In a direct shear test, the soil is subjected to more stress at the ______.

Asked in SSC JE 15/11/2022 (Shift-III)

Detailed Explanation

In the shear box, one half of the specimen is pushed against the other. The rigid walls of the box press on the soil near the ends of the shearing plane, so shearing does not develop evenly across the plane.

Why “edges” Is Correct

Stress concentrations develop at the edges (the leading and trailing ends of the box) while the centre carries less. Failure therefore starts at the edges and progresses inwards (progressive failure). This non-uniform stress distribution is one of the main drawbacks of the direct shear test.

Direct shear box. The lower half is pushed sideways while the upper half is held by the proving ring, which measures the shear load. Failure is forced along the horizontal plane between the two halves, and stress is concentrated near the edges of the box.

Why the Other Options Are Wrong

A. top: The top surface carries the applied normal load uniformly through the loading plate; the issue is along the shear plane.
B. bottom: The bottom is fixed to the moving box half; it is not where the stress concentrates.
C. centre: The centre carries less stress than the edges.

Key Point

Direct shear test drawbacks: non-uniform stress (high at edges), fixed failure plane, no drainage control, no pore pressure measurement.

Key Concepts for Students

Progressive failure means peak strength is not mobilised at the same time across the plane.
The area of contact also reduces as shearing proceeds; a corrected area can be used.
Triaxial tests give a much more uniform stress state.

Practice more: this is question 14 in Shear Strength of Soils MCQs – Part 1. Try the whole set and check every answer instantly.