A 50-storey Commercial Building is to be Supported on Bored Piles (1 m diameter): Foundation Engineering Assignment, PSB, Singapore

Assignment Overview:

A 50-storey commercial building is to be supported on bored piles (1 m diameter) at a site with a soil profile as shown in Figure Q4. The sand fill has an average N-value of 10 and a total unit weight of 20 kN/m3 above and below the water table. The total unit weight of the marine clay is 16 kN/m3. The water table is at 3 m below the ground surface. The horizontal pressure coefficient, Ks, the interface friction angle of the sand fill, 8, and the limiting shaft friction in the siltstone, fs, are as indicated in Figure Q4.

(a) A consolidation test was conducted on a sample taken from 20 m depth (Point “A”). The preconsolidation pressure obtained from the test is 195 kPa. Base on the test result, do you anticipate any future consolidation settlement?

(b) It is anticipated that the water table will be permanently lowered by 2 m. Hence, there will be future settlements inducing a large downdrag on the piles. Where is the likely location of the neutral point? Compute the maximum downdrag on a single pile. The clay has a (cu/σ’v)nc ratio of 0.22, where cu = undrained shear strength, σ’v= effective vertical stress, and “NC” denotes normally consolidated.

(c) For piles subjected to downdrag, it is common not to include the transient live load in design. Explain the rationale behind it using a sketch showing the load distribution curve of a pile subjected to downdrag with and without the transient live load.

(d) Assume Grade 40 concrete and a factor of safety of 4.0. Compute the working load Qc for this pile using the following criteria:

Qc + ηQnsf <= Qall,strut

Qc + ηQnsf <= Qpsf / 1.5

where Qnsf is downdrag, Opsf is positive shaft resistance and η is a reduction factor with η = 1 for a single pile. The toe resistance is ignored because of the soft toe condition.