Shear Strength MCQ with answer and solution

Concept

Direct shear test mechanism and stress measurement on the failure plane.

Step-by-step solution

1. Understand the setup of the direct shear test: a soil sample is placed in a square or circular metal shear box divided horizontally into two halves. 2. A normal load is applied vertically, and a horizontal shear force is applied to cause failure along the predetermined horizontal plane between the two box halves. 3. Both the applied normal load and the resisting shear load are measured directly using load measuring devices (like proving rings). 4. The stresses are calculated directly on this known failure plane area, unlike triaxial tests where principal stresses are measured and Mohr's circle is used to find stresses on inclined planes.

Explanation

In a direct shear test, the soil sample is forced to fail along a predetermined horizontal shear plane. Because both the normal force and the shear force applied to this specific plane are directly measured by proving rings or load cells, the normal stress and shear stress on the failure plane are measured directly.

Why the other options are wrong

A: In CD triaxial test, principal stresses are measured and failure plane stresses are computed using Mohr's circle. B: In UU triaxial test, total principal stresses are measured, not stresses on the actual failure plane directly. D: In CU triaxial test, principal stresses are measured and pore pressures are recorded, but stresses on the failure plane are calculated.

Shortcut

Direct shear test has a 'direct' failure plane where both normal and shear stresses are directly known.

Common mistake

Confusing direct shear test with triaxial tests, where only principal stresses are measured directly and stresses on failure planes must be computed using Mohr-Coulomb failure criteria or Mohr's circle.