16-Civ-B3 Geotechnical Design · December 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format. National Examinations, December 2017 — 16-Civ-B3 Geotechnical Design; three hours, open book, any non-communicating calculator. Section A holds five discussion questions worth 7 marks each of which four are marked; Section B holds four design questions worth 24 marks each of which three are marked, so 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 marked script.
Reference texts. B. M. Das, Principles of Foundation Engineering, 8th–9th ed. (Cengage); B. M. Das, Principles of Geotechnical Engineering, 9th ed.; R. F. Craig / J. Knappett, Craig's Soil Mechanics, 9th ed.; Canadian Geotechnical Society, Canadian Foundation Engineering Manual (CFEM), 4th ed.; D. P. Coduto, Foundation Design: Principles and Practices; J. E. Bowles, Foundation Analysis and Design; ASTM D1586 (SPT), D5778 (CPTu), D2573 (field vane).
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.
The choice between spread footings and piles for a five-storey hotel is not made on the magnitude of the load — a five-storey framed building imposes only about 10 to 12 kPa per storey, so interior column loads are typically 1500 to 3000 kN and a raft pressure of 50 to 70 kPa. It is made on two independent properties of the soil profile within the depth influenced by the foundation: its strength, which decides whether a shear failure mechanism can form beneath the footing, and its deformation behaviour, which decides how much the building will settle and, more importantly, how unevenly.
The strength argument. A spread footing is viable only if the stratum immediately below founding level can develop an ultimate bearing capacity giving a net allowable pressure of roughly 200 kPa or more at a factor of safety of 3. Dense to very dense sands and gravels, glacial till, stiff to hard clays with an undrained strength above about 100 kPa, and weathered rock all satisfy this comfortably at a founding depth of 1.5 to 3 m. Loose sands, normally consolidated silts and clays, peat, organic silts and uncontrolled fill do not: their ultimate capacity may be 100 kPa or less, so a footing large enough to work becomes a raft, and even the raft may not close the gap. A deep foundation solves the strength problem by bypassing the weak material entirely and mobilising shaft friction plus end bearing in a competent stratum, so the governing strength is that of a soil the footing would never have reached.
The deformation argument, which usually governs. For a building of this height the ultimate limit state is rarely critical; serviceability is. Total settlement can be tolerated in large amounts if it is uniform, but differential settlement drives cracking, door and window distortion, and out-of-level lift guides. The conventional Canadian criteria (CFEM, Ch. 8) are a total settlement of 25 mm on sand or 50 to 65 mm on clay and an angular distortion no worse than 1/500 for a framed building with masonry infill. In granular soils settlement is immediate and is estimated from Schmertmann's strain-influence method or an elastic solution; in clays it is the sum of an immediate component and a time-dependent consolidation component that may continue for decades, and it is the long consolidation tail that most often disqualifies footings. A five-star hotel raises this bar further: stone floor finishes, full-height glazing, pools and spas, and long unbroken corridors are all intolerant of distortion, and the deep-foundation route is often chosen purely to buy settlement uniformity even where footings would be safe against collapse.
The recommendation, and the middle ground. I would recommend individual column footings when boreholes show a competent, laterally continuous stratum within about 3 m of grade, the water table is below or only slightly above founding level, the compressible thickness within the stress bulb (roughly two footing widths deep) is small, and the computed differential settlement is comfortably inside 1/500. I would recommend piles — driven or bored, socketed into till or rock — when the near-surface profile is soft or loose, when a compressible clay layer lies within the stress bulb, when the site contains fill of unknown quality or organic deposits, when the sand is loose and saturated and therefore liquefiable under the design seismic event (a live issue on the west coast, where the NBCC design earthquake governs), or when scour, frost, expansive clay or adjacent deep excavations make a shallow bearing level unreliable. Between the two sits the raft: on a uniformly compressible but not weak profile a rigid mat spreads the load, averages the deformation and can be made partly compensated by matching the excavated weight to the building weight, and it is frequently the most economical answer for exactly this size of structure. The honest engineering answer is therefore that the decision is made after the site investigation, from a settlement calculation, not from the number of storeys.
