Shear Failure and Brittle to Ductile Transition in Shales from P-Wave Velocity

This paper present an experimental study on brittle-ductile transition and shear strength behavior of shales. The experimental study is based on triaxial tests on different shales sheared at different levels of effective confining stresses. Based on the results of the tests, it is shown that the brittle-to-ductile transition can be related to the apparent overconsolidation ratio of the shale. A relationship between normalized undrained shear strength and apparent overconsolidation ratio is established. Correlations of the apparent preconsolidation stress with unconfined compressive strength and compressional wave velocity are proposed. These correlations were used to adequately predict the normalized undrained shear strength of shales whose apparent preconsolidation stresses or apparent OCR values are not known. In the absence of material-specific data, the different correlations can be used to obtain preliminary estimates of the undrained shear strength and ductility/brittleness of shales