16-Civ-B3 Geotechnical Design · May 2014
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. Professional Engineers Ontario / Engineers Canada National Examinations, May 2014 — 98-Civ-B3 Geotechnical Design. Three hours, OPEN BOOK, 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 (16-Civ-B3 / 98-Civ-B3 Geotechnical Design).
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. The values imported into the solutions below are, in full: bearing-capacity factors Nc = 5.7 (Terzaghi strip, phi = 0) and 5.14 (Meyerhof / Prandtl, phi = 0), Das Foundation Engineering Table 3.1 and Eq. 3.19; shape and depth factors from De Beer and Hansen as tabulated in Das Table 3.4; the adhesion factor alpha = 0.45 for bored piles in stiff clay, Skempton (1959) as reproduced in CFEM Ch. 18; the end-bearing coefficient Nc* = 9 for piles in clay, Skempton (1951); the compression index correlation Cc = 0.009(LL − 10), Terzaghi and Peck (1967); and a specific gravity Gs = 2.70 where a void ratio had to be back-figured. Each is repeated at the point of use.
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.
Recommendation. Use an anchored (tied-back) embedded wall — a secant or contiguous bored-pile wall for a permanent, water-tight, low-vibration solution, or a soldier-pile-and-lagging wall where the ground is dry, cohesive and the retention is temporary. Both are installed from ground level before excavation begins, both occupy only the thickness of the wall itself, and both take their lateral support from ground anchors drilled beneath the neighbouring property rather than from a sloping backfill or a wide base.
Why the conventional walls fail the brief. A gravity wall for a 5 m retained height needs a base of roughly 0.5 to 0.7 H, i.e. 2.5 to 3.5 m, and it needs that width excavated and then backfilled — on a restricted site there is nowhere to put the temporary batter, and the neighbouring foundation would have to be underpinned. A cantilever wall is slimmer but still needs a 0.4 to 0.7 H base slab and the same open excavation behind it. A mechanically stabilised earth (MSE) wall is worse: its reinforcement straps extend 0.7 to 0.8 H behind the face and it is built bottom-up from a fully excavated cut. All three are bottom-up systems, and the site has no room to make the cut.
Key construction and design details. Install the piles at 600 to 900 mm diameter on the property line, hard against the boundary; in a secant wall the unreinforced female piles are bored first and the reinforced male piles are then cut into them, giving a continuous, essentially water-tight face. Cast a reinforced-concrete capping beam over the pile heads to tie the wall into one unit and to spread any eccentric loading. Excavate in stages of 2 to 2.5 m; at each stage install a row of ground anchors through a waler at 2 to 3 m horizontal centres, inclined 15 to 30 degrees below horizontal, with the grouted bond length carried well beyond the active wedge (a line at 45 + phi'/2 from the base of the wall) so that the anchor takes reaction from undisturbed ground. For a 5 m wall one anchor level at roughly 1.5 m below the capping beam is usually enough; two levels are used where movement limits are tight. Stress every anchor to 1.25 to 1.5 times working load, proof-test to the ASTM D4435 / CFEM procedure, and lock off at working load. Carry the pile toe 1.5 to 2 m below formation to develop passive resistance, and check the wall for free-earth-support equilibrium, for the bending moment at the anchor level, and for the anchor forces.
Serviceability and legal points. The controlling criterion next to an existing building is not stability but movement: keep the predicted wall deflection below about 0.2 to 0.3 per cent of H, monitor with inclinometers in the piles and with optical targets or automated total-station prisms on the neighbouring facade, and set trigger levels before work starts. Provide drainage — weep holes with a geocomposite drain behind a soldier-pile wall, or a drained cavity wall in front of a secant wall — because a wall designed for earth pressure alone will fail under full hydrostatic pressure. Finally, ground anchors cross the property boundary, so a licence or easement from the neighbour is required; removable or de-stressable anchors are normally specified for this reason. Where anchors are refused, the same wall can be propped internally with raking shores or a corner brace, or a soil-nailed wall can be used if the ground stands unsupported for a 1.5 m lift.
