16-Civ-B3 Geotechnical Design · December 2014

Question 4 of 10: Site-investigation plan for a piled hotel on clay, and the properties required

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

Paper format. Professional Engineers Ontario / Engineers Canada National Examinations, December 2014 — 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 five design questions of 24 marks each (answer any three); the examinable total is 4 × 7 + 3 × 24 = 100 marks. All ten 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).

B. M. Das, Principles of Foundation Engineering, 9th ed. — bearing capacity (Ch. 3), stress increase in a soil mass (Ch. 6), retaining walls (Ch. 8), pile foundations (Ch. 11).
B. M. Das, Principles of Geotechnical Engineering, 9th ed. — lateral earth pressure (Ch. 13), shear strength (Ch. 12), slope stability (Ch. 15), subsurface exploration (Ch. 17).
Canadian Geotechnical Society, Canadian Foundation Engineering Manual (CFEM), 4th ed. — the governing Canadian practice document for site investigation, bearing resistance, deep foundations and earth-retaining structures.
R. F. Craig, Craig's Soil Mechanics, 8th ed. — earth pressure theory and slope stability.
D. P. Coduto, Foundation Design: Principles and Practices, 2nd ed. — in-situ testing and SPT correlations.
J. E. Bowles, Foundation Analysis and Design, 5th ed. — bearing capacity factors and retaining-wall stability tables.

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. Each chart reading and each assumption below is therefore named where it is used, and the values assumed in the absence of data are collected here:

Q6 — adhesion factor α from Das, Principles of Foundation Engineering, Table 11.6 (Terzaghi, Peck & Mesri form, α against $c_u/p_a$); $\lambda$ from Vijayvergiya & Focht (1972) as tabulated by Das, Table 11.7.
Q7 — overburden correction $C_N$ from Liao & Whitman (1986); $\phi'$ from Wolff (1989) and from Hatanaka & Uchida (1996), both reproduced in Das, Ch. 2; settlement-controlled bearing pressure from Meyerhof (1965) as given by Das, Ch. 5, used only as a serviceability check because the question forbids direct correlations of bearing capacity to penetration index. Table I prints the blow counts as field values $N_f$; with no hammer data they are converted as $N_{60} = N_f$, i.e. a safety hammer at the reference 60 per cent energy ratio with borehole, sampler and rod-length factors of 1 (Das, Ch. 2, hammer-efficiency and correction-factor tables).
Q8 — embankment influence factor from Osterberg (1957), reproduced as Das Fig. 6.24; the closed form of that chart is used so the reading carries no chart-scaling error.
Q9 — Meyerhof general bearing-capacity equation with the shape factors of De Beer (1970) and the depth factors of Hansen (1970), as set out in Das, Ch. 3.
Q10 — Coulomb active earth-pressure coefficient, Das Eq. 13.31; unit weight of the mass-concrete wall assumed $\gamma_c = 24\ \text{kN/m}^3$ (CFEM 4th ed., normal-density concrete), the only value the figure does not supply.

Question 4: Site-investigation plan for a piled hotel on clay, and the properties required (7 marks)

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 collect the surficial and bedrock geology maps (Geological Survey of Canada and the provincial survey), the water-well records, air photographs and historical fire-insurance plans, and any borehole logs from adjacent buildings. For a hotel this also means establishing the previous land use, because contaminated or filled ground changes both the drilling programme and the pile material. A walkover then confirms access, overhead and buried services, and any evidence of soft ground, old watercourses or slope movement.

Stage 2 — scope the investigation to the structure. The programme is sized by the foundation, not by the site area. For a piled multi-storey building CFEM recommends boreholes at a spacing of the order of 15 to 30 m across the footprint, with a minimum of one borehole per major column group and never fewer than three or four for a building of this size, plus at least one borehole outside the footprint for the tower crane and access ramps. The critical decision is depth: each borehole must extend to at least the anticipated pile toe plus a further depth sufficient to cover the stress bulb of the group, conventionally the toe level plus one and a half times the least plan dimension of the group, or to proven bedrock with a minimum of 3 m of coring to confirm that the rock is not a boulder.

Stage 3 — field work. The programme in a clay deposit would combine:

Continuous CPTu soundings between the boreholes, to profile the deposit continuously, locate granular seams and soft layers, and measure pore-pressure dissipation for $c_h$.
Cased boreholes with continuous sampling: thin-wall Shelby tubes in the cohesive layers for laboratory strength and consolidation testing, split-spoon SPT in any granular seams and in the bearing stratum.
Field vane tests at 1 to 1.5 m centres through soft and firm clay, with remoulded readings for sensitivity.
Piezometers (at least two, in different strata) to establish the groundwater regime and whether an artesian pressure exists in a granular layer beneath the clay — a common and dangerous surprise under a driven-pile site.
A preliminary pile load test where the loads justify it: a static compression test to failure on a sacrificial pile, and dynamic testing with a pile driving analyser plus CAPWAP on production piles, with restrike tests to quantify set-up.

Key properties required for the pile design. The design of a pile in clay needs the following, and the programme above is built to deliver each of them:

Stratigraphy and the level and continuity of the bearing stratum across the footprint — this determines pile length and whether the toe is in a competent layer or floating.
Undrained shear strength profile $c_u$ with depth, from field vane, CPTu ($q_{net}/N_{kt}$) and laboratory UU triaxial — this drives both the $\alpha$-method shaft resistance $f = \alpha c_u$ and the end bearing $q_p = 9 c_u$.
Sensitivity $S_t$ (peak over remoulded vane strength) — it governs the loss of capacity during driving and the magnitude of set-up afterwards.
Effective-stress parameters $c'$, $\phi'$ and the overconsolidation ratio / preconsolidation pressure $\sigma'_c$, from consolidated-undrained triaxial tests with pore-pressure measurement and from the oedometer — needed for the $\beta$-method long-term shaft resistance and for negative skin friction.
Compressibility: $C_c$, $C_r$, $c_v$ — for the settlement of the pile group as an equivalent raft, and for the time over which any downdrag develops.
Unit weight, water content, Atterberg limits and grain-size for classification, effective stress profiles and correlation checks.
Groundwater level and pore-pressure regime, including any artesian head.
Chemical aggressivity — sulphate, chloride and pH — to set the concrete exposure class under CSA A23.1 and to assess corrosion of steel piles, and organic content where peat is suspected.
Obstructions and drivability: cobbles, boulders, buried structures, and the presence of any dense crust that would refuse a driven pile.

Finally, for a hotel the investigation must also answer the construction questions that sit alongside the design: whether driving vibration and noise are acceptable next to existing buildings (which may force bored or CFA piles), and whether the excavation for pile caps will need dewatering or support.

Topic: planning a subsurface investigation for a deep-foundation project in cohesive soil.

Key relations: shaft resistance $Q_s = \sum \alpha c_u p \Delta L$ (total stress) or $\sum \beta \sigma'_v p \Delta L$ (effective stress); end bearing $Q_p = 9 c_u A_p$; equivalent-raft group settlement using $C_c$ and $\sigma'_c$ from the oedometer.

Why this works: the investigation is scoped backwards from the design equations. Every term in the capacity and settlement equations must be traceable to a measurement, and the borehole depth is set by the depth over which those equations are sensitive — which for a pile group is far deeper than for a single pile.

Common pitfall: terminating boreholes at the anticipated pile toe. The group stress bulb extends well below the toe, and a compressible layer sitting just beneath a competent bearing stratum will produce group settlement that no single-pile load test can reveal.

Source: CFEM 4th ed., Ch. 4 (Site investigation) and Ch. 18 (Deep foundations); B. M. Das, Principles of Foundation Engineering, 9th ed., Ch. 2 and Ch. 11; CSA A23.1 for concrete exposure classes.

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