1189 A THEORITICAL APPROACH TO THE STUDY OF COMPACTION BANDS IN POROUS ROCKS ARGHYA DAS, GIANG D. NGUYEN, ITAI EINAV School of Civil Engineering The University of sydney Sydney NSW 2006, Australia email: arghya.das@sydney.edu.au, giang.nguyen@sydney.edu.au, itai.einav@sydney.edu.au, Web page: http://sydney.edu.au/ Key words: Compaction band, Breakage mechanics, Localization analysis, Grain crushing, Rate dependency, Boundary value problem. Summary. The formation and propagation of compaction bands in high porosity sandstones is theoretically investigated in this paper using a new constitutive model based on the recently developed continuum breakage mechanics theory [1,2]. This model possesses a micromechanics-based link between the evolving grain size distribution (gsd) and the macroscopic stress strain relationship, through an internal variable called Breakage. This is an advanced feature over many existing plasticity based models in the literature, helping to faithfully track the evolving gsd and its related physics (e.g. permeability reduction). A localization analysis based on the acoustic tensor [3] is performed to determine both the onset and orientation of compaction bands due to grain crushing. It is shown that the model used is able to capture well both the material behaviour and formation of compaction band experimentally observed. An enhancement using rate-dependent regularization is applied to the model to deal with instability issues in the analysis of Boundary Value Problems. Based on the regularised model, the formation and propagation of compaction bands due to grain crushing is analysed through a numerical experiment on a porous rock specimen under triaxial loading condition. Good agreement between numerical predictions and experimental observations demonstrates the capability of the new model. 1 INTRODUCTION The formation of localization bands in high porosity sandstones involves several micromechanical processes such as grain crushing, grain sliding, bond breaking and pore collapse [4,5]. Shearing at low confining pressures facilitates the fracture of grain bonding cement, allowing the grains to rotate and slip, which could be followed by the flow of granulated material. This bond breaking also reduces the mobilized shear strength, observed through the shear stress drop in experiments. In contrast, shearing at high confining pressures leads to grain crushing followed by pore collapse. During this process, the contacting grains tend to crush under the pressure, leading to the rearrangement of fragments, which further reduces the porosity and consequently hardens the material [4,6]. In this sense, pore collapse acts as a passive mechanism facilitated by a grain-crushing event. At a macroscopic level, XI International Conference on Computational Plasticity. Fundamentals and Applications COMPLAS XI E. Oñate, D.R.J. Owen, D. Peric and B. Suárez (Eds)