Method of Trepanate Drilling of Deep and Large Diameter Holes in Hard Rocks

Existing methods for the tunneling and deep drilling of hard rock rely on blasting techniques to separate the material and produce cavities. As a result of this process microcracks are introduced into the surface of the rock material. For special applications high quality large diameter holes are required that are free from such microcracking. Since high pressure abrasive waterjet cutting was introduced five years ago it has become possible to concentrate the energy of cutting on much smaller surface areas. This material removal method is based both on the microchipping action of the abrasives and the formation of hydraulic wedges within the rock structure. Together, these mechanisms minimize the risk of crack propagation to the outer surfaces of the hole being drilled. The total thrust exerted on the material is also significantly reduced. The presence of abrasives in the jet ensures that stubborn pieces of aggregate in the parent rock can also be cut away. This technique can be applied successfully in the drilling of large deep diameter holes. Test results for the cutting of granite samples by abrasive waterjet, over the pressure range of 70 to 280 MPa, are presented in this paper. These results are used in the compilation of a technological data base for the proposed trepanate method of high efficiency, microcrack free drilling of hard rocks