Why the Triaxial Shear Test is Preferred Over Direct Shear Which statement best explains why the triaxial test is generally preferred to the direct shear test for determining soil shear strength?

Introduction / Context:
Laboratory shear tests aim to measure strength parameters under controlled stress and drainage conditions. The triaxial test apparatus allows flexible control and measurement compared with the simpler direct shear box, providing more reliable parameters for design of foundations, slopes, and retaining structures.

Given Data / Assumptions:

• Specimen is cylindrical and jacketed (triaxial cell).
• Drainage and pore pressure can be controlled/monitored.
• Failure plane is not pre-imposed, allowing natural failure to develop.

Concept / Approach:

Triaxial testing supports undrained–unconsolidated (UU), consolidated–undrained (CU with pore pressure measurement), and consolidated–drained (CD) conditions. It permits measurement of volume change and pore pressure (in CU) and yields well-defined stress paths and principal stress states. In contrast, the direct shear test enforces a horizontal failure surface and suffers from stress non-uniformity along the plane, offering limited drainage control.

Step-by-Step Solution:

Verification / Alternative check:

Design manuals recommend triaxial parameters (CU with pore pressure; CD for sands) when accurate undrained/drained strengths are needed.

Why Other Options Are Wrong:

(b) is false—triaxial enables such measurements; (c) describes a drawback of the direct shear test, not a benefit.

Common Pitfalls:

Using direct shear results for conditions requiring pore pressure control; misinterpreting UU results for long-term behavior.

Final Answer:

It can be performed under all three drainage conditions (UU, CU, CD) with good control of stresses and drainage