07-Str-B1 · May 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Examinations — May 2015 — 07-Str-B1 Geotechnical Design. Three-hour, OPEN-BOOK exam; any non-communicating calculator permitted (the candidate must record its make and model). Format: Section A carries five discussion questions of 7 marks each, of which any FOUR are to be answered; Section B carries four design problems of 24 marks each, of which any THREE are to be answered — a marked total of 100. The paper instructs candidates to state any interpretive assumptions, to identify the source of every design chart and assumed value, and to exercise sound engineering judgment where data are absent. All nine printed questions are worked below, because the set is intended as a study resource.
Reference texts: Das, B.M., Principles of Foundation Engineering (9th ed., Cengage) — general bearing-capacity equation, pile and pile-group capacity, consolidation settlement of footings, retaining walls; Das, B.M., Principles of Geotechnical Engineering (9th ed., Cengage) — lateral earth pressure, effective stress, consolidation theory; Canadian Geotechnical Society, Canadian Foundation Engineering Manual (CFEM, 4th ed., 2006) — Canadian practice for site investigation, SPT/CPT interpretation, pile design and tolerable settlement; Craig, R.F. / Knappett, J.A., Craig's Soil Mechanics (8th ed., CRC Press) — shear strength and earth-pressure theory; Duncan, J.M., Wright, S.G. & Brandon, T.L., Soil Strength and Slope Stability (2nd ed., Wiley) — fully softened and residual strengths for fissured and expansive clays; Fredlund, D.G., Rahardjo, H. & Fredlund, M.D., Unsaturated Soil Mechanics in Engineering Practice (Wiley) — swelling soils and matric suction.
Note — Figure 2 is printed over a coarse halftone. The soil-property annotations inside the photograph-style Figure 2 (Question 8) are printed over a coarse dot screen. The values used below are read from the printed figure and are: upper sand $\gamma = 15\ \text{kN/m}^3$ over 1.5 m, lower sand $\gamma_{sat} = 18\ \text{kN/m}^3$ over 1.5 m, normally consolidated clay 2.5 m thick with $w = 35\%$ and $LL = 48$, over sand; groundwater table at the underside of the footing.
Assumptions declared once, applied throughout. $\gamma_w = 9.81\ \text{kN/m}^3$; reinforced concrete $\gamma_c = 24\ \text{kN/m}^3$; specific gravity of soil solids $G_s = 2.70$ where a void ratio must be back-figured from water content; loads are vertical and concentric unless stated. Every assumption that changes a numerical answer is repeated in the question where it is used.
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, assemble the regional surficial geology and bedrock maps, seismic hazard values for the site from the National Building Code of Canada, historical air photographs, records of adjacent construction and any archived borehole logs (in British Columbia the provincial water-well database and municipal records are the usual sources). Walk the site: note surface water, existing fill, settlement of neighbouring structures, access and utility constraints, and the presence of any deleterious ground such as buried services or old foundations that will obstruct piling.
Stage 2 — investigation layout. For a single ten-storey building CFEM guidance and normal practice give a minimum of four to six boreholes, one near each corner and one under the core, on a grid of roughly 15 to 30 m, with additional holes at any planned tower crane, basement or transfer structure. Depth is governed by the pile, not the building footprint: extend the holes at least 5 to 10 m (or $1.5B_{group}$) below the anticipated pile toe, and in any case to a depth at which the group stress increase has fallen below 10 % of the in-situ effective vertical stress. For a ten-storey structure on piles that is typically 30 to 40 m. At least one hole should be taken deeper, or to refusal on a dense/bedrock horizon, to prove that no compressible layer underlies the bearing stratum.
Stage 3 — in-situ testing. In sand, undisturbed sampling is impracticable, so the field tests are the strength measurement. Run the standard penetration test at 1.5 m intervals with a calibrated automatic hammer and record the energy ratio so that $N_{60}$ and $(N_1)_{60}$ can be computed. Supplement with electric piezocone soundings (CPTu), which are the preferred tool here: continuous $q_c$, $f_s$ and $u_2$ profiles resolve thin silt or clay seams that boreholes miss, give a direct dissipation measurement of the piezometric head, and can be interpreted for both shaft and toe resistance. Add seismic CPT or a downhole shear-wave survey for the small-strain modulus $G_{max}$ needed for seismic design, and a pressuremeter test or two if lateral pile capacity governs.
Stage 4 — groundwater and chemistry. Because the table is only 2 m down, install standpipe piezometers in at least two holes and monitor over a full seasonal cycle; a hotel basement or pile cap below the water table needs the design head, not the drilling-day head. Measure hydraulic conductivity for dewatering design, and test the groundwater and soil for sulphate, chloride and pH to specify concrete exposure class and to assess corrosion of steel piles.
Stage 5 — laboratory and field pile testing. Grain-size distribution and fines content on all SPT samples, Atterberg limits on any cohesive seams, maximum and minimum density tests to convert $N$ and $q_c$ to relative density, and specific gravity. Finally, budget for a preliminary test-pile programme with static load testing or a bidirectional (Osterberg) cell, plus dynamic monitoring (PDA with CAPWAP) on production piles; on sand, load testing is the only reliable way to confirm the shaft and toe resistances assumed in design.
Key properties required for pile design. Relative density and its variation with depth, expressed through $(N_1)_{60}$ and $q_c$; the effective friction angle $\phi'$ and interface friction angle $\delta$ derived from them; the depth, thickness and continuity of the bearing stratum and any weak layer beneath it; the design groundwater level and pore-pressure profile, since shaft and toe resistance are effective-stress quantities; soil modulus for settlement of the pile group; the seismic site class and the liquefaction susceptibility of loose saturated sand above the toe, which controls both downdrag and lateral demand; drivability parameters and the presence of cobbles or boulders; and the chemical aggressiveness of soil and groundwater.
