or Cherts lies above the impermeable rocks, rotational landslides are very likely to happen. Water is able to percolate through the porous rocks but it cannot pass through the impermeable layer. As a result, water accumulated at the base of the impermeable rock layer, which leads to the increase in weight, lubricating effect between rock layers and reduced friction between soil particles and between rock layers.

3) Earthquakes
Earthquakes causes vibration and thus trigger slope instability. The vigorous shaking if an already-unstable slope by seismic waves may cause slope failure. The higher the magnitude of an earthquake, the more vigorous the shaking will be and thus the more mass wasting will occur.

4) Human Activities
Construction of roads and houses causes oversteepening of slopes as such activities results in the removal of toes which effectively reduces the resisting mass, leading to slope instability and in turn, slope failure. Building of houses on slopes will also add weight to slope, adding on to the fact that human activities introduces a lot of ground water beneath the homes, lubricating effect and weakening of the soil/rock layer beneath occurs.

This picture shows a slumping failure.