In this paper, a numerical investigation is performed for the size and stress gradient effects on the fracture initiation and propagation around single pre-existing cavities in brittle rock. A distinct elements code is used to perform the numerical simulations. To investigate the rock fracture around cavities and to assess the potential of the numerical model to simulate this behavior, published laboratory physical models on plaster and granite are simulated numerically. The numerical models are presented and the calibration of the BPM microparameters is described. Then, the calibrated numerical models are used to investigate the effect of the size of the cavity on the primary, secondary and side wall fracturing, as well as on the fracturing modes.
The bonded-particle model BPM (Potyondy & Cundal 2004) has extensively been used over the past decade to simulate the mechanical behavior and fracture of rock under a variety of loading configurations. In the BPM, the intact rock is represented by a dense packing of rigid spheres (in 3D) or disks (in 2D) bonded together at their contacts. The model is implemented in the Particle Flow Code PFC (Itasca 2014). The BPM has been extended by Potyondy (2010) to form the Grain Based Model (GBM) in order to simulate a rock grain structure of deformable, breakable or not, polygonal grains, cemented along their adjoining sides. The GBM has been successfully used to describe the crack initiation, crack damage and peak strength of Dionyssos marble specimens under uniaxial compression (Karatza et al. 2012). However, more computational effort and extended calibration procedures are normally required to match the macroscopic rock behavior with the GBM. With the recent addition of the flat-joint contact logic in the BPM (Potyondy 2012) particle interlocking and friction resistance at the contact are imposed, restricting the relative movement of particles, and thus attaining the advantages of simulating the rock structure.
In this study, the BPM is used to model the fracture initiation and damage around cylindrical openings in compression. Published laboratory physical models on plaster (Lajtai 1971) and granite (Carter et al. 1991) are simulated numerically with the PFC2D (v. 5.00.11) code. Both the BPM parallel bond and the flat-joint contact models are used.