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16-Civ-B3 Geotechnical Design · May 2013

Question 2 of 10: Practical limitations of the SPT in foundation design

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

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.

Note on the question numbering. The printed paper numbers two different Section B questions as “Question 9” — the retaining wall on page 5 and the drilled pier on pages 5–6 — and its Section B heading says “any three of the following four questions” while five questions are actually printed. The drilled-pier question is treated here as Question 10 so that every printed question has a unique number; no wording has been changed.

Question 2: Practical limitations of the SPT in foundation design (7 marks)

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 standard penetration test survives because it is cheap, rugged and supported by seventy years of correlations, not because it is a good test. Its limitations fall into three groups.

The measurement itself is not standard. The blow count depends on how much of the hammer's potential energy actually reaches the sampler. Donut, safety and automatic-trip hammers deliver roughly 45, 60 and 80 per cent respectively, so a raw N value is meaningless until it is corrected to $N_{60}$. Further corrections are needed for rod length below about 10 m, for borehole diameter above 115 mm, and for samplers designed for liners but used without them. Because N also grows with confining stress, an overburden correction $C_N$ is required before N values from different depths can be compared. Repeat tests by different crews on the same deposit commonly differ by 30 to 50 per cent.

Whole soil types are outside its range. In gravels and cobbles an oversize particle jammed in the shoe inflates N without telling anything about the deposit. In soft and sensitive clays N is 0 to 2, so the test resolves nothing, and the sampling disturbance it causes destroys the structure that governs the strength. In fine saturated silts and silty sands the transient pore pressures generated by driving make N depend on dilatancy rather than on density.

It is the wrong kind of test for a settlement calculation. The SPT is a dynamic, large-strain, fully disturbed test. It yields no stress–strain curve, no modulus, no pore pressure and no permeability, so every design quantity — relative density, friction angle, undrained strength, elastic modulus — comes from an empirical correlation with wide scatter and regional bias. Sampling is also discontinuous, usually at 1.5 m centres, so a thin soft seam that will govern settlement can pass unnoticed between two samples.

Canadian practice therefore treats the SPT as a profiling and index tool. The CFEM recommends the cone penetration test, and the seismic CPT with pore-pressure measurement, wherever deformation properties matter, with the SPT reserved for coarse granular soils, for correlation and for recovering a disturbed sample.