16-Civ-A4 Geotechnical Materials and Analysis · May 2018

Question 2 of 6: Short-Answer Soil Behaviour

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

Paper format: National Examinations — 16-Civ-A4 Geotechnical Materials and Analysis, May 2018. Closed book, 3 hours, 100 marks. Answer all six questions. All required charts (rectangular-area influence chart, Newmark chart) and a formula sheet are provided at the back of the exam.

Reference texts: R.F. Craig & J. Knappett, Craig’s Soil Mechanics (8th ed.); B.M. Das, Principles of Geotechnical Engineering; R.D. Holtz, W.D. Kovacs & T.C. Sheahan, An Introduction to Geotechnical Engineering (2nd ed.); M. Budhu, Soil Mechanics and Foundations. Take $\gamma_w = 9.81\ \text{kN/m}^3$ throughout.

Question 2: Short-Answer Soil Behaviour (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 five parts each require the choice and its reason:

1 — Greater peak strength in a CD test on sand: (ii) dense sand B. A dense sand must dilate to shear, and the work done against the confining stress during dilation shows up as extra shear resistance — a distinct peak friction angle above the constant-volume (critical-state) value. A loose sand simply contracts to critical state with no peak.
2 — Greater peak strength in a CU test on clay: (ii) over-consolidated clay D (OCR = 10). The heavily over-consolidated clay carries a locked-in stress history: on shearing it tends to dilate, generating negative excess pore pressure that raises the effective stress and hence the peak undrained strength. The NC clay C generates positive excess pore pressure, lowering effective stress and strength.
3 — Higher shear strength for identical NC clay, E vs F, one CU one CD: the CD (drained) sample. In the drained test no excess pore pressure develops, so the whole normal-stress increment becomes effective stress and the sample fails at a higher effective stress and higher shear strength. In the undrained test the NC clay generates positive excess pore pressure, reducing effective stress at failure. Hence the drained specimen is stronger.
4 — Volume of a saturated NC clay during drained (CD) shear: (iii) reduces. A normally consolidated clay is contractive; when drainage is allowed, it densifies (expels water) as it is sheared, so its volume decreases.
5 — Pore-water pressure along the phreatic surface (top flow line BC) of the earth dam: (ii) atmospheric, i.e. zero. By definition, the phreatic surface (top flow line) is the locus of points where the pore-water pressure equals atmospheric pressure ($u=0$); above it the soil is unsaturated and below it $u\gt 0$. On this line total head equals elevation head.

Question 2 — choices
Part	1	2	3	4	5
Answer	(ii) dense B	(ii) OC clay D	CD (drained)	(iii) reduces	(ii) zero

Topic: Drained vs. undrained shearing; dilatancy; the phreatic surface in seepage.

Key relations: $\tau_f=c'+(\sigma-u)\tan\phi'$; Skempton $\Delta u=B[\Delta\sigma_3+A(\Delta\sigma_1-\Delta\sigma_3)]$; on the phreatic line $u=0$ so total head $=$ elevation head.

Why this works: Every part turns on whether the soil wants to contract (positive $\Delta u$, lower effective stress) or dilate (negative $\Delta u$, higher effective stress) and on whether drainage is permitted. Dense sand and OC clay dilate and are strong at peak; loose sand and NC clay contract. Allowing drainage in an NC clay lets it densify, so CD strength exceeds CU strength.

Common pitfall: Confusing total-stress and effective-stress strength — the choices must be argued in effective stress, because it is the pore-pressure response that separates the cases.

Source: Das, Principles of Geotechnical Engineering, ch. 12; Holtz, Kovacs & Sheahan, chs. 11–12.

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