Fractures are ubiquitous on all scales in crustal rocks. The investigation of fractures and their influence on physical and transport properties of rocks is therefore essential for understanding of many key problems in seismology, volcanology and rock engineering. In crystalline rocks, pore water is primarily stored in and migrates through networks of cracks and fractures at all scales. It is therefore essential to know how fluid flow in such fracture networks responds to the elevated pressures found at depth. Here, we report results from an investigation of changes in fluid permeability, and associated changes in P-wave and S-wave velocities, at elevated effective pressure for intact, macro-fractured and micro-fractured samples of Seljadur basalt. In all cases, permeability decreases and both wave velocities increase with increasing effective pressure. Permeability decreases were smallest in the intact material (from approximately 10-19 m 2 to 3×10-20 m 2), intermediate in the micro-fractured material (from approximately 5×10-17 m 2 to 1×10-17 m 2) and largest in the macro-fractured material (from approximately 3×10-15 m 2 to 9×10-19 m 2). For material containing both micro-fractures and macro-fractures, the closure of macro-fractures dominated the permeability reduction at low pressure, with the closure of micro-fractures exerting an increasing influence at higher pressure.

In order to ensure long-term stability of structures in a rock mass, the study of time-dependent fracturing is essential. The influences of the surrounding environmental conditions and rock fabric on subcritical crack growth in sedimentary rocks in air are yet to be clarified, while the nature of subcritical crack growth in igneous rocks has been studied well. In this study, the influences of temperature and relative humidity on subcritical crack growth in Berea sandstone, Shirahama sandstone and Kushiro sandstone were investigated in air. The load relaxation method of Double Torsion (DT) testing method was used to measure both crack velocity and stress intensity factor under a controlled temperature and relative humidity. Results show that the change of the crack velocity at a given stress intensity factor was unclear when the temperature increased under a constant relative humidity in air. On the other hand, we show that the crack velocity increased by several orders of magnitude when the relative humidity increased threefold or fourfold under a constant temperature at a given stress intensity factor. This increase is much larger than that expected from the conventional concept based on the theory of stress corrosion. It is therefore necessary to consider the additional mechanisms for subcritical crack growth in sandstone. The increase of the crack velocity was larger for sandstone which contained larger amount of clays. We conclude that subcritical crack growth in sandstone in air is affected remarkably by the relative humidity and the amount of clays in rock

a b s t r a c tInformation relating to the fracture toughness of geomaterials is critical to our understanding of tensile fracturing, and in particular in geological and rock engineering projects that are subjected to elevated moisture levels. In this study, we conducted a comprehensive set of fracture toughness tests on a suite of key rock types in air under different relative humidities and at constant temperature in order to investigate the influence of relative humidity on fracture toughness. Three sandstones and two igneous rocks were chosen for this purpose. We show that the value of fracture toughness decreases with increasing relative humidity. In addition, we find that the decrease in fracture toughness was more significant when the expansive clay such as smectite was included in rock. Since smectite is prone to expanding in the presence of water, the strength and thus crack growth resistance decrease when relative humidity is high. Therefore, we interpret the decreasing fracture toughness upon the degradation of expansive clays with increasing water content. It was also shown that the decrease of the fracture toughness with increasing humidity is less significant than the concomitant decrease in the measured value of the subcritical stress intensity factor. This was likely as a result of stress corrosion having little influence on the fracture toughness. We conclude that crack growth in rock is affected by humidity, and that clay content is an important contributing factor to changes in fracture toughness and subcritical stress intensity factor.

No abstract

The effects of relative humidity and temperature on subcritical crack growth in igneous rock have been investigated experimentally on samples of Kumamoto andesite and Oshima granite. Stress intensity factors and crack velocities were measured using the double-torsion technique, and all experiments were conducted in moist air. Our results show that, in experiments conducted under the same relative humidity, crack velocity increased with increasing temperature, in agreement with previous studies. Our results also show that, in experiments conducted at the same temperature, crack velocity increased dramatically with increasing relative humidity. A three-to four-fold increase in relative humidity resulted in an increase in crack velocity of between one and four orders of magnitude. Such an increase is larger than that predicted by classical stress corrosion theory. It is suggested that capillary condensation of water vapour close to crack tips of small aperture influences the rate of crack growth. It is concluded that relative humidity needs to be controlled to avoid time-dependent weakening and extend the lifetime of structures in a rock mass.

Understanding of time-dependent deformation and fracture propagation in rock is essential, since the knowledge of the long-term integrity of rock is required for many subsurface structures excavated in a rock mass. Time-dependent fracture propagation has been invoked as a potential key mechanism responsible for the increase in seismicity preceding earthquake ruptures and volcanic eruptions. In engineering projects, and in preventing natural hazards, the study of subcritical crack growth and the long-term strength of rock is necessary. Since the long-term strength is affected by the values of the subcritical crack growth parameters, it is important to know the influence of the surrounding environment on the subcritical crack growth parameters and long-term strength. The influence of the surrounding environment on the subcritical crack growth parameters, however, has not been completely clarified yet. In this study, the subcritical crack growth parameters were estimated under various environmental conditions on igneous rocks (andesite and granite) using the Double-Torsion method. Based on the results of subcritical crack growth parameters estimations, we calculated the long-term strength of rock. It was shown that the subcritical crack growth parameters were affected by the environmental conditions such as the temperature, humidity and existence of water. Especially, it was shown that the subcritical crack growth index in water was smaller than that in air. When the relative humidity of the air was higher, subcritical crack growth index tended to be smaller. The subcritical crack growth index at 90 per cent relative humidity was close to the value in water. By the calculation based on the results of our subcritical crack growth parameters estimation, it was shown that long-term strength decreased under the conditions of higher temperature, humidity in air and in water. It is concluded that the subcritical crack growth parameters and long-term strength are affected by the surrounding environment, that is, by the temperature, relative humidity and water. To ensure the long-term stability of rock, dry conditions, at low temperature, are most suitable.

High-strength and ultra low-permeability concrete (HSULPC) is a strong candidate for a radioactive waste package containing transuranic radionuclides (TRU waste) for geological disposal. Knowledge of the time-dependent fracturing of HSULPC and surrounding rock mass is essential to assess the long-term stability of such underground repositories. We have measured crack velocity in andesite and HSULPC both in air and water to examine subcritical crack growth by the Double-Torsion method. In air, the crack velocity in andesite increased when the temperature and relative humidity increased. On the other hand, the temperature and relative humidity had little effect on the crack velocity in HSULPC in air. In water, the crack velocity increased when the temperature was higher for both andesite and HSULPC. Using these experimental results, the longterm strength was estimated. It was shown that the long-term strength of HSULPC was higher than that of andesite. In air, the long-term strength for andesite was affected by the temperature and relative humidity. The long-term strength for andesite decreased when the temperature or relative humidity increased. For HSULPC, the change of the long-term strength with varying temperature or relative humidity was smaller than andesite in air. In water, the long-term strength for both materials decreased with increasing the temperature. Comparing the longterm strength of andesite and HSULPC at the same environmental conditions, it was recognized that the decrease of the long-term strength of HSULPC is smaller than that of andesite. The long-term strength in water was smaller than that in air for both materials.

No abstract