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Which of the following is a disadvantage of the box shear test?
Stress condition of soil is complex
The test cannot be used for coarse grained soil
No control on the drainage of soil
The box shear test is more complex test
Stress condition of soil is complex
The primary disadvantage of the box shear test (direct shear test) is that the failure plane is predetermined as a horizontal plane. This constraint prevents the soil from failing along its natural weakest plane, leading to a complex and non-uniform stress distribution within the specimen.
The primary disadvantage of the box shear test (direct shear test) is that the failure plane is predetermined as a horizontal plane. This constraint prevents the soil from failing along its natural weakest plane, leading to a complex and non-uniform stress distribution within the specimen.
τ=AF — shear stress calculation where F is horizontal force and A is the area of shear plane.
τ=c+σtan(ϕ) — Mohr-Coulomb failure criterion used to derive shear parameters.
In a direct shear test, a soil specimen is placed in a split shear box and subjected to a normal load, while a shear force is applied to cause horizontal displacement. Because the movement is restricted to a horizontal interface, the stress concentration is non-uniform, and the specimen cannot undergo rotation of principal stresses, unlike in a Triaxial test.
The failure plane is artificially forced to be horizontal.
It is difficult to control pore water drainage precisely during the test.
The test does not allow for measurement of pore water pressure.
Stress conditions at the edges are not uniform compared to the center.
Simple and easy to perform.
Suitable for testing sandy soils.
Relatively inexpensive equipment.
Forced failure plane.
Non-uniform stress distribution.
Cannot measure pore water pressure.
Rotation of principal planes is not possible.
Determination of shear strength parameters for sandy soils.
Quick estimation of soil friction angles.
Option B is incorrect as the test is specifically suitable for coarse-grained soils.
Option C is partially true but less significant than the fundamental geometric constraint of the failure plane (Option A).
The box shear test is generally considered 'simpler' than the Triaxial test, making Option D factually incorrect.
A is correct — The box shear test forces a predetermined failure plane, resulting in non-uniform stress distribution and an inability to track principal stress rotation.
Always remember that for research-grade precision in geotechnical projects, Triaxial tests are preferred over Direct Shear tests because Triaxial tests allow for controlled drainage and pore pressure measurement.