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5. (a) For the existing retaining wall as shown below (Figure 3), assume the wall is retaining material that has a shear strength of c'= 0 and $'= 35°, with ys = 18 kN/m?; the friction angle along the retaining wall base = 38°; groundwater level is 3.5m below the ground surface behind the wall and coincides with the wall base in front of the wall. You may ignore the contribution of the passive pressure. State clearly other assumptions as necessary for the calculation. Use the approach recommended in the First Edition of Geoguide 1 (16 marks). 20kN/m2 TV 3m 0.5m 3.5m 5m v 0.5m 7V 5m Figure 3 * Pmin Pmax Note: Soil bearing pressure, P = (17) where Ry’ is the effective vertical reaction force acting on the wall base, B is wall base width, and e is eccentricity of the point of action of the reaction force on the wall base (i) What are the maximum and minimum bearing pressure beneath the wall base? (6 marks) (ii) Is the reaction force at the wall base within the middle third” of the base? (2 marks) (iii) What is the factor of safety against sliding failure of the above retaining wall? (4 marks) (iv) What is the factor of safety against overturning failure of the above retaining wall? (4 marks) (b) List the key difference between the two limit states in the design of retaining wall. (4 marks) Page 4 of 5