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3. (a) Explain, with the aid of diagrams, how the earth pressure on a vertical retaining wall is related to the movement of the wall relative to the retained soil. In your answer, you should distinguish between the at rest active, and passive states (15 marks) (b) It is proposed to support an excavation 3.0m deep in sand using a 70m long sheet pile retaining wall, as shown in Figure 3(b). The sand has effective stress shear strength parameters of op' = 30' and ' - 0 kPa. The unit weight of the sand above the water table is 17 kN/m. and the saturated unit weight is 20 kN/m'. Take the unit weight of water as 10 kN/m The water table is at a depth of 4.0m bgl on the active side and 1.5m bg on the passive side, as shown in Figure 23(b). Neglecting factors of safety and partiallactors, and assuming Rankine's theory is appropriate, carry out an appropriate check and state if the design of the wall is satisfactory Sheetple 3.0m Sand 0 = 30 d=OkPa V 17 kNm Y20 km 15m WT Fig. 23 (b) (75 marks) Alde Memoire: K = (1-sino(1+sin') K = (1+sino W(1-sino") (c) Would you advise a contractor to construct this design as is? In your answer consider a margin for uncertainty and suggest two ways that such a margin could be applied to the design. (15 marks)