Question 1 of 6: True/False with justification

Question 1: True/False with justification (5 × 4 = 20 marks)

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) TRUE. The Proctor compaction response is controlled by grain-size and mineralogy. Well-graded granular (sandy) soils reach a comparatively high maximum dry unit weight because their rounded, frictional grains pack efficiently and need only a thin lubricating water film to reduce inter-particle friction; that film is achieved at a low optimum moisture content (typically 8–12 %). Clays, by contrast, carry adsorbed double-layer water on plate-shaped particles, so a larger water content (often 18–28 %) is required before the particles can slide into their densest arrangement, and even then the maximum dry unit weight is lower because the platey fabric is inherently more porous. Hence sandy soils give a higher γ_d,max at a lower OMC — both clauses of the statement are correct.

(ii) TRUE. The plasticity index I_p = LL − PL scales with the clay-mineral activity and specific surface area. Bentonite is dominated by sodium montmorillonite, an expansive 2:1 clay with an enormous specific surface (≈ 800 m²/g) and a liquid limit that can exceed 300–500 %, giving plasticity indices of the order of 100–500. A sandy clay is a low-activity mixture in which a large sand fraction dilutes the plastic fines, so its I_p is typically only 10–25. Bentonite therefore has a far greater plasticity index — the statement is true, and this contrast is exactly why bentonite is chosen for slurry walls and drilling muds.

(iii) TRUE. For the same load increment and the same layer thickness, the compressibility of the clay decides the settlement. A normally-consolidated (NC) clay sits on its virgin compression line, so the full increment is resisted by the large compression index C_c. An over-consolidated (OC) clay of the same thickness first responds along the much flatter recompression line (index C_r ≈ C_c/5 to C_c/10) until the pre-consolidation pressure is exceeded, so for the same Δσ' it compresses far less. The dam founded on the NC clay therefore settles more — the statement is true.

(iv) FALSE. When a consolidated-undrained (CU) test is interpreted without pore-pressure measurement, the failure envelope is plotted in terms of total stress and yields the total-stress friction angle φ_cu. Because a normally- or lightly-over-consolidated clay generates positive excess pore pressure on shearing, the total-stress Mohr circles are pushed to the left relative to the effective-stress circles, so the total-stress envelope is flatter: φ_cu is only about one-half to two-thirds of the drained (effective-stress) angle φ'. The statement claims φ_cu is always greater than φ', which is the reverse of reality — it is false. (Only a heavily over-consolidated clay, which dilates and develops negative pore pressure, can push φ_cu above φ', and even then not “always”.)

(v) TRUE. Coefficient of permeability falls steeply as the fines content and plasticity rise, because plastic clay minerals present tortuous, sub-micron pore throats and hold immobile adsorbed water. A soil with I_p = 0 is non-plastic — a clean sand or silt whose interconnected pores give a permeability many orders of magnitude larger (k of 10⁻² to 10⁻⁵ m/s) than a plastic clay (k of 10⁻⁹ to 10⁻¹¹ m/s). Of the three soils, Soil A (I_p = 0) is the most permeable — the statement is true.