6.4.1 Water pressure
The effect of water pressure in reducing effective stresses is very important in slope stability analysis. As the effect of various assumptions regarding specification of pore pressure distributions in slopes is not well understood, two methods are commonly used to specify pore pressure distributions within slopes. The most rigorous method is to perform a complete flow analysis, and use the resultant pore pressure in the stability analyses (figure 16). A less rigorous but more common method is to specify a water table, and the resulting pore pressures are given by the product of the vertical depth below the water table, the water density and gravity.
The water table method under-predicts the actual pore pressure concentrations near toe of a slope, and slightly over-predicts the pore pressure behind toe by ignoring the inclination of equipotential lines. Seepage forces must also be considered in the analysis. Flow analysis automatically accounts for seepage forces resulting due to difference in hydraulic gradient between any two points at the elevation.
Figure 16: Simulation of rain water infiltration and generation of water table