Section A

1. Attempt this entire question. Each part is worth 8 marks.

(a) Figure A1a shows a borehole log and undrained shear strength profile. It is proposed to excavate to 1m below ground level and construct a 2m square, 0.5m deep concrete foundation carrying a total load of 200kN and a variable load of 100kN (Figure A1b). The excavation is not backfilled. The groundwater level is 1.5m below ground level. Using Design Approach 1 Combination 1, determine whether the foundation is acceptable.

Figure A1a The soil and soil property profiles

Figure A1b Cross-section of foundation

(b) 0.3 diameter CFA piles will be used to support the slurry tank shown in Figure A1b. The tank will be a piled raft supported by 20 piles at 1m centers. The ground level around the building will be raised by 2m using engineered fill after the tank is built. Assume that the walls and floor of the tank are 0.2m thick and the density of the slurry is 1100kg/m3. Produce a diagram showing the forces acting on the pile group and calculate.

Figure A1b Cross-section showing the soil profile and location of tank supported by piles

(c) A 15m high, 8m diameter steel grain silo sits on a square reinforced concrete foundation, which rests on stiff clay. The foundation is 1.5m deep with the formation level at 1m below ground level. The mass of the silo including the weight of the foundation is 45 tonnes. The silo stores grain with a density of 790kg/m3. The
maximum wind load produces a horizontal force of 30kN acting at 10m above ground level. Determine the maximum and minimum contact stress between the base of the foundation and the soil when the silo is full.

(d) A 15m long, 400mm diameter steel circular pile was driven through 1m of firm clay, 5m of normally consolidated soft clay into dense sand. The pile was installed in winter when the groundwater level was at 1m below ground level. The following summer there is a drought. This results in the groundwater level falling to 2m below groundwater level.

Determine what the effect would be on the load acting on the pile. Assume that the unit weight of the firm clay is 18kN/m3 and the ratio of its undrained strength to the vertical effective stress is 0.5. The unit weight of the soft clay is 16kN/m3 and the ratio of its undrained strength to the vertical effective stress is 0.3. The SPTN60 of the dense sand is 15.

(e) 4.5m diameter, 3m high steel water tank sits on a 5m diameter, 0.5m thick concrete foundation with a formation level at 1m below ground level as shown in Figure A1e.

The soil profile shown in Figure A1e is taken from the ground investigation. Calculate the time it takes for the foundation to settle.

At the time of construction, the firm to stiff clay was found to extend to 10m below ground level. Would this change the time for the settlement to occur and, if so, how long would it take?