18-Geol-A6 Soil Mechanics, December 2018: worked solutions

National Exams — December 2018 — 18-Geol-A6 Soil Mechanics. Three-hour, closed-book exam; a Casio or Sharp approved calculator, a compass and a ruler are permitted. Six questions constitute the complete 100-mark paper: Questions 1–5 (10+15+20+20+20 marks) are compulsory; Question 6 offers eight 5-mark optional items of which the source asks for three.

Reference texts: Das, Principles of Geotechnical Engineering, 9th ed. — USCS classification, phase relations, compaction, permeability/seepage, consolidation; Craig's Soil Mechanics (Craig & Knappett), 8th ed. — effective stress, shear strength, seepage/flow nets, consolidation theory.

Each reading is cross-checked below by an independent physical-consistency test (Q2: the 12%-point saturation comes out to $\approx100\%$, matching its plotted position essentially on the $S=100\%$ curve; Q4: the two digitized curves in Figure Q4-2 are treated as the overconsolidated-clay sample (upper, steep-kneed curve, per the question's own description) and an unused second dataset (lower curve, not needed for a single-sample problem); Q5: the resulting pore pressures give positive effective stress at every point and a linear uplift diagram, the expected signature of a consistent equipotential count). Assumed $G_s=2.70$ (not given) for Q2(c)'s phase relations — typical for a silty/clayey compacted fill and independently supported by the $S\approx100\%$ check above.

18-Geol-A6 Soil Mechanics: December 2018