16-Civ-B3 Geotechnical Design · May 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. EGBC / Engineers Canada National Examination 16-Civ-B3 Geotechnical Design, May 2018. Three hours, open book, any non-communicating calculator. Section A — five discussion questions of 7 marks each, answer any four. Section B — four design questions of 24 marks each, answer any three. Examinable total $4\times 7 + 3\times 24 = 100$ marks. Page 1 Note 6 requires the candidate to identify clearly the source of every design chart and assumed value used, so the provenance of each correlation is named where it is used, not only in the concept notes. All nine questions are solved below, because the set is a study resource rather than a three-hour sitting.
Reference texts for this subject. B. M. Das, Principles of Foundation Engineering, 9th ed. (Cengage) — Ch. 3 (subsurface exploration and SPT corrections), Ch. 4 (bearing capacity), Ch. 5 (settlement, Schmertmann), Ch. 8 (retaining walls), Ch. 11–12 (pile foundations and drilled shafts); B. M. Das, Principles of Geotechnical Engineering, 9th ed. — Ch. 8 (shear strength), Ch. 15 (slope stability); R. D. Holtz, W. D. Kovacs & T. C. Sheahan, An Introduction to Geotechnical Engineering, 2nd ed.; Canadian Geotechnical Society, Canadian Foundation Engineering Manual (CFEM), 4th ed. — the Canadian design authority for the factors of safety and serviceability limits quoted here; L. C. Reese & M. W. O'Neill, Drilled Shafts: Construction Procedures and Design Methods (FHWA-HI-88-042).
Check — figure readings. Two dimensions are read from the drawings, as follows. (1) In Figure 2 the “1 m” dimension is the height of the bell: its arrows point inward at the flare, and scaling against the 4 m dimension on the same figure puts the bell base exactly on the 12 m line. The pile is therefore $L = 12$ m long with the bell top at 11 m, not 11 m long with its base floating 1 m clear of the layer base. (2) In Figure 3 the “0.35 m” label carries extension lines from the top and bottom corners of the base slab, so it is the base thickness; the “0.5 m” at the left is measured to the base underside, so only 0.15 m of soil covers the toe. The toe projection is not dimensioned and follows from the printed values as $4.0 - 0.3 - 2.0 = 1.7$ m. Figure 3 is not drawn to scale — its toe is drawn about half its dimensioned length — so the printed numbers govern, as page 1 Note 1 anticipates.
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.
[Figure not reproduced: Figure 1 (redrawn). The six investigation families offered by the question. Settlement of a sand is a stiffness problem, so the two recommended methods are those that measure stress and strain directly in the ground. See the official exam paper.]
Settlement of a foundation on sand is governed by the stiffness of the deposit, not by its strength, and it is essentially immediate: the sand is free draining, so the whole of the movement occurs as the load is applied and there is no consolidation stage to be predicted from a laboratory compressibility test. That single fact disqualifies most of the toolbox in Figure 1 at once. A clean sand cannot be sampled undisturbed by any routine method — driving or pushing a tube destroys the very fabric, density and interlocking that control stiffness — so the sampling line of investigation cannot deliver a defensible modulus. Piezometers measure pore pressure, which fixes the effective stress at which the sand is working but says nothing about how it deforms. Geophysical methods (seismic refraction, cross-hole or MASW shear-wave surveys) do measure a modulus, but it is the very-small-strain value $G_{max}$ at strains below $10^{-5}$, roughly three to five times the operational secant modulus under a working footing; using it directly would under-predict settlement badly, and converting it needs a modulus-degradation curve the survey does not supply.
The two methods I would recommend are the pressuremeter test (PMT) and the plate load test (PLT), with the cone penetrometer as the routine work-horse that both of them should calibrate.
The pressuremeter expands a cylindrical membrane against the borehole wall and records pressure against cavity volume. It therefore produces a genuine in-situ stress–strain curve, from which the pressuremeter modulus $E_{PMT}$ is obtained over the same strain range that a foundation imposes, together with the limit pressure $p_L$ that bounds bearing capacity. Because the test is run at the working depth and at the in-situ effective stress, it needs no correction for sampling disturbance, and a self-boring pressuremeter in particular disturbs the cavity wall very little. Its weaknesses are that the result is sensitive to the quality of the borehole and to the operator, and that the expansion is horizontal while foundation settlement is a vertical, largely one-dimensional problem, so an anisotropy correction (typically an $\alpha$ rheological factor after Ménard) is required.
The plate load test is the most direct measurement of all: a rigid plate is loaded in increments on the sand at the proposed founding level and the load–settlement curve is measured. Nothing has to be assumed about the constitutive behaviour of the sand, and the test is carried out at the correct stress level in the correct material. Its one serious limitation is the scale effect, because the depth of the stressed zone scales with the loaded width: settlement of a real footing is extrapolated from the plate by the Terzaghi–Peck relation $S_F = S_P\left[\dfrac{2B_F}{B_F+B_P}\right]^2$, and the extrapolation becomes unreliable when $B_F/B_P$ exceeds about five, or when the sand is layered within the footing's influence zone but outside the plate's. A plate test must therefore always be accompanied by a profile of the deposit — which is exactly what the cone penetrometer supplies. The CPT gives a continuous $q_c$ record that feeds Schmertmann's strain-influence method through $E_s = 2.5\,q_c$ (square footings) or $3.5\,q_c$ (strip footings), and it is the only one of the six that both locates the soft layers and quantifies them.
Recommendation. Use the pressuremeter and the plate load test to fix the deformation modulus at the founding level, and profile the whole influence zone (a depth of at least $4B$ below the base for a strip footing) with the cone penetrometer so that the two point measurements can be extended over the site. Sources: Das, Principles of Foundation Engineering, 9th ed., §3.13–3.18; CFEM 4th ed., §4 and §11.
