16-Civ-A4 Geotechnical Materials and Analysis · December 2013
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper: National Examinations — December 2013 · 98-Civ-A4 Geotechnical Materials and Analysis · 3 hours, closed book · 100 marks · answer all six questions. Charts (m–n influence, Newmark) and a formula sheet are supplied at the back of the paper.
Reference texts. R. F. Craig / Knappett & Craig, Craig’s Soil Mechanics (8th ed.); B. M. Das, Principles of Geotechnical Engineering; Holtz, Kovacs & Sheahan, An Introduction to Geotechnical Engineering; M. Budhu, Soil Mechanics and Foundations. Canadian practice: Canadian Geotechnical Society, Canadian Foundation Engineering Manual (CFEM, 4th ed.). Unit weight of water taken as $\gamma_w = 9.81\ \text{kN/m}^3$ throughout.
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.
On a standard Proctor curve the dry unit weight rises with water content to a single peak — the maximum dry unit weight $\gamma_{d,\max}$ at the optimum moisture content (OMC) — then falls. The rising limb (water contents below OMC) is the dry side of optimum; the falling limb (water contents above OMC) is the wet side.
Wet/dry side in ≤30 words: Below OMC, added water lubricates grains so they pack denser; above OMC, water fills voids and pushes grains apart, so extra water lowers dry density despite the same compaction effort.
Unconfined shear strength. Samples compacted dry of optimum develop a flocculated fabric with high matric suction, so they are stiffer and stronger in an unconfined compression test — higher peak $q_u$, but brittle and more prone to swelling if later wetted. Samples compacted wet of optimum take a dispersed, oriented fabric; they are weaker and softer in unconfined compression (lower $q_u$), but more ductile, less permeable and less expansive. Thus, at equal dry density the dry-side sample gives the greater unconfined shear strength, which is why strength-controlled fills are placed slightly dry of optimum while low-permeability cores (e.g. dam clay cores) are placed slightly wet of optimum.
