16-Civ-B3 Geotechnical Design · December 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Examinations, December 2018 — 16-Civ-B3 Geotechnical Design. Three hours, open book, any non-communicating calculator. Section A holds five discussion questions worth 7 marks each (answer any four); Section B holds four design questions worth 24 marks each (answer any three). The examinable total is therefore 4 × 7 + 3 × 24 = 100 marks. Page-1 Note 6 requires the candidate to name the source of every design chart and of every assumed value, so each chart read and each assumption below is attributed where it is used. All nine questions are solved here, because the set is a study resource rather than a timed sitting.
Reference texts. B. M. Das, Principles of Foundation Engineering, 9th ed. (bearing capacity, settlement, retaining walls, pile foundations); B. M. Das, Principles of Geotechnical Engineering, 9th ed. (shear strength, lateral earth pressure); Canadian Geotechnical Society, Canadian Foundation Engineering Manual (CFEM), 4th ed. (Canadian practice, factors of safety, site investigation); R. F. Craig, Craig's Soil Mechanics, 9th ed. (effective stress, slope stability); D. P. Coduto, Foundation Design: Principles and Practices, 2nd ed. (SPT interpretation, shallow foundation design).
Check — conventions used throughout this paper. Unit weights printed on the figures are taken as bulk (saturated below a water table) values; effective unit weights use γw = 9.81 kN/m3. Where the exam omits a number that the solution needs, the assumption is stated in the question where it is used, with its source, as page-1 Notes 1, 6 and 7 direct.
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.
Definition. The active zone is the depth interval, measured from the ground surface, over which the soil water content changes seasonally by enough to produce significant volume change. Below it the water content is effectively constant year to year, so the soil neither swells nor shrinks. Its depth is a property of climate, vegetation, drainage and soil fabric rather than of the clay mineralogy alone: in the semi-arid Canadian prairies, where the Lake Agassiz and Regina clays are strongly expansive, it commonly extends 3 to 4 m, and desiccation by mature trees can drive it well past 5 m. It is identified in practice from the water-content and suction profiles of a series of boreholes drilled in different seasons; the depth at which those profiles converge is the base of the active zone.
Why it matters. Expansive clay in the active zone can generate swelling pressures of several hundred kilopascals — far more than the bearing pressure under a light residential footing or a slab-on-grade — and the resulting heave is differential, because the perimeter of a heated building dries and shrinks while the centre stays wet. Distortion of this kind is the single largest source of foundation damage claims in Regina, Winnipeg and much of southern Saskatchewan. The engineering consequences run through the whole design:
Definition. Negative skin friction, or downdrag, is the downward shear stress applied to a pile shaft by soil that is settling more than the pile. Instead of helping to carry the structural load, that length of shaft adds to it. The classic setting is a compressible clay or peat layer that is consolidating under a recently placed fill, with the pile driven through the fill and the clay to a firm bearing stratum; lowering of the groundwater table, which increases the effective stress in every layer, and consolidation of a soft layer under its own weight in a recent deposit, produce the same effect.
The neutral plane. The depth at which the settlement of the soil equals the settlement of the pile is the neutral plane. Above it the friction is negative, below it positive, and the axial force in the pile reaches its maximum there — not at the pile head. Recognising this is the practical heart of the concept: the structural check on the pile section, and the check on the concrete or steel stress, must be made at the neutral plane using the sum of the structural load and the accumulated downdrag.
Engineering significance. The consequences are of two distinct kinds and must be checked separately. Downdrag is a settlement problem and a structural problem, but it is not, strictly, a bearing-capacity problem: at the moment the pile plunges, the soil is moving up relative to the pile everywhere and the friction is positive over the full length, so the ultimate geotechnical capacity is unaffected. What downdrag does is (i) increase the maximum axial force the pile section must carry, and (ii) drag the pile down by whatever settlement is needed to reach equilibrium at the neutral plane. For a group, the downdrag is limited by the weight of the fill enclosed within the group perimeter plus the friction on the outside of the block, which is often much less than the sum of the individual pile values, so the block mechanism should always be checked. Mitigation in Canadian practice includes preloading or surcharging the fill with wick drains before piling, coating the shaft with bitumen slip layers within the settling zone, sleeving the pile through the fill, and reducing shaft area within the settling layer while founding on a competent stratum.
