Landslide analysis in Geographic Information Systems
Infiltration and drainage model
The rainfall-induced failures are the common occurrences in soil slopes. Depending upon the intensity and duration of rainfall and permeability characteristics of the slope, infiltration causes development of pore pressures. Rise in piezometric level in hillside slopes, decreases the effective stress, eventually leading to failure of soil mass. Hence prediction of piezometric levels constitutes an important element in the evaluation of landslide hazards. Suitable hydrologic model predicting the level of pore pressures induced for a particular intensity of rainfall and for the given soil, hydraulic and slope conditions should be coupled with the stability model. The lumped parameter model given by Wu et al (1996), gives the recharge of the groundwater as a function of rainfall characteristics and soil properties. Then a finite-difference solution should be used to calculate the pressure head at different points in a slope. Stability model Slope failures triggered by any causal factors, involves movement of mass or slide. The exact configuration of the movement and its mass is difficult to predict. Though, depending upon the material characteristics, slope conditions, suitable theoretical models could be used for analysis. Limit equilibrium solutions are available for stability analysis, depending on the assumption of the failure plane, whether it is plane failure of finite/ infinite slope or circular failures. However, in actual slope failures, the assumption of single slope failure is not realistic. It may include a slide of mass with circular failure surface and also with plane surface failure, as multiple slope failures are possible. Hence, the stability model should capture both the types of failures according to the conditions of its occurrences. Appropriate criterion could be established for identifying which type of failure would occur for the given conditions.  As Figure 3(a) shows a slope of height H, inclined at angle y, with a back slope a, composed of rock/soil with cohesion c, friction angle j, and unit weight g (Christian et al. 1997). For a failure surface (Figure 3(b)) inclined at an angle q, the static factor of safety F is  Ignoring the effect of the vertical acceleration on the calculated static factor of safety, the above equation becomes F*, while the ground acceleration is ah and the amplification factor in the slope is A,  The infinite slope model (Figure) is also used for plane slopes, where g, gs, gw are the unit weights of unsaturated and saturated soils and water, respectively; hs = hp/cos2b is the water level in the slope; b is the slope angle; cr and c’ are the cohesion due to roots and effective cohesion; j’ is the effective angle of internal friction; and z is the depth.  The stability analysis of a plane slope does not account for the possibility of failure where the local slope angles are smaller than the general slope angle. Hence, from the method of slices could be used to calculate the safety factor Fc, for failure along the circular arc. 
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