ABSTRACT:
Anisotropy plays a key role in fracture propagation models and rock fragmentation processes by rock drilling and cutting. Drilling tests with a percussive rock drill have been performed to examine the influence of anisotropy and inhomogeneity on fracture propagation in different rock types. Subsequently, thin sections of the bottom of the borehole were analyzed to get the associated crack patterns. Based on that, the attempt was made to simulate the drilling process in a specific rock material with the Particle Flow Code (PFC). Since the code is based on the discontinous mechanical approch, a rock sample is converted into an assembly of spheres, where the particles are able to interact with each other and fractures are able to propagate. Different rock cutting tools were applied such as button bits or disc cutters to penetrate different rock samples.
1 INTRODUCTION
Geological parameters such as foliation, acting as weakness planes in the rock, may have a high impact on the fragmentation process underneath rock cutting tools. Extensive field and laboratory studies have been performed to examine drilling or cutting progress in correlation with the orientation of foliation and other rock properties (Thuro 1997, 2002, 2003, Thuro & Spaun 1996, Thuro et al. 2002). It has been found, that the drilling velocity as well as the disc cutter penetration is best orthogonal to foliation or schistosity and poor parallel to it. Between those angles, the drilling or cutting rates decrease significantly. After these field studies it seemed promising to investigate the obtained microscopic fracture patterns underneath rock cutting tools in different rock types and to perform a numerical simulation of it. Using the Particle Flow Code “PFC”, which is based on discontinuous mechanical properties, virtual rock samples were modeled and virtual drilling and cutting tests were performed.
