16-Civ-B3 Geotechnical Design · December 2014

Question 5 of 10: Retaining wall for highway slope stabilisation in 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, December 2014 — 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 five design questions of 24 marks each (answer any three); the examinable total is 4 × 7 + 3 × 24 = 100 marks. All ten 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).
B. M. Das, Principles of Geotechnical Engineering, 9th ed. — lateral earth pressure (Ch. 13), shear strength (Ch. 12), slope stability (Ch. 15), subsurface exploration (Ch. 17).
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, 8th ed. — earth pressure theory and slope stability.
D. P. Coduto, Foundation Design: Principles and Practices, 2nd ed. — in-situ testing and SPT correlations.
J. E. Bowles, Foundation Analysis and Design, 5th ed. — bearing capacity factors and retaining-wall stability tables.

Sources of charts and assumed values (page-1 Note 6). Note 6 of this paper requires the candidate to identify the source of every design chart and every assumed value. Each chart reading and each assumption below is therefore named where it is used, and the values assumed in the absence of data are collected here:

Q6 — adhesion factor α from Das, Principles of Foundation Engineering, Table 11.6 (Terzaghi, Peck & Mesri form, α against $c_u/p_a$); $\lambda$ from Vijayvergiya & Focht (1972) as tabulated by Das, Table 11.7.
Q7 — overburden correction $C_N$ from Liao & Whitman (1986); $\phi'$ from Wolff (1989) and from Hatanaka & Uchida (1996), both reproduced in Das, Ch. 2; settlement-controlled bearing pressure from Meyerhof (1965) as given by Das, Ch. 5, used only as a serviceability check because the question forbids direct correlations of bearing capacity to penetration index. Table I prints the blow counts as field values $N_f$; with no hammer data they are converted as $N_{60} = N_f$, i.e. a safety hammer at the reference 60 per cent energy ratio with borehole, sampler and rod-length factors of 1 (Das, Ch. 2, hammer-efficiency and correction-factor tables).
Q8 — embankment influence factor from Osterberg (1957), reproduced as Das Fig. 6.24; the closed form of that chart is used so the reading carries no chart-scaling error.
Q9 — Meyerhof general bearing-capacity equation with the shape factors of De Beer (1970) and the depth factors of Hansen (1970), as set out in Das, Ch. 3.
Q10 — Coulomb active earth-pressure coefficient, Das Eq. 13.31; unit weight of the mass-concrete wall assumed $\gamma_c = 24\ \text{kN/m}^3$ (CFEM 4th ed., normal-density concrete), the only value the figure does not supply.

Question 5: Retaining wall for highway slope stabilisation in 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.

Parameters to be considered. The design of a slope-stabilising wall in clay is controlled by a wider set of parameters than a conventional backfill-retaining wall, because the wall must change the stability of a soil mass that is already marginal. The parameters are:

Geometry — existing and final slope angles, wall height, the depth and shape of the critical slip surface from a back-analysis of the existing slope, and the available right-of-way.
Stratigraphy and strength — the clay profile, the depth to a competent stratum or bedrock into which the wall can be keyed, undrained strength $c_u$ for the construction case, and drained parameters for the long term. In a stiff fissured clay these must be the fully softened values, and on any pre-sheared surface the residual values.
Groundwater and seepage — the piezometric surface, perched water, and the pore pressures on the slip surface. Water is both a load and a strength reducer and is the dominant design variable in a clay slope.
Loads — highway live load surcharge, the weight of the pavement structure, snow-clearing and maintenance vehicles, guardrail impact, and the down-slope creep pressure from the mass above the wall.
Seismic — the design spectral accelerations from the National Building Code of Canada for the site, and the resulting pseudo-static horizontal coefficient for a Mononobe–Okabe check.
Serviceability and environment — tolerable movement of the pavement, frost penetration depth and frost-susceptibility of the retained soil, freeze–thaw and de-icing salt exposure of the concrete (CSA A23.1 class C-2 or C-4), and drainage provision.
Constructability — access on a steep slope, the stability of any temporary excavation, and whether traffic must be maintained during construction.

Type of wall proposed, and the justification. For a highway cut or sidelong fill on a steep clay slope I would propose a discrete bored-pile (soldier pile or contiguous / tangent pile) wall, socketed below the critical slip surface, with a reinforced-concrete capping beam and, if the height requires it, one row of ground anchors.

The justification is that the governing limit state here is global stability, not the local overturning or sliding of a wall stem. A gravity or cantilever wall resists only the earth pressure acting on its own back face; it does nothing for a slip surface that passes beneath its base, and its own weight is an additional driving load on the slope. A mechanically stabilised earth wall is worse still on this site, because it requires a wide excavation into the slope and a large volume of imported granular fill, both of which destabilise the slope during construction. By contrast a pile wall keyed 3 to 5 m below the critical circle adds a real resisting shear force across that surface, and it is built top-down from the existing bench with no bulk excavation, so the slope is never temporarily unsupported. The piles can be installed with a small track rig on a narrow bench, which suits difficult access, and the wall face can be finished with shotcrete or precast panels between the piles. Where the retained height is modest and access is good, a cantilever reinforced-concrete wall with a deep shear key is a reasonable and cheaper alternative, and a soil-nailed wall is a good choice in a cut where the clay is stiff and can stand unsupported for the height of one nail lift.

Hazard scenarios. The design must be checked against, at minimum:

Deep-seated global instability passing beneath or behind the wall — the single most important check, and the one a conventional wall-stability calculation does not cover.
Build-up of water pressure behind the wall from a blocked or under-designed drain. This is historically the commonest cause of retaining-wall failure, and it must be addressed by a granular drainage blanket or geocomposite drain, weep holes, and a maintainable outlet.
Long-term softening of the clay from peak towards fully softened strength, and progressive failure along a strain-softening slip surface.
Seismic loading, including the inertial force on the wall and the increased active thrust.
Construction-stage instability — the temporary excavation, surcharge from plant, and the period before the capping beam or anchors are stressed.
Toe erosion or scour where a watercourse runs at the base of the slope, and future excavation in front of the wall (service trenches) removing passive resistance.
Frost heave and freeze–thaw, including ice-lensing pressures on the wall face if a frost-susceptible soil is retained without drainage.
Downslope creep of the mass above the wall, which imposes a lateral load independent of the classical active wedge.
Differential settlement of the wall and the adjoining pavement, and the resulting bump at the wall.
Durability — corrosion of anchors and reinforcement under de-icing salt exposure, and the consequence of an anchor losing its capacity late in the design life.

Topic: selection and design of an earth-retaining structure whose purpose is slope stabilisation rather than backfill retention.

Key relations: global stability $FS = \dfrac{\sum \left[c'\ell + (W\cos\alpha - u\ell)\tan\phi'\right] + R_{wall}} {\sum W \sin\alpha}$; the wall contributes $R_{wall}$ as an additional resisting force across the slip surface, which is why it must be keyed below that surface.

Why this works: a stabilising wall increases the factor of safety only through the shear it can transmit across the failure plane. A structure that sits entirely above the plane transmits none, however heavy it is; a pile socketed through the plane transmits its full plastic shear capacity. Choosing the wall type is therefore a question of where the slip surface is, which is why the back-analysis comes first.

Common pitfall: designing the wall for overturning, sliding and bearing, declaring the factors of safety adequate, and never running a global stability analysis. Slope-stabilising walls fail globally, not by overturning.

Source: CFEM 4th ed., Ch. 6 (Slope stability) and Ch. 26/27 (Earth-retaining structures); B. M. Das, Principles of Foundation Engineering, 9th ed., Ch. 8; National Building Code of Canada for seismic hazard values; CSA A23.1 for concrete exposure.

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