16-Civ-B3 Geotechnical Design · May 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Examinations, May 2013 — 98-Civ-B3 Geotechnical Design. Three hours, open book, any non-communicating calculator. Section A holds five 7-mark questions (answer any four); Section B holds the long 24-mark design questions (answer any three). Candidates are asked to identify the source of every design chart and assumed value used. Every question is answered here, because the set is a study resource rather than a sitting.
Reference texts. B. M. Das, Principles of Foundation Engineering (9th ed.) and Principles of Geotechnical Engineering (9th ed.); Canadian Geotechnical Society, Canadian Foundation Engineering Manual (CFEM, 4th ed.) — the governing Canadian reference for foundation practice; D. P. Coduto, Foundation Design: Principles and Practices; R. F. Craig, Craig's Soil Mechanics.
Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.
The statement is a design convention rather than a statement of physics: some shaft resistance certainly does develop in the weaker layers, but a prudent designer refuses to count on it. Four arguments justify that refusal.
Strain compatibility. Shaft friction and end bearing are mobilized at very different displacements. Skin friction reaches its peak at a relative pile–soil movement of only 5 to 10 mm, essentially independent of pile diameter, whereas end bearing on a dense gravel needs a tip settlement of roughly 10 per cent of the pile diameter. When the toe rests on a very stiff stratum the pile behaves almost as a rigid column: the toe barely moves, so the shaft above it cannot slip far enough relative to the weak soil to develop much friction at working load. The load simply travels down the shaft to the bearing layer.
The weak layers are not dependable. Soft, compressible strata are exactly the soils whose properties change with time and with construction. Driving or boring remoulds them; excess pore pressures generated during installation dissipate over months; a nearby excavation, a dewatering scheme, scour at a river crossing or a new surcharge fill can all remove or reverse the friction that a load test measured on day one.
Negative skin friction. If the weak layers consolidate under fill, groundwater lowering or their own self-weight, they move downwards relative to the pile. The shaft resistance then reverses sign and becomes a downdrag load that must be added to the structural load rather than subtracted from it. Assuming zero contribution from those layers is the first step towards recognising that hazard; the second is to check the neutral plane explicitly.
Consistency with the capacity check. The ultimate capacity of an end-bearing pile is dominated by the bearing stratum; adding a small, uncertain friction term changes the answer by a few per cent while introducing a large uncertainty. The Canadian Foundation Engineering Manual accordingly recommends that a pile driven through compressible soils to a dense bearing stratum be designed as an end-bearing pile, with shaft resistance in the weak soils neglected and downdrag assessed separately.
