16-Civ-A4 Geotechnical Materials and Analysis · May 2016
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Examination 98-Civ-A4 Geotechnical Materials and Analysis — May 2016. Closed book; 3 hours; total 100 marks; answer ALL six questions. Influence charts (m–n and Newmark) and a formula sheet are supplied with the paper.
Reference texts: B.M. Das & K. Sobhan, Principles of Geotechnical Engineering, 9th ed. (Cengage); R.F. Craig, Craig's Soil Mechanics, 8th ed. (Spon Press); Holtz, Kovacs & Sheahan, An Introduction to Geotechnical Engineering, 2nd ed. (Pearson).
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.
Peat and organic soils are extremely compressible and, unlike inorganic clays, undergo very large and prolonged secondary (creep) compression. The estimate must therefore capture both primary consolidation and a secondary component that can be as large as, or larger than, the primary.
Tests to undertake. The governing laboratory test is the one-dimensional consolidation (oedometer) test on undisturbed samples, run long enough at each load increment to define the secondary (creep) branch. It is supplemented by classification and in-situ work: natural water content (often 200–800% in peat), organic content by loss on ignition, fibre content and unit weight; and in the field, in-situ vane shear and piezocone (CPTu) soundings for stratigraphy and strength, plus settlement plates / instrumentation for the monitored fill.
Key parameters derived. From the oedometer $e\text{–}\log\sigma'$ and $e\text{–}\log t$ plots: the compression index $C_c$ and recompression index $C_r$, the preconsolidation pressure $\sigma_p'$, the coefficient of consolidation $c_v$, the coefficient of volume compressibility $m_v$, the initial void ratio $e_0$, and — critically for peat — the secondary compression index $C_\alpha$ and the ratio $C_\alpha/C_c$.
How to proceed with the analysis. (1) Establish the initial vertical effective stress profile $\sigma_0'$ and the stress increase $\Delta\sigma$ from the structure (Boussinesq/2:1). (2) Compute the primary consolidation settlement; for a normally consolidated layer
using $C_r$ for the part of the load below $\sigma_p'$ if the soil is over-consolidated. (3) Obtain the time–settlement history from $c_v$ and the Terzaghi time factor $T_v$. (4) Add the secondary compression, which for peat is decisive,
evaluated over the design life $t_2$ from the end of primary $t_1$. (5) Because total settlements are large and slow, recommend mitigation — preloading/surcharge with prefabricated vertical drains, staged construction, lightweight fill, or transferring load to a pile/raft foundation.
| Component | Parameter (test) | Expression |
|---|---|---|
| Primary | $C_c,\ C_r,\ \sigma_p',\ e_0$ (oedometer) | $S_c=\frac{C_c}{1+e_0}H\log\frac{\sigma_0'+\Delta\sigma}{\sigma_0'}$ |
| Rate | $c_v$ (oedometer) | $t=T_v H_{dr}^2/c_v$ |
| Secondary (dominant) | $C_\alpha$ (oedometer, long stage) | $S_s=\frac{C_\alpha}{1+e_p}H\log\frac{t_2}{t_1}$ |
