Automated Multi-Stage Triaxial Testing of Loose and Dense Sands

The goal of this project was to develop criteria to signify when a soil specimen is just on the verge of failure when tested in a consolidated-drained triaxial test. By identifying the imminent failure of the specimen, a multi-stage triaxial test can be automated, allowing for soil strength properties to be determined from a single specimen. The purpose of identification of imminent failure of the specimen is for purposes of development of automated multi-stage test operation of a single specimen for determination of soil properties. Currently, shear strength parameters for a soil could either be calculated from at least three separate triaxial tests or a multi-stage test where each stage would end based on the operator's judgement. By developing generalized criteria that would signify failure, and therefore the need to move on to the next stage of a multi-state test, a computer program could be used to automatically end one loading stage and begin the next. This automation would allow for a wider use of multi-stage tests, which are faster and therefore less expensive to run than three standard triaxial tests. Triaxial tests were performed on loose and dense sand specimens. During standard testing, the loose sand had a friction angle of 29.61o and the dense sand had a friction angle of 38.63o. Using a zero tangent modulus as the stage-end criteria, the loose sand had a friction angle of 27.69o and the dense sand had a friction angle of 37.03o. Using the maximum volumetric strain as the stage-end criteria, the loose sand had a friction angle of 25.16o. The multi-stage shear strength parameters were reasonable compared to the single-stage test parameters, if slightly conservative. This suggests that computer automation of multi-stage triaxial tests will produce results that can be used in analysis and design by geotechnical engineers. However, more research will be required to confirm this initial assumptions for a wider range of sand gradations as well as for other soil types and testing conditions.