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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.

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.

The effect of the humidity on subcritical crack growth 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 crack velocity and the stress intensity factor. All experiments 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 crack velocity strongly. The crack velocity increased with the increase of the humidity. The increases of the crack velocity were much larger than that estimated from the classical stress corrosion theory. The increases of the crack velocity in Shirahama sandstone and Kushiro sandstone were larger than that in Berea sandstone by several orders of magnitude. This difference may be due to the difference of the rock fabric. Shirahama sandstone and Kushiro sandstone include clay minerals much more than Berea sandstone. It is probable that the fracture resistance of clay decreased due to the increase of the humidity. Consequently, large increases of the crack velocity occurred in Shirahama sandstone and Kushiro sandstone. It is concluded that the humidity has the strong effect on subcritical crack growth in sandstone and the control of the humidity is important to prevent the weakening of rock.

The effect of the humidity on subcritical crack growth in igneous rock in air was investigated experimentally. Rocks used in this study were Kumamoto andesite and Oshima granite. Double-Torsion method was used in this study to measure the crack velocity and the stress intensity factor. All experiments 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 crack velocity strongly. The crack velocity increased with the increase of the humidity. The crack velocity increased by 1 ∼ 4 orders of magnitude when the humidity increased threefold or fourfold. This increase is larger than that estimated from the classical stress corrosion theory. It is concluded that the humidity has the strong effect on subcritical crack growth in rock and the control of the humidity is important to prevent the weakening of rock.

It is well-known that granite has anisotropy due to the preferred orientation of micro-cracks. Therefore, it is important to investigate the orientation of micro-cracks in granite to discuss the anisotropy. It has been postulated that the orientation of micro-cracks in rock can be determined by DSCA (Differential Strain Curve Analysis). In the case of DSCA, however, it is necessary to use a lot of specimens to determine the orientation of micro-cracks precisely. Additionally, it is impossible to evaluate the orientation of micro-cracks if the rock contains few oriented micro-cracks.The seismic wave velocity shows the extreme values in the direction of principal axes of elasticity. Therefore, it is possible to obtain the information of the orientation of micro-cracks in rock by the measurement of the wave velocities. In this study, longitudinal wave velocities in various directions were measured to determine the orientation of micro-cracks in granite by using polyhedral specimens.From the measurement of the longitudinal wave velocities, the three-dimensional distribution of the velocities in granite could be obtained. By using the associated Legendre function, it was shown that the three-dimensional distribution of the longitudinal wave velocity could be approximated within 2nd order terms of the associated Legendre function. This result means that the wave distribution function can be expressed with the quadric surface and the coefficients can be expressed as the symmetric tensor of the 2nd rank. This shows the existence of the principal axes and the principal values which can be expressed with the eigen values and the eigen vector same as the cases of the stress and the strain. It is concluded that the directions of the principal axes and the values of the longitudinal wave velocities in granite can be determined by the calculation as evaluating the principal stress or the principal strain from the stress tensor or the strain tensor.

A review of studies of subcritical crack growth in rock is presented. First, theoretical background of subcritical crack growth is presented. It has been considered that the main mechanism of subcritical crack growth in silicate materials is stress corrosion under low homologous temperature and pressure. Next, theoretical background of stress corrosion is presented. Especially, the theoretical relation between the crack velocity and the stress intensity factor is described. Then, progress of the study of subcritical crack growth in silicate materials is presented by reviewing previous studies for glass, quartz, and silicate rocks. It has been clarified that the reactive species of stress corrosion is water and hydroxyl ion. For anisotropic rock, the crack velocity was dependent on the propagation direction and opening direction even at the same stress intensity factor.Finally, recent results of subcritical crack growth in rock are presented. It was clarified that the relation between the crack velocity and the stress intensity factor was decided by the crack opening direction. The crack velocity was higher when the temperature and the humidity were higher. It was shown that the activation energy became smaller when the crack path was smooth from the observation of the crack path.