16-Civ-B3 Geotechnical Design · December 2015

Question 2 of 9: The Adhesion Factor in Augered Cast-in-Place Pile Capacity

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 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. — subsurface exploration (Ch. 2), bearing capacity (Ch. 3), settlement of shallow foundations (Ch. 5), 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. — consolidation (Ch. 11), 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, earth-retaining structures and expansive soils.
R. F. Craig, Craig's Soil Mechanics, 9th ed. — effective stress, earth pressure theory and slope stability.
D. P. Coduto, Foundation Design: Principles and Practices, 3rd ed. — in-situ testing correlations and settlement of shallow foundations.
M. J. Tomlinson & J. Woodward, Pile Design and Construction Practice, 6th ed. — shaft adhesion in clay, bored and augered piles, 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 — Rankine active coefficient for a sloping backfill, Das, Principles of Foundation Engineering, Eq. (8.5); base friction and adhesion mobilisation factors $k_1 = k_2 = \tfrac{2}{3}$ after Das §8.5; Rankine passive coefficient $K_p = \tan^2(45^{\circ} + \phi'_2/2)$. Assumed: stem height $H = 10.0$ m (the exam omits it — see the callout in Q6); reinforced concrete $\gamma_c = 24$ kN/m$^3$ (CFEM §4; CSA A23.3 normal-density concrete); the backfill is fully drained so no water force acts.
Q7 — overburden correction $C_N$ after Liao & Whitman (1986); $\phi'$ from $(N_1)_{60}$ after Peck, Hanson & Thornburn (1974) as fitted by Wolff (1989), cross-checked against Hatanaka & Uchida (1996); bearing capacity factors from Das 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; settlement from Meyerhof's (1965) SPT expression, Das Eq. (5.42), cross-checked by Schmertmann's strain-influence method with $E_s = 500(N_{60}+15)$ kPa. Assumed: founding depth $D_f = 2.0$ m; the sand is uniform to at least $2B$ below the base; tolerable settlement 25 mm.
Q8 — compression index from Terzaghi & Peck (1967), $C_c = 0.009(LL-10)$; stress increase by the 2:1 method (Das §6.2) with a Boussinesq rectangular-area cross-check (Das Table 6.6); Simpson weighting of $\Delta\sigma'$ prescribed on the exam paper itself. Assumed: $\gamma_w = 9.81$ kN/m$^3$; the clay is saturated so $e_0 = wG_s$; the sand layers are incompressible relative to the clay.
Q9 — undrained ($\phi_u = 0$) mass procedure, Das, Principles of Geotechnical Engineering, §15.5; drained comparison by the ordinary method of slices and Bishop's simplified method, Das §15.11–15.12, and by the infinite-slope criterion, Das Eq. (15.10). Assumed: no external water force and no seismic loading; the sliding mass is homogeneous.

Section A — discussion questions (7 marks each; answer any four)

Question 2: The Adhesion Factor in Augered Cast-in-Place Pile Capacity (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.

In the α method the unit shaft resistance of a pile in clay is written as $f_s = \alpha c_u$, in which $c_u$ is the undrained shear strength of the surrounding clay and α is an empirical adhesion factor. The purpose of α is to bridge the gap between the strength that the ground had before the pile existed and the shear resistance that the pile–soil interface can actually deliver after construction. That gap has several physical sources, and α lumps them into one number because they cannot practicably be separated on a routine job.

For an augered cast-in-place pile the sources are specific and mostly reducing. Boring the hole unloads the clay laterally, so the horizontal effective stress against which friction can act is far lower than the at-rest value that the undrained strength implicitly reflects. The auger smears and remoulds a thin annulus of clay at the shaft wall, destroying whatever structure contributed to $c_u$. The clay adjacent to the hole then draws water from the fluid concrete and from the surrounding soil and softens, and in a stiff fissured clay this softening can be severe. Working the other way, the concrete is placed fluid and exerts a hydrostatic pressure that partially restores lateral stress, and it forms a rough, interlocked contact rather than a smooth one. The net effect for augered piles in stiff clay is a value of α near 0.4 to 0.5, falling further in heavily fissured clays and rising toward 1.0 in soft normally consolidated clays where there is little structure to destroy and little strength difference between the annulus and the mass. A second role of α is statistical: because the correlation was calibrated against load tests, using it carries the observed scatter of real piles into the design, which is why CFEM and the codes attach a resistance factor to shaft resistance in addition to the α value itself.

The choice between α, β and λ is a choice about which stress state controls the answer.

Prefer the α method when the governing condition is short-term and undrained — a pile in a saturated clay loaded soon after installation, in soft to stiff clays where a reliable $c_u$ profile is available from vane, cone or triaxial testing, and where local load-test experience allows α to be chosen with confidence. It is also the natural method whenever the design is being checked against a database of load tests in the same clay, because that is the form in which such databases are published. Its great practical advantage is that it needs only $c_u$, which is the parameter most reliably measured in a clay.
Prefer the β method when the governing condition is long-term and drained: piles in sands and non-plastic silts, piles in stiff clays that will carry sustained load for decades, and any situation where excess pore pressures generated by installation will have dissipated well before the design load is applied. Since $f_s = \beta \sigma'_v = K \tan\delta' \cdot \sigma'_v$, it is written in effective stresses and is therefore the physically correct statement of what the interface can sustain once drainage is complete; it requires $\phi'$, the earth-pressure coefficient $K$ appropriate to the installation method, and a reliable vertical effective-stress profile.
Prefer the λ method for long driven piles in clay, particularly in the marine and offshore setting for which Vijayvergiya and Focht developed it. Because $f_{s,av} = \lambda(\bar{\sigma}'_v + 2\bar{c}_u)$ mixes an effective stress term and an undrained term, and because $\lambda$ falls with embedded length, it captures the length effect that a constant α misses. It is best used as a check on a long uniform pile rather than as the primary method for a short bored pile.

Good practice on any significant job is to compute the capacity by at least two of the three, treat the spread as a measure of the uncertainty, and resolve it with a static load test — conventionally on the order of one pile in a hundred for routine work and one in ten where the ground is variable or the α value is being pushed.

Topic. Shaft resistance of piles in clay and the selection of a capacity method.

Key relations. $Q_s = \sum f_s \, p \, \Delta L$ with $f_s = \alpha c_u$ (total stress), $f_s = \beta \sigma'_v = K\tan\delta' \sigma'_v$ (effective stress), and $f_{s,av} = \lambda(\bar{\sigma}'_v + 2\bar{c}_u)$ (Vijayvergiya & Focht, 1972).

Why this works. All three methods are statements of Coulomb friction on the shaft; they differ only in which stress measure is used as the proxy for the normal effective stress acting on the interface at the moment the pile is loaded. Immediately after installation the normal stress is unknown but is correlated with $c_u$, so an undrained formulation is the practical one. After full dissipation the normal stress is knowable from $\sigma'_v$ and $K$, so an effective-stress formulation is both correct and computable. The λ form is a deliberate hybrid that reproduces the observed decay of average unit friction with embedded length.

Common pitfall. Carrying an α value calibrated for driven displacement piles across to a bored or augered pile. Displacement installation increases lateral stress; augering removes it. The same clay can support $\alpha \approx 0.8$ on a driven pile and only $\alpha \approx 0.45$ on an augered one, so the transfer is unconservative by close to a factor of two.

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

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