16-Civ-B3 Geotechnical Design · December 2015

Question 5 of 9: Site Investigation and Design Criteria for a Highway Embankment in Expansive Soil

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 5: Site Investigation and Design Criteria for a Highway Embankment in Expansive Soil (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 project has two expansive-soil problems, not one: a foundation of expansive clay beneath the embankment, and an embankment built out of the same material. The investigation must therefore characterise the deposit both as a foundation and as a construction material, and the design must recognise that the critical condition for an expansive-clay fill slope is not the end of construction but the fully softened, seasonally wetted long-term state.

Field investigation. Begin with a desk study of surficial geology, airphotos and any existing highway performance in the same formation — recurrent shallow slips and pavement cracking in nearby cuts and fills are the single most valuable piece of evidence available. Follow with continuously sampled boreholes along the alignment at a spacing appropriate to the variability, extended at least to the depth of any potential deep-seated failure surface beneath the toe, complemented by CPTu soundings for stratigraphy and pore-pressure response and by test pits that expose the near-surface fabric. Two field observations matter more here than anywhere else. The first is the fissure and slickenside pattern: highly expansive clays are almost always fissured, and pre-existing polished shear surfaces from earlier movement control the strength on any surface that follows them. The second is the moisture regime: install standpipe and vibrating-wire piezometers to define the groundwater table and any perched water, and take water-content profiles in both a wet and a dry season so that the depth of the active zone and the desiccated crust are known rather than assumed. Field vane tests give an undrained profile in the softer material, and a borrow investigation of the proposed fill source is required in parallel.

Shear strength parameters. Stability of a highly expansive, fissured clay slope must be assessed in effective stresses for the long term, and at a strength well below the intact peak. Specifically:

Consolidated–undrained triaxial tests with pore-pressure measurement (CU-bar) on undisturbed foundation samples and on samples of the fill recompacted to the specified placement density and water content, giving $c'$ and $\phi'$ directly and also the pore-pressure parameters needed for the construction case.
Consolidated–drained (CD) direct shear or triaxial tests as the primary long-term reference, since they measure the drained envelope without relying on pore-pressure corrections.
Ring shear tests to obtain the residual angle $\phi'_r$. This is the critical measurement in this soil. Where slickensides or a pre-existing slip surface are found, the strength available on that surface is the residual, which in a high-plasticity clay can be 8 to 14 degrees against a peak $\phi'$ of 20 to 25 degrees. Designing on peak strength where residual conditions exist is the classic cause of expansive-clay embankment failure.
Fully softened strength on normally consolidated remoulded specimens. For a compacted expansive fill, seasonal wetting and drying progressively destroys the compaction-induced structure, and within a few years the mobilised strength in the outer few metres approaches the fully softened value with $c' \approx 0$. This, not the as-compacted strength, is the correct parameter for shallow surficial stability.
Unconsolidated–undrained (UU) triaxial and field vane for the end-of-construction check on the soft foundation clay, in the total-stress ($\phi_u = 0$) form.
Oedometer and swell–consolidation tests for the swelling pressure, free-swell strain, $C_c$ and $c_v$, which feed the settlement and the active-zone assessments.

Other design criteria. Stability alone will not deliver a serviceable embankment in this material. The design must also address:

Shallow surficial instability — the dominant failure mode for expansive fill slopes. Flatten the slope (3H:1V or flatter is common in high-plasticity clay), or place a granular or lime-treated outer cover layer 1.0 to 1.5 m thick over an expansive core so that the wetting-drying zone is not the expansive material.
Moisture and drainage control — the mechanism that drives everything else. Crown the subgrade, seal the pavement edges, install intercepting subdrains and a drainage blanket at the fill/foundation contact, and vegetate the face promptly. Cracking on drying admits water directly to depth in the next wet season.
Placement specification — compact at 1 to 3 percent above optimum water content to a moderate density. Expansive clay compacted dry of optimum to a high density has the greatest swelling potential; wetter placement at lower density trades a little strength for a large reduction in swell.
Lime or lime–fly-ash stabilisation of the fill, which reduces plasticity index and swell dramatically and is often the decisive measure when no better borrow exists. It must be validated by a mix design including a sulphate check, since sulphate-bearing clays can heave on lime treatment.
Settlement and construction rate — consolidation settlement of the foundation clay, its time rate from $c_v$, and staged construction with piezometric monitoring if the end-of-construction undrained case is marginal.
Pavement and structure interfaces — differential movement at bridge approaches and culverts, a non-expansive subgrade capping layer, and geosynthetic reinforcement or berms where the geometry is constrained.
Long-term monitoring and maintenance — slope inclinometers and piezometers on the higher fills, with an explicit maintenance expectation, since these slopes deteriorate over decades rather than failing suddenly.

Target factors of safety follow normal Canadian highway practice: of the order of 1.3 for the end-of-construction undrained case with monitoring, and 1.5 for the long-term drained case, with the long-term case evaluated at fully softened or residual strength as the fissuring evidence dictates.

Topic. Site characterisation and slope design for high-plasticity expansive soils used both as foundation and as fill.

Key relations. Long-term slope stability from $\tau_f = c' + (\sigma - u)\tan\phi'$, with $c' \to 0$ and $\phi' \to \phi'_{fs}$ or $\phi'_r$ on pre-sheared surfaces; infinite-slope criterion for shallow surficial failure, $F = \dfrac{c' + (\gamma z\cos^2\beta - u)\tan\phi'}{\gamma z \sin\beta\cos\beta}$.

Why this works. Strength in an expansive clay is not a fixed property but a function of the shear strain the soil has already experienced and of how much water it has taken up. Peak strength exists only in intact, unwetted material; a fissured or previously sheared clay offers residual strength, and a seasonally wetted compacted fill drifts toward the fully softened value. Choosing the parameter therefore means choosing the state the soil will actually be in when it is asked to perform, which for a highway embankment is many wet-dry cycles after construction.

Common pitfall. Running a suite of UU triaxial tests on the fill, using the resulting high as-compacted strength in a long-term stability model, and obtaining a comfortable factor of safety for a slope that will suffer shallow slips within five years. The second pitfall is treating swelling as a pavement problem only, when it is in fact the mechanism that opens the cracks through which the water arrives.

Source. CFEM 4th ed., §4 (investigation), §6 (slopes and embankments) and §16 (swelling soils); Das, Principles of Geotechnical Engineering, 9th ed., Ch. 15; Craig's Soil Mechanics, 9th ed., §5.6 and Ch. 9.

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