Direct Shear Test
In a Direct Shear Test, a soil specimen of rectangular or circular cross-section is prepared and placed in the shear box of the testing machine. A normal load is applied to the specimen through the loading platform, and then the shear load is applied horizontally to the specimen at a controlled rate. The displacement of the specimen and the load applied are recorded continuously, and the test is continued until the sample fails.
The Direct Shear Test can be conducted under drained or undrained conditions. In undrained conditions, the drainage of water is prevented, and the pore pressures are allowed to build up during the test. In drained conditions, the specimen is allowed to drain during the test.
The test results in a shear stress vs. shear displacement or shear strain curve, and the peak shear stress value obtained from the curve is used to determine the shear strength of the soil specimen. The test can be performed at various normal loads and under different drainage conditions to determine the shear strength parameters of the soil, such as cohesion and friction angle.
Overall, the Direct Shear Test is a simple, quick, and inexpensive method for determining the shear strength of soils and is commonly used in geotechnical engineering practice and research.
Test equipment
The specimen is placed between the two metallic plates, which are usually lined with a layer of porous stones to allow water to drain out during the test. The plates are then clamped together and a normal load is applied to the top plate using a loading cap.
The horizontal shear force is applied to the specimen by displacing one of the plates horizontally relative to the other. This causes the soil to deform and eventually fail along a shear plane, which is typically at an angle to the horizontal.
The shear stress is calculated by dividing the applied shear force by the area of the specimen. The shear strain is calculated as the displacement of the moving plate divided by the initial height of the specimen.
The test is typically repeated at different normal stresses to generate a shear strength envelope that can be used to characterize the shear strength of the soil material.
Test Procedure
The following is a step-by-step procedure for conducting a direct shear test:
Weigh the soil sample and record its mass.
Measure the height and diameter of the shear box.
Place the shear box in the shearing device.
Stabilize the bottom part of the shear box by fixing the two screws.
Assemble the shear box by putting the porous stones and the gripper disk together.
Place the soil sample inside the shear box and put a filter paper, a porous stone, and the loading cap on top of it.
Weigh the remaining soil to derive the mass of the sample.
Null the initial dial gauges (horizontal and vertical).
Apply a direct normal pressure at the top of the sample using a leverage weight or pneumatic system to initiate the consolidation stage. Incrementally apply the normal load until the desired stress level is reached. At this stage, the metallic plates should be screwed together.
Before the shearing stage begins, slightly raise one metallic plate to ensure that the normal and shear loads are transmitted only through the specimen.
Apply a shearing stress along the pre-determined horizontal plane until the specimen fails. The shearing device should apply a constant displacement rate with 5% accuracy tolerance. The rate should be relatively small to prevent pore pressure buildup and allow for sufficient drainage. It is controlled by a servomotor device and a gear box assembly.
Record the vertical and horizontal deformations using dial gauges.
The shearing device is not equipped with sensors to measure pore pressure during the test. Therefore, it is crucial that the sample is saturated and the consolidation process is completed before the shearing begins. Otherwise, if pore water pressure increases during the test, the shear strength of the sample will be underestimated.
Common errors that occur during direct shear testing include disturbing the sample before the test, not allowing it to fully consolidate, or selecting a shearing rate that is too high. These errors can affect the sample's structure, reduce its strength, and lead to inaccurate test results.
Take reading measures frequently depending on the shearing rate until the shear stress peaks and falls or until the horizontal deformation reaches 10-15% of the initial diameter.
Calculation
τ = c + σn tan(φ)
where τ is the shear stress, c is the cohesion, σn is the normal effective stress, and φ is the friction angle.