The triaxial test is arguably the most widely used test method in investigating soils’ stress-strain behavior in geotechnical engineering. Its popularity is partially due to the well-understood state of stress during the test, as well as its versatility. In a triaxial test, the specimen is encased in a membrane with drainage from top and bottom (and sides if filter paper is used). The specimen is confined to specific stress through a pressurized fluid (such as water) inside the test chamber. In many cases, the specimen is backpressure saturated before being consolidated to desired stresses. Pumps or other sources of pressure and volume control can be used to generate confining stress and backpressure. Once the specimen is at the target stresses, deviator stress is gradually applied, usually axially, via a load frame, until the specimen fails, or the desired strain or stress is achieved.
The most common triaxial tests are Consolidated Drained (CD), Consolidated Undrained (CU), and Unconsolidated Undrained (UU) conducted in isotropic condition and with deviator stress applied with a constant rate of axial strain. Using the results of several tests at different confining stresses, one can develop shear strength parameters using a failure criterion like Mohr-Coulomb. Shear strength parameters can then be used in the process of designing geostructures.
Soils are unusual materials in that their engineering properties are stress path-dependent. This means their strength and stiffness depend on past, present, and future stress conditions. For instance, an in-place soil element will have different strength and stiffness if the stresses increase in horizontal direction compared to stresses increased in the vertical direction.
In some cases, the underground soil is not isotropically consolidated but is in at-rest (K0) condition. The soil elements change in axial and radial stresses that may be increased or decreased. The geostructure is subjected to axial or radial unloading rather than loading, or there may be a pore-pressure decrease or increase due to the changes in the hydraulic equilibrium and condition of the area such as heavy rain. These complicated stress paths and conditions have traditionally been hard to simulate in the laboratory. For example, to conduct a triaxial test under the K0 condition, one must maintain the radial strain at zero while applying the target axial stress to the specimen by constantly changing the cell pressure. This requires a fast closed-loop controller which renders manual testing impractical.
The introduction of computer-controlled equipment with high-speed closed-loop and adaptive control systems has revolutionized the quality, reliability, and speed at which stress path tests can be conducted. A modern computer-controlled triaxial testing apparatus with advanced adaptive control can:
- Easily back-pressure saturate specimens, conduct B-checks and move on to the next step automatically when saturation is complete
- Consolidate the specimen at any stress state (isotropic or anisotropic), automatically
- Determine the K0 value for the applied axial stress (for K0 tests)
- Ensure completion of the consolidation before automatically moving on to the next step (after a predefined time or number of log cycles are past the 100% primary consolidation)
- Easily impose various stress paths on the specimen
Such advanced systems enable engineers and researchers to go beyond standard tests and study the specimen’s behavior in more realistic stress states, however complicated they might be. They can simulate different scenarios or possible failure mechanisms, with a high level of accuracy.
Another benefit of the triaxial test is that it can be fitted with additional measurement sensors to expand the capabilities of this test method further. For example, by adding piezoelectric sensors at the end caps or sides of the specimen, one can determine the S- and P- wave velocities in different directions from which the low-strain shear and P- wave modulus can be measured at any stage of the test. The resulting data can provide valuable insight into the low-strain characteristics of the specimen at any given stress state and be used to investigate possible levels of disturbance in the specimen. End caps can also be retrofitted to measure the electrical resistivity of the specimen or measure and apply matric suction for unsaturated testing.
With the addition of a cyclic loading unit, the cyclic strength, liquefaction susceptibility, cyclic strength, cyclic mobility characteristics, and dynamic properties of soil specimens under triaxial loading conditions can be investigated. More advanced equipment can even apply the actual Cyclic Stress Ratio (CSR) or strain time history of an earthquake or other vibration sources at any given effective stress on the specimen for a more realistic study of its behavior.
Geocomp pioneered the introduction of fully automated testing equipment in production laboratories with the 1985 installation of three fully automated consolidometers in the USACE materials testing facility in Dallas, TX. Since then, it has continuously improved and expanded its line of fully automated geotechnical testing equipment to include triaxial, consolidation, direct shear, direct simple shear, permeability, resilient modulus, resonant column torsional shear, cyclic triaxial, cyclic direct simple shear, and cyclic torsional shear tests. Nowadays, engineers have many affordable options in their tool chests thanks to the technological advances and expanded capabilities of automated testing machines. This is in a sense the golden age of geotechnical engineering. We can put aside conservative approaches and take on data-driven best practices to manage risks and save money and lives at the same time.
Kaveh Zehtab is the Director of Research, Development, and Production at Geocomp. Kaveh has a wide array of experience in investigating soil behavior, experimental geomechanics, and automated testing.