16-Civ-B3 Geotechnical Design · May 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. Professional Engineers Ontario / Engineers Canada National Examinations, May 2014 — 98-Civ-B3 Geotechnical Design. Three hours, OPEN BOOK, 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 (16-Civ-B3 / 98-Civ-B3 Geotechnical Design).
Sources of charts and assumed values (page-1 Note 6). Note 6 of this paper requires the candidate to identify the source of every design chart and every assumed value. The values imported into the solutions below are, in full: bearing-capacity factors Nc = 5.7 (Terzaghi strip, phi = 0) and 5.14 (Meyerhof / Prandtl, phi = 0), Das Foundation Engineering Table 3.1 and Eq. 3.19; shape and depth factors from De Beer and Hansen as tabulated in Das Table 3.4; the adhesion factor alpha = 0.45 for bored piles in stiff clay, Skempton (1959) as reproduced in CFEM Ch. 18; the end-bearing coefficient Nc* = 9 for piles in clay, Skempton (1951); the compression index correlation Cc = 0.009(LL − 10), Terzaghi and Peck (1967); and a specific gravity Gs = 2.70 where a void ratio had to be back-figured. Each is repeated at the point of use.
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.
Two features of the brief dominate the programme. The founding stratum is sand, so undisturbed sampling is impractical and the investigation must be built around in-situ tests; and the building has an underground garage, so the structure is partly compensated — the pressure that causes settlement is the net increase over the stress removed by the excavation, and unloading, heave and re-loading all have to be measured, not assumed.
Extent and depth. A 10-storey hotel on a raft or on pad footings will have a loaded width of the order of 20 to 30 m, so the significant depth is 1.5 to 2 times that width below founding level, not below ground level. Plan a grid of boreholes at roughly 20 to 30 m centres with at least one hole under each corner and one at the centre, taken to 40 to 50 m or to refusal on bedrock, whichever comes first. Every hole must prove that no compressible clay or peat lens lurks below the sand: a 1 m silt seam at 25 m depth would control the settlement completely and is invisible from the surface.
In-situ testing — the core of the work. Run the standard penetration test at 1.5 m intervals with a calibrated automatic hammer, and record the energy ratio so that field N can be reduced to $N_{60}$ and then to $(N_1)_{60}$ using the overburden correction $C_N = \sqrt{95.76/\sigma_0^{\prime}}$ (Liao and Whitman). Complement, and preferably lead with, electric cone penetration testing: the CPT gives a continuous qc profile, detects thin layers the SPT steps over, and feeds directly into Schmertmann's strain-influence method through $E_s \approx 2.5 q_c$ for square footings. Add flat-dilatometer (DMT) or pressuremeter (PMT) soundings in the most heavily loaded areas to obtain a measured modulus rather than a correlated one, and a cross-hole or MASW seismic survey for the small-strain modulus G0, which is the right stiffness for the small strains a serviceability check actually involves. If a large raft is proposed, one or two plate load tests at founding level are worthwhile, with the explicit understanding that plate results must be extrapolated to the real width using Terzaghi and Peck's relation $S_F = S_P\left[\dfrac{2B_F}{B_F + B_P}\right]^{2}$, since settlement in sand — unlike bearing capacity in clay — is strongly size dependent.
The excavation and the water table. Establish the groundwater level and its seasonal range with standpipe piezometers, and run at least one pumping or rising-head test to get the permeability, because the garage excavation will require dewatering and the drawdown itself causes settlement of adjacent property. Sample the groundwater for sulphates and chlorides for concrete durability. Record the density state of the sand carefully: settlement of a loose sand under a construction vibration regime can exceed the static settlement entirely.
Laboratory and analysis. Laboratory work on sand is limited but not optional: grain-size distribution and fines content on bulk samples (they control both the liquefaction screening and the reliability of the CPT correlations), maximum and minimum density to convert N or qc to relative density, and specific gravity. Settlement is then estimated by at least two independent routes — Schmertmann's strain-influence method using the CPT profile and the Burland and Burbidge method using $(N_1)_{60}$ — with the net foundation pressure $q_{net} = q_{applied} - \gamma D_f$ reflecting the garage excavation, and with differential settlement, not total settlement, checked against the tolerable limits (typically 25 mm differential and an angular distortion of 1/500 for a framed building).
