16-Civ-A4 Geotechnical Materials and Analysis · December 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Examination 98-Civ-A4 Geotechnical Materials and Analysis — December 2016. Closed book; 3 hours; total 100 marks; answer ALL five questions. Newmark / m–n influence charts and a formula sheet are supplied with the paper.
Reference texts: B.M. Das & K. Sobhan, Principles of Geotechnical Engineering, 9th ed. (Cengage); R.F. Craig, Craig's Soil Mechanics, 8th ed. (Spon Press); Holtz, Kovacs & Sheahan, An Introduction to Geotechnical Engineering, 2nd ed. (Pearson).
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.
Each part is answered with the correct choice and the reasoning required for marks.
(i) The zero-air-voids (ZAV) line. Statement (a) is correct. The ZAV line is the locus of dry unit weights at 100 % saturation, $\gamma_{d,\text{zav}} = \dfrac{G_s\,\gamma_w}{1 + wG_s}$, so it depends only on the specific gravity $G_s$ and water content — it can be plotted analytically without any compaction test. Because field compaction never expels all air, every point on a real compaction curve lies to the left of (below) the ZAV line, which therefore forms an upper bound the data approach but never cross. Statement (b) is wrong: no test is needed and the curve cannot lie above the ZAV line.
(ii) Structure and permeability about the optimum. Compacting wet of optimum produces a dispersed (oriented) fabric — the extra pore water lets the plate-shaped clay particles slide into a parallel arrangement. Compacting dry of optimum produces a flocculated fabric with an open, edge-to-face particle arrangement and larger interconnected voids; consequently the coefficient of permeability is higher when the clay is compacted dry of optimum than when the same clay is compacted wet of optimum at the same dry density. This is why clay liners and cores are placed slightly wet of optimum — to obtain the low, dispersed-fabric permeability.
(iii) GW versus CH permeability. A well-graded gravel (GW) has by far the higher coefficient of permeability. Permeability is governed by the size of the flow channels, which scale with the effective grain size $D_{10}$ (Hazen: $k \approx C\,D_{10}^{2}$). A gravel has $D_{10}$ of the order of millimetres, whereas a high-plasticity clay (CH) has particles finer than 2 µm with tortuous, sub-micron voids and strongly adsorbed water. Typical values differ by roughly eight to ten orders of magnitude ($k_{GW}\sim10^{-2}\,$m/s versus $k_{CH}\sim10^{-10}\,$m/s).
(iv) Sand A ($C_u = 4$) versus Sand B ($C_u = 1$). These two properties point to different soils. Sand B ($C_u = 1$) is uniform (essentially single-sized) and has the greater coefficient of permeability, because its pore network is open and unobstructed by finer particles. Sand A ($C_u = 4$) is the better-graded soil, so finer grains fill the voids between coarser ones, giving denser packing, more interparticle contacts and greater interlocking — hence the greater effective angle of internal friction $\phi'$. Better grading lowers permeability but raises shear strength.
(v) Undrained friction angle from triaxial tests. The statement that all soils share the same $\phi_u$ is false. For saturated fine-grained soils tested undrained (UU), the total-stress envelope is horizontal, so $\phi_u \approx 0$ — this applies to the expansive clay, the clayey glacial till and, essentially, the silt. Sand (Soil D), however, is free-draining: it cannot sustain excess pore pressure during a normal triaxial shear, so it responds in effectively drained terms and mobilises its real friction angle ($\phi' \approx 30\text{--}40^\circ$). Soil D (sand) has the highest “undrained” friction angle; the soils do not all have the same $\phi_u$.
