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16-Civ-A4 Geotechnical Materials and Analysis · Undated paper

Question 2 of 6: Short-Answer Concept Questions

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

Notes on this paper

National Examinations — May 2019  |  16-Civ-A4 Geotechnical Materials and Analysis  |  3 hours, open book  |  100 marks  |  Answer ALL questions (Q1–Q6).

Reference texts: Das & Sobhan, Principles of Geotechnical Engineering, 9th ed. (Cengage); Craig, Craig's Soil Mechanics, 8th ed.; Holtz, Kovacs & Sheahan, An Introduction to Geotechnical Engineering, 2nd ed. Canadian practice frame (CFEM 4th ed., EGBC).

Source-quality note. Flow-net field counts carry the usual ±½-field hand-sketch tolerance (Exam Note 3).

Question 2: Short-Answer Concept Questions (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 items:

  1. Which sample — A (overconsolidated) or B (normally consolidated) — shows the greater peak strength in a consolidated-drained (CD) test?
  2. Which sample — C (normally consolidated clay) or D (clay with an OCR of 1) — shows the greater peak strength in a CU test on saturated clay? (i) C  (ii) D  (iii) the same.
  3. Samples E and F are identical normally-consolidated-clay specimens tested under CU and CD conditions. Which has the higher shear strength? Give your reason.
  4. In a CD test on saturated dense sand the sample volume during shear: (i) increases  (ii) unchanged  (iii) reduces.
  5. The pore-water pressure at C (top of the horizontal drain) of a homogeneous earth dam (Figure 1) is: (i) negative  (ii) atmospheric / zero  (iii) positive.
Reservoirphreatic surface (u = 0)C (top of drain)horizontal drain
Figure 1 — homogeneous earth dam. The phreatic (free) surface is the uppermost flow line, on which the pore pressure equals atmospheric. Point C sits where it discharges into the toe drain.

Answers with reasons.

1 → Sample A (overconsolidated). A dense / heavily overconsolidated soil is interlocked; it must dilate to shear, mobilising a higher peak strength before softening toward the critical state. The looser, normally consolidated sample contracts and reaches a lower peak.

2 → (iii) They are the same. An overconsolidation ratio of 1 is the normally consolidated state. Sample D at OCR = 1 is in exactly the same stress history as the normally consolidated Sample C, so at equal confining stress the two give identical peak strengths.

3 → The CD specimen is stronger. For normally consolidated clay, undrained (CU) shear generates positive excess pore pressure, which lowers the effective stress and therefore the mobilised strength ($\tau_f=\sigma'\tan\phi'$). In the drained (CD) test no excess pressure builds up, the effective stress at failure is higher, and the measured shear strength is greater.

4 → (i) Increases. Dense sand is dilatant — grains ride up over one another during shear — so at constant effective stress the drained specimen expands until it reaches the critical-state (constant-volume) condition.

5 → (ii) Atmospheric (zero). C lies on the phreatic surface, which is the uppermost flow line and a surface of zero (atmospheric) gauge pore pressure. Where that free surface exits into the horizontal drain, the pore-water pressure is therefore zero.