Usability Assessment of Different Contact Models for Modelling the Failure Behaviour of Sedimentary Rocks Under Unconfined Stress Conditions

The methods applied in rock material and rock mass studies related to rock mechanics have begun to differentiate with computational and software technology development in recent years. Particle Flow Code software, which is based on the discrete element method, is used in many studies on rock mechanics. Using Particle Flow Code, rock behaviour under different stress conditions can be modelled and analysed in both two and three dimensions. Rocks can be presented by an assembly of disks in two dimensions and spherical particles in three dimensions. These particles are bonded to each other by contact models with different micro-mechanical properties. Model failure occurs as a result of the fracturing with the rupture of these bonds. Using parameters such as density, strength, deformation, which can be defined as the mathematical expression of the natural properties of rock samples, a representative models can be created by following multi-stage calibration procedure and used for different research purposes. In order to have a representative model, chosen contact models should be mimic the failure behaviour of the rock material. For this purpose, calibrated models were created for both PBM (Parallel-bonded model) and FJM (Flat-jointed model) using the experimental results of the unconfined compressive strength test of Hawkesbury sandstone in the laboratory. In the calibration phase, the results obtained from the models were compared against micromechanical data obtained from the laboratory experiments such as elasticity modulus, uniaxial compressive strength, tensile strength, and Poisson ratio. Results were evaluated by considering the displacement vectors, force chains, and crack parameters in terms of resolution and representativeness of the failure behaviour. The results of PBM and FJM model results were similar for displacement vector direction and orientation and distribution of the forces effective in breaking the bonds which is force chain. However, in terms of fracture resolution and localization, FJM provides detailed results, of which could be very useful for comprehensive studies.