The triaxial soil test measures the mechanical properties of a soil.
The triaxial soil test may also be called the triaxial shear test. It measures the mechanical properties of deformable soils such as soil mixtures, sand, clay and gravel mixtures. A solid is theoretically defined as a material that can support shear stress without moving. But there is always an upper limit as to how much shear stress it can support.
Shear Strength
This is the maximum stress a material can withstand before it ruptures and deforms. The theory behind the concept of a triaxial soil test suggests that the stress applied vertically can be different from the stress applied in the horizontal direction. The test measures the upper shear stress limits of a soil sample. It predicts slope failure due to shear stress.
Test Setup
The technician packs loose granular materials in a cylindrical latex sleeve with a flat, circular metal plate across the bottom and top ends, and places the cylinder in a water bath. The water doesn't mix into the sample, but provides consistent pressure along the sides of the cylinder. The water bath contains a known volume of water and this will provide data about any volume changes in the test cylinder. As the piston pushes downward or upward, it pushes the test sample against the stationary plate on the opposing end of the cylinder. The plate remains stationary during the test as the piston pushes the sample against it.
Test Evaluation
The distance the piston moves is a function of the force required to move it. With the pressure increasing as cylinder compresses, the volume of the test sample decreases as the void spaces in the soil sample collapse. Any water or air present in the sample squeezes out as the soil sample consolidates under the pressure from the piston. Measurements of the water movement in the surrounding water bath detects the net volume change in the test cylinder.
Test Cylinder Failure
The technician continues to apply increasing amounts of pressure through the piston's force on the axial sample until the test cylinder ruptures or fails. Failure results when the axial pressure is so great that lateral sliding regions form near the center of the cylinder. It first bulges and then breaks or fails. The top of the sample cylinder shifts laterally in one direction, while the bottom region shifts in the opposite direction. This is the failure plane.
Test Results
As the failure becomes imminent, the soil sample cylinder bulges out at the sides and gets shorter. The test data reveal the fundamental material character or parameters, which indicate the apparent cohesion of the particles in the soil sample and the angle of shearing resistance. Test results predict the soil stability of a slope. Triaxial soil tests are useful in making engineering predictions about slope failure.
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