Katsuhiko Sugawara

The Strength of Rock under Water Vapor Pressure.

In order to investigate the environmental dependence on strength of rock, uniaxial compression test and Brazilian test under water vapor environment were conducted on Kumamoto andesite and Kitagishima granite. Tests were carried out under various water vapor pressures, which are controlled in special chambers, at a constant strain rate. The results obtained by the uniaxial compression test and Brazilian test are follows:The Youngs moduli are almost constant with the change of water vapor pressure. On the other hand, the water vapor pressure largely affects the uniaxial compressive strength and the tensile strength of rock. Namely, the strengths of rock increase with decreasing water vapor pressure.The relationships between uniaxial compressive strength Sc , tensile strength St and water vapor pressure p can be represents by the following equation: long Sc ∝ -Nc log p and long St ∝ -Nt log p, where Nc and Nt are the inclination of lines.Comparing the above equations based on the test results with equation (5), the stress corrosion indexes are obtained as 24 in Kumamoto andesite and 62 in Kitagishima granite on uniaxial compression test, then 58 in Kumamoto andesite on Brazilian test. It is discussed that the difference between the stress corrosion indexes obtained from uniaxial compression test and Brazilian test is caused by the stress state within rock specimen in each test. The estimation method of long term-strength of rock is shown according to equation (8), then the long-term strength of Kumamoto andesite and Kitagishima granite can be estimated concretely, that is, uniaxial compressive strengths of Kumamoto andesite and Kitagishima granite after 1000 years are estimated 75% and 89% of uniaxial compressive strengths at the present time respectively.

Measurement of in situ rock stress by hemispherical-ended borehole technique

Abstract The paper presents a new technique to measure the complete state of in-situ rock stress in a single borehole. The stress tensor is computed from 16 strains on the hemispherical bottom surface of a borehole. An arrangement of strain gauges to determine the stress tensor with high accuracy is described. Details concerning relevant equations to be used in practice, a spherical strain cell to bond the strain gauges directly to the bottom surface at the designated stations and the system for in-situ measurement by the stress relief technique are presented and discussed.

Analysis of the Effective Compliance. Homogenization analysis of multi joint set problem by the Stress Compensation-Displacement Discontinuity Method(SC-DDM). (2nd Report).

Theory and practice for the effective compliance analysis are presented and discussed, along with successful case examples, in order to make clear that the Stress Compensation-Displacement Discontinuity Method (SC-DDM) is an useful tool available for the homogenization problem of multi joint sets.The fundamental concept of the homogenization of joint sets and the SC-DDM, proposed in the previous paper, are firstly summarized, and it is shown that an infinite structure of rock mass is homogenized using a unit cell which is a representative structure of microscopic, and that the macroscopic characteristics is able to be defined and analyzed using the fundamental response of the unit cell computed numerically by the Displacement Discontinuity Method. Subsequently the stress compensation scheme available for the analysis of the effective compliance is proposed, showing successful case studies using a latticed model and an alternative model of a single open joint set. It is shown how the restrict stress acting on the unit cell can be removed to compensate the boundary stress condition required, and how the macroscopic strain of the unit cell can be computed from the displacement distribution on the sides of a representative unit cell. Additionally, the relation between the joint density and the effective compliance is discussed, comparing with the approximation by a parallel pillar model.

Mixed mode crack propagation of homogenized cracks by the two-dimensional DDM analysis

Abstract The crack propagation process in rock mass is numerically estimated by two-dimensional stress analysis. Rock mass typically contains numerous cracks within, with varying extent and orientation. In order to analyze the mechanical interaction of cracks, the homogenization method is introduced too in this study. In the analysis, SC-DDM (stress compensation-displacement discontinuity method) and DDCM (displacement discontinuity comparison method) are applied to the crack homogenization model, and the stress intensity factors are calculated. Then the crack propagation of the model is analyzed, using the maximum circumferential stress criterion. The influences of the mechanical interaction between cracks to the crack propagation is discussed.

Rock stress interpretations from Mt. Torigata (Japan) based on calcite strain gauge and differential strain curve analysis

Abstract In order to interpret stress history at the Torigata Limestone Mine (Japan), calcite strain gauge (CSG) and differential strain curve analyses (DSCA) were performed. The maximum principal shortening direction determined by CSG and DSCA is oriented to the NW–SE. However, the published values for the maximum principal directions from these two methods are different from the NW–SE-directed maximum principal compression direction determined by the overcoring method. Accordingly, the directions of in-situ rock stress calculated by DSCA are considered to reflect the paleostress directions. The results by DSCA reflect the direction of pre-existing cracks developed in Torigata limesone, not microcracks induced due to stress relief. This is probably due to shallow depths of sample collection, where stared elastic strains are immeasurable using the DSCA method. Considering the stress fields found in this study area, the maximum principal direction of the paleostress is consistent with the fracture system near the study area and the late Mesozoic and Paleogene movement of the Philippine Sea plate.

In-situ rock stress measurement at an open-pit limestone mine using the CCBO technique

In-situ rock stress measurement is performed to estimate the state of stress at the Torigata open-pit limestone mine in Japan using the compact conica-ended borehole overcoring (CCBO) technique. The significance of the state of in-situ rock stress to evaluate the stability analysis of rock slope is then discussed. The CCBO technique was applied to the two horizontal boreholes, Borehole BH-1 at location I and Borehole BH-2 at location II. As a result of in-situ rock stress measured from the two horizontal boreholes using the CCBO technique, the maximum horizontal principal stress is in the NNE-SSW direction at present. The measured average vertical normal stress (−6.50 MPa) for Boreholes BH-1 and BH-2 is in good agreement with the estimated average overburden stress (−6.60 MPa). The horizontal normal stresses σxx (−5.10 MPa) and σyy (−6.00 MPa) are 78.5–92.3% of the vertical normal stress σzz (−6.50 MPa) suggesting that large horizontal stresses exist in Mount Torigata. The ratio of the horizontal normal stress to the vertical normal stress is relatively large at the measured location. Accordingly, for an appropriate evaluation of the mining-induced rock slope stability, not only the vertical stress components but also the horizontal stress components component should be considered.

Specific Energy of Continuous Chipping upon Water Jet Drilling in Rock.

Specific energy of the chipping upon water jet drilling is discussed from a latticed multi-point drilling experiment, to make clear the utility of chipping as an excavation measure in hard rock. The experiment is conducted using the full-size specimen of Amakusa sandstone of the uniaxial compressive strength: SC = 84 MPa, and Granite of SC = 190 MPa. The optimum drilling interval is evaluated as well as the specific energy Es, after consideration to the relation between the crater depth: h and the excavation volume: V. It is clarified that the dimension ratio of m = SC / Es is in a range of from 0.40 to 0.55 in the case of the driving pressure: pI = 90 MPa. A model named the repetition model is proposed, to forecast the specific energy of the continuous chipping by the Third Power Method, and its validity is successfully verified by comparing the computation to the experimental results. Subsequently, it is discussed that the continuous chipping results in an excavation of higher efficiency than both of the pure water jet cutting and the abrasive water jet cutting.

Elastoplastic deformation around underground openings under biaxial initial stress field

Estimation of elastoplastic deformation around an underground opening induced by the excavation of it, especially displacement and strain field in plastic region, is presented in this paper, as well as the formulation for calculating the displacement and strain in the plastic region around the underground opening by the coupled Boundary Element Method - Characteristics Method (BEM-CM). In this method, the non-associated flow rule is adopted to calculate the displacement and strain field in the plastic region, which is determined by the integration of the displacement along characteristics lines under the boundary condition of the elastic displacement on an elastoplastic interface analysed. It is shown that this method is one of the accurate and effective methods for estimating not only the shape and extent of the plastic region but also the state of the displacement and strain in the plastic region around the underground opening, comparing the theoretical solution with numerical results by this method for a circular opening under hydrostatic initial stress condition. Furthermore, this method is applied to rectangular and horse-shoe shaped openings and the characteristics of the strain field in the plastic region are discussed.

Analysis of Mechanical and Hydraulic Properties of Cracked Structure by the Ratio of Crack Opening Dependency (RCOD)

Abstract The Ratio of Crack Opening Dependency (RCOD) was proposed as a new index to simultaneously evaluate the mechanical and hydraulic properties of cracked structure which contains many open cracks within. The RCOD is an index which is defined as the ratio of total amount of crack opening and the total deformation of cracked structure. The efficiency of RCOD as an index to evaluate the coupled problem was numerically analyzed by applying SCDDM (stress compensation-displacement discontinuity method) to the two dimensional multi crack model. It was shown that RCOD is an useful index which can directly evaluate the deformability and the permeability of the cracked structure without any parameters such as stresses or crack size. The possibility of application of RCOD to the field measurements was also discussed.

Analysis of Mode I Crack Opening Behavior of Homogenized Multi-crack by Two Dimensional Displacement Discontinuity Method.

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 Youngs 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 Youngs 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 Youngs modulus. When the cracks get closer to each other in the normal direction, the value of effective Youngs 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.