To ensure the long-term integrity of a rock mass, understanding the time-dependent fracturing is essential. For this reason, the study of subcritical crack growth in rock has been conducted by various researchers. It has been clarified that the influence of water is significant for the increase of the crack velocity in rock. Even though the surrounding environment is alkali, the crack velocity in rock decreased if the concentration of calcium ion in water is high. This phenomenon is basically microscopic. Even though subcritical crack growth happens in a rock mass, it is impossible to detect this phenomenon, because the aperture of the crack is on the order of micrometer. Since the study of subcritical crack growth is fundamental, it is often considered that its application to rock engineering project is difficult. However, industry-academia collaboration related to radioactive waste disposal has been started after obtaining the result with high reproducibility and clarifying the influence of surrounding environment on the change of the crack velocity in various rock materials. In addition, some international collaboration has been started to research the influence of subcritical crack growth on the weathering in rock and time-dependent deformation in a rock mass under excavation. It is suggested that the future development is possible even though we conduct some fundamental studies which may be useless apparently for rock engineering projects and natural phenomena.

Percussion rock drills have been widely used mainly for blast hole drilling in mineral extraction, tunnel excavation, and underground development. To advance the performance and efficiency of rock drills, it is essential to understand the complex mechanisms of percussive rock drilling: hydraulic mechanisms in a drill body, elastic wave propagation and attenuation in rods and rod joints, dynamic bit penetration into rock, and the effects of thrust, rotation, and flushing. This paper focused on and detailed the bit penetration into rock. Some theories concerning the penetration characteristics of rock drill bits were presented such as elastic contact theory, slip line field theory, and fracture theory. Recent authors’ studies on static and dynamic penetration tests with a single button were introduced as an elementary drilling process of a bit in which a number of buttons are embedded. Then, recent authors’ studies on dynamic penetration tests with a button bit were introduced with the calculation method of force and penetration depth at the bit tip and with the results of force-penetration curves. The loading and unloading phases of the curves were modeled by convex functions, and their variation was discussed with the relation between the force-penetration curves of a bit and buttons. Important future research issues on the bit penetration into rock were presented for the further progress of percussive rock drilling.

Underground Coal Gasification (UCG) is a promising process for recovering energy from potential underground coal resources as combustible gases such as hydrogen and carbon monoxide. The gasification process that occurs underground is an invisible phenomenon, and the temperature in the underground gasification reaction zone can be over 1,000 °C. Therefore, the visualization/monitoring of the gasification area of UCG plays an important role in the development of a highly efficient, safe , and low environmental impact UCG system. This article reviews the possibility of monitoring high-temperature gasification reaction zones in UCG using Acoustic Emission (AE) technique. AE monitoring can be used as a real time monitoring technique because AE can be detected at the same time when the cracks occur due to the gasification, indicating that it gives an advance warning immediately if the extensive damage occurs. The challenges and future prospects for estimating the gasification reaction area by AE technique are also addressed at the end of article.

After the 1995 Hyogo-ken Nanbu Earthquake in Japan, rock structures, including rock slopes and underground cavities, were required to be evaluated for seismic resistance. Understanding the dynamic deformation properties of hard rock is essential for performing dynamic response analysis of large deformations of rock structures. This paper discusses the effect of confining pressure on the strain level dependence of the Young’s modulus and damping ratio of water-saturated sandstone based on the results of cyclic triaxial tests. The relationship between the axial strain and equivalent Young’s modulus ratio is different for quasi-elastic deformation below 2,000 με and elastoplastic deformation above 2,000 με and hence can be approximated by two logarithmic functions. The relationship at the quasi-elastic strain level can be approximated by the same function regardless of the confining pressure. However, at the elastoplastic strain level, the slope of the logarithmic function decreases with increasing confining pressure. Furthermore, the relationship between the axial strain and damping ratio is distributed on the same curve regardless of the confining pressure in the axial strain range below 4,000 με. However, when the axial strain exceeds 4,000 με, this relationship deviates from the curve and the damping ratio rapidly increases with decreasing confining pressure. The experimental results for water-saturated sandstone were compared with those for dry sandstone presented in a previous paper. At the quasi-elastic strain level, the relationship between the axial strain and equivalent Young’s modulus ratio can be approximated by the same function regardless of the water content. However, at the elastoplastic strain level, the slope of the logarithmic function is greater for water-saturated sandstone than for dry sandstone. Meanwhile, the relationship between the axial strain and damping ratio, regardless of the water content, is distributed on the same curve in the range from quasi-elastic to elastoplastic strain.

The development and utilization of deep underground is known to cause induced seismicity that could inflict devastating damage and pose a serious risk for safety and production. Notwithstanding the effort made in the past, a difficulty still lies in estimating the risk for induced seismicity especially taking place in geological structures. To address this problem, the present study numerically investigates stress states of multi-scale geological structures, namely kilometer-scale sedimentary basin and meter-scale fractured rock mass in a fault damage zone. The numerical simulation result of the sedimentary basin revealed that when the bedrock stiffness is higher than that of the reservoir, the differential stress of the reservoir noticeably increases, hence rendering its mechanical stability lower. In contrast, when the stiffness is lower compared to the reservoir, stress concentration takes place in the caprock because the stiffness of caprock is generally higher than that of the reservoir, causing the increase in the differential stress and thus decreasing its stability. The small-scale analysis of the fractured rock mass in a fault damage zone indicated that the complex, fracture-induced stiffness heterogeneity generates a meter-scale heterogeneous stress state on the fault plane. It was further demonstrated that the shear stress distribution on the fault plane particularly plays a key role in the occurrence of induced seismicity rather than the normal stress because of the wide variation ratio of the shear stress. By combining the results, it was then concluded that the meter-scale heterogeneity could cause seismic events with magnitudes of up to 1.0 at a depth of 2000 m, whilst more intense seismic events necessitate stress concentration affected by large-scale geological structures. However, a further study is needed to explore and quantify characteristics of seismicity taking place in multi-scale geological structures, including the dynamic behaviour and faulting type.

Polycrystalline diamond compact (PDC) bits, which are widely used in oil and gas well drilling, have begun to be used in geothermal well drilling. Mitsubishi Materials Corporation, K. Maikai Co., Ltd, and National Institute of Advanced Industrial Science and Technology started collaborative research project commissioned by Japan Organization for Metals and Energy Security with the aim of development of PDC bits that have satisfying efficiency to drill hard, inhomogeneous and fractured formations as is common in the geothermal fields. In the present report, the outline of the project is provided, including the development of PDC cutters and bits and the performance evaluation of PDC bits through laboratory and field drilling tests. Three predetermined numerical goals concerning the drilling performance, namely drilling speed, bit life, and gage loss, were achieved in the project. The ROP model proposed by Miyazaki et al. (2022), which was constructed based on a series of laboratory drilling tests under controlled conditions conducted as part of the project, is also reviewed. The model considers the effect of bit wear quantitatively and has an extremely simple form compared with earlier models. The applicability of the model to field drilling data and several future issues are also discussed.

The final slope of Une mine experienced sudden increases in displacement in specific sections during the typhoons of 2007 and 2011, as observed through long-term monitoring since the excavation from the top. The rise in groundwater due to increased rainfall was identified as a significant factor leading to measures specifically targeting rainfall. Notably, despite previous instances of heavy rainfall, sudden displacements occurred without clear triggers, emphasizing the need for a comprehensive understanding of the behavior of the rock slope. Future maintenance efforts rely on elucidating these mechanisms, often solely attributed to rainfall. To analyze factors beyond rainfall affecting the final slope's displacement, this study introduces the concept of "unit rainfall displacement," representing displacement per unit rainfall. The unit rainfall displacement is defined as the annual final slope displacement divided by the cumulative effective annual rainfall. The cumulative effective annual rainfall is determined by cumulating rainfall for events with 100 mm or more rainfall for a year, based on previous studies indicating that the final slope exhibits behavior in such conditions. Focusing on the W4, a key fracture zone of Mt.Buko, this study explores the relationship between unit rainfall displacement and the exposure status of W4 on the final slope. ’ and the exposure status of W4 on the final slope. As a result, this study revealed an overview of W4's impact on the variations in unit rainfall displacement, providing insight into the rock slope displacement attributed to W4.

In the geological disposal of radioactive waste, it is imperative to perform long-term assessments of the shielding efficacy and mechanical stability of the surrounding rock. Numerous investigations into the timedependent characteristics of various rock types have been undertaken to predict the long-term mechanical response of rocks. However, the time-dependent behaviors of diatomaceous earth, such as the strain-rate dependency of strength and creep behavior, remain unclear. In this study, constant-strain-rate and creep tests on diatomaceous earth specimens under undrained conditions and a confining pressure of 10 MPa, at both ambient (25 °C) and elevated (80 °C) temperatures were conducted. Although variations in strength (maximum differential stress) were observed between tests conducted at different temperatures, it is inconclusive as to whether the strength variations were primarily induced by temperature variations, as the unintentional deviation in water content between specimens used in the constant-strain-rate tests at 80 °C and 25 °C might have exerted some effects. The strain-rate dependency of strength and the creep behavior of diatomaceous earth were obtained, and a constitutive equation of the variable-compliance type, which accounts for the time-dependent behaviors of rocks, was applied to the obtained experimental results. Formulating an improved constitutive equation through further triaxial compression tests is an issue in the future.