Triaxial Soil Testing


Some soils such as gravel or clay are theoretically solid, able to withstand more force without moving than looser soil compounds. This isn't always the case. With pressure coming from multiple directions, sloped soils are often less tolerant of building constructions. Triaxial soil testing is to determine the solidity of the soil and the limit of stress it can support.


Triaxial soil testing is based on the principle that soil structures will erupt at different rates when horizontal stress is applied versus vertical stress. The test is designed to measure the soil's upper limits the materials can withstand before falling apart. Stress is constantly measured and increased upon a soil-filled cylinder until its metal plates are unable to support the force and cause slope failure.


Technicians take soil samples and fill a cylinder from top to bottom, then place it in water. The water is able to apply constant pressure without mixing into the sample, providing data as the volume within the cylinder changes. A metal piston presses the sample into a stationary plate at the end of the cylinder, increasing the density of the material by packing it together.


The cylinder increases in pressure as the sample compresses from the force of the piston.The soil will push out any trapped air or water into the water bath surrounding the cylinder. This water movement is detected as the bath increases in overall volume. This helps give accurate measurements of the soil's density as pressure increases.

Cylinder Failure

The technician will continue to apply pressure through the piston into the sample until the cylinder fails and soil erupts through the metal. The results of the failure through axial pressure take into account the shifting of the cylinder in one direction, while the outside region shifts in the opposite. These results are considered the failure plane of the soil sample and are equivalent to the maximum force where the soil will buckle on a slope.


The soil sample will bulge at the sides as the cylinder begins to fail. This data will reveal the fundamental parameters of the material's cohesion and support. Test results can predict the stability of the soil at a specific shearing slope. This is useful mainly to engineers who need accurate predictions of slope failure concerning their future constructions.

Keywords: triaxial soil testing, geotechnical soil basics, triaxial soil strength

About this Author

Jonathan Budzinski started his writing career in 2007. His work appears on websites such as eHow and WordGigs. Budzinski specializes in nonprofit topics, as he spent two years with Basic Rights Oregon and WomanSpace. He has received recognition as a Shining Star Talent Scholar in English while studying English at the University of Oregon.