SOIL MECHANICS
Q1. Shown in Figure 3 below is a concrete dam constructed on a uniform layer of soil underlain by an impermeable stratum. An impermeable sheet pile is situated at the upstream face of the dam and extends for a depth of 15 m into the soil layer. The depth of water behind the dam is 18 m and the coefficient of permeability of the soil is k = 169 x 10-4 cm/sec. Calculate (a) the seepage loss and (b) the distribution of uplift pressure on the base of the dam. (0.94 m3/d perm)
For a cut-off wall located at the midpoint of the base of the dam (Figure 3), re-compute (a) and (b). Comment on the effect of the position of the cut-off wall on the seepage loss and uplift pressure. (1.01 m3/d perm)
Q2. Figure 4 shows a cross-section through a long cofferdam. It is proposed to dewater the cofferdam by lowering the water levels inside it to the floor of the excavation. Investigate the suitability of this proposal by means of a carefully sketched flow net on an appropriately transformed cross-section. How might the stability of the base be ensured?
Q3. A retaining wall 5 m high with a vertical back is provided with a drain as shown in Figure 5. It rests on an impermeable stratum. The backfill is saturated, with the water table established at its upper horizontal surface and rain falls at such a rate that this water level is maintained. Draw the flow net and compare the horizontal thrust on the wall by the water with the case of no drain.
Q4) A quantity of 2000 ml of water required 20 min to flow through a sand sample, the cross-sectional area of which was 60 cm2. The void ratio of the sand was0.71. Compute the velocity of water moving through the soil and the actual (interstitial ortrue) velocity.