Compact Conical-ended Borehole Overcoring Research Articles

Re-Opening and Shut-in Behaviors under a Large Ratio of Principal Stresses in a Hydraulic Fracturing Test

In-situ stress measurements were carried out under a 290m overburden in the Kamaishi mine in Iwate Prefecture of Japan. The geology of the measurement site consists of granite. The Hydraulic Fracturing (HF) technique and the Compact Conical-ended Borehole Overcoring (CCBO) technique were applied in the same horizontal borehole. The three-dimensional stress state as measured by CCBO is explained below. The directions of the maximum principal stress (σ1) and the intermediate principal stress (σ2) are almost horizontal. The direction of σ1 is perpendicular to the borehole axis, and the direction of σ2 is parallel to the borehole axis. The direction of the minimum principal stress (σ3) is approximately vertical. In this case, σ1/σ3 ratio is about 5 which is considered to be beyond the scope of the application of the HF technique. To observe the behavior of re-opening and shut-in under a large principal stress ratio, we tried to apply the HF test under these conditions. While, the re-opening pressure could not be evaluated, the shut-in pressure was observed. The results confirmed that σ3 estimated by HF is substantially equal to the vertical stress evaluated by CCBO.

Stress State Monitoring in the Surroundings of the Roadway Ahead of Longwall Mining

Accurate knowledge of the stress-strain state of rock mass, not only in their vicinity but also in the wide surroundings of mine workings, is absolutely critical for precise support designing. Investigation of the rock stress is usually carried out by interpretation of the rock mass deformation processes, which can be relatively precisely observed and measured.In order to verify the stress state of the rock mass and changes in it induced by longwall mining, monitoring of changes in the rock mass stress in connection with the mine out of the longwall No. 371 202 was carried out. The seam extracted by monitored longwall has a thickness of approximately 2 m at a depth about 1100 m and lies within the Czech part of the Upper Silesian Coal Basin. Interpretation of the initial rock mass stress tensor and verification of its changes during longwall mining were the aims of this stress monitoring. A total of five probes were installed on the roof rocks of the main gate. Two compact conical-ended borehole overcoring probes were installed to obtain the pre-mining full stress tensor and afterwards three compact conical-ended borehole monitoring probes were installed to continuously monitor the stress state in the rock mass ahead of the advancing longwall. The monitored stress development contributes to our knowledge of stress distribution and its changes during excavation at great depth in multi-seam sedimentary deposits of the Upper Silesian Coal Basin.

Determination of Stress State in Rock Mass Using Strain Gauge Probes CCBO

The strain gauge probes of different construction are typically used for determination of stress state rock mass. The modified overcoring method known as the Compact Conical ended Borehole Overcoring method (CCBO) for stress state determination in rock mass was designed in Institute of Geonics of the CAS (IGN) in cooperation with Kumamoto University in Japan. The implemented adjustment of the overcoring method consists mainly in omitting the overcoring phase (stress relief phase). The probe is glued directly to the conically shaped end of a borehole. The data logger located within the conical probe enables continual strain monitoring directly in the conically shaped end of the borehohole during the overcoring procedure. The conical probe used to monitor stress changes, named Compact Conical ended Borehole Monitoring (CCBM), can continuously monitor rock strain changes in key locations due to mining.Many stress measurements using both strain gauge probes CCBO and CCBM were carried out in the last decade. These measurements were performed in varied rock mass adjacent to mine excavations. Most of the stress measurements were carried out in Carboniferous sedimentary rocks as part of the experimental work in the Czech part of the Upper Silesian Coal Basin (USCB). Several stress measurements were carried out during the mine development operations and associated geotechnical exploration work while constructing the Milasín – Bukov underground gas storage (BUGS) [1], as well as the Bukov Underground Research Laboratory (BURL) [2]. Both underground facilities were designed within Rožná and Olší uranium deposits situated on the north-eastern margins of the Strážek Unit consisting of the metamorphic rock formations. Several measurements were carried out in granitic environments (igneous rocks) as part of the international “Large-Scale Monitoring” (LASMO) project in Grimsel (Switzerland) and in Josef underground laboratory (Bohemian massif). The article presents the basic principles and the methodology of stress measurements in rock mass using strain gauge probes and the data analysis from the variable rock environments.

Rotation of principal axes and changes of stress due to mine-induced stresses

This research project comprises stress monitoring, primarily of the changes induced by longwall mining or destress blasting, which was realized in a mine of the Ostrava-Karviná Coalfields (Upper Silesian Coal Basin). Verification of the stress field response to stresses induced by different geological or mining factors was the main aim of the research project. Stress change monitoring in the region of the advanced longwall was carried out using seven stress probes. In all, seven boreholes were instrumented with compact conical ended borehole monitoring (CCBM) probes for continuous stress monitoring of the stress–strain changes due to mining and the destress blasting effect. Compact conical-ended borehole overcoring (CCBO) and CCBM probe installation were carried out before the start of the longwall mining. Data of the stress changes in all seven probes were recorded weekly and, after the longwall advanced, daily. Analysis of the stress changes was based on the idea of original stress distribution, which is based on long-term experiences in rock mass behavior in similar geological and mining conditions. For all probes, the magnitude of stress changes and principal axis direction changes were investigated. Analysis results to date indicate that stress changes ahead of the longwall face present not only the changes of stress magnitude, but the rotation of the axes of the stress components as well. Rotations of the principal axes emerged as very important factors of the stress tensor changes and their influence on rock mass deformation and stability of the workings. In addition, results show that the vertical to horizontal stress component ratio converges to 1 and in many cases horizontal stresses are higher than vertical ones. This of course relates to the impact of mine-induced stresses. This knowledge is basic for the purpose of prevention of rock burst because the actual stress state is fundamental to its proper and operative planning.

わが国で開発・改良された初期地圧測定法の基準化と国際化

The techniques of initial stress measurement were developed and improved in Japan. It is recommended to standardize some dominant methods in domestic market at first, and then encourage related organizations to promote them to international standard, in order to take an advantageous position of the methods in the international market. Currently, stress relief method and hydraulic fracturing method are considered to be valuable and practical methods. The compact conical-ended borehole overcoring method, one of the stress relief methods, was assigned as the standard of the Japanese Geotechnical Society in 2012. In addition, the new committee has been organized recently for standardizing the high stiffness testing method, an improved hydraulic fracturing technique. If international standard methods that are not the method used in Japan were assigned in future, we may be at a disadvantage for international bids. We may lose opportunities to apply our valuable technical resources such as engineers, instruments, established standards, and know-how in international market. And our techniques may become irrelevant in the international market. Our methods havebeen applied in many domestic foundation investigation fields such as construction of underground power plants, nuclear power plants, and large scale tunnels. It is in our best interest to have our techniques become the international standard as we expect an increase of requirement of those techniques in developing countries in future.

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.

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.

下向き円錐孔底ひずみ法による原位置応力測定

In-situ stress measurement by means of the Downward Compact Conical-ended Borehole Overcoring (DCCBO) technique, which one of the overcoring method, was carried out at Kamaishi mine to aim at practical use of this technique. Prior to in-situ stress measurement, measuring apparatus (conical bit, strain cell insertion tool and data logging equipment built-in tilt-azimuth meter) for HQ-size borehole were developed by improving apparatus for PQ-size borehole. Then, the measurement method based on the determination of strain coefficients depending on the estimation of the elastic properties of the adhesive under in-situ measured temperature was proposed.Performing in-situ tests, we verified that improved measuring apparatus for HQ-size borehole works well in a water-filled vertical borehole. Moreover, it was confirmed that the proposed measurement method could estimate the in-situ stresses relatively consistent with those by other measurement method near this test site. It was concluded that in-situ stresses measured by the proposed measurement method have sufficient reliability.

下向き円錐孔底ひずみ法の測定精度に及ぼす接着剤の影響と地圧評価の高精度化

By performing experiments in the laboratory, we verified how the measuring accuracy of the Downward Compact Conical-ended Borehole Overcoring (DCCBO) technique is affected by the time dependence of the mechanical characteristics of an adhesive to bond the strain cell together. In order to quantify the mechanical characteristics of the adhesive by the correct evaluation of experimental results, we determined the strain coefficients using numerical model including models of adhesive and strain cell.Consequently, we found that the best evaluation of experimental results measured two hours after affixing the strain cell, was achieved when Young's modulus was 1.45 MPa and Poisson's ratio was 0.00. We also demonstrated that proposed method was capable of highly accurate rock stress evaluation.Performing in-situ tests, we clarified the requirements for the general characteristics of the adhesive and the strain cell (epoxy section). Young's modulus of adhesive and strain cell (epoxy section) should be set to about 100 MPa for moderately hard or hard rocks having a Young's modulus of about 10 GPa or greater and to about 10 MPa for soft rocks having a Young's modulus of about 1.0 GPa.

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.

深部地圧計測のための下向き円錐孔底ひずみ法の開発と室内実証試験

Accurate determination of rock stresses at great depth is one of the most important subjects that should be resolved in the fields such as deep geological disposal of nuclear fuel waste and underground sequestration of carbon dioxide. To determine complete three-dimensional in situ stress at great depth with a high accuracy, the Downward Compact Conical-ended Borehole Overcoring (DCCBO) technique, which is one of stress relief methods, was developed. DCCBO technique can be applied to a water-filled vertical/sub-vertical borehole. The main features of this technique are as follows: 1) This technique is applicable to a water-filled borehole by using an appropriate adhesive.2) A diving-bell-type bonding device keeps a strain cell dry until the moment of installation.3) Just before the strain cell reaches the bottom of the borehole, high-pressure nitrogen gas cleans the borehole bottom, where cuttings may exist.4) The process described above proceeds within a PQ bit which overcores the strain cell.5) The data during the overcoring is recorded by a multi-channel data logger which is installed at the top of the strain cell.6) A system consisting of a small video camera and compass for single shots measure the direction of the strain cell.In laboratory test, a measuring system consisting of a bonding device, a cleaning system and a multi-channel data logger were verified to work well in a water-filled borehole with cuttings on the bottom. Reasonable results were able to be obtained by measurement sensitivity test using a specimen of rock block. Thus, we confirmed the usability of DCCBO 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.

Updating the use of the CCBO cell in Japan: overcoring case studies

Abstract The Compact Conical Ended Borehole Overcoring (CCBO) device for stress measurement was the subject of an ISRM suggested method published in 1999 (Int. J. Rock Mech. Min. Sci. 36 (3) (1999) 307). In this paper, recent case studies using the CCBO device are described. The six case examples involve both natural and perturbed stress states—near a fault, within a mountain, around a tunnel and cavern, over a wide region, and in a high stress field. The use of the measurement results and the characteristics of the states of stress were clarified successfully from these measurements.

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.

円錐孔底ひずみ法におけるコアディスキング現象に関する基礎的研究

Core disking is sometimes observed in high stress region where rock stress measurements are performed by the Compact Conical-ended Borehole Overcoring (CCBO) Technique, which is one of the stress relief methods. When the tensile stress induced on the borehole bottom surface becomes larger than the tensile strength of the rock, a failure from the borehole bottom surface will be imitated. Therefore, if the stress changes on the borehole bottom surface can be analyzed during overcoring, the rock stress prior to boring may be estimated.In this paper, assuming that the tensile stress on a borehole bottom surface induced due to overcoring is one of factors of generating core disking, the mechanism of the core disking in the CCBO is discussed as well as the location of failure initiation and the type of failure. Firstly, the characteristics of core disking in the CCBO is shown. Secondly, simulating the overcoring of the CCBO under three dimensional initial stress state in which the axis of a principal stress coincides with that of a borehole, the failure initiated on the borehole bottom surface during the overcoring is discussed, based on the results calculated by a semi-analytical boundary element process for axisymmetric elasticity with arbitrary boundary element conditions. From the results, the criterion of core disking is suggested and the applicable limitation of the CCBO is made clear. Furthermore, the X-ray CT method is applied to the recovered core with core disking, and then the state of failure within the core is visualized. Finally, it is discussed that the rock stress prior to boring is estimated by the suggested criterion of the core disking, comparing the neighboring rock stress measurement and tensile strength obtained from cores.

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.