16-Civ-B3 Geotechnical Design · December 2016

Question 1 of 9: Foundation choice for a shopping complex on 5 m of peat over glacial till

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 2016 — 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).

B. M. Das, Principles of Foundation Engineering, 9th ed. — subsurface exploration (Ch. 2), bearing capacity of shallow foundations (Ch. 3), settlement of shallow foundations including Schmertmann's strain-influence method (Ch. 5), retaining walls (Ch. 8), pile foundations (Ch. 11), drilled shafts (Ch. 12).
B. M. Das, Principles of Geotechnical Engineering, 9th ed. — shear strength (Ch. 12), lateral earth pressure (Ch. 13), slope stability (Ch. 15).
Canadian Geotechnical Society, Canadian Foundation Engineering Manual (CFEM), 4th ed. — the governing Canadian practice document for site investigation, in-situ testing, bearing resistance, deep foundations and earth-retaining structures.
R. F. Craig, Craig's Soil Mechanics, 9th ed. — effective stress, undrained versus drained behaviour, earth pressure, and the total-stress circular-arc slope analysis.
M. J. Tomlinson & J. Woodward, Pile Design and Construction Practice, 6th ed. — shaft adhesion in clay and under-reamed (belled) bored piles.
P. K. Robertson & K. L. Cabal, Guide to Cone Penetration Testing, 6th ed. — CPT interpretation and CPT-versus-SPT selection.

Sources of design charts and assumed values (page-1 Note 6). Note 6 requires the candidate to identify the source of every design chart and of every value assumed where the paper supplies none. They are named at the point of use and collected here:

Adhesion factor α = 0.55 for a drilled shaft in clay (Q6) — O'Neill and Reese (1999), reproduced in Das, Principles of Foundation Engineering, 9th ed., Section 12.9; the exclusion of the top 1.5 m and of one shaft diameter above the bell comes from the same source.
Bearing-capacity factor N_c^* = 9 (Q6) — Skempton (1951), as tabulated in Das, Section 11.11.
Overburden correction C_N = √(p_a/σ'_o) (Q7) — Liao and Whitman (1986), Das Principles of Geotechnical Engineering, Section 17.6.
SPT-to-friction-angle correlation (Q7) — Peck, Hanson and Thornburn (1974) as fitted by Wolff (1989); cross-checked against Kulhawy and Mayne (1990). Both are tabulated in Das, Principles of Foundation Engineering, 9th ed., Section 2.9.
Bearing-capacity factors and shape/depth factors (Q7) — Vesic (1973) and De Beer (1970), Das Sections 3.6 and 3.7.
Strain-influence diagram and the C₁, C₂ factors (Q7) — Schmertmann, Hartman and Brown (1978), Das Section 5.6; the modulus correlation E_s = 500(N₆₀ + 15) kPa is Bowles (1996), reproduced in the same section.
Rankine active coefficient for an inclined backfill (Q9) — Das, Principles of Geotechnical Engineering, 9th ed., Eq. (13.35); the base friction and adhesion reductions k₁ = k₂ = 2/3 are Das Section 8.4.
Unit weight of the submerged backfill (Q9) — assumed equal to the printed moist unit weight, 18 kN/m³, in the absence of a saturated value; the consequence of that assumption is bounded in the Q9 callout.

Section A

Question 1: Foundation choice for a shopping complex on 5 m of peat over glacial till (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.

The recommended foundation is a deep foundation — driven precast or steel piles, or bored (cast-in-place) piles — carried right through the peat and socketed a few diameters into the glacial till, with the ground-floor slab suspended from the pile caps rather than cast on grade. For a multi-storey shopping complex with heavy, closely spaced column loads and long, settlement-sensitive floor plates, no shallow alternative can be defended.

The decision is driven entirely by the peat. Peat is a fibrous organic soil with water contents that routinely exceed 500 per cent, void ratios above 5, and bulk unit weights close to that of water. Its undrained shear strength is typically 5 to 20 kPa, so its bearing resistance is an order of magnitude below what a mid-rise column needs. Worse than the strength is the compressibility: the compression index of peat is very large, its coefficient of consolidation is high so primary consolidation is quickly over, and it is then followed by secondary compression at a rate one to two orders of magnitude greater than in an inorganic clay. Secondary compression in peat does not stop — it continues for the whole life of the structure, and no waiting period during construction removes it. A raft bearing on peat would therefore keep settling for decades. Peat also oxidises and decomposes when the water table is drawn down, adding a further, irreversible loss of thickness that has nothing to do with the applied load.

The glacial till beneath is the opposite in every respect: dense to very dense, heavily overconsolidated by ice loading, of low compressibility and high shear strength, and continuing to great depth so there is no risk of punching through it into something weaker. A pile toe founded in till mobilises a high end-bearing resistance over a short penetration, and the 5 m of peat is short enough that pile lengths of roughly 7 to 10 m will do — well within the reach of ordinary driven piling plant.

Two design consequences follow and must be stated with the recommendation. First, negative skin friction (downdrag). The peat will continue to compress under its own secondary compression and under any site grading fill, so it moves down relative to the piles and hangs on them. That downdrag is a load, not a resistance: it must be added to the structural load on the pile and on the pile as a column, and the shaft resistance within the peat must be taken as zero or negative. It is usually mitigated by a bitumen slip coating or a sleeve over the peat thickness. Second, the floor slab. A slab-on-grade would settle away from the piled columns and crack; the ground floor must be a structural slab spanning between pile caps, with a void former or compressible layer beneath it.

Alternatives worth naming, and why they lose. Excavating and replacing 5 m of peat is technically sound and is sometimes economic for lightly loaded outbuildings and pavements, but for a multi-storey footprint the excavation volume, dewatering, and disposal of organic spoil make it expensive, and it does not by itself solve the bearing problem for heavy columns. Preloading with surcharge and wick drains accelerates primary consolidation but cannot pre-empt secondary compression, which is the governing settlement mechanism in peat. Stone columns and other granular inclusions perform poorly in fibrous peat because the surrounding soil offers almost no lateral confinement and the columns bulge. A raft sized for a low contact pressure would still settle unacceptably and differentially.

In Canadian practice this recommendation is made under the Canadian Foundation Engineering Manual, which treats organic soils as unsuitable founding strata for buildings and directs the designer to a deep foundation with an explicit downdrag assessment. The investigation supporting the design should confirm the peat thickness across the whole footprint (it is rarely uniform), its organic content and water content, and the elevation and density of the till surface at each column line.

Topic. Foundation selection over a highly organic, highly compressible surface deposit overlying a competent bearing stratum.

Key relations. Ultimate pile capacity $Q_u = Q_p + Q_s$; with a peat layer the shaft term over that layer is set to zero and a downdrag load $Q_{nf}$ is added to the applied load, so the design condition is $Q_{\text{applied}} + Q_{nf} \le Q_u / FS$. Secondary compression settlement $S_s = C_{\alpha}' H \log(t_2/t_1)$ shows why waiting does not help: it is unbounded in $\log t$.

Why this works. A foundation must satisfy two independent requirements — adequate bearing resistance and tolerable settlement — and in organic soils the second is decisive by a wide margin. Peat can carry a light load without collapsing, so a strength-only check misleads; it is the creep component of its compression that makes any bearing-on-peat scheme untenable for a permanent building. Transferring load to the till converts a settlement problem in a soil with an enormous $C_{\alpha}$ into a settlement problem in a dense overconsolidated soil with a very small one.

Common pitfall. Two recur. Students recommend piles but then forget that the settling peat loads the piles downwards, so they size the pile for the column load alone; and they specify a slab-on-grade, which then hangs from the columns and cracks. A third, subtler error is to assume the till surface is level — glacial till surfaces are irregular, and pile lengths must be confirmed per column line.

Source. Das, Principles of Foundation Engineering, 9th ed., Sections 11.2 and 11.17 (pile types and negative skin friction); Canadian Geotechnical Society, CFEM, 4th ed., Ch. 18 (deep foundations) and Ch. 11 (settlement of organic soils).

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