封圧下における岩石の破壊靱性

Abstract

In order to determine the fracture toughness of rocks at confining pressure, the splitting tests in a triaxial vessel are undertaken on four kinds of rocks, namely OGINO tuff, AKIYOSHI marble, KIMACHI sandstone and INADA granite.The rock plates of 200mm×150mm×20mm which have a slot are prepared in this test. The slot produces a stress concentration around the tip of slot when the splitting force in the triaxial vessel is applied to the slot, and a crack occurs from the end of slot into the rock plate. The relationships among splitting load, crack opening displacement and crack length during the fracture process of rock plate are studied in this test, and the values of fracture toughness KIC of various rock specimens at confining pressure are determined with the compliance method.The outline of this study is as follows;(1) To measure a crack length of rock plate splitted in triaxial vessel, the silver conducting 7 grid lines are drawn on rock plate as shown in Fig. 3. The position of crack-tip propagating in rock plate is recorded on a pen-recorder when the silver conducting grid lines are broken by the crack.(2) The splitting load-crack opening displacement curves of various rock specimens are obtained at atmospheric and confined pressure as shown in Fig. 4. From the figure, it is clear that the rising part of splitting load-crack opening displacement curves at atmospheric pressure is relatively linear, but as confining pressure increases, the rising part of these curves becomes non-linear and the splitting load increases.(3) From the study of the effect of confining pressure on the fracture toughness, it becomes clear that the fracture toughness of rocks increases with increasing confining pressure. For instance, compared with the fracture toughness at confining pressure of 240kg/cm2 and that at atmospheric pressure, an increasing rate of the fracture toughness of OGINO tuff is about 70%, AKIYOSHI marble about 50%, KIMACHI sandstone about 150% and INADA granite about 100%.