ABSTRACT:

In the literature, the point of maximum secant modulus (MSM) (low stress) on the loading cycle or point of maximum co mpressive strength (MCS) (high stress) are often used as the unloading point in a Multi-Stage Triaxial (MST) test. We investigated the use of an alternate point of unloading known as point of positive dilatancy (PPD) where the tangent strain ratio (the tangent value of the ratio of radial strain over axial strain) reaches 0.5. Acoustic Emission (AE) measurements were used to correlate amount of “accumulated damage” related to irrecoverable strain at MSM, PPD and MCS of a MST test. Measurements were made on a weakly consolidated Miocene sandstone. Less than 2% of total number of AE events were recorded up to the MSM suggesting that minimal plastic strain has occurred, and the yield surface has not been approached. In contrast, for the MCS, 80% of AE events are observed after the PPD. The MST test with PPD as unloading point is therefore interpreted to be a useful compromise between MSM and MCS. It probes the yield surface without causing a large number of AE events (irrecoverable strain) while still allowing a simple correction factor to the MCS.

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