Abstract

Nowadays a multistage triaxial test using one specimen has been widely implemented in laboratory of rock mechanics as a substitution of the single stage (conventional) triaxial test which requires three or more rock specimens. However, the result of multistage triaxial test sometimes different with the single stage triaxial test. It is suspected that the second stage and the next stage have been affected by the loading path (stress path) of previous stage.

This research was carried out in laboratory to investigate whether different stress paths will give significant influence on the multistage triaxial testing results. Two different stress paths were proposed in transition period from on going stage to the next stage. The first scenario was both axial load and confining pressure simultaneously increased. The second scenario was an axial load is kept constant while a confining pressure increased. Measured stress-strain rate during the multistage triaxial test is used to identify fracture initiation for each stage. The test results were evaluated in term of strength envelope and stress-strain curve. Numerical simulation was performed to validate the test result.

Increasing of axial loading and confining pressure simultaneously in first scenario prevents more cracks propagation. It has been proven by comparing the ultrasonic velocity while concrete samples loaded in multistage triaxial test. The first scenario stress paths result shows higher triaxial compressive strength. Generated failure envelopes were also closer to the conventional triaxial test than the second scenario. Excessive sample damage of the second scenario can be seen both directly and from stress-strain curve that showing longer axial strain. Modified modulus used in numerical modeling by validation of stress-strain curve has been compatible with lab test result whereas the first stress paths scenario made sample behave stiffer. This description analysis will expectively enrich multistage triaxial testing references.

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