16-Civ-B3 Geotechnical Design · May 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. Professional Engineers Ontario / Engineers Canada National Examinations, May 2015 — 98-Civ-B3 Geotechnical Design. Three hours, OPEN BOOK, any non-communicating calculator. Section A carries five discussion questions of 7 marks each (answer any four); Section B carries four design questions of 24 marks each (answer any three); 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 timed attempt.
Reference texts (98-Civ-B3 / 16-Civ-B3 Geotechnical Design).
Sources of design charts and assumed values (page-1 Note 6). Note 6 of this paper requires the candidate to identify the source of every design chart used and of every value assumed in the absence of data. They are named where used and collected here:
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.
Stage 1 — desk study and reconnaissance. Before any drilling I would assemble the surficial and bedrock geology maps, water-well records, any previous investigations on adjacent lots, historical air photographs and fire-insurance plans to identify made ground, buried watercourses and former industrial use, and the seismic hazard values for the site from the National Building Code of Canada. A ten-storey hotel on sand with the water table at 2 m immediately raises three questions that the whole programme must be designed to answer: how deep is the competent bearing stratum, is the sand liquefiable, and how much total and differential settlement will the structure see.
Stage 2 — the field programme. For a building of this size CFEM guidance and normal Canadian practice indicate a minimum of four to six boreholes, one near each corner of the tower footprint plus one at the centre and one at any tower-crane or basement feature, at a spacing of roughly 20 to 30 m. Because the piles will be end-bearing or shaft-friction piles in sand, the boreholes must extend to at least five diameters, and preferably $1.5B_g$ (where $B_g$ is the plan width of the pile group), below the anticipated toe level, or until the induced vertical stress is below 10 per cent of the in-situ effective overburden, or to refusal on sound bedrock — whichever is shallower. Two of the holes should be taken deeper than the rest to prove that no compressible clay or peat layer lies beneath the bearing stratum.
The in-situ testing programme should combine methods, because no single test gives everything. Standard Penetration Tests at 1.5 m intervals give the classic $N$ profile and, crucially, disturbed samples for grain size and classification; the hammer energy must be measured so that $N$ can be corrected to $N_{60}$ and then to $(N_1)_{60}$ using an overburden factor such as $C_N = \sqrt{p_a/\sigma'_v}$ (Liao & Whitman). Piezocone soundings (CPTu) are the primary tool in sand: they give a continuous $q_c$, $f_s$ and $u_2$ profile, they locate thin silt and clay seams that the SPT steps straight over, they yield $\phi'$, relative density and pile unit resistances by direct correlation, and the dissipation tests give the true piezometric level. A pressuremeter or dilatometer test in one hole provides an independent modulus for settlement, and a seismic downhole or MASW survey gives the small-strain shear modulus $G_{max}$ and the $V_{s30}$ needed for the NBCC seismic site class. Standpipe piezometers should be installed in at least two holes and read seasonally, and one falling-head or pumping test run for permeability, since the water table at 2 m governs both dewatering for the basement and the effective stresses used in design. Finally, groundwater and soil samples should be tested for sulphate, chloride, pH and resistivity to specify concrete exposure class and to assess corrosion of steel piles, and I would specify a preliminary static load test with instrumentation on at least one pile, with dynamic testing on roughly one pile in ten during production, since correlations in sand carry real uncertainty.
Key properties required for pile design. The programme is aimed squarely at the following: the stratigraphy and the depth, thickness and continuity of the competent bearing stratum; the groundwater regime, its seasonal range and its permeability; unit weights above and below the water table; the profile of $(N_1)_{60}$ and $q_c$ from which the effective friction angle $\phi'$ and the relative density $D_r$ are derived; the in-situ horizontal stress ratio $K_0$ and hence $\beta = K\tan\delta$ for shaft resistance; the bearing capacity factor $N_q^{*}$ or the CPT-derived unit end resistance $q_p$ for the toe; the drained modulus $E_s$ or the constrained modulus $M$ for settlement of the pile group by the equivalent-raft method; the compressibility ($C_c$, $C_r$, $c_v$, $\sigma'_c$) of any clay layer found beneath the toe; the cyclic resistance ratio and the fines content needed for a liquefaction assessment, which a saturated sand under a ten-storey building in any Canadian seismic zone demands; and the chemical aggressivity of the ground. Together these fix the pile type, diameter, length, group layout and the predicted load-settlement behaviour.
