Rock fracture initiation and propagation by mechanical and hydraulic impact

Abstract

Abstract Rock fracture due to high-pressure fluid jetting and insert indentation was experimentally investigated. In the frame of this article, indentation designates the process of pressing a tungsten carbide insert (TCI) for roller cone bits displacement-controlled into the rock surface. Under atmospheric conditions, several crystalline and one sedimentary rock type were tested. Depending on the size, type and bonding of the mineral grains, distinct differences in fracture behaviour were observed. The influence of elevated ambient pressure regimes on the evolving cracks and fractures in the tested granite revealed that microcracking ceases with increasing ambient pressure, while the specific energy is also significantly affected. Under similar conditions, indentation experiments were performed on undamaged samples and on samples containing jetted kerfs. A distinct variation in the measured maximum force and removed volume indicated different underlying rock fracture mechanisms. By applying methods for crack visualization and three-dimensional rendering, it turned out that the dimensions of the kerf significantly affected the mechanism of fracture initiation and propagation as well as the extent of the associated fracture pattern. Furthermore, the common practice of characterizing the rock excavation process only via measurable quantities such as the specific energy, without considering the created fracture pattern, yields misleading results.