NivaarExam PrepOfficial exam papers ↗

16-Civ-B3 Geotechnical Design · December 2013

Question 1 of 9: Selecting in-situ equipment for bearing capacity and settlement in sand

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

Notes on this paper

Paper format. National Examinations, December 2013 — 98-Civ-B3 Geotechnical Design. Three hours, open book, any non-communicating calculator. Section A holds five 7-mark discussion questions (answer any four); Section B holds four 24-mark design questions (answer any three), so the examinable total is 4 × 7 + 3 × 24 = 100 marks. Every one of the nine questions 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; R. F. Craig, Craig's Soil Mechanics (9th ed.); D. P. Coduto, Foundation Design: Principles and Practices (3rd ed.).

Sources of design charts and assumed values (paper Note 6). The paper requires every chart and assumed value to be identified. Bearing-capacity factors are Terzaghi’s (Das, Foundation Engineering, Table 3.1, with Nγ after Kumbhojkar 1993); the pile end-bearing factor Nq* is read from Meyerhof’s chart (Das Fig. 11.14) and cross-checked against Janbu’s closed-form expression; the adhesion factor α is Das Table 11.5 (Terzaghi, Peck & Mesri); the strain-influence distribution is Schmertmann, Hartman & Brown (1978). Assumed values — specific gravity of solids Gs = 2.65 for the Question 8 backfill, base friction δ = ⅔φ′ and base adhesion ca = ⅔c′ (CFEM §24), and a driving-parameter value K = 1.4K0 for a high-displacement driven pile (Das §11.11) — are flagged where they are used.
Check — Question 6 text and figure disagree. The printed text of Question 6 states a 1 m × 1 m square footing with γ = 20 kN/m³ and φ = 36°, while Figure 3 is drawn for a 2.5 m footing with γ = 18 kN/m³ and φ′ = 38°. The question text governs (it is the instruction to the candidate); the figure is used only for the embedment Df = 1.5 m and for the CPT modulus profile, which the text does not restate. The alternative reading is noted at the end of the answer.

Question 1: Selecting in-situ equipment for bearing capacity and settlement in sand (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.

Sand cannot be sampled undisturbed at reasonable cost, so both the bearing capacity and the settlement of a shallow foundation in sand are obtained from in-situ tests rather than from laboratory strength testing. From the equipment shown I would use the cone penetration test (the penetrometer) as the primary tool, supported by the drill rig with split-spoon sampling (SPT) for stratigraphy and index classification, and I would add a pressuremeter test at foundation level in one or two boreholes as an independent measure of stiffness. A plate load test would be run only if a full-scale check of the settlement prediction is wanted before construction. Because the water table is deeper than 15 m — far below any stress bulb of a condominium footing — the piezometer is not required, and geophysics would add nothing beyond a rapid check on depth to bedrock or on lateral continuity between boreholes.

The CPT is chosen because it is continuous, repeatable and operator-independent. It gives cone resistance qc at 10–20 mm intervals, from which the friction angle follows through published correlations (Robertson & Campanella) and the drained modulus follows as Es ≈ 2.5–3.5 qc, feeding directly into Schmertmann’s strain-influence settlement method. Its limitations are real and must be stated: it recovers no sample, so the soil is classified only by inference from the friction ratio; it cannot be pushed through gravel, cobbles or cemented layers without refusal or damage; the correlations to φ′ and Es carry a scatter of roughly ±2° and ±50 % respectively; and the measured resistance reflects a small, rapidly loaded failure zone that is not the same as the deep, slow failure mechanism under a real footing.

The SPT complements it by returning a disturbed sample for grain-size and visual classification and by working in gravelly horizons where the cone refuses. Its weakness is that the raw blow count is dominated by equipment: hammer energy, rod length, borehole diameter and sampler liner all have to be corrected out (N60) before the overburden correction CN is applied, and even then it gives no direct measure of stiffness. The pressuremeter measures a genuine stress–strain curve in place and so gives a defensible modulus, but it is slow, expensive, sensitive to borehole disturbance, and gives data only at the depths tested. The plate load test loads the soil in the correct sense but influences only about twice the plate width, so for a condominium raft or a 2–3 m footing it samples a fraction of the stressed depth and its results must be scaled by an empirical size rule that is itself uncertain.

The practical programme is therefore boreholes with SPT to establish the profile, CPT soundings between them for continuous strength and stiffness, and pressuremeter tests at founding level for confirmation — a combination that brackets the answer rather than relying on any one correlation.

← Paper overview