APPLICATION OF THE METHOD OF SLICES
The shear strength parameters are of paramount importance in slope stability calculations. The soils at the slip surface are at or near the critical or the residual state. You should use f9 5 f9cs in all slope stability calculations except for fi ssured overconsolidated clays. Progressive failure usually occurs in fissured overconsolidated clays. The appropriate value of f9 to use is f9r—the residual friction angle.
The measured undrained shear strength is often unreliable. You should use conservative values of su for a TSA. These values should ideally come from direct simple shear tests. Tension cracks in fi ne-grained soils tend to develop on the crest and the face of slopes in fi negrained soils. There are three important effects of tension cracks. First, they modify the slip surface. The slip surface does not intersect the ground surface but stops at the base of the tension crack (Figure 16.8). Recall from Equation that the depth of a tension crack is zcr 5 2su/g. Second, the tension crack may be fi lled with water. In this case, the critical depth is z9cr 5 2su/g9 and a hydrostatic pressure is applied along the depth of the crack. The net effect is a reduction in the factor of safety because the disturbing moment is increased. The additional disturbing moment from the hydrostatic pressure iswhere zs is the vertical distance from the top of the tension crack to the center of rotation (Figure). The factor of safety using Bishop’s simplifi ed method becomes
Third, the tension crack provides a channel for water to reach underlying soil layers. The water can introduce seepage forces and weaken these layers. The locations of the tension cracks and the critical slip plane are not sensitive to the location of the phreatic surface. Dams and cuts supporting reservoirs can be subjected to rapid drawdown. Consider the earth dam shown in Figure When the reservoir is full, the groundwater level within the dam will equilibrate with the reservoir water level. If water is withdrawn rapidly, the water level in the reservoir will drop, but very little change in the groundwater level in the dam will occur. In fi ne-grained soils, a few weeks of drawdown can be rapid because of the low permeability of these soils. Because the restraining lateral force of the water in the reservoir is no longer present and the porewater pressure in the dam is high,
Effect of tension
crack on the slip surface.
in a reservoir.
the FS will be reduced. The worst-case scenario is rapid, complete drawdown. If a partial drawdown occurs and is maintained, then the phreatic surface will keep changing and seepage forces (resulting from porewater pressure gradients) would be present in addition to the porewater pressures.