16-Civ-A4 Geotechnical Materials and Analysis · December 2017
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
Paper format: National Examinations — 16-Civ-A4 Geotechnical Materials and Analysis, December 2017. Closed book, 3 hours, 100 marks. Answer all six questions. 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.
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 ten statements to be judged (from the exam’s True/False table):
1. The angle of internal friction of normally consolidated clay determined from consolidated drained (CD) triaxial shear tests is higher than the angle of internal friction determined from consolidated undrained (CU) tests without pore-water pressure measurements.
2. At the liquid limit the shear strength of a silty clay (LL = 42%) is the same as the shear strength of a clayey silt (LL = 25.5%).
3. The pore-water pressure can be either negative or positive in an over-consolidated clay depending on the applied normal stress.
4. The angle of internal friction of an expansive clay tested under residual conditions is lower than the peak friction value.
5. The undrained cohesion cu from an unconfined compression test on a saturated clayey sample is approximately the same as the cu from an unconsolidated-undrained (UU) test.
6. The vane shear test can be used to determine the shear strength of soft saturated clays under drained loading conditions.
7. The direct shear test apparatus can be used to determine the effective shear strength parameters c′ and φ′ of a saturated clay.
8. The strength parameters must be determined under consolidated-drained loading conditions to estimate the long-term stability of a homogeneous earth dam constructed of homogeneous clay.
9. Sand typically liquefies when the hydraulic gradient reaches the critical condition.
10. A well-graded sand has a lower angle of friction than a gap-graded soil.
Each statement is judged against soil-mechanics first principles; the reasoning matters more than the verdict, so a one- or two-line justification is given for each.
| # | Answer | Governing principle |
|---|---|---|
| 1 | True | CD gives φ′; CU-total gives φcu. For NC clay φ′ > φcu. |
| 2 | True | All soils have ~the same undrained strength (≈1.7–2 kPa) at their liquid limit. |
| 3 | True | OC clay dilates (−u) at low stress but generates +u at high stress. |
| 4 | True | Residual strength < peak after large shear displacement (particle re-alignment). |
| 5 | True | Both are φ = 0 undrained tests; for saturated clay cu is independent of confinement. |
| 6 | False | The vane shear test is a rapid undrained test — it gives cu, not drained strength. |
| 7 | True | Sheared slowly (fully drained), direct shear gives effective c′, φ′ (u ≈ 0). |
| 8 | True | Long-term = drained ⇒ use effective (CD) parameters c′, φ′. |
| 9 | True | At i = icr effective stress → 0 (boiling / quick condition). |
| 10 | False | Well-graded sand packs denser and interlocks better ⇒ higher φ. |
Selected justifications. (1) In a CU test on NC clay, positive excess pore pressure develops during shear; when it is not measured the total-stress angle φcu is reported, and this is always smaller than the effective-stress angle φ′ obtained from a fully drained CD test (see Question 5, where φcu = 13.3° against φ′ = 30°). (2) The liquid limit is defined at an essentially constant undrained strength of roughly 1.7 kPa, so two remoulded soils each at their own liquid limit have the same strength even though their limits differ. (6) The field vane is rotated in seconds; in a low-permeability clay no drainage occurs, so it measures the undrained strength — the statement’s claim of “drained” loading is what makes it false. (10) A well-graded (broadly-distributed) sand fills its voids, achieves a denser packing and greater grain interlock, and therefore develops a higher friction angle than a uniform or gap-graded sand.
