Inada Granite Research Articles

異方性岩石の引張強度と破壊靱性について

The purpose of this report is to obtain orientation dependence on tensile strength and fracture toughness in anisotropic rocks. Tensile tests and fracture toughness tests by bending and hydraulic fracturing were conducted for two types of crack orientation, which one was normal to the rift planes (type I) and the other was in the rift planes (type II).The ratio of tensile strength for type I and II was 1.68 for Inada granite, 0.98 for Kofu andesite and 1.70 for Emochi tuffaceous andesite. The results show pronounced orientation dependence on tensile strength in Inada granite and Emochi tuffaceous andesite, and suggest that the orientation of hydro-fracture is liable to be depended on the influence of rock anisotropy.The ratio of fracture toughness for type I and II was 1.14 for Inada granite, 1.33 for Emochi tuffaceous andesite by bending test, and 1.10 for Inada granite, 1.07 forKofu andesite by hydraulic fracturing test. Fracture toughness values by hydraulic fracturing under permeable condition were about 1.5 times of the values by bending. These resultsshow that the influence of rock anisotropy is more important problem in fracture initiation than in fracture extension

Effects of zeta potential on microcrack growth in rock under relatively low uniaxial compression

The seismic velocity, attenuation, and acoustic emission were measured to detect and monitor microcrack growth in samples of Inada granite as they were held under constant uniaxial compression. The samples were saturated with two kinds of aqueous solutions, aluminum nitrate solutions and potassium nitrate solutions, and the ζ potentials at the rock (mineral)‐water interface were varied with different concentration of the salts in the solutions. The axial stress was held constant at 94 MPa, that is, the stress at the onset of dilatancy for the present sample. At this stress level, the development of microcracks, which was indicated mainly by the decrease in velocity (>10%) of elastic waves propagating in the direction perpendicular to the axis of stress, was found to occur after certain time has elapsed and to be strongly affected by the ζ potential. An “incubation period” for the microcrack growth (i.e., brittle creep) becomes shorter, and subcritical crack growth rate becomes higher as the ζ potential approaches zero. A model of tensile cracking in a rock loaded in compression (circular hole model) was proposed to explain the observed time‐dependent microcracking, and some chemomechanical mechanisms of the ζ potential effects are discussed.

岩石の熱疲労に関する実験的研究

It is well known that rocks are more or less deteriorated by sudden cooling after being heated. Furthermore, by repeating the cycle of heating-cooling, rocks might be expected to be weakened more severely.In this paper, measuring the changes of the physical and mechanical properties of rocks including apparent specific gravity, P-wave velocity, Young's modulus and uniaxial compressive strength, the thermal fatigue process of rocks is characterized for four kinds of rocks, namely, OGINO tuff, EMOCHI welded tuff, AKIYOSHI marble and INADA granite. The cylindrical specimens are suddenly submerged into water after being heated and the cycle is automatically repeated in the testing machine.The maximum temperature and the maximum cycles in the experiment are 600°C and 54, respectively.The main results obtained are as follows:(1) The main failure mechanism is different between the crystalline rock and the sedimentary rock. The failure of the former takes place by the thermal interaction between minerals and that of the latter by the transient thermal stresses. As the result, crystalline rocks collapse to be particles or powders and sedimentary rocks are fractured initiating regular thermal cracks (Fig.6).(2) The strengths of the rocks except welded tuff decrease remarkably within 5 cycles if the temperature is sufficiently high and the cooling time is larger enough (Fig.3).(3) The strengths of the rocks except marble decrease as the cooling time increases. However, the additional effect is very small if the cooling time is larger than that needed for the specimens to be perfectly cooled (Fig.4).(4) The cycles at which the specimens collapse exponentially increase as the temperature decreases (Fig.5).

封圧下における岩石の破壊靱性

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%.

小型熱削孔機の開発と削孔速度について

Thermal fracturing is one of the most useful methods for breaking hard rocks such as granite and quartzite. In our laboratory a few types of small air-kerosene thermodrills (45mm in diameter) were designed and constructed for the practical use, and the drilling tests for Inada granite were carried out. The thermal drilling rate was about 15cm/min and the borehole diameter was 80mm.The authors examined the process of thermal drilling theoretically and derived an equation of thermal drilling rate (v) as follows;v=√a/πtfe-(h/λ)2atf/erfc(h/λ√atf)-ah/λtfsatisfies the following equationθf=θj{1-e(h/λ)2atf·erfc(h/λ√atf)}where h: heat transfer coefficientα: thermal diffusivity of rockλthermal conductivity of rockθj: effective temperature of jet flameθf: surface temperature of rock at the time of fractureThe calculated results by the above equation closely coincide with the experimental ones. This equation showed also that the parameters which affected the thermal drilling rate were effective temperature index defined by authors and the heat transfer coefficient.More rapid drilling rate is estimated with the increment of combustion chamber pressure.