Section 4: Soil and Water Conservation Engineering
Soil Shear Strength & Mohr-Coulomb Theory
Subtopic: Direct shear vs triaxial test comparison
Question Statement:
Which of the following statements regarding laboratory shear strength testing of soils is/are TRUE?
Options:
(A) In a direct shear test, the failure plane is pre-determined horizontally and is not necessarily the weakest plane
(B) In an unconfined compression test, the cell pressure is zero ($\sigma_3 = 0$), so the unconfined compressive strength $q_u = 2 c_u$ for saturated clay under undrained condition ($\phi_u = 0$)
(C) In a triaxial test, pore water pressure can be measured during undrained shearing
(D) In a standard Proctor compaction test, zero-air-voids line represents $100\%$ saturation
Answer Key:
A, B, C, D
Step-by-Step Solution & Explanation
1. Direct shear forces shear along the split-box interface horizontally (A is true).
2. For $\phi_u = 0$ undrained clay, $\sigma_1 = \sigma_3 + 2c_u = 0 + 2c_u \implies q_u = 2 c_u$ (B is true).
3. Triaxial apparatus permits controlled drainage and pore pressure transducer measurement (C is true).
4. Theoretical compaction maximum occurs at zero air voids ($S_r = 100\%$) (D is true).
All statements A, B, C, D are correct.