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.
Short-term means undrained. A natural slope in soft clay is at its most critical immediately after any change in geometry or loading, because the clay has had no time to drain and the strength available is the undrained shear strength cu at the in-situ water content. The whole investigation is therefore aimed at one number, the profile of cu with depth, and at the geometry of the potential slip surface. The analysis that follows is a total-stress (phiu = 0) circular-arc analysis, so the programme below must deliver cu, the unit weight, and the location of any weak layer.
Desk study and reconnaissance. Before any drilling, assemble air photographs, geological maps and any historic aerial imagery, and walk the slope. Soft-clay slopes carry their history on the surface: arcuate scarps, hummocky toe bulges, tension cracks along the crest, tilted trees and seepage lines all say the slope has moved before, and a back-analysis of a previous failure is worth more than any laboratory test. In British Columbia and the St Lawrence lowlands, check specifically for sensitive marine clay, which behaves completely differently once remoulded.
Boreholes and in-situ testing. Put down a minimum of three boreholes on a section through the steepest part of the slope — one behind the crest, one on the face, one beyond the toe — taken to at least 1.5 times the slope height below the toe so that a deep-seated circle cannot escape below the investigated depth. In soft clay the SPT is useless; use the field vane at 0.5 to 1.0 m intervals to get a continuous cu profile, with the remoulded vane reading at each depth to obtain the sensitivity. Correct the vane strength for plasticity using Bjerrum's correction factor mu (roughly 1.0 at PI = 20 falling to about 0.6 at PI = 100), because uncorrected vane strengths systematically over-predict field behaviour. A piezocone (CPTu) profile alongside the boreholes gives a continuous stratigraphic record, identifies thin silt or sand seams that would otherwise be missed, and, from the dissipation tests, gives the in-situ pore pressure and the coefficient of consolidation.
Sampling and laboratory work. Recover 75 mm thin-walled Shelby tube samples continuously through the soft clay, kept upright, sealed and tested quickly. Run unconsolidated-undrained (UU) triaxial tests to confirm the vane profile, unconfined-compression tests as an economical check, and Atterberg limits, water content, bulk density and organic content on every sample so that the strength profile can be normalised and any anomalous reading explained. One or two consolidated-undrained (CU) triaxial tests with pore-pressure measurement should also be run: they give the effective strength parameters needed later for the long-term case and confirm the stress history (over-consolidation ratio) that controls whether cu should increase with depth.
Groundwater and instrumentation. Install standpipe piezometers, and vibrating-wire piezometers in the low-permeability zones, monitored over at least one wet season. Install inclinometers through the suspected slip zone: a few millimetres of movement at a distinct depth locates the failure surface far more reliably than any calculation.
Analysis and reporting. Build the section from the borehole record, adopt the corrected cu profile, and run a total-stress circular-arc analysis (Taylor's chart for a first estimate, then Bishop's simplified method with a search for the critical circle), including a tension crack of depth $z_0 = 2c_u/\gamma$ filled with water if cracking is credible. Report the factor of safety, the sensitivity of that factor to the vane correction, and a monitoring regime.
