(v) Steady Seepage with Sustained Rainfall If the downstream shell is relatively pervious, the pore pressures can increase appreciably by the penetration of rain water. In such situations, the gravity flownet should be constructed assuming that the dam crest and the downstream slope are sources of supply for seepage water (4) (Fig. 15.26). The downstream slope is analysed for stability assuming that partial
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saturation occurs due to rainfall. The saturation for the downstream shell is taken as 50 per cent (for coefficient of permeability of the shell less than 10–4 cm/s and 0 per cent for coefficient of permeability of the shell more than 10–2 cm/s) or a suitable value between 0 and 50 per cent depending upon the value of the coefficient of permeability of the shell material (12).
