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What Is a Triaxial Test?

Triaxial tests use soil samples to determine how much pressure the soil can withstand.
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  • Written By: Mary McMahon
  • Edited By: Shereen Skola
  • Last Modified Date: 29 October 2014
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A triaxial test measures soil’s ability to resist shearing forces. This can be important in project planning, where soils need to meet certain standards to support buildings and other structures. Soil testing may be ordered early in site evaluation to determine if a location is appropriate for a project. If the soil doesn’t meet standards, it may be necessary to modify the design with tools like anchoring buildings in bedrock to provide the necessary support. Alternatively, the project could be relocated to a better site.

This test involves placing a sample of the soil into a tube made from rubber or another thin material that creates a membrane. The tube is sealed and dropped into a container filled with fluid. Liquid exerts strain on the material, pressing in from all sides. The operator can apply pressure to the container to squeeze the soil from above. When it starts to give, it has succumbed to the pressure and reached its shear point.

Test equipment includes tubing that measures the amount of pressure exerted on the soil. This allows the person performing the triaxial test to quantify the shear point. Multiple tests can be used to confirm the result before issuing a formal opinion on soil quality. If it gives under low pressure, it may be too loose to support large or unstable structures. Certain kinds of buildings, like gas refineries, might not be permissible on the soil at all due to safety concerns.

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Laboratories performing triaxial tests can choose from several methods. Some use consolidated soil which has been packed down, while others rely on unconsolidated soil in a sample which is as close to conditions on the site as possible. Soil can be drained or undrained for the test, depending on what is being tested for and the desired results. The condition of the sample is always noted in the triaxial test documentation to provide context for readers who may be curious to know how the test was performed.

Site evaluation may include testing by several independent labs. If triaxial test results do not match, technicians can explore the conditions and results to determine the cause of the disparity and develop a plan for a new triaxial test that will hopefully provide a more definitive answer. Labs follow strict protocols for all testing to maintain consistency and reliability, but is possible for them to get different or conflicting results due to unique protocols, variations in equipment, and other factors.

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