Direct shear test — limitations and drawbacks: Which of the following is a principal disadvantage of the direct shear (shear box) test for soils?

Introduction / Context:
The direct shear test is widely used for rapid estimation of drained shear strength parameters, especially for sands. However, it has several inherent limitations compared with triaxial testing, which can influence the interpretation of measured shear strength and deformational characteristics.

Given Data / Assumptions:

• A square or circular specimen is split horizontally and sheared along a predefined plane.
• Normal stress is applied by a vertical load system.
• Drainage is usually allowed (drained test).

Concept / Approach:
Because the shear plane is forced and the upper half moves relative to the lower half, the actual area engaged in shear reduces as displacement increases (the halves separate laterally). This changing area complicates the calculation of shear stress (τ = P/A), requiring area corrections to avoid overestimating strength. Rapid drainage and pore-pressure dissipation are typically advantages, not disadvantages, in drained testing.

Step-by-Step Solution:

Verification / Alternative check:
Standards often include area-correction procedures; results differ from triaxial tests where the failure plane develops naturally and stress state is better controlled.

Why Other Options Are Wrong:

• Options A and B describe features that are generally beneficial for drained testing.
• Option E is incorrect; the test does apply and control normal stress.

Common Pitfalls:
Neglecting area correction; assuming drained results apply to undrained behavior; ignoring nonuniform stress distribution near the box boundaries.

Final Answer:
The plan area actually under shear changes progressively during the test