16-Civ-B3 Geotechnical Design · May 2015

Question 1 of 9: Shaft Adhesion in Soft Clay versus Stiff Clay

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

Notes on this paper

Paper format. Professional Engineers Ontario / Engineers Canada National Examinations, May 2015 — 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. — bearing capacity (Ch. 3), stress increase in a soil mass (Ch. 6), retaining walls (Ch. 8), pile foundations (Ch. 11), subsurface exploration (Ch. 2).
B. M. Das, Principles of Geotechnical Engineering, 9th ed. — consolidation (Ch. 11), shear strength (Ch. 12), lateral earth pressure (Ch. 13).
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, 9th ed. — earth pressure theory, effective stress and slope stability.
D. P. Coduto, Foundation Design: Principles and Practices, 3rd ed. — in-situ testing, settlement of shallow and deep foundations.
M. J. Tomlinson & J. Woodward, Pile Design and Construction Practice, 6th ed. — shaft adhesion in clay, pile-group behaviour, load testing.

Sources of design charts and assumed values (page-1 Note 6). Note 6 of this paper requires the candidate to identify the source of every design chart used and of every value assumed in the absence of data. They are named where used and collected here:

Q6 — bearing capacity factors $N_c$, $N_q$, $N_{\gamma}$ from Das, Principles of Foundation Engineering, Table 3.3 (Prandtl–Reissner $N_q$, Vesic $N_{\gamma}=2(N_q+1)\tan\phi'$); shape factors after De Beer (1970) and depth factors after Hansen (1970), Das Table 3.4. Assumed: unit weight of water $\gamma_w = 9.81$ kN/m$^3$; general shear failure; the sand extends at least $2B$ below the base.
Q7 — adhesion factor $\alpha = 1.0$ for soft clay ($c_u \le 50$ kPa) from NAVFAC DM-7.2 Fig. 1 and Tomlinson & Woodward Table 4.6; $\lambda = 0.24$ at an embedded length of 12 m from Vijayvergiya & Focht (1972) as tabulated by Das, Table 11.7; bearing factor $N_c^{*}=9$ for $L/D \ge 4$ (Skempton). Assumed: pile spacing $s = 3d = 1.5$ m centre to centre, driven closed-end concrete piles, clay $\gamma_{sat} = 17$ kN/m$^3$ with the water table at ground level.
Q8 — compression index from the Skempton correlation $C_c = 0.009\,(LL-10)$, Das Principles of Geotechnical Engineering Eq. (11.42); $2{:}1$ stress distribution, Das Eq. (6.31); Boussinesq rectangular influence factor (Das Table 6.6) used as the cross-check.
Q9 — Coulomb active pressure coefficient, Das Principles of Foundation Engineering Eq. (8.13) with the wall friction angle prescribed by the question, $\delta = 0.6\phi' = 18^{\circ}$. Assumed: unit weight of reinforced concrete $\gamma_c = 24$ kN/m$^3$ (CSA A23.3 nominal); passive resistance in front of the toe neglected; the backfill surface is horizontal and carries no surcharge.

Question 1: Shaft Adhesion in Soft Clay versus Stiff Clay (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 observation is the empirical basis of the alpha method, in which the limiting unit shaft resistance in a clay is written $f_s = \alpha\,c_u$. Back-analysis of load tests gives $\alpha \approx 1.0$ for soft normally consolidated clays ($c_u \lesssim 50$ kPa) but $\alpha \approx 0.3$ to $0.5$ for stiff overconsolidated clays. The reasons are all consequences of what pile installation does to the soil immediately around the shaft, and of what the shaft-soil interface can subsequently recover.

Installation disturbance and reconsolidation. Driving a displacement pile completely remoulds an annulus of clay one to two diameters thick and generates large excess pore pressures. In a soft, normally consolidated clay this excess pore pressure dissipates radially over weeks to months, the remoulded annulus reconsolidates against the pile, and the horizontal effective stress on the shaft returns to something close to, or even above, its pre-driving value. Because a soft clay is only lightly structured, remoulding destroys little strength that reconsolidation and thixotropic regain do not restore; the shaft therefore ends up with an interface strength close to the intact $c_u$, hence $\alpha \approx 1$. In a stiff overconsolidated clay the same driving process is far more damaging: the clay is strongly structured, often cemented and always fissured, and remoulding at large strain destroys a bonding that reconsolidation cannot rebuild.

Negative pore pressures and softening. Shearing a heavily overconsolidated clay is dilatant, so driving generates negative excess pore pressures near the shaft. As those negative pressures dissipate, water migrates toward the pile and the annulus softens — its water content rises and its undrained strength falls permanently. The long-term shaft resistance is therefore governed by a softened, near-normally-consolidated material, not by the stiff intact clay from which $c_u$ was measured.

The gap, and the mismatch of stress level. A stiff clay stands unsupported, so a driven or bored pile in stiff clay commonly leaves an open annular gap over the upper several diameters, or a slurry-smeared, low-strength film in the case of a bored pile; over that length the shaft resistance is essentially zero. More fundamentally, the shaft resistance is really an effective-stress phenomenon, $f_s = \beta\,\sigma'_v = K\tan\delta\,\sigma'_v$. A stiff clay owes its high $c_u$ to a past overburden that no longer exists, so the current vertical effective stress is modest while $c_u$ is large; the ratio $f_s/c_u$ must therefore be small. In a soft normally consolidated clay $c_u/\sigma'_v \approx 0.22$ to $0.25$ throughout, so the same effective-stress mechanism produces $f_s$ of the same order as $c_u$.

Brittleness and progressive failure. Finally, a stiff clay is strain-softening: its peak strength is mobilised at small displacement and falls toward a residual value beyond it. A 20 m pile does not slip uniformly — the top of the shaft reaches peak and passes into the softening branch while the toe is still loading elastically — so the peak $c_u$ is never available simultaneously over the full shaft. Soft clay is essentially ductile and does not suffer this progressive-failure loss. For Canadian practice, CFEM 4th ed. Ch. 18 recommends exactly this treatment: $\alpha$ near unity in soft sensitive clays such as the Champlain Sea deposits, with much lower values in the stiff glacial tills and Cretaceous clays of the Prairies, and confirmation by static load test.

Topic. Shaft resistance of piles in cohesive soil — the $\alpha$, $\beta$ and $\lambda$ methods.

Key relations. Total-stress: $Q_s = \sum \alpha\,c_u\,p\,\Delta L$. Effective-stress: $Q_s = \sum \beta\,\sigma'_v\,p\,\Delta L$ with $\beta = K\tan\delta$. Mixed: $f_{av} = \lambda(\bar\sigma'_v + 2\bar c_u)$. For a normally consolidated clay $c_u/\sigma'_v \approx 0.11 + 0.0037\,PI$ (Skempton).

Why this works. All three methods are attempts to predict the same physical quantity: the shear stress that can be sustained on a slip surface a few millimetres from the pile after installation and reconsolidation are complete. That surface is governed by the horizontal effective stress acting on it and by the friction angle of the remoulded interface material. In a soft normally consolidated clay the post-installation horizontal effective stress is close to what it was before driving, and the remoulded strength is close to the intact strength, so the total-stress shortcut $f_s = c_u$ happens to work. In a stiff overconsolidated clay neither condition holds, so the shortcut over-predicts badly and must be corrected by a small $\alpha$.

Common pitfall. Applying $\alpha = 1.0$ to a stiff till because it is called a "clay"; and quoting a shaft capacity computed from an end-of-driving load test as though it were the long-term value, when set-up in soft clay can double the capacity over several months while relaxation in stiff clay can reduce it.

Source. Das, Principles of Foundation Engineering 9th ed. §11.12–11.14; Tomlinson & Woodward, Pile Design and Construction Practice 6th ed. §4.2; CFEM 4th ed. Ch. 18.

← Paper overview Next: Question 2 →