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16-Civ-B3 Geotechnical Design · May 2013

Question 4 of 10: Short-term versus long-term slope stability of a clay embankment dam

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

Notes on this paper

Paper format. National Examinations, May 2013 — 98-Civ-B3 Geotechnical Design. Three hours, open book, any non-communicating calculator. Section A holds five 7-mark questions (answer any four); Section B holds the long 24-mark design questions (answer any three). Candidates are asked to identify the source of every design chart and assumed value used. Every question is answered here, because the set is a study resource rather than a sitting.

Reference texts. B. M. Das, Principles of Foundation Engineering (9th ed.) and Principles of Geotechnical Engineering (9th ed.); Canadian Geotechnical Society, Canadian Foundation Engineering Manual (CFEM, 4th ed.) — the governing Canadian reference for foundation practice; D. P. Coduto, Foundation Design: Principles and Practices; R. F. Craig, Craig's Soil Mechanics.

Note on the question numbering. The printed paper numbers two different Section B questions as “Question 9” — the retaining wall on page 5 and the drilled pier on pages 5–6 — and its Section B heading says “any three of the following four questions” while five questions are actually printed. The drilled-pier question is treated here as Question 10 so that every printed question has a unique number; no wording has been changed.

Question 4: Short-term versus long-term slope stability of a clay embankment dam (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.

Why the short-term case is given the larger margin. The end-of-construction analysis of a clay embankment is a total-stress analysis, and it is the more uncertain of the two calculations in every respect that matters. The undrained strength is a measured quantity rather than a computed one, and it is notoriously sensitive to sampling disturbance, to specimen orientation (anisotropy), to strain rate, and to fissuring in the parent clay; strengths from different test types on the same deposit routinely differ by 30 per cent. The construction-induced pore pressures that the total-stress analysis quietly assumes are equally uncertain, because they depend on placement moisture content, lift thickness and rate of raising, none of which is fixed at design time.

The consequences are also worse. Undrained failure of a saturated clay is rapid, and in a sensitive Canadian clay it can be brittle and retrogressive, giving little warning and no opportunity to intervene. A long-term drained failure under steady seepage develops slowly, is preceded by measurable crest movement and piezometric change, and can be arrested by a berm or by drawdown. Finally, for an embankment on a clay foundation the factor of safety is at its minimum at the end of construction and then increases as the foundation consolidates and gains strength, so an error made at that instant is never recovered. A larger nominal factor of safety is the cheapest way to absorb all of this.

Check — convention. The question's premise is adopted as stated. Note that several agencies, notably USACE EM 1110-2-1902, run the opposite convention and accept a lower minimum factor of safety at end of construction (about 1.3) than for long-term steady seepage (1.5), on the argument that the undrained condition is transient and closely monitored. Whichever convention a project adopts, both conditions must be analysed and the governing one designed for.

Parameters and properties needed for the short-term analysis. For the saturated clay foundation, a total-stress ($\phi_u = 0$) analysis using the undrained shear strength $c_u$ with depth, together with its anisotropy and sensitivity. For the compacted clay fill, which is only partially saturated as placed, the total-stress envelope has both intercept and slope, so $c_u$ and $\phi_u$ are both required. In addition: the bulk (moist) unit weight at the specified placement density and moisture content; the compaction specification itself (standard or modified Proctor maximum dry density and optimum moisture content); Atterberg limits and grain-size distribution for classification and for judging susceptibility to strength loss; the degree of saturation; the pore-pressure parameters $A$ and $B$ if the analysis is to be run in effective stresses with predicted pore pressures; and the coefficient of consolidation $c_v$ to establish how quickly the short-term condition gives way to the long-term one, which in turn sets the permissible rate of raising.

Recommended tests. Unconsolidated-undrained (UU) triaxial tests on undisturbed foundation samples and on fill specimens compacted to the specified density and moisture content, to obtain $c_u$ and $\phi_u$ directly in the as-placed state. Field vane shear tests through the soft foundation clay for a continuous $c_u$ profile, with remoulded vane readings giving the sensitivity. Unconfined compression tests as a rapid index check ($c_u = q_u/2$). Consolidated-undrained (CU) triaxial tests with pore-pressure measurement to obtain $c'$, $\phi'$ and Skempton's $A$ parameter, which serve both the effective-stress check and the long-term case. Supporting index work: standard or modified Proctor compaction, moisture content and density, Atterberg limits, sieve and hydrometer analysis, and oedometer consolidation tests for $c_v$.