The importance of plasticity on rock mechanical behavior has been illustrated in several previous studies. Elastoplastic models have been developed to describe the irrecoverable plastic deformation of the rock under an applied stress. However, many of the elastoplastic models are either too simple and can't describe the actual rock deformation or are too complex with many fitting parameters and are therefore difficult to calibrate. In this study the effect of plastic deformation on mudrocks mechanical behavior has been compared to laboratory multi-stage triaxial experiments on resediment mud-rocks. The calibration parameters are stress dependent, meaning the parameters change depending on the mean stress of the experiment. A more complicated "double yield surface" was also required to describe these ductile rock types.

1. INTRODUCTION

The original cam clay model was developed by (Roscoe & Schofield, 1963) to describe soil elastoplastic properties. The model suggests a yield surface shape, a plastic potential, and a hardening rule to calculate the plastic strains. The original model suggested that the yield surface had a cusp shape at the mean stress intercept on the p:q plot, this leads to issues in determining the plastic potential at this point. The Modified Cam Clay model (MCC) was introduced by (Roscoe & Burland, 1968) that assumes an elliptical yield surface so that there is no shear plastic strain at the mean stress intercept. The Modified Cam Clay is a relatively simple model with a reasonable number of parameters (five). This makes it relatively easy to calibrate using laboratory experiments such as one-dimensional compression to obtain the loading parameters, and triaxial tests to quantify the yield surface. It has been mostly successful in geotechnical applications that involves constructions on clay (Wood, 1990). This model includes all the important stress-strain behaviors such as hardening, softening, and dilation. This model has formed the basis of several studies including thick walled cylinder tests, fault activation, and the sequestration (Dunayevsky, Myers, & Bennett, 2012), (Dunayevsky, Myers, & Bennett, 2012) and (Myers, Dunayevsky, & Bennett, 2013).

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