Question

Write the effects of water on slope stability

Answer 1

Soft, saturated foundation soils or ground water seepage plays a very important role in geotechnical failures. They act as a major factor affecting stability of slopes. The effect of water on slope stability is discussed below.

Effect on Cohesionless Soils - In case of Cohesionless soils, water does not affect the angle of internal friction. The effect of water on cohesionless soils below the water table is to decrease the intergranular stress between soil grains, which in turn decreases the frictional shearing resistance.

Effect on Cohesive Soils - Routine seasonal fluctuations in the G.W.T. (Ground Water Table) do not usually influence either the amount of water in the pore spaces between soil grains or cohesion. The forces of attraction between soil particles prevent water absorption unless external forces such as pile driving, disrupt the structure.

If absorbed moisture increases, it causes decrease in strength of cohesive soils. Water absorbed by clay minerals causes increased water content that decrease the cohesion of clayey soils.

Clay Fills - Excess pore water pressure are created when fills are placed on clay. As pore water pressure increases excessively, consolidation occurs and shear strength of clay increases being directly proportional to time, unless applied loads do not cause undrained shear strength. Simultaneously, the factor of safety increases as well.

Slaking - It's a process where sudden increase of moisture in rocks can produce pore pressure increase in trapped pore air.It is accompanied by local expansion and strength decrease. This mechanism causes disintegration of hard shales or claystones.

Cuts in clay - Cuts in clay reduces effective stress. This allows the clay to expand and absorb water, which leads to the decrease of the strength of the clay directly proportional to tine. As a result, the factor of safety of a cut slope decreases with time.