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A numerical time-dependent deformation model of rock by incorporating the stress corrosion model into a three-dimensional discrete element code was established to investigate time-dependent deformation and fracture evolution of surrounding rock around underground excavations with circular, inverted U-shaped and rectangular cross-sections. The mesoscale model input parameters were calibrated to replicate the mechanical behavior and failure patterns of the rock observed in laboratory. Numerical simulations on the time-independent and -dependent deformation and fracturing evolution of the rock around underground excavations with different shapes under hydrostatic stress were studied. The results show that the maximum time-independent displacements of circular, inverted U-shaped and rectangular openings are 11, 15, and 34 mm at left roof, right floor and left side, respectively. It shows that the surrounding rocks around the circular, inverted U-shaped openings are much more stable than that around the rectangular opening. The cracks within the surrounding rock around different shaped openings gradually increases as time goes by, and the tensile cracks are dominant. The progressive failure process of the circular openings under varying lateral pressures was further modeled. The results show the failure zone at the roof wall and floor wall becomes more evident with an increase in the lateral pressure coefficient.

High strength and ultra low permeability concrete (HSULPC) is being considered as a material used to package transuranic (TRU) waste for disposal in geological repositories. Therefore, information on the permeability of HSULPC is essential. Permeability tests need to be highly accurate to determine the hydraulic conductivity of HSULPC because of its ultralow permeability. In our study, we measured the permeability of HSULPC samples using the transient pulse method. The temperature of the concrete was finely controlled and held constant. The hydraulic conductivities were determined from the measurements to be around 10(-13) to 10(-12) m/s for confining pressures between 2 and 10 MPa. The pore pressure was a constant 1 MPa. The results further showed that the permeability of HSULPC had a hysteretic dependence on the effective confining pressure. We found that the hydraulic conductivity of HSULPC is comparable to or less than that of intact Toki granite obtained from Gifu Prefecture in central Japan. It was also considered that the hydraulic conductivity of HSULPC stabilized at around 10(-13) m/s after being buried and stressed. The high density and impermeability of HSULPC would enable it to effectively confine C-14 radionuclides found in TRU waste.

It is essential to understand the long-term migration of radionuclides when considering rock engineering projects such as the geological disposal of radioactive waste. The network of fractures and pores in a rock mass plays a major role in fluid migration as it provides a pathway for fluid flow. The geometry of the network can change due to fracture sealing by some fine-grained materials over long-term periods. Groundwater usually contains fine-grained minerals such as clay minerals. Therefore, it is possible that the accumulation of such fine-grained minerals occurs within a rock fracture under groundwater flow. In this case, the aperture of a fracture may decrease, which brings about the decrease of the permeability. It is therefore essential to conduct permeability measurements using water including fine-grained minerals in order to understand the permeability characteristics of a rock. However, this has not been investigated well. In this study, we use a macro-fractured granite sample to investigate the temporal change of the permeability that occurs under the flow of water that includes two different amounts of clay.It was shown that the clay accumulated in the macro-fracture and that the permeability of the macro-fractured granite sample decreased over time. It was also recognized that the decrease of the permeability was more significant under the water flow with the higher clay content. As a result of the observation using microscope, it was recognized that the clay minerals accumulated in the macro-fracture in the granite sample, which decreased the aperture of the fracture. We concluded that the accumulation of clay minerals in the fracture decreased the permeability of the rock. Furthermore, it is concluded that the filling and closure of fractures in rock is possible under the flow of groundwater including clay minerals.

Clay-mineral type and content, bulk mineralogical composition and alteration of bentonite are very important factors for the ultra-long-term stabilization of barriers and backfills in radioactive waste disposal. This study investigates the effects of clay-mineral type and content on the swelling characteristics and permeability of bentonite–sand mixtures with clay minerals using one-dimensional swelling-pressure and constant-pressure permeability tests. The hydraulic conductivity of bentonite–sand–clay mineral mixtures increased with increasing content of non-swelling alteration products of montmorillonite. Furthermore, hydraulic conductivity was comparable to that determined with the Kozeny–Carman equation for a specific surface area, suggesting that hydraulic conductivity may be estimated based on the abundance of expected alteration products of montmorillonite. This study provides a basis for evaluation of the hydraulic conductivity of bentonite–sand mixtures with known quantities of expected alteration products of montmorillonite., This paper was presented during the ‘7th International Conference on Clays in Natural and Engineered Barriers for Radioactive Waste Confinement’, September 2017.

In this study, the precipitation of minerals onto rock surfaces was investigated to consider whether sealing of pores and cracks in rock can be accelerated. Cylindrical specimens were prepared and then kept in purified water with powders of high-strength and ultra-low-permeability concrete (HSULPC), which will be used to confine transuranic wastes in Japan. Then, the rock specimens were weighed and the surfaces of rock specimens were inspected under a microscope. It was recognized that precipitation occurred on the surface of the rock specimens. It was also shown that precipitation did not occur on rock specimens kept in water without HSULPC. The weight of all specimens stored in HSULPC increased, and the observed weight change was larger for rocks with higher porosities. It is concluded that precipitation of minerals occurs on the rock surface when the rock is kept in water with HSULPC powders. From the results obtained in this study, it is suggested that the sealing of pores and cracks in rock can be accelerated by the precipitation of calcium compounds using HSULPC. It is concluded that HSULPC is useful for underground radioactive waste disposal.

Information on the permeability of rock is essential for various geoengineering projects, such as geological disposal of radioactive wastes, hydrocarbon extraction, and natural hazard risk mitigation. It is especially important to investigate how fractures and pores influence the physical and transport properties of rock. Infiltration of groundwater through the damage zone fills fractures in granite with fine-grained minerals. However, the permeability of rock possessing a fracture naturally filled with fine-grained mineral grains has yet to be investigated. In this study, the permeabilities of granite samples, including a macro-fracture filled with clay and a mineral vein, are investigated. The permeability of granite with a fine-grained mineral vein agrees well with that of the intact sample, whereas the permeability of granite possessing a macro-fracture filled with clay is lower than that of the macro-fractured sample. The decrease in the permeability is due to the filling of fine-grained minerals and clay in the macro-fracture. It is concluded that the permeability of granite increases due to the existence of the fractures, but decreases upon filling them with fine-grained minerals.

Investigating the time-dependent fracturing in sandstone is crucial to consider the long-term integrity of a sedimentary rock mass surrounding structures. Subcritical crack growth is one of the main causes of the time-dependent fracturing in rock materials. For the long-term integrity, it is essential to investigate subcritical crack growth in rock, which is influenced by the surrounding environmental conditions. Numerous cementitious materials are used to construct various structures such as underground repositories for radioactive waste, underground power plants, etc. In these cases, the calcium ion concentration in the ground water in a rock mass surrounding these structures is high. Therefore, investigation on the subcritical crack growth in rock in water with a high calcium ion concentration is essential. In this study, subcritical crack growth in sandstone is measured in distilled water and calcium hydroxide solution (Ca(OH)2) to investigate the influence of calcium ions on the time-dependent fracturing in rock. Berea sandstone was used as a rock sample. The load-relaxation method of the double-torsion test was used to measure the crack velocity and the stress intensity factor. All measurements have been carried out under controlled temperature and relative humidity. It was shown that the crack velocity in a calcium hydroxide solution is lower than that in distilled water even though the pH was high. In previous researches, it has been considered that the crack velocity in rock increased when the pH was high from the measurements in water and sodium hydroxide solution (NaOH). Therefore, it is considered that calcium ion affects the decrease of the crack velocity. It is concluded that a water environment with a high calcium ion concentration is suitable for the long-term integrity of a sedimentary rock mass.

In the present paper, we investigate how water acts to weaken rock in two complementary ways: mechanically through the generalized effective stress principle, and chemically through time-dependent rock-fluid reactions that allow subcritical crack growth. These processes, together with capillary suction and stress corrosion , were incorporated into a three-dimensional discrete element grain-based model to investigate both the time-independent and the time-dependent mechanical behavior of partially saturated sandstone at the mesoscale . The capillary parameters related to capillary suction and subcritical parameters related to stress corrosion in the model were calibrated to match the deformation behavior of partially saturated sandstone observed in laboratory. Following this calibration, numerical simulations of partially saturated sandstone with different levels of saturation were performed in uniaxial compression. The simulations show that both the peak strength and elastic modulus of the sandstone decrease as a function of increasing saturation and that the relationships between these properties can be expressed by negative exponential functions. The simulations are in good agreement with the experimental results. Second, the long-term brittle deformation of partially saturated sandstone with different levels of saturation under a constant stress level was modeled. The results show that time-to-failure during brittle creep decreases, and the initial strain and the minimum creep strain rate increases, as a function of increasing saturation, as also observed in laboratory. The simulations also highlight the formation of tensile cracks as the main deformation mechanism during brittle creep. Finally, brittle creep in partially saturated sandstone samples with different levels of saturation was studied under different stress levels. These simulations show that the minimum creep strain rate and the time-to-failure as a function of stress can be well described by exponential relations. We conclude that the proposed model permits a deeper understanding of time-independent and time-dependent deformation and failure of partially saturated sandstone at the mesoscale.

Understanding subcritical crack growth in rock is essential for determining appropriate measures to ensure the long-term integrity of rock masses surrounding structures and for construction from rock material. In this study, subcritical crack growth in marble was investigated experimentally, focusing on the influence of the surrounding environment on the relationship between the crack velocity and stress intensity factor. The crack velocity increased with increasing temperature and/or relative humidity. In all cases, the crack velocity increased with increasing stress intensity factor. However, for Carrara marble (CM) in air, we observed a region in which the crack velocity still increased with temperature, but the increase in the crack velocity with increasing stress intensity factor was not significant. This is similar to Region II of subcritical crack growth observed in glass in air. Region II in glass is controlled by mass transport to the crack tip. In the case of rock, the transport of water to the crack tip is important. In general, Region II is not observed for subcritical crack growth in rock materials, because rocks contain water. Because the porosity of CM is very low, the amount of water contained in the marble is also very small. Therefore, our results imply that we observed Region II in CM. Because the crack velocity increased in both water and air with increasing temperature and humidity, we concluded that dry conditions at low temperature are desirable for the long-term integrity of a carbonate rock mass. Additionally, mass transport to the crack tip is an important process for subcritical crack growth in rock with low porosity.

It is important to understand time-dependent deformation and fracturing in rock to evaluate its long-term strength (LTS); subcritical crack growth (SCG) provides insight into the weathering of a rock mass over the long term. The LTS of rock is commonly evaluated under the same environmental conditions. However, in practice, the environment is constantly changing and must be accounted for in evaluating the LTS of rock. In this study, we developed a method to evaluate LTS under changing environmental conditions, with a focus on the influence of water on the LTS of rock. LTS decreased rapidly when the environmental conditions changed from air to water. In a case where the environmental conditions changed repeatedly from air to water at various duration intervals, the value of the LTS was similar to that in a continuous water environment. Because a dramatic decrease in the LTS occurred when the environmental conditions changed from air to water, we conclude that the effect of water on the acceleration of SCG in rock should be considered in the long-term use of rock structures.

Information of subcritical crack growth in rock is essential to ensure the long-term integrity of a rock mass surrounding various structures and various architectures made of rock materials. In this study, using Carrara marble as a rock sample, subcritical crack growth was investigated experimentally. Especially, the load relaxation method of the double-torsion test was used for all measurement under controlled temperature and relative humidity. It was shown that the crack velocity in air increased with increasing temperature. The crack velocity increased with increasing the stress intensity factor. However, we found a region where the increase of the crack velocity with stress intensity factor was not significant. It is also found that the crack velocity in this region increased with temperature. This is similar to Region II of subcritical crack growth observed in glass in air. The Region II in glass is controlled by the mass-transport to the crack tip. In the case of rock, the transport of water to crack tip is important. In general, Region II is not observed for subcritical crack growth in rock materials, because rocks contain water. Since the porosity of Carrara marble is very low, the contained amount of water is also very small. Therefore, it is considered that Region II is observed in Carrara marble. Since the crack velocity increased in the environment with higher temperature, it is concluded that the condition with low temperature in air is desirable for the long-term integrity of a carbonate rock mass. Additionally, it is concluded that mass-transport is an important process for subcritical crack growth in rock with low porosity.

The physics of water imbibition into initially unsaturated sandstone is critical to the understanding of displacement processes and fluid transport in the vadose zone. The distribution of water within rock is important due to its significant influence on rock mechanical behavior. Here, therefore, we used nuclear magnetic resonance (NMR) technology to visualize and quantify the dynamics of water infiltration and distribution in initially unsaturated sandstone. The progression of water imbibition in sandstone specimens under the following two conditions were analyzed: (1) specimens soaked in water for different durations and (2) specimens soaked in water for the same duration and then held in this state for different durations. An analytic function was developed to estimate the sandstone moisture profile and to determine the unsaturated flow within the sandstone when the water distribution matched laboratory observations. Finally, a three-dimensional discrete element grain-based model was formulated that incorporates the local parallel-plate method, the unsaturated flow function, and the generalized effective stress principle. We used this model to effectively reproduce the process of water imbibition in the laboratory sandstone specimens. The effect of water imbibition on the mechanical properties of the studied sandstone was also evaluated. These results show that the strength of the core of the sample was reduced as water migrated from its surface to its center, resulting in a decrease in bulk sample strength as standing duration (i.e. water distribution uniformity) increased. The results of this study aid in our understanding of the influence of water imbibition on the mechanical behavior of sandstone, which is important for rock slope stability assessments following rainfall.

A numerical time-dependent model is established by incorporating the stress corrosion model into the three-dimensional discrete element grain-based model (3DEC-GBM) to analyze the fracture evolution and instability mechanism of the surrounding rock mass around a circular tunnel. The surrounding rock of the tunnel is assumed to be made from intact Yunnan sandstone formation, whose material properties were found using laboratory experiments (ignoring scale effects). The mesoscale mechanical stress corrosion parameters in the model are calibrated to reproduce the time-independent and -dependent deformation and failure of Yunnan sandstone observed in a laboratory. The model is validated against laboratory data and then further applied to analyze the time-independent and -dependent deformation of the surrounding rock of the tunnel. The fracture evolution around the tunnel instability is numerically visualized. The failure zone around the tunnel gradually extends deeper into the surrounding rock with time, and the location of failure at the roof and floor of the circular tunnel becomes more evident as the lateral pressure coefficient increases. We conclude that the model is not only able to simulate the location of the concentrated tensile and shear stress around the tunnel but also has important theoretical guidance and practical significance for the long-term stability study of tunnels.

We performed hydraulic fracturing experiments on cylindrical cores of anisotropic shale obtained by drilling normal to the sedimentary plane. Experiments were conducted under ambient condition and uniaxial stresses, using three types of fracturing fluid: viscous oil, water, and liquid carbon dioxide (L-CO2). In the experiments using water and oil, cracks extended along the loading direction normal to the sedimentary plane under the uniaxial loading and extended along the sedimentary plane without loading. These results suggest that the direction of crack extension is strongly affected by in situ stress conditions. Fluorescent microscopy revealed that hydraulic fracturing with viscous oil produced linear cracks with few branches, whereas that with water produced cracks with many branches inclining from the loading axis. Statistical analysis of P wave polarity of acoustic emission waveforms showed that viscous oil tended to induce Mode I fracture, whereas both water and L-CO2 tended to induce Mode II fracture. Crack extension upon injection of L-CO2 was independent of loading condition unlike extension for the other two fluids. This result seemed attributable to the low viscosity of L-CO2 and was consistent with previous observations for granite specimens that low-viscosity fluids like CO2 tend to induce widely extending cracks with many branches, with Mode II fractures being dominant. These features are more advantageous for shale gas production than those induced by injection of conventional slick water.

The direction of auxin transport changes in gravistimulated roots, causing auxin accumulation in the lower side of horizontally reoriented roots. This study found that auxin was similarly involved in hydrotropism and gravitropism in rice and pea roots, but hydrotropism in Lotus japonicus roots was independent of both auxin transport and response. Application of either auxin transport inhibitors or an auxin response inhibitor decreased both hydrotropism and gravitropism in rice roots, and reduced hydrotropism in pea roots. However, Lotus roots treated with these inhibitors showed reduced gravitropism but an unaltered or an enhanced hydrotropic response. Inhibiting auxin biosynthesis substantially reduced both tropisms in rice and Lotus roots. Removing the final 0.2 mm (including the root cap) from the root tip inhibited gravitropism but not hydrotropism in rice seedling roots. These results suggested that modes of auxin involvement in hydrotropism differed between plant species. In rice roots, although auxin transport and responses were required for both gravitropism and hydrotropism, the root cap was involved in the auxin regulation of gravitropism but not hydrotropism. Hydrotropism in Lotus roots, however, may be regulated by a novel mechanism that is independent of both auxin transport and the TIR1/AFBs auxin response pathway.

Information on subcritical crack growth in rock is essential to ensure the long-term stability of structures in a rock mass. Subcritical crack growth in rock is known to be affected by its surrounding environment. In most cases, rock found underground is saturated by water. The underground water can be fresh water or salt water with variable electrolyte concentrations. However, the influence of electrolyte concentration on subcritical crack growth in rock has not been fully elucidated. In this study, subcritical crack growth in Berea sandstone and Shirahama sandstone was measured when saturated with distilled water and with sodium chloride solutions of differing concentrations. Crack velocity in Berea sandstone changed very little with different electrolyte concentrations. By contrast, in the Shirahama sandstone, crack velocity became lower with increased electrolyte concentrations up to 1.0 mol/l. With increasing electrolyte concentration, the width of the electric double layer on the surface of mineral grains decreased, which caused a decrease in the repulsive force acting on the crack surface, leading to a decrease in crack velocity up to concentrations of 1.0 mol/l. However, when the electrolyte concentration was higher than 1.0 mol/l, crack velocity increased due to the nucleation of microscopic defects on the boundary between clays and stiff mineral grains. Thus, electrolyte concentration affects subcritical crack growth by decreasing the width of the electric double layer and the condensation of clay minerals.

Fracture sealing by precipitation is known to occur in high-strength and ultra-low-permeability concrete (HSULPC) immersed in water. Because a high ability to retard radionuclide migration is required for HSULPC, understanding both the sealing process and the composition of sealing deposits is important to identify optimum conditions for significant sealing. In this study, sealing of a macro-fractured HSULPC specimen with initial aperture of approximately 0.1 mm was investigated in simulated seawater over 49 days. The composition of sealing deposits at 49 days after immersion was clarified by scanning electron microscopy and energy dispersive X-ray spectroscopy (SEM/EDX), and the progress of sealing during the 49 days was clarified by image analysis with micro-focus X-ray computed tomography (X-rayCT). Both the SEM/EDX and X-rayCT results showed that significant sealing was attained only near the outermost part of the specimen. The SEM/EDX results showed that a thin brucite layer formed on the entire specimen surface over which significant precipitation of calcium carbonate occurred and sealed the macro-fracture only near the outermost part of the specimen. The X-rayCT results indicated that the amount of sealing deposits in the macro-fracture ( $$P_\mathrm{seal})$$ reached 70 % in the mostly sealed region at 49 days and the rate of change in $$P_\mathrm{seal}$$ became maximum $$(3.7\,\% \hbox { day}^{-1})$$ during 7–21 days after immersion, then decreased. In conclusion, the findings in this study represent an important clue in the search for optimum conditions to achieve fracture sealing in HSUPLC.

Granite rock mass is used for various rock engineering purposes. To ensure long-term stability, information about the subcritical crack growth (SCG) and an estimate of the long-term strength (LTS) of the rock are necessary. The influence of the anisotropy of granite on its LTS has not yet been clarified. In this study, the anisotropy of the long-term rock strength was investigated for two types of granite rocks, Oshima granite and Inada granite. Specifically, the effect of the anisotropy in crack propagation on the LTS was examined. The results showed that the LTS of granite is anisotropic, as are the fracture toughness and Brazilian tensile strength measured in this study. The LTS was lowest when crack propagation occurred parallel to the rift plane, where most of the microcracks occur. For Inada granite, which has an anisotropic SCG index, the degree of anisotropy of the LTS increased as the time-to-failure increased. This suggests that the LTS of granite is anisotropic.

For cementitious composites and materials, the sealing of fractures can occur in water by the precipitation of calcium compounds. In this study, the sealing behavior in a macro-fractured high-strength and ultra-low-permeability concrete (HSULPC) specimen was investigated in simulated seawater using micro-focus X-ray computed tomography (CT). In particular, the influence of fracture width (0.10 and 0.25 mm) on fracture sealing was investigated. Precipitation occurred mainly at the outermost parts of the fractured surface of the specimen for both fracture widths. While significant sealing was observed for the fracture width of 0.10 mm, sealing was not attained for the fracture width of 0.25 mm within the observation period (49 days). Examination of the sealed regions on the macro-fracture was performed using a three-dimensional image registration technique and applying image subtraction between the CT images of the HSULPC specimen before and after maintaining the specimen in simulated seawater. The temporal change of the sealing deposits for the fracture width of 0.10 mm was much larger than that for the fracture width of 0.25 mm. Therefore, it is concluded that the sealability of the fracture in the HSULPC is affected by the fracture width.

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.

Information 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.

It is essential to investigate the influence of pores and fractures on physical and transport properties of rocks for understanding many key problems in seismology, volcanology and rock engineering. In rocks, water is primarily stored in and migrates through networks of pores and fractures at all scales. It is therefore important to know how fluid flow in such pores and 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 both intact and macro-fractured sandstone samples. Specifically, we used two types of sandstone; Clashach sandstone from Scotland and Shirahama sandstone from Japan, which have different porosities and pore size distributions. Wave velocities increased with increasing pressure for all cases. Additionally, wave velocities in macro-fractured samples were lower than those in intact samples. For Shirahama sandstone, permeability decreased with increasing pressure. By contrast, permeability of Clashach sandstone remained essentially constant over the whole pressure range. This lack of change is interpreted as being due to the presence of a well-connected network of large pores with high aspect ratios. It was also found that the difference in permeability between intact and macro-fractured sandstone samples was small. From these results, it is concluded that the networks of pores control the permeability in sandstone. © 2012 The Society of Materials Science, Japan.

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 x 10(-20) m(2)), intermediate in the micro-fractured material (from approximately 5 x 10(-17) m(2) to 1 x 10(-17) m(2)) and largest in the macro-fractured material (from approximately 3 x 10(-15) m(2) to 9 x 10(-19) m(2)). For a 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. (C) 2010 Elsevier B.V. All rights reserved.

Durability evaluation of rocks subjected to freezing-and-thawing is important for stability assessment of a rock fill dam as well as rock slopes, especially in cold weather regions. In this study, freezing-and-thawing tests up to 300 cycles were carried out on twelve kinds of rock samples in order to characterize fracture propagation by freezing-and-thawing and evaluate the durability, and fracture propagation was observed. The fracturing process was characterized by macroscopic observation by digital camera and changes in physical properties. Fracture initiation and propagation in the specimen were also observed by using X-ray CT. It was found that the fracturing process depends on the rock type. For rocks which possess preferred orientation of pre-existing fractures and anisotropy of tensile strength, only one fracture propagated dominantly and split the rock sample in half. On the other hand, for isotropic rock, propagation of several fractures occurred and rock samples were split into several parts. Additionally, spalling continuously progressed once the loss of weight exceeded 5% for isotropic rocks because fractures had developed throughout the specimen. This result indicates that freezing-and-thawing cycles at 5% weight loss is assumed to be the spalling initiation. Therefore, we can conclude that the number of freezing-and-thawing cycles to the spalling initiation is the important indicator of the durability evaluation. It was also pointed out that both supply capacity of water to the crack tip and crack growth resistance affect the durability, because the tensile strength divided by the percentage of water absorption is in good correlation with the number of cycles to the spalling initiation. It is concluded that the durability can be easily evaluated by both tensile strength and the percentage of water absorption.

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 fourfold 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.

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 long-term 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 long-term 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.

Study of subcritical crack growth is important in estimating the long-term strength of rock and in ensuring the long-term stability of structures in a rock mass. The long-term strength is affected by the value of subcritical crack growth index (SCGI). However, the effect of environmental conditions on SCGI has not been clarified. In this study, we have estimated SCGI under various environmental conditions on a sample of Kumamoto andesite, using the Double-Torsion method. We show that the SCGI decreased when the temperature is higher in both air and water; the SCGI in water is smaller than that in air. When the relative humidity of the air increased, SCGI tended to be smaller and at 90 % relative humidity was close to the value in water. Under alkali conditions, the SCGI decreased slightly. Using the values of SCGI, the long-term strength was estimated and it is shown that the long-term strength decreased when SCGI was smaller. It is concluded that SCGI and the long-term strength is affected by the surrounding environment, specifically by the temperature and water. To ensure the long-term stability of rock, the control of the temperature and water migration is important.

The effect of the humidity on the fracture toughness in sandstone in air was investigated experimentally. Rocks used in this study were Berea sandstone, Shirahama sandstone and sandstone obtained at Kushiro coalmine (Kushiro sandstone). Double-Torsion method was used in this study to measure the fracture toughness to compare the results to those of subcritical crack growth. All measurements were conducted under the same temperature and different relative humidity to investigate the effect of the humidity.It was shown that the humidity affected the fracture toughness. The fracture toughness decreased with the increase of the humidity. The decrease of the fracture toughness was more remarkable when the content of clays in rock was higher. It is considered that the amount of clays had a significant effect on the change of the fracture toughness of sandstone. Degradation of clays with increasing the water content is considered to affect the decrease of the fracture toughness. It was shown that the decrease of the fracture toughness with increasing the humidity was less than the decrease of the stress intensity factor for subcritical crack growth. This was caused because stress corrosion had less effect on the fracture toughness. It is concluded that the crack growth in sandstone is affected by the humidity and the content of clays.

Subcritical crack growth is one of the main causes of time-dependent fracturing in rock. In this study, subcritical crack growth in rock was investigated in distilled water (pH=6) and aqueous solution of sodium hydroxide (NaOHaq, pH=12). Especially, comparing the results in water to those in air, the effect of water was investigated. Additionally, the effect of pH in aqueous environment was also investigated. Rock sample were andesite and granite. The relation between the crack velocity and the stress intensity factor was determined by Double-Torsion method. All experiments were conducted with controlling temperature. It was shown that the crack velocities in water were higher than those in air. These results are in accordance with the results by many researchers who reported that the crack velocity increased in water. Comparing the results in NaOHaq to those in water, however, it was shown that the crack velocity at the same stress intensity factor didn?t change even though pH of surrounding environment changed. This result doesn?t agree with conventional concept that the hydroxyl ion accelerates subcritical crack growth in silicate materials. It is concluded that water accelerates the crack velocity and pH has little effect within the environmental conditions in this study.