16-Civ-B3 Geotechnical Design · December 2016

Question 4 of 9: Site-investigation plan for shallow foundations in clay with punching shear

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 4: Site-investigation plan for shallow foundations in clay with punching shear (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.

Punching shear is the failure mode of a soft, loose or highly compressible deposit: the footing drives a wedge downwards with large vertical compression of the soil beneath it and no well-defined slip surface reaching the ground surface, and the load-settlement curve never shows a peak. The expectation of punching shear therefore tells the investigation what to look for before a single borehole is drilled — a soft clay, or a firm surface crust underlain by a softer stratum — and it tells the designer that settlement, not bearing capacity, will govern. The plan below is built around that.

Stage 1: desk study and reconnaissance. Surficial and bedrock geology maps, historical air photographs, water-well records, previous reports for adjacent buildings, and a site walkover. In eastern Canada this stage would flag sensitive Champlain Sea clay; in the west, glaciolacustrine or estuarine deposits. It costs almost nothing and it sets the borehole depths.

Stage 2: field investigation. For a hotel footprint a reasonable programme is boreholes on a 15 to 30 m grid, with at least one borehole at each corner and one near the centre, and one at every heavily loaded core or tower position; CFEM guidance for a building of this size is typically four to six boreholes as a minimum, increased where the desk study suggests variability. Each borehole must be taken to whichever is deeper: (i) the depth at which the stress increase from the loaded area falls below about 10 per cent of the existing effective overburden pressure — for a raft or a group of footings this is judged from the equivalent loaded area, not from a single footing, and is often 1.5 to 2 times the least plan dimension; or (ii) through the entire soft stratum into a demonstrably competent one. Stopping in the soft clay is the single most expensive mistake available on this project.

In-situ testing should be led by the field vane, at 1.0 to 1.5 m intervals, giving a continuous profile of c_u and, from the remoulded reading, the sensitivity. A piezocone (CPTu) sounding beside two or three of the boreholes is strongly recommended: it profiles thin layers continuously, identifies a firm crust over softer material, and its dissipation tests give the in-situ pore pressure and the horizontal coefficient of consolidation. The SPT is of limited value in soft clay and should not be relied on for strength. Piezometers or standpipes must be installed and read on at least two occasions, because the groundwater regime controls every effective stress in the design.

Stage 3: sampling and laboratory testing. Continuous or closely spaced undisturbed thin-walled (Shelby) sampling is essential — a soft clay tested from a disturbed sample gives a compressibility that is both too low in the recompression range and too high beyond the preconsolidation pressure. Oedometer tests on samples from each identifiable sub-layer give the preconsolidation pressure σ'_p and hence the overconsolidation ratio, the compression and recompression indices C_c and C_r, the secondary compression index C_α, and the coefficient of consolidation c_v. Consolidated-undrained triaxial tests with pore-pressure measurement give c' and φ' for long-term checks and confirm the c_u profile. Classification tests — water content, Atterberg limits, unit weight, organic content — are run on every sample.

Key properties to be determined. Ranked by their effect on this design: the profile of undrained shear strength c_u with depth and its sensitivity; the preconsolidation pressure and OCR, which decide whether the foundation loading stays on the recompression line or pushes the clay onto the virgin curve; the compressibility indices C_c, C_r and C_α; the coefficient of consolidation c_v, which sets the settlement rate and hence how much of it happens before the finishes go on; the undrained modulus E_u for immediate settlement; the total and effective unit weights; the effective strength parameters c' and φ'; the thickness and continuity of every layer including any firm crust; and the groundwater level and its seasonal range.

Two design consequences should be stated back to the client. Because punching shear is expected, the bearing-capacity calculation must use Vesic's local shear reduction (strength parameters reduced to about two-thirds) or Vesic's compressibility factors rather than the general-shear equation, and the result must then be checked against a tolerable-settlement criterion which will almost certainly control. And where a firm crust overlies softer clay, an explicit punch-through check of the crust must be made in addition to the conventional bearing check.

Topic. Planning a site investigation for shallow foundations in a compressible clay, and the properties that a punching-shear regime makes critical.

Key relations. Local-shear bearing capacity uses $c_u^{*} = \tfrac{2}{3}c_u$ and $\tan\phi^{*} = \tfrac{2}{3}\tan\phi$ in the standard equation. Borehole depth follows from $\Delta\sigma_z \le 0.10\,\sigma'_{vo}$. Consolidation settlement is $S_c = \dfrac{C_c H}{1+e_0}\log\dfrac{\sigma'_{o}+\Delta\sigma'}{\sigma'_{o}}$ on the virgin curve, with $C_r$ replacing $C_c$ while $\sigma'_{o}+\Delta\sigma' \le \sigma'_p$.

Why this works. The mode of failure a soil will show is a property of its compressibility, and compressibility is exactly what an oedometer measures. Building the investigation around consolidation testing therefore answers both questions at once — it confirms that punching is the right model and it supplies the numbers for the settlement calculation that will govern the design.

Common pitfall. Terminating boreholes at a nominal depth such as 1.5B below a single footing when the building is really a group of closely spaced footings or a raft: the stress bulb of the combined loaded area reaches far deeper, and a soft layer just below the boreholes will dominate the settlement. A second pitfall is relying on SPT N values in soft clay, where they are near zero and carry no useful information.

Source. Das, Principles of Foundation Engineering, 9th ed., Ch. 2 (subsurface exploration; borehole spacing and depth, Tables 2.1 and 2.2) and Section 3.5 (local shear and Vesic's compressibility factors); CFEM, 4th ed., Ch. 4 (planning site investigations) and Ch. 5 (in-situ testing).

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