NivaarExam PrepOfficial exam papers ↗

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

Question 2 of 6: Multiple Choice with Reasoning

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

Notes on this paper

Paper: National Examinations — December 2018 · 16-Civ-A4 Geotechnical Materials and Analysis · closed book, 3 hours, 100 marks · answer ALL six questions.

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: Multiple Choice with Reasoning (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 exam lists parts numbered 1, 2, 4 and 5 (its own numbering skips 3). Each is answered with the required justification.

  1. Part 1 — Higher $\phi'$: (i) Sample A, the well-graded sand. A well-graded sand contains a range of sizes that pack densely, so grains interlock and dilate against one another during shear. That raises the angle of internal friction above that of a uniform (single-size) sand, whose looser, more compressible fabric mobilises a smaller $\phi'$.
  2. Part 2 — Higher pore-water pressure at failure: (ii) Sample D ($\sigma_3'=300$ kPa). The pre-consolidation pressure is 200 kPa. Sample C ($\sigma_3'=100\lt200$) is over-consolidated, so it tends to dilate and develops low — even negative — shear-induced pore pressure. Sample D ($\sigma_3'=300\gt200$) is normally consolidated, so it contracts and generates a large positive pore pressure at failure.
  3. Part 4 — Volume of a saturated NC clay during a CD shear: (iii) is reduced. A normally consolidated clay is contractive; with drainage permitted (CD), that contractive tendency is realised as a decrease in specimen volume as shear proceeds.
  4. Part 5 — Pore pressure below the phreatic surface in an earth dam: (iii) positive value. The phreatic (top flow) line is the surface of atmospheric ($u=0$) pore pressure. Below it the soil is saturated and under a positive seepage pressure head, so $u\gt0$; above it (capillary zone) the pore pressure would be negative.