1. Failure of the sample occurs along a predetermined failure plane. The test sample is not allowed to fail along the weakest plane.
2. The test sample fails along the horizontal plane progressively. It starts from the edge and moves towards the center. So, the entire shear stress of the plane is not distributed simultaneously.
3. The distribution of shear stress over the plane of failure is not uniform.
4. In a direct shear test, it is implausible to control drainage from the sample. Measuring pore water pressure and volume change are impossible. Total stress can be measured, except when the shear rate is kept slow enough to ensure no rise in porewater pressure, for example in cohesionless soil.
5. Normal stress cannot vary easily. The area under both normal and shear load does not remain constant.
6. During shearing, the sidewalls of the shear box cause lateral restraint on the edges of the sample.
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