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.
Bearing capacity and settlement are two different limit states, and they are reached at very different pressures. A shallow footing on a competent soil usually reaches an unacceptable settlement long before it approaches a shear failure, so the serviceability limit state, not the ultimate one, sets the allowable pressure.
On sands. A medium-dense to dense sand has a friction angle of 35 to 40 degrees, which puts $N_q$ and $N_\gamma$ in the tens or hundreds; the ultimate bearing capacity of even a modest footing runs to several megapascals, and dividing it by a factor of three still leaves an allowable value far above what a structure can be built to. Meanwhile the sand compresses immediately under load, and the classical Terzaghi and Peck design charts, along with every modern SPT- or CPT-based method, are written the other way round: they return the pressure that produces 25 mm of settlement. Settlement also grows with footing width at constant pressure, so on a large raft the serviceability limit becomes even more dominant.
On clays. Here the argument is about time rather than magnitude. A stiff clay easily provides an adequate bearing capacity, but the same load produces primary consolidation settlement that continues for years and is followed by secondary compression. For a wide foundation the stressed zone extends to a depth of the order of the footing width, so a large raft can settle hundreds of millimetres on a deposit that shows no sign of distress in a bearing capacity calculation. Only for a narrow, heavily loaded footing on a soft, normally consolidated clay does bearing capacity genuinely control.
What actually damages structures. Buildings are not damaged by uniform settlement; they are damaged by differential settlement and by the angular distortion it produces. Cladding cracks and doors bind at angular distortions near 1/500, and structural distress follows near 1/150 — both far below any state approaching collapse. Because soil properties, footing sizes and column loads all vary across a site, differential settlement is typically half to three quarters of the maximum total settlement, so limiting total settlement is the practical way of limiting distortion. Services, drainage falls and lift guides impose their own limits. All of this argues for a design driven by deformation, with the bearing capacity check retained as the guarantee against the catastrophic, not the everyday, failure.
