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.
The standard penetration test is the method commonly used for sandy soils, and it remains the test on which most routine sand design in Canada is actually carried out. Question 7 of this same paper is a demonstration of the point: it hands the candidate a profile of corrected N60 values in sand and asks for a footing size and a bearing capacity from them.
Sand cannot be sampled undisturbed, so an in-situ index is unavoidable. A clean sand loses its fabric and its state the moment a tube sampler enters it, and below the water table it will not stay in the tube at all. There is no economic way to bring a representative specimen of sand to a laboratory, so the strength and compressibility of the deposit have to be inferred from a penetration resistance measured in place. That leaves the SPT and the CPT, and the SPT has the advantage of being performed inside a borehole that is being advanced anyway.
The SPT returns a sample. The split-spoon recovers a disturbed but genuine specimen at every test depth, so the material can be classified, its grading and fines content determined, and the presence of silt seams, organics, marine bioclastic debris or cobbles recorded. The CPT returns only qc, fs and pore pressure, from which soil type must be inferred through a behaviour-type chart. On a site where the deposit is described as "erratic" — as in Question 9 of this paper — having the actual soil in hand matters.
The correlation base is overwhelmingly SPT-based. Relative density (Skempton, Terzaghi and Peck), friction angle (Peck–Hanson–Thornburn, Wolff, Kulhawy and Mayne), allowable bearing pressure for a tolerable settlement (Meyerhof, Bowles, Burland and Burbidge), the soil modulus used in a Schmertmann calculation, and liquefaction triggering (the Youd–Idriss NCEER framework, which underpins Canadian seismic practice) are all expressed in terms of N60 or (N1)60. Decades of case records give these correlations a reliability that no purely theoretical route can match.
The SPT works where the cone will not go. Canadian granular deposits are commonly glacial: sands with gravel, cobbles, and dense till layers. A cone will refuse, deviate, or suffer damage in such material, whereas the split-spoon can be driven and, if refusal occurs, the borehole can simply be advanced past the obstruction. The SPT is also available anywhere a drill rig can reach; the CPT needs a heavy reaction mass, which sloping, wooded or restricted-access sites often cannot accommodate.
Where the CPT is better, and why the answer is still the SPT. The cone is unquestionably the superior instrument in several respects: it is continuous rather than at 1.5 m intervals, so thin loose layers are not missed; it is repeatable and almost operator-independent, whereas SPT blow counts vary with hammer energy, rod length, borehole diameter, liner use and drilling technique, and require energy correction to the 60 per cent standard; and the CPT gives direct input to Schmertmann's settlement method through Es ≈ 2.5 qc. In loose, fine, uniform sands and hydraulic fills, and for any careful liquefaction assessment, the CPT is the better tool and modern Canadian practice increasingly pairs the two — CPT for the continuous profile, a few boreholes with SPT for sampling and calibration. But asked which is commonly used for sandy soils, the SPT is the honest answer: it is cheaper, more widely available, tolerant of coarse material, and connected to the correlation set that sand design still depends on.
