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.
Silt is the awkward material in this list of tools, and the choice of equipment follows directly from why it is awkward. A Toronto silt sits between the two regimes our standard tests were calibrated for: its permeability, of the order of 10−6 to 10−8 m/s, is low enough that a footing loaded in days responds close to undrained, yet high enough that appreciable consolidation occurs during construction, so neither a purely undrained nor a purely drained idealisation is safe on its own. Silt is also the material most damaged by sampling: it has little cohesion to hold a specimen together, it is prone to dilatant pore-pressure response in a rapid test, and the tube sampling of a loose silt can change its state entirely. A condominium complex adds two further requirements. The loaded area is wide, so the stressed zone extends tens of metres below founding level and cannot be characterised from a shallow test; and a downtown site almost certainly involves a multi-level basement, which means excavation, dewatering and a rebound-recompression settlement path rather than simple virgin loading.
Recommended programme. I would use four of the six families shown, and would decline the other two.
1. Sampling (boreholes). Sampling is not optional: it is the only way to obtain classification, grain-size distribution, Atterberg limits and, above all, one-dimensional consolidation (oedometer) data. Settlement of a condominium on silt is a compressibility problem, and the compression index, recompression index and preconsolidation pressure cannot be inferred reliably from any in-situ test. I would specify continuous-flight-auger or mud-rotary holes with 76 mm thin-walled fixed-piston (Shelby) samples in the cohesive silt, and split-spoon samples for index testing and for a Standard Penetration Test profile that gives continuity with the local database. Two or three deep holes to at least twice the raft width below founding level, plus shallower holes at the corners.
2. Penetrometers, specifically the piezocone (CPTu). This is the single most valuable test on the site. The cone gives a continuous record at 10 to 20 mm intervals, so it finds the thin sand partings and clayey laminations that a 1.5 m SPT interval steps straight over, and those laminations control drainage and therefore the rate of settlement. The measured pore pressure u2 distinguishes drained, partially drained and undrained cone response, which in a silt is exactly the ambiguity we need resolved; pore-pressure dissipation tests at selected depths give the horizontal coefficient of consolidation directly. Cone tip resistance feeds a settlement calculation through Schmertmann's method and gives an undrained strength through the cone factor for a bearing-capacity check. Specifying a seismic cone (SCPTu) adds a small-strain shear modulus profile and a shear-wave velocity for the seismic site classification required by the National Building Code, for perhaps ten per cent extra cost.
3. Pressuremeter. One or two pressuremeter profiles in the founding stratum are worth the cost precisely because silt cannot be sampled undisturbed. The pressuremeter loads the ground in place through a well-defined boundary-value problem (expansion of a cylindrical cavity), so its modulus and limit pressure are measured rather than correlated, and a self-boring device minimises the installation disturbance that would otherwise dominate a loose silt. It is the natural cross-check on the stiffness that governs the settlement prediction.
4. Piezometers. Vibrating-wire piezometers in each stratum are essential and inexpensive. Effective stress, and therefore both bearing capacity and settlement, depends on the pore-water pressure regime; a downtown site may have an artesian or perched condition, seasonal variation, and drawdown from neighbouring construction dewatering. Piezometers also monitor the excavation and provide the baseline against which any dewatering-induced settlement of adjacent structures is judged.
Declined, with reasons. Plate load tests should not be used here. The stressed zone beneath a 300 to 750 mm plate reaches only about twice the plate width, whereas a condominium raft stresses the ground to a depth of order the raft width — tens of metres. The plate therefore tests the wrong soil volume, and in a silt the plate test also drains at a completely different rate from the prototype, so both the strength and the stiffness it measures are unrepresentative. Stand-alone geophysical surveys (seismic refraction, MASW, resistivity) give useful stratigraphic continuity between boreholes and a rapid estimate of depth to till or bedrock, but they measure the very-small-strain response and give no bearing capacity and no settlement at working strain. On a congested downtown site they are also degraded by traffic noise and buried services. Their value is captured far more cheaply by the seismic module on the cone, which is why the SCPTu was recommended above rather than a separate geophysical spread.
Programme in summary. Piezocone soundings on a grid across the footprint to define stratigraphy and drainage; three deep sampled boreholes with piston samples for oedometer and triaxial testing; one or two pressuremeter profiles in the founding stratum; vibrating-wire piezometers left in place. That combination determines the bearing capacity from measured strength, the magnitude of settlement from measured compressibility, and the rate of settlement from measured consolidation characteristics — the three things the client actually needs — without paying for tests that answer a question the project does not ask.
