16-Civ-A4 Geotechnical Materials and Analysis · May 2015
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Reference texts: Das & Sobhan, Principles of Geotechnical Engineering (Cengage); Craig, Soil Mechanics; Holtz, Kovacs & Sheahan, An Introduction to Geotechnical Engineering. Canadian practice: effective-stress, consolidation and Rankine methods as summarised in the Canadian Foundation Engineering Manual (CFEM, 4th ed.).
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.
(i) False (as stated). The claim conflates two separate ideas. Effective stress does have a clear physical meaning: it is the average inter-granular (grain-to-grain contact) stress carried by the soil skeleton, and it is the stress that actually controls shear strength and volume change — Terzaghi’s principle, $\sigma' = \sigma - u$. What is true is only the second clause: effective stress cannot be measured with a single instrument, because we can measure total stress $\sigma$ and pore-water pressure $u$ but not the contact forces directly — $\sigma'$ is obtained by subtraction. So the statement is best marked False: effective stress is physically meaningful even though it is determined indirectly.
(ii) Sand C. All three sketches show rounded grains, so angularity is not the discriminator; gradation is. Sand A is a single, uniform grain size (poorly graded), Sand B has only a few sizes (gap/poorly graded), while Sand C shows a continuous range from coarse to fine (well graded). A well-graded sand packs to a higher relative density because the finer grains fill the voids between the coarser ones, producing more inter-particle contacts and greater interlocking. Higher density and interlocking give the highest angle of internal friction $\phi'$, so Sand C governs.
(iii) GW. Saturated permeability is dominated by the size of the smallest continuous pore channels, which scales with the effective grain size: Hazen’s relation $k \approx C\,D_{10}^{2}$. A well-graded gravel (GW) has by far the largest $D_{10}$ and the largest, best-connected voids; a low-plasticity silt (ML) is much finer; a high-plasticity clay (CH) has the smallest pores and adsorbed water, giving $k$ several orders of magnitude lower. Ranking $k$: GW $\gg$ ML $\gg$ CH, so GW has the highest saturated coefficient of permeability.
(iv) Soil A — the normally consolidated clay. A normally consolidated clay has never carried a stress greater than its present overburden, so any load increment moves it along the steep virgin compression line with the full compression index $C_c$. An over-consolidated clay has previously been loaded to a higher stress; below its pre-consolidation pressure it responds on the much flatter recompression line with $C_r \ll C_c$. For the same load increment the NC clay therefore compresses far more, so it has the higher compression index.
(v) Flocculated structure. When a fine-grained clay is compacted dry of optimum there is too little water to develop the diffuse double layers around the particles; the net edge-to-face electrical attraction dominates and the platelets arrange in a random, edge-to-face flocculated fabric (Figure 2A). This fabric gives higher unconfined strength, higher permeability and more brittle, swell-prone behaviour than the parallel, face-to-face dispersed fabric produced wet of optimum. (This is why clay liners, which need low permeability, are compacted slightly wet of optimum.)
