Compact Conical-ended Borehole Overcoring Technique Research Articles

円錐孔底ひずみ法の直交異方性岩盤への適用

Stress-measurement methods based on the overcoring method assume that the rock mass is linearly elastic, isotropic, continuous, and homogeneous. However, a rock mass is actually anisotropic to some degree. In this study, we carried out theoretical, numerical and experimental studies to apply the Downward Compact Conical-ended Borehole Overcoring technique to an orthotropic rock. Numerical experiments with the use of a 3D-FEM analysis were conducted to confirm the applicability and efficiency of the proposed measurement theory.The results showed that the measurement includes a non-negligible error if we do not consider the anisotropy of rock when such anisotropy is strong.For example, the isotropic assumption method gives an error of more than 20 % in the estimation of stress when the degree of anisotropy in the direction of the maximum applied stress exceeds 20 %. On the other hand, the error is less than 5 % in the orthotropic assumption method. Finally, laboratory experiments for four kinds of orthotropic rocks using a true-triaxial compressive apparatus were carried out to verify the efficiency of the proposed measurement theory. These experiments also verified that the proposed method is suitable for use in orthotropic rock.

Evaluation of core disking rock stress and tensile strength via the compact conical-ended borehole overcoring technique

To evaluate core disking rock stress and tensile strength via the compact conical-ended borehole overcoring (CCBO) technique, the mechanism of core disking by means of the CCBO technique is discussed. It is simulated that overcoring of the CCBO under three-dimensional initial stress state coincides with borehole axis and is one of principal directions. The characteristics of induced stress and initiated failure due to tensile stress on the borehole bottom surface during overcoring are discussed. They are based on the results calculated by a semi-analytical boundary element process for axisymmetric elasticity with arbitrary boundary element conditions. From the results, an initiation criterion of core disking is suggested. It is shown that the failure is initiated from outside surface of core and that the failure initiation criterion on the longitudinal tensile stress outside of core can define the criterion of core disking. Secondly, since tensile strength of rock is closely related to core disking, an estimation procedure of it is suggested using strains during compact overcoring. It is shown that representative rock volume for tensile strength estimated by the filed measurement data is smaller than that by Brazilian test, and that the fracture initiation criterion may estimate rock stress using tensile strength obtained by Brazilian test with small volume. Finally, an estimation procedure of rock stress is suggested by means of core disking phenomenon on the CCBO technique.

Measurements of induced stress and strength in the near-field around a tunnel and associated estimation of the Mohr–Coulomb parameters for rock mass strength

Abstract The state of induced stress measured by the compact conical-ended borehole overcoring technique in the immediate roof of an approach tunnel excavated under high rock stress is described. During the measurements, core disking was observed. An X-ray Computed Tomography (CT) scanner was used to select strain data uninfluenced by the core disking; then the induced rock stress was estimated from selected strain data. From these results, it is shown that the non-destructive investigation using X-ray CT is effective for visualization of the fracturing within cores and the selection of strains measured during overcoring. Furthermore, the Mohr–Coulomb failure criterion parameters for the rock mass were estimated by comparing the measured stresses with the shear strength of in situ rock and the uniaxial compressive strength determined in laboratory tests.

Clarification of regional and local in situ stresses using the compact conical‐ended borehole overcoring technique and numerical analysis

Abstract Stress measurement is performed to estimate the states of in situ rock stress at the Torigata open‐pit limestone mine in Japan using the compact conical‐ended borehole overcoring (CCBO) technique. A set of back and forward analyses are then carried out to evaluate the states of regional and local in situ rock stresses and the mine‐induced rock slope stability using a 3‐D finite element model. The maximum horizontal local in situ rock stress measured by the CCBO technique acts in the northeast–southwest direction. The horizontal regional tectonic stresses obtained by the back analysis are in good agreement with those of the horizontal local in situ rock stress measured by the CCBO technique. However, the horizontal regional tectonic stress is more compressive than the horizontal local in situ rock stress. This is because 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 rock stress measured by the CCBO technique. The local stress obtained by the forward analysis, especially its horizontal components, is in good agreement with the horizontal local in situ rock stress measured by the CCBO technique, and 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 nearest excavation level and decreases with depth. This means that the stress field within the mine‐induced rock slope is affected by the horizontal components of the local in situ rock stress.

Estimation of horizontal regional stresses at an open-pit limestone mine by back analysis

A set of back and forward analyses is carried out to evaluate the states of regional and local in-situ rock stresses and the mining-induced rock slope stability using a three-dimensional finite element model and the state of local in-situ rock stress measured by the compact conical-ended borehole overcoring (CCBO) technique. 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 rock stress measured by the CCBO technique. The magnitudes of the horizontal regional tectonic stress are more compressive than those of the horizontal local in-situ rock stress. This presumably results from the fact that the horizontal regional stress due to the gravity is not considered in the back-analyzed horizontal regional tectonic stress while it is included in the local in-situ rock stress measured by the CCBO technique. The local stress obtained by the forward analysis is also in good agreement with the horizontal local in-situ rock stress measured by the CCBO technique, and the magnitudes of the vertical normal stress increase more rapidly than those of the horizontal normal stresses with depth. The ratio of the horizontal normal stress to the vertical normal stress is largest at the current excavation level and decreases with depth. This suggests that the stress field within the mining-induced rock slope is affected by the horizontal components of the local in-situ rock stress.

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

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.