Water is the most important factor in most of the slope stability analysis. Pore water in soil can strongly influence the physical interaction among soil grains. Changes in pore pressures can directly impact the effective stresses, which in turn, affect both the shear strength and consolidation behaviour of soil. Therefore, analysis of pore fluid seepage plays an important role in the solution of many geotechnical problems, especially those concerning the stability analysis of slopes and retaining structures.
Failure of soil slopes, both natural and man-made, during or shortly after rainfall is a commonly occuring phenomena. Such rainfall related failures are often associated with tropical areas, where intense rainfall may occur seasonally, and the soils are residual soils derived from the underlying rock. Under these conditions, infiltration may result in large volume of water entering into unsaturated soil slope. Such infiltration may lead to the soil becoming fully saturated, or an increased degree of saturation, without full saturation being achieved.
Whenever loads are applied to the surface of a soil, they set up stresses within it. As the pore-fluid has a low compressibility, it will not easily change its volume. As a result, pore-fluid pressures are set up. In soils of low permeability, these excess pore-fluid pressures cannot escape except after the passage of much time, and are therefore likely to have a major influence on the behavior of the soil. Conversely, in soils of high permeability, the excess pore-fluid pressures escape so readily that to all intents and purposes they may be ignored. The term 'drained' is used here to denote absence of excess or stress change induced by pore water pressures, whereas, the term 'undrained' is used to denote their presence. It is also possible to have partly drained conditions after the escape of some of the excess pore fluid pressures.
There are two types of water flow in surface or subsurface: steady state flow and transient flow. In the steady state flow, the pore water pressure is constant whereas, it is always chaining in transient flow. Transient pore pressure is developed in response to short but intense rainfall and plays an important role in slope failure occurrences.