Yuzo Obara

ISRM-Suggested Method for Determining the Mode I Static Fracture Toughness Using Semi-Circular Bend Specimen

The International Society for Rock Mechanics has so far developed two standard methods for the determination of static fracture toughness of rock. They used three different core-based specimens and tests were to be performed on a typical laboratory compression or tension load frame. Another method to determine the mode I fracture toughness of rock using semi-circular bend specimen is herein presented. The specimen is semi-circular in shape and made from typical cores taken from the rock with any relative material directions noted. The specimens are tested in three-point bending using a laboratory compression test instrument. The failure load along with its dimensions is used to determine the fracture toughness. Most sedimentary rocks which are layered in structure may exhibit fracture properties that depend on the orientation and therefore measurements in more than one material direction may be necessary. The fracture toughness measurements are expected to yield a size-independent material property if certain minimum specimen size requirements are satisfied.

Visualization of failure in a pull-out test of cable bolts using X-ray CT

Abstract The patterns of failure in a pull-out test are visualized by an X-ray CT scanner to supply the basic data on the bond failure of rock bolts and cable bolts. In the pull-out test, a comparison was made between two types of cable bolts and rock bolts. From the results, it is shown that the microscopic failure mechanism is classified as the different failure mode. Then the supporting effect of rock bolts and cable bolts are discussed.

Measurement of local stress and estimation of regional stress associated with stability assessment of an open-pit rock slope

This paper discusses the concept of a new methodology for rock slope stability assessment. Then, results on rock stress measurement using the compact conical-ended borehole overcoring (CCBO) technique at Torigata limestone mine in Japan are presented. A procedure for back analysis of the regional strain and stress field with the 3-D finite element method, using the measured local stress, is suggested and demonstrated successfully in relation to Torigata limestone mine. Finally, to estimate the state of stress at the mine excavation level, 3-D finite element analyses were performed using boundary conditions from the analyzed regional strain and stress field. It is shown that the horizontal stress at the present excavation level is not reduced, and that the horizontal stress component cannot be disregarded in estimating the stability of rock slopes at this location, even though the mine is located near the top of a mountain.

Estimation of Fracture Toughness of Anisotropic Rocks by Semi-Circular Bend (SCB) Tests Under Water Vapor Pressure

In order to investigate the influence of water vapor pressure in the surrounding environment on mode I fracture toughness (KIc) of rocks, semi-circular bend (SCB) tests under various water vapor pressures were conducted. Water vapor is one of the most effective agents which promote stress corrosion of rocks. The range of water vapor pressure used was 10−2 to 103 Pa, and two anisotropic rock types, African granodiorite and Korean granite, were used in this work. The measurement of elastic wave velocity and observation of thin sections of these rocks were performed to investigate the microstructures of the rocks. It was found that the distribution of inherent microcracks and grains have a preferred orientation. Two types of specimens in different orientations, namely Type-1 and Type-3, were prepared based on the anisotropy identified by the differences in the elastic wave velocity. KIc of both rock types was dependent on the water vapor pressure in the surrounding environment and decreased with increasing water vapor pressure. It was found that the degree of the dependence is influenced by the orientation and density of inherent microcracks. The experimental results also showed that KIc depended on the material anisotropy. A fracture process was discussed on the basis of the geometry of fractures within fractured specimens visualized by the X-ray computed tomography (CT) method. It was concluded that the dominant factor causing the anisotropy of KIc is the distribution of grains rather than inherent microcracks in these rocks.

Comparison of stresses obtained from Acoustic Emission and Compact Conical-Ended Borehole Overcoring techniques and an evaluation of the Kaiser Effect level

The purpose of the study was to compare stress values obtained from Acoustic Emission and Compact Conical-Ended Borehole Overcoring stress measurement techniques applied at an underground limestone mine in Japan, and to investigate the effect of rock anisotropy and confining pressure on the Kaiser Effect level which has been used to determine in situ stresses by the Acoustic Emission technique. Initially, Acoustic Emission tests were carried out on limestone cores extracted from horizontal boreholes in a pillar and hanging wall in the underground mine. The stress values obtained were two or three times greater than those obtained by the Compact Conical-Ended Borehole Overcoring method. In the second stage, the anisotropy of a granite block was determined by P-wave measurements. Core specimens extracted from two different directions were pre-loaded under axisymetric triaxial conditions by applying a series of differential stresses. The cores were then re-loaded under uniaxial conditions and the Kaiser Effect levels were determined. It was concluded that both the anisotropy and confining pressure have an important influence on the Kaiser Effect level.RésuméLe but de l’étude était de comparer les valeurs de contraintes obtenues à partir de la technique de mesure par Emission acoustique (AE) et une technique de Sur-carottage (CCBO), mises en œuvre dans une mine souterraine de calcaire au Japon. De plus, les effets de l’anisotropie de la roche et de la pression de confinement sur l’effet Kaiser défini dans les techniques d’émission acoustique ont été étudiés. Initialement les essais d’émission acoustique ont été réalisés sur des carottes de calcaire issues de forages horizontaux réalisés dans un pilier et une couche de toit de la mine souterraine. Les valeurs de contraintes obtenues étaient deux ou trois fois plus grandes que celles obtenues par la méthode de sur-carottage. Dans une deuxième étape, l’anisotropie d’un bloc de granite a été déterminée par des mesures de vitesses d’ondes P. Des échantillons carottés suivant deux directions différentes furent pré-chargés sous conditions triaxiales symétriques en appliquant différents niveaux de contraintes. Les échantillons furent re-chargés sous conditions uni-axiales et les valeurs d’effet Kaiser déterminées. On a conclu que l’anisotropie et la pression de confinement avaient tous deux une influence importante sur les valeurs d’effet Kaiser.

Stress Intensity Factors of Semi-Circular Bend Specimens with Straight-Through and Chevron Notches

Semi-circular bend specimen is one of the useful test specimens for determining fracture toughness of rock and geo-materials. Generally, in rock test specimens, initial cracks are produced in two shapes: straight-edge cracks and chevron notches. In this study, the minimum dimensionless stress intensity factors of semi-circular bend specimen (SCB) with straight-through and chevron notches are calculated. First, using finite element analysis, a suitable relation for the dimensionless stress intensity factor of SCB with straight-through crack is presented based on the normalized crack length and half-distance between supports. For evaluating the validity and accuracy of this relation, the obtained results are then compared with numerical and experimental results reported in the literature. Subsequently, by performing some experiments and also finite element analysis of the SCB specimen with chevron notch, the minimum dimensionless stress intensity factor of this specimen is obtained. Using the new equation for the dimensionless stress intensity factor of SCB with straight-through crack and an analytical method, i.e., Bluhm’s slice synthesis method, the minimum (critical) dimensionless stress intensity factor of chevron notched semi-circular bend specimens is calculated. Good agreement is observed between the results of two mentioned methods.

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.

Rock stress measurements and the state of stress at an open-pit limestone mine in Japan

Abstract A series of rock stress measurements is performed to estimate the states of local paleostress and present (in situ) stress at the Torigata open-pit limestone mine in Japan using the calcite strain gauge (CSG) technique and the compact conical-ended borehole overcoring (CCBO) technique, respectively. A set of back and forward numerical analyses is then carried out to evaluate the states of regional and local present stresses and the mine-induced rock slope stability using a three-dimensional finite element model and the state of local present stress measured by the CCBO technique. The results of rock stress measurements from this and other studies suggest that the maximum principal stress direction in the study area has rotated counterclockwise from the southeast (NW–SE trend) to the northeast (NE–SW trend) since the late Cretaceous period up to the present time. The results of the back analysis show that the directions of the horizontal principal regional tectonic stresses obtained by the back analysis are in good agreement with those of the horizontal principal local in situ stresses measured by the CCBO technique. However, the horizontal regional tectonic stresses are more compressive than the horizontal local in situ stresses. It results from the fact that the horizontal regional stress due to gravity is not considered in the back-analyzed horizontal regional tectonic stress, but it is included in the local in situ stress measured by the CCBO technique. The results of the forward analysis show that the local stress obtained by the forward analysis, especially its horizontal components, is in good agreement with the horizontal local in situ stress measured by the CCBO technique, and that the magnitude of the vertical normal stress increases more rapidly than those of the horizontal normal stresses with depth. As a result, the ratio of the horizontal normal stress to the vertical normal stress is largest at the current excavation level (ground surface) and decreases with depth. It means that the stress field within the mine-induced rock slope is strongly affected by the horizontal components of the local in situ stress.

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.

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.