16-Civ-B3 Geotechnical Design · May 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Examinations, May 2017 — 16-Civ-B3 Geotechnical Design; three hours, open book, any non-communicating calculator. Section A holds five discussion questions worth 7 marks each of which four are marked; Section B holds four design questions worth 24 marks each of which three are marked, so the examinable total is 4 × 7 + 3 × 24 = 100 marks. All nine questions are worked below, because the set is a study resource rather than a marked script.
Reference texts. B. M. Das, Principles of Foundation Engineering, 7th–9th ed. (Cengage); B. M. Das, Principles of Geotechnical Engineering; R. F. Craig, Craig's Soil Mechanics, 8th ed. (Knappett & Craig); Canadian Geotechnical Society, Canadian Foundation Engineering Manual (CFEM), 4th ed.; J. E. Bowles, Foundation Analysis and Design; ASTM D1586 (SPT) and D5778 (CPT).
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 cone penetration test and the standard penetration test answer different questions well, and the preference follows from what the project needs rather than from any general superiority. The cone is a continuous, repeatable, quantitative measurement made by a machine; the SPT is an intermittent, operator-sensitive, dynamic test that brings back a sample. Where the design depends on how the ground varies and on quantitative strength or stiffness, the cone wins; where the design depends on knowing what the material actually is, or where the ground would damage a cone, the SPT wins.
Situations in which I would prefer the CPT.
Soft to firm clays, silts and loose to medium sands. These are the materials in which the SPT has poor resolution — blow counts of 0 to 4 in a soft clay carry almost no information, and weight-of-rod penetration is common — while the cone measures tip resistance and sleeve friction continuously with excellent sensitivity. Precisely the Toronto silt of Question 1 is the case in point.
When stratigraphy controls the design. The cone records at 10 to 20 mm intervals and so detects thin sand seams, clay laminae and the exact level of a stratum change. An SPT sampled at 1.5 m intervals recovers 450 mm of every 1500 mm and can step straight over a 200 mm drainage layer that halves the consolidation time of the deposit, or over a soft band that governs a slope.
When pore pressure matters. The piezocone measures u2 behind the tip, which distinguishes drained, partially drained and undrained penetration, identifies fine-grained layers unambiguously, and through a dissipation test at a chosen depth gives the horizontal coefficient of consolidation ch — a parameter no SPT can supply and one that controls the design of preloading, wick drains and dewatering.
When repeatability and operator independence matter. Cone results are largely free of the energy-ratio, rod-length and borehole-diameter effects that dominate the SPT, so soundings by different crews can be compared directly and a specification can be written around measured values. This matters for quality control of ground improvement (dynamic compaction, vibro-replacement, preloading), where before-and-after comparison is the acceptance test.
When quantitative settlement, liquefaction or pile design is required. Schmertmann's settlement method takes the elastic modulus directly from tip resistance (Es of order 2.5 qc for a square footing and 3.5 qc for a strip). Simplified liquefaction assessment is now expressed primarily in terms of the normalised clean-sand-equivalent tip resistance qc1N,cs, and the Canadian Foundation Engineering Manual and the direct European methods (LCPC / Bustamante and Gianeselli) give pile shaft and base resistances directly from qc, avoiding two layers of correlation. A seismic cone adds the shear-wave velocity profile needed for the seismic site class in the National Building Code of Canada.
When speed, cost per metre or site constraints dominate. A cone rig can complete 100 to 150 m of sounding a day against perhaps 30 m of sampled borehole, produces no spoil — a decisive advantage on a contaminated or landfill site where drill cuttings become regulated waste — and needs no drilling fluid.
Situations in which I would still prefer, or at least add, the SPT. Gravels, cobbles, construction rubble and glacial till with clasts will refuse or damage a cone, and the SPT, though crude in such ground, at least survives it; the same applies to very dense or weakly cemented sands and to weathered rock, where the cone reaches its capacity of 15 to 20 tonnes at shallow depth. The SPT also returns a physical sample, which is the only way to obtain classification, Atterberg limits, grain-size distribution, organic content or contaminant chemistry; a cone infers soil type from a behaviour-type chart and can misclassify a sensitive clay or an organic silt. Where a drill rig must be on site anyway for deep boreholes or for undisturbed sampling, the marginal cost of SPT testing is small. Finally, a great deal of local practice, including much municipal and provincial precedent, is written in terms of N values, and a design that must be checked against that precedent needs N values to be checked against.
The practical answer for most conventional projects is not one or the other but a paired programme: cone soundings to define the stratigraphy and to provide continuous quantitative profiles, with a smaller number of sampled boreholes with SPT testing to identify the materials, to calibrate the cone correlations for the site, and to supply specimens for laboratory testing. Where budget forces a single choice, the deciding question is whether the design risk lies in the variability of the ground (choose the cone) or in the identity of the ground (choose the borehole).
