3. A steel pile of hollow tubular section (with a base plate), 10 m long, 600 mm external diameter, and 16 mm wall thickness is driven to full-length penetration into the soil profile given in Table Q3.1. Table Q3.1 Depth (m) Description Unit weight y (kN/m3) Angle of friction ¢' (°) 0-2.3 loose silty sand (partially saturated) (17.4+Z) (30+X) 2.3-7 loose silty sand (saturated) (18.2+Z) (30+X) 7-13 dense sandy gravel (19.3+Z) 36 Underlying the sandy gravel is firm to stiff silty clay of unknown thickness, and the water table is 2.3m below ground level. Interface tests indicate that 8'/ o' for the steel pile can be taken as 0.75. The variables X and Z are defined on the cover page. (a) Showing all of your calculations, estimate the ultimate load capacity of the pile, stating any assumptions made. [22] (b) How much force would be required to pull the pile out of the ground? Again, show all calculations, state any assumptions, and take the density of steel as 7850 kg/m3. [5] (c) It is proposed to drive a group of 28 of these steel tubular piles in a 7 × 4 rectangular grid, at 3 m spacing in both directions, to support a foundation raft for an industrial building. Explain how the single pile capacity obtained in (a) can be used to estimate the capacity of the group, and what design checks need to be carried out. [6]

Fig: 1