Landslide causes and slip surface techniques

A landslide is the downward and outward movement of rock, soil or debris down a slope along a slip surface, under gravity. It occurs when the driving shear stress exceeds the shear strength of the slope material.

Causes

Natural causes

• Steep slope, weak or weathered rock, and adverse orientation of joints and bedding.
• Heavy rainfall and rise of the groundwater table (increase of pore-water pressure).
• Earthquake shaking and volcanic activity.
• Undercutting of the toe by river or wave erosion.
• Loss of vegetation due to forest fire.

Man-made causes

• Cutting of the slope or toe for roads (the main cause in Nepal).
• Loading the top by buildings, dumps and reservoirs.
• Deforestation and poor drainage, leaking pipes and irrigation canals.
• Blasting and vibration, excessive mining or quarrying.

Techniques to determine the slip surface

The slip (failure) surface is the base of the moving mass; its depth and shape are needed to design stabilisation.

1. Surface mapping: location of the main scarp, cracks, bulges at the toe, and tilted trees or poles. It gives the approximate outline and shape of the slide.
2. Test pits and trenches: excavation exposes shear surfaces directly, shown by polished, slickensided or remoulded clay layers. Suitable for shallow slides.
3. Boreholes and core logging: drilling through the slide shows the change from disturbed to intact material; soft and sheared zones, loss of core, and different moisture identify the slip zone.
4. Inclinometer: a casing in a borehole is read at intervals; the depth at which the casing bends or shifts gives the slip surface and rate of movement. It is the most reliable method.
5. Slip indicators and shear strips: a flexible tube, a wire or a column of rings or beads in a borehole is sheared at the slip depth.
6. Time domain reflectometry (TDR): a coaxial cable in a borehole gives a signal change when it is sheared.
7. Geophysical surveys: seismic refraction, electrical resistivity imaging and ground penetrating radar show the contrast between the slide mass and the stable ground beneath.
8. Piezometers: water pressure readings help locate the water-bearing zone along the slip surface.
9. Back analysis: a trial slip surface is found by stability analysis for $FS = 1$ using the known strength and geometry.

Using more than one method gives better confidence.