Question 1 of 6: Multiple-Choice Concept Bank

Question 1: Multiple-Choice Concept Bank (10 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.

The ten items and their correct choices:

1. The maximum size of a clay particle is: (A) 2×10⁻⁶ m  (B) 75×10⁻⁶ m  (C) 100×10⁻⁶ m  (D) 200×10⁻⁶ m
2. The optimum moisture content of a soil is close to the soil's: (A) Shrinkage limit  (B) Plastic limit  (C) Liquid limit  (D) Plasticity index
3. One soil property that decreases due to compaction: (A) Strength  (B) Permeability  (C) Bearing capacity
4. The permeability of fine-grained soils is determined by the: (A) Falling-head test  (B) Constant-head test  (C) Piezometer test
5. Settlement in saturated clays is mainly attributed to: (A) Deformation of grains  (B) Compression of pore water  (C) Expulsion of water from the voids  (D) Expulsion of soil grains
6. The stress–strain behaviour of an overconsolidated clay is similar to that of a dense sand: (A) True  (B) False
7. CU triaxial results with pore-pressure measurement can only be used to estimate effective shear-strength parameters: (A) True  (B) False
8. Which statement(s) is/are INCORRECT: (A) OC clay has effective cohesion equal to zero  (B) Pore pressure can be negative during shearing of certain saturated clays  (C) Volume reduction is expected during an undrained triaxial shear test  (D) CD results can analyse long-term embankment stability
9. Which is/are NOT an assumption of Terzaghi 1-D consolidation theory: (A) Homogeneous & 100% saturated  (B) m_v and k vary with effective stress  (C) Drainage at both top and bottom  (D) Grains and water incompressible  (E) Unique volume-change / effective-stress relationship
10. Which test has a different drainage condition from the others: (A) CD triaxial on saturated clay  (B) Vane shear on saturated clay  (C) Unconfined compression on saturated clay

Answers with reasons.

1 → (A) 2×10⁻⁶ m. The clay fraction is defined (Unified / CFEM) as particles finer than 0.002 mm = 2 µm = 2×10⁻⁶ m; the coarser sizes listed are silt.

2 → (B) Plastic limit. For most fine soils the standard-Proctor optimum water content plots close to the plastic limit, where the soil is workable but not yet a paste.

3 → (B) Permeability. Compaction expels air and reduces the void ratio, so hydraulic conductivity drops (often by orders of magnitude); strength and bearing capacity increase.

4 → (A) Falling-head test. Fine-grained, low-permeability soils pass too little flow to time accurately in a constant-head cell; the falling-head permeameter measures the small discharge through the drop of a standpipe. (The piezometer is a field device, not a lab permeability test.)

5 → (C) Expulsion of water from the voids. Saturated-clay settlement is consolidation: the excess pore pressure dissipates and pore water is squeezed out over time. Grains and water are treated as incompressible.

6 → (A) True. Overconsolidated clay, like dense sand, dilates on shearing and shows a distinct peak followed by strain-softening toward a critical state.

7 → (B) False. A CU test with pore-pressure measurement gives both the total-stress parameters (from the total-stress circles) and the effective-stress parameters (using the measured u). The word “only” makes the statement false.

8 → (A) and (C) are INCORRECT. (A) is wrong: overconsolidated clay develops an apparent effective cohesion $c' > 0$; it is the normally consolidated clay whose envelope passes through the origin ($c' \approx 0$). (C) is wrong: an undrained test is a constant-volume test — no drainage, hence no volume reduction. Statements (B) (dilatant clays generate negative pore pressure) and (D) (CD parameters govern long-term, fully-drained stability) are correct.

9 → (B) is NOT an assumption (and (C) is a boundary condition, not a theory assumption). Terzaghi's theory assumes the coefficient of volume change $m_v$ and permeability $k$ remain constant over the small stress increment (so that $c_v$ is constant); (B) states the opposite. Whether the layer drains from one or both faces is a boundary condition the theory accommodates, not a founding assumption — so (C) also does not belong. Items (A), (D), (E) are genuine assumptions.

10 → (A) CD triaxial. The CD test is drained; both the vane-shear and the unconfined-compression tests on saturated clay are performed rapidly with no drainage (undrained). The CD test is therefore the odd one out.