This paper presents a general view of the recent progress in the studies on in situ rock stress measurements. Knowledge of rock stress is of fundamental importance in a wide range of rock engineering activities, including the safe and economic excavation design in civil and mining engineering, stress-related instability problems of underground openings, dams, slopes, stopes, tunnels and deep wellbores, the interpretation for hot dry rock geothermal energy projects and radioactive waste disposal, and the prediction research of earthquakes. Methods and techniques for in situ rock stress measurements are focussed on, and the recent researches are reviewed to discuss the subject for a future study and the matter to be solved at present.

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Experimental studies have been undertaken to explore the applicability of acoustic emission (AE) method and deformation rate analysis (DRA) to in situ stress estimation. Firstly, the paper discusses the influence of delay time and confining pressure on the recollection of the maximum previous stress based on laboratory experiments of Inada granite previously stressed under uniaxial or confining pressure. The main conclusions are: 1) The elapsed time, up to 7 years, does not produce any detrimental effects on the recollection of the maximum previous stress, and the stress could be estimated from AE method and DRA within 10% difference. 2) The specimen, previously stressed under the confining pressure, produced the significant change in AE and strain behaviors at the stress level which is close to the previous stress axially applied to the rock specimen. So, it was concluded that AE method and DRA allow us to estimate the previous stress along the loading axis of a specimen. Secondly, to show the applicability of the technique to in situ stress measurement, the paper discusses the results on drilled core rock obtained from two mines in Australia. Rock cores were obtained from two mines in Australia. The stresses were estimated using the AE method and DRA in different directions. The main conclusions are : 3) Although the uniaxial compression tests were carried out 30 to 40 days or 60 to 70 days after the drilling, the Kaiser effect in AE occurrence was clearly observed in the most tests. 4) The estimated stresses from the AE method were well consistent with that from DRA except one kind of rock core specimen, and also agreed well with the results obtained by hollow inclusion cell developed by CSIRO.

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Three point bending tests were conducted to investigate the effect of the chemical and wet environments on the fracture toughness of Inada granite, Kimachi sandstone, and Shin-Komatsu andesite. The new testing method was also suggested here so that the chemical solution or water constantly can exist at the crack tip during the tests. The fracture toughness was evaluated by both ISRM suggested method and AE (Acoustic Emission) technique. In AE technique AE hypocenter distribution was determined to pinpoint the crack front and process zone, then stress intensity factor was calculated using the crack length evaluated by AE hypocenters distribution. Fracture toughness of the granite, in which significant decrease has not been reported before even in wet condition, dropped 25 % in wet condition when compared to the one in dry condition, and also 37 % down in Kimachi sandstone. In Shin-Komatsu andesite, fracture toughness in wet or chemical condition dropped about 10 % from dry condition. It was found that the form of crack, estimated by AE hypocenters, have two patterns in Inada Granite. But in other rocks, there are only one pattern infracture loading. The AE behaviors in wet condition implies that the fracture mechanism could be changed from brittle manner to possibly ductile one.

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Deformation and failure of a rock mass are strongly connected with the geological discontinuities such as cracks or faults. It is very important to estimate the effects of these defects and their interactions in order to evaluate the mechanical properties of rock mass such as elastic modulus. In order to evaluate the behavior of the rock mass which contains many small cracks, we applied two dimensional displacement discontinuity method (2D-DDM) and linear fracture mechanics to homogenized multi-crack model, and estimated the effective Young's modulus. The homogenized multi-crack model is the one that all cracks are parallel and infinitely arranged at same intervals in the all directions, and that the crack arrangement around a crack is always same at any part of the model. The behavior of each crack, therefore, is always equal and analysis of an elemental region which contains one crack is enough to estimate the properties of the whole mass. We also estimated the effective Young's modulus of the homogenized multi-crack model by Self Consistent Method and Equivalent Volume Defect Method, and discussed the validity of those methods. It was found that the interaction of cracks is crucial for the reduction of effective Young's modulus. When the cracks get closer to each other in the normal direction, the value of effective Young's modulus dramatically drops and the usage of the conventional methods is limited in the narrow range in which the effect of crack interaction is relatively small.

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There are a considerable number of sheeting joints at a V-shaped valley in Japan, which is a construction site for a P-Dam. Sheeting joints could cause water leakage and reduce the strength of rock mass as a dam foundation. The mechanical backgrounds of sheeting joints were therefore investigated. In-situ stress measurement by the stress relief method revealed that maximum horizontal stress, which is almost perpendicular to the P-River, is about 12 MPa. This result coincides with results obtained by the Deformation Rate Analysis method. It was also clarified that the temperature in rock mass shows large day-variation as deep as 30 cm, and that a significant temperature gradient appears under direct sunlight. Through numerical analyses, it was determined that an inclined crack near a free surface can grow much longer than that in an infinite medium. From these points of view, it was considered that the sheeting joints are exfoliation as a result of crack propagation near the free surface under horizontal stress concentrated by the geometry of the V-shaped valley and periodically-acting horizontal thermal stress.

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Most of the overcoring method for rock stress measurements use a large diameter drilling, what is called the overcoring, to relieve the stress around a pilot hole in which a transducer is installed. When the overcoring is advanced into hard rock subjected to high rock stress, a phenomenon of a core disking is sometimes observed. As the recovered cores which have a constant thickness have a center hole like a doughnut, this core disking is called a hollow core disking. In this paper, simulating the overcoring under three dimensional initial stress state in which the axis of a principal stress coincides with that of a borehole, the hollow core disking initiated during the overcoring is discussed, based on the results calculated by a semi-analytical boundary element process for axisymmetric elasticity with arbitrary boundary conditions. Firstly, the surface stress intensity coefficients in the tangential and rotational directions of the borehole are analyzed under the conditions in which each principal stress of initial stress is subjected independently. It is clarified that the coefficients is not changed beyond the advance of the overcoring more than the radius of the borehole. Secondly, using the surface stress intensity coefficients, the surface stresses of the borehole are analyzed under three dimensional initial stress state, then the fracture criterion which represents the fracture initiation due to the tensile stress induced by the overcoring is formulated as a function of the advance of the overcoring. Finally, analyzing the surface stress of the borehole induced by the overcoring, it is made clear that the location of the fracture initiation, the type of the fracture and the thickness of disk are dependent upon initial stress state.

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Crack Projection Method has been proposed for the crack opening measurement of rocks, using the 3-dimensional data of the linear absorption coefficient given by the X-rays Computerized Tomography. This is a promising method to measure the crack opening from the X-rays projection normal to the crack surface. The fundamental formulae for the crack opening measurement have been firstly described as well as the necessary regional averaging technique and the data processing for measurement of accuracy to micron meters, and subsequently the crack model experiments using a couple of half-moon aluminum plates has been conducted to examine the accuracy and reliability of the method. As a case example of the Crack Projection Method, the Short Rod testing has been presented and discussed,that is a suggested testing method of the International Society for Rock Mechanics for determining the fracture toughness of rock. Crack growth characteristics with pull-loading has been described by showing the 2-dimensional images of the X-rays Computerized Tomography. The permanent opening displacement of the extended crack has been successfully examined by the subtraction of the crack opening profiles measured before and after the Short Rod testing, and the elastic opening of the extended crack has been also examined by analyzing the change of the crack opening profile by reloading. Moreover, the dimension of the fracture process zone has been estimated as well as the permanent strain in this zone.

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The fundamental governing equation for the transient flow in homogeneous and isotropic poroelasticity has been used to obtain the permeability and storage coefficient of specimen under axi-symmetric triaxial stress state. Rigorous analytical solutions of Constant Head, Falling Head, Transient Pulse and Constant Flow Pump Tests are presented. They were also adopted to compare the distributions of hydraulic head and hydraulic gradient within the specimen among the four permeability tests. It was found that the time-dependent variations of hydraulic head and hydraulic gradient within specimen are rather complicated. Three dimensional storage coefficient (S') of homogeneous porous media is greater than the specific storage (S). Special attention should be paid when estimating the storage coefficients because different storage coefficients are defined by different boundary conditions based on the poroelasticity.

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Compression and Brazilian tests were carried out under confining pressure without sealing the specimen in order to know the effect of pore water pressure on the strengths of Ogino tuff and Kimachi sandstone. Main results are summarized as follows: (1) The maximum differential stress and tensile strength of Kimachi sandstone when load was applied immediately after setting confining pressure were almost the same as those of under atmospheric pressure. On the other hand, the maximum differential stress and tensile strength of Ogino tuff increased with confining pressure. (2) Unconfined tensile strength of Ogino tuff was not influenced by the confining pressure which was applied and unloaded before the test. Namely, the increase in tensile strength of Ogino tuff, described in (1), was not due to permanent change of the microstructure. (3) In the case of Ogino tuff, the maximum differential stress decreased with the exposed time of confining pressure, and the tensile strength approached to the value obtained under atmospheric pressure with the exposed time. (4) In Ogino tuff, tensile strength parallel to the bedding plane was greater than that vertical to the bedding plane in dry conditions. However, in wet conditions, tensile strength parallel to the bedding plane was smaller than that vertical to the bedding plane. (5) The maximum differential stress and tensile strength of Ogino tuff were smaller in 1N NaCl solution than those in distilled water. Furthermore, the anisotropy in tensile strength observed in distilled water disappeared in the 1N NaCl solution.

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The triaxial compression test was conducted for Akiyoshi marble, Shirahama sandstone and Berea sandstone with the three kinds of confining pressure, 15 MPa, 25 MPa and 35 MPa. During the test, the radial displacement of the rock specimen was measured by the twelve displacement meters of cantilever type set around the cylindrical rock specimen in every 30° at the center cross section. In order to understand the behavior of the radial deformation with increasing the axial strain in the post-failure region, the measured radial deformation was analyzed by decomposing it into four modes of the deformation. They were isotropic deformation, anisotropic deformation, parallel movement of the specimen and sliding along the generated fault plane. Consequently, for the different rock types and for the different confining pressures, it was cleared that the major deformation mode changed from isotropic deformation to sliding along the generated fault plane, and the point when the faulting is initiated was also cleared. Furthermore, it was cleared that the relationship between the maximum radial strain and the axial strain is almost linear in the post-failure region.

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Physical properties of rock, such as strength, elastic wave velocities and permeability, depend largely on microcrack geometry. In particular, anisotropy of these properties is intimately related to the distribution of microcrack orientations. In this paper, an estimating method for three-dimensional distribution of microcrack orientations in rock is introduced by using the associated Legendre function. If a microcrack is expressed by a penny shaped disk, the distributions of crack orientations and diameters are expressed using probability density functions respectively and the intersection of cracks with a plane is geometrically modeled. Based on the model, the density function of three-dimensional orientations is characterized by observing traces of cracks, which have two-dimensional orientations, on some surfaces of a rock. A microscopic examination using a digital microscope under ultraviolet light is employed for visualization of microcracks on Inada granite filled with methylmetaacrylate combined with fluorescent paint. After a few steps of image processing, line elements on the digital images are automatically extracted as traces of cracks using the Segment Tracing Algorithm that is a lineament identification method. Once line elements are obtained, it is easy to characterize their two-dimensional orientations. An observation equation is constructed by using these two-dimensional orientations on three orthogonal surfaces of the granite, and then the probability density function of three-dimensional microcrack orientations is obtained by solving the equation. P-wave velocities in the Inada granite are also measured in the study. Anisotropy of the P-wave velocity is in harmony with the distribution of microcrack orientations. The orientation of the rift plane of the granite approximately agrees with the primary orientation of microcracks, while there is no obvious difference between grain and hardway planes.

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In underground construction it is important to obtain a discontinuities' model of rocks for simulations to evaluate rock stability around the working space and fluid flow in the rocks. In this study, an uniaxial compression test on a prism-shaped specimen of granite was conducted to develop a more feasible modeling method of cracks in rocks based on AE (acoustic emission) monitoring data and the fractal properties of the AE source distribution. 1. From this experiment, it was found that the AE source distribution was fractal and that the relationship between amplitude and frequency of AE was also fractal. 2. Crack distributions in the specimen and on the surface were both fractal. 3. A 3-D crack distribution model was constructed using AE monitoring data obtained from the experiment. In this model, the crack was described by a disk, and the fractal dimension of sectional lines that appeared on the surface of a specimen was adjusted to coincide with the dimension of lineaments extracted from a real crack image of the same surface of the specimen by correcting the AE parameters. That is, the relative AE energy for each event were altered iteratively until both fractal dimensions were matched. It is thought that this new modeling method for cracks in rocks can be directly applied to field studies of underground construction in the future.

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The behavior of rock in tension is considered to be as important as in compression for designing underground structures, especially large ones. But few results of tensile tests have been reported. The servo-controlled testing machine was used in this study, and uniaxial tension tests were conducted on six rocks (one granite, one andesite, one tuff and three sandstones). Complete stress-strain curves were successfully obtained, and compliance and irrecoverable strain were evaluated by unloading-reloading test. The main results can be summarized as follows; 1) Compliance increases through the whole process. But in the pre-failure region, the increase rate is relatively slight. On the other hand, compliance in the post-failure region increases rapidly with decrease of the load bearing capacity. 2) Irrecoverable strain is closely correlated with compliance change, i. e. with crack growth. The failure process of rock was discussed in detail based on the evaluation of compliance as well as irrecoverable strain. 3) The shape of stress-elastic strain curves in uniaxial tension tests are similar to those in uniaxial compression tests.

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