INTRODUCTION
A pairs of deep longitudinal slots were made by abrasive water jet from the inside of the borehole which is drilled in the large mortar specimens. Confining pressures were applied to the specimens by flatjacks and a loading frame. Hydraulic fracturing tests were carried out in the sections including the slots and the fractures on the plane of prolongation of the slots were produced. Shut-in pressures of the fractures were then detected. Numerical simulation of fracture extension by internal pressurization at the borehole section in an infinite elastic solid were also carried out. In this report, such experimental and numerical results are presented.
INVESTIGATIONS PREVIOUS LY MADE
Previously, the authors carried out numerical computations by using boundary element method in two dimension and laboratory experiment by using small scale specimen of mortar. The results of those are reported in the references[I,2].
Bock et all3] has proposed a new stress relief technique for in-situ stress measurement. The authors followed his idea and manufactured a borehole slotter in which a diamond blade cutter driven by compressed air is equipped and applied it to in-situ experiment[4]. However, any slot made by the slotter is not deep enough, i.e., it is difficult to make a slot by diamond blade as that ratio of slot depth to borehole diameter is larger than 0.8.
Hence, they developed a new slotting machine in which abrasive water jet system is equipped. At present study, the authors confirm good performance of the new slotting machine and carry out hydraulic fracturing after slotting.
PERFORMANCE TESTS OF ABRASIVE WATER JET
The double nozzle is kept at the center of borehole by the centerizer and two slots on the opposite side of the borehole wall can be made at the same time. As the nozzle is inserted into a borehole, it must be compact. Therefore, the type of prior mixing of water and abrasive is employed.
The specimen is a rock mass of granite. Size of the specimen is about 360x210x300 inm and two boreholes of 76 mm diameter are previously drilled at the plane of 360x210 mm. In the performance tests, nozzle distance, spout pressure and abrasive content of water-abrasive mixture are varied.
After performance tests finished, the specimen was cut perpendicular to the borehole axes into the slices and the shapes of the slots were observed. It is found that deep slot can be made by the abrasive water jet system developed by the authors although slot obtained from any test condition widens at the bottom and slot depth shortens with cutting progress. The latter phenomenon may be caused by that water jet hits longer time the part of slot cut in advance than the part just cut.
