A 900 mm diameter bored cast-in-place reinforced concrete pile is to be installed in the following ground profile: Depth below GL (m) 0-4.5 4.5-34.0 >34.0 Table Q3.1 Description Weathered stiff silty clay Unweathered stiff to very stiff silty clay Unknown - boreholes terminated at this depth Undrained shear strength c (kPa) (45 + X) (75 + X) + 5z' where z' is the depth below the top of the unweathered layer (i.e. depth below ground level - 4.5 m). (a) Assuming an adhesion factor of 0.80 in the weathered clay and 0.45 in the unweathered clay, calculate the minimum length of pile required to carry an ultimate load of 4500 kN. Describe and justify all other assumptions made in your calculation, and comment on your answer. [21] (b) Sketch the typical expected variation of (i) shaft load, (ii) base load, and (iii) total load against settlement, for a pile of this type. Use your sketch to explain why (in a lumped factor approach) it is appropriate to use different load factors on the shaft and base components when estimating the safe working load of the pile. [4] (c) A group of 20 of the piles described in part (a) is to be installed and connected by a reinforced concrete pile cap at ground level, to provide foundation support to a structure which will impose a total load of 32 MN. In plan the group will be formed of 5 rows x 4 columns, at a centre-to-centre spacing of 3 m in both directions. An efficiency factor of 0.9 should be assumed. Estimate the ultimate undrained capacity of the group against (i) single pile failure, and (ii) block failure, and comment on your answers. [8]