DEM Simulation Of Transversely Isotropic Rocks Under True Triaxial Stress Condition
Saad FAIZI1,2#+, Chung Yee KWOK1, Kang DUAN3
1The University of Hong Kong, Hong Kong SAR, 2Ove Arup & Partners Hong Kong Ltd, Hong Kong SAR, 3Shandong University, China

The response of transversely isotropic rocks under true triaxial testing is studied using the discrete element method (DEM). Numerical rock specimens of four different orientations of beddings (i.e. β = 0o, 30o, 60o, 90o) are created and three series of test simulations (i.e. σ3 =10, 50, 100 MPa) are conducted on each of the rock models, with five different values, ranging from σ2 = σ3 to σstress states. The impact of the bedding orientation and the intermediate stress on the peak strength and degree of anisotropy are subsequently explored and explained through underlying micromechanics. An ascending then descending variation of the peak strength, in form of a ‘fan-shaped’ curve, is noted against σ2. The initial ascend is because of the increase in stress asymmetry, reducing the number of failure planes, as well as the suppression of sliding of weak planes, while the strength eventually descends as σbecomes large enough to induce cracks in the pre-peak region. Similarly, varying degrees of ‘tilting’ of the fan-shaped curves for the different bedding orientations are noted. This is owing to the difference in the degree of suppression of the weak plane slippage, indicating the effect of β on peak compressive strengths. The U-shaped variation of peak strength with anisotropy angle is also observed that flattens under higher σvalues, due to the suppression of the slipping of weak planes, indicating the reduced effects of anisotropy when σis increased. For numerical models of Posidonia Shale under σnormalized by σ*, which is the conventional triaxial compression strength obtained for β = 0o at a specific state, the anisotropy effect, in fact, ceases to influence when σ/ σ* is higher than 0.76. This study, hence, provides a comprehensive understanding of the mechanical behavior of anisotropic rocks under polyaxial stress conditions. The data and findings from this study can be utilized in not only evaluating the effectiveness of various proposed failure criteria but also modifying or formulating a new failure criterion for rocks that can model the effects of both polyaxiality and anisotropy together.