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Abstract The stress measurements determined by both the overcoring (OC) and hydraulic fracturing (HF) methods in the Shuichang iron mine and Sanshandao gold mine were compared and evaluated, respectively. The results indicate that the independent OC and HF data in the two mines reveal the same dominant faulting stress regime. The σ H orientations derived from the OC and HF methods in the Shuichang iron mine are dominantly oriented in the N81.1°W–N89.4°W and N77.0°E–N88.0°E, respectively, and the σ H orientations yielded from the OC and HF techniques in the Sanshandao gold mine are predominantly in the N30°W–N90°W and N55.5°W–N60.4°W, respectively; hence, the σ H orientations obtained by the two different methods in the two mines are comparatively similar. In addition, the shapes of the probability density diagrams using an improved Bayesian regression approach of the three principal stresses measured by the OC and HF methods in the same mine are quite similar, and all the obtained Kolmogorov–Smirnov test p-values are larger than the selected significance level of 0.01, indicating that the stress data interpreted by the two methods approximately follow the same distribution law. Thus, the performance of the two techniques and the reliability of the measured data are satisfactory.

Adopting 47 sets of in situ stress measurements taken at depths ranging from 417 to 1123 m in a coal mining district in central China by the overcoring method, we characterize the magnitude and orientation of the current stress field and its relationship to tectonism in this district to determine the controls on in situ stress in underground coal energy exploration and seismogeology. The measurements reveal that this district is in a relatively high horizontal stress environment. The stress regimes are generally favorable for strike-slip faulting and reverse faulting and are characterized by σH> σv> σhand σH> σh> σv, revealing that the regional stress field is absolutely dominated by horizontal tectonic stress and reflects a typical tectonic stress field type. The maximum horizontal principal stress is predominantly oriented between the ENE–WSW and WNW–ESE directions and averages nearly E–W,​ which is closely related to the tectonic plate movement and crustal deformation and agrees well with the direction derived from focal mechanism solutions and other stress indicators in and around the region. The current stress field in this district generally inherited the characteristics of the tectonic stress field in the third stage and partially retained the traces generated by the tectonic stress field in the first and second stages. In addition, the frictional parameter μm, which is calculated using the derived stress data, is used as a strong predictor to reflect the capability of the level of crustal stress accumulation. This region is in a state of critical or subcritical equilibrium, and regional faults at shallow depths have a medium to slightly high frictional strength, indicating that some well-oriented faults may theoretically approach or reach frictional equilibrium and that frictional sliding may occur.

The contemporary stress field in and around a gold mine area (GMA) and its seismological implications were investigated based on overcoring stress measurements and focal mechanism solutions. The results indicate that the stress level of the GMA appears to be relatively high, and stress states are characterized by reverse and strike-slip faulting stress regimes, which vary somewhat with depth. The stress field is governed by the maximum horizontal stress, which is well oriented along the WNW–ESE or approximately E-W directions, revealing a uniform stress direction. This dominant stress orientation is identical to the regional tectonic stress field interpreted by various independent geological indicators, and the stress field is mainly caused by the combined actions of far-field, near-field, and local forces. The stress conditions suggest that this region is in the subcritical equilibrium state, so frictional sliding probably occurs along the pre-existing and cohesionless faults that are optimally oriented to the stress field. Moreover, the ratio of the maximum shear stress to the effective mean principal stress (μm) values of 0.3 to 0.5 can be used as the lower and upper critical bounds for future geodynamic-related prediction analysis in and around the GMA and other possible regions. [ABSTRACT FROM AUTHOR]

There are various methods to determine in situ stress parameters, but each method is having its own advantages and limitations. Among the methods available, the hydraulic fracturing method is the most commonly adopted procedure for in situ stress measurements because of its simplicity, reliability and economic viability. But, the limitations with this technique are that some assumptions implicit for this method such as that the rock mass is continuous and elastic. Moreover, the reliability and validity of this method becomes questionable in porous rocks that may encounter in underground tunnels. These limitations are experienced ever since the introduction of this method, especially in porous rocks. In general, the recognition of fracture initiation in the hydraulic fracturing test has not proved difficult. The relatively slow rates of pressurisation have ensured that when fracture initiation occurs, the sudden increase in volume has led to a marked drop in pressure in the porous section, which is easily recognised from the pressure record (Sjoberg et al. in Int J Rock Mech 40:999–1010, 2003). But the drop-in pressure in a porous rock is far more difficult to recognise. This is since pressure cannot be developed if the rate of leakage in the formation is equal to or higher than the flow rate applied for fracture initiation. This problem of non-generation of water pressure can be tackled by use of a high viscosity fluid. Stress measurement were conducted by hydraulic fracturing method by using high viscous liquid in porous rocks. The stresses evaluated by this technique were correlated with stress measured by overcoring method. The stress measured by overcoring method was used as bench-mark as this method does not suffer from the presence of porosity of the rock. This new technique will be helpful in conducting the stress measurements in porous rocks, which will be highly beneficial to both mining and hydropower related excavation. [ABSTRACT FROM AUTHOR]

The in-situ stress is a crucial parameter in rock engineering. CSIRO in-situ stress measurements are one of the four suggested direct measurement methods and have been widely used worldwide. With the development of exploration and excavation in deep earth, temperature disturbances during overcoring and the nonlinear behavior of rock under high stress levels increase the error of CSIRO in-situ stress measurements. To satisfy the requirements of in-situ stress measurements in depth, a new local digital wireless CSIRO cell was developed and is introduced in this paper. The twin temperature compensation method is illustrated, and deviations based on several data processing methods are compared as well. The thermal output of a rock-gauge set is discussed, and a series of self-compensating strain gauges are selected for the manufacturing of the CSIRO cell. Two sites for in-situ stress measurements were utilized at the Sanshandao gold mine (795 m deep) and Lilou iron mine (300 m deep). Strains resulting from stress relief are analyzed, and temperature deviations occurring in the sensor system and data logging system are calculated by the twin temperature compensation method. To eliminate the theoretical limitation of the linear behavior assumption, nonlinear relations of the mean stress vs. bulk and shear modulus are proposed for the biaxial pressurization test. A high confining pressure apparatus was developed to reach a similar stress level as deep rock; in addition, associated equations to describe nonlinear features of rock core are deduced. Biaxial testing at high pressure was conducted, and both linear and nonlinear fits were applied to overcoring cores of the Sanshandao and Lilou mines. The results show that the proposed nonlinear method achieves an extremely good approximation and higher precision. [ABSTRACT FROM AUTHOR]

An extensive campaign to characterize rock stresses on the decameter scale was carried out in three 18-24 m long boreholes drilled from a tunnel in foliated granite at the Grimsel Test Site, Switzerland. The survey combined stress relief methods with hydrofracturing (HF) tests and concomitant monitoring of induced seismicity. Hydrofracture traces at the borehole wall were visualized with impression packer tests. The microseismic clouds indicate sub-vertical south-dipping HFs. Initial inversion of the overcoring strains with an isotropic rock model yielded stress tensors that disagreed with the HF and microseismic results. The discrepancy was eliminated using a transversely isotropic rock model, parametrized by a novel method that used numerical modelling of the in situ biaxial cell data to determine the requisite five independent elastic parameters. The results show that stress is reasonably uniform in the rock volume that lies to the south of a shear zone that cuts the NNW of the study volume. Stress in this volume is considered to be unperturbed by structures, and has principal stress magnitudes of 13.1-14.4 MPa for σ1, 9.2-10.2 MPa for σ2, and 8.6-9.7 MPa for σ3 with σ1 plunging to the east at 30-40°. To the NNW of the uniform stress regime, the minimum principal stress declines and the principal axes rotate as the shear zone is approached. The stress perturbation is clearly associated with the shear zone, and may reflect the presence of more fragmented rock acting as a compliant inclusion, or remnant stresses arising from slip on the shear zone in the past. [ABSTRACT FROM AUTHOR]

• Relationship between cross-sectional borehole deformation parameters and geo-stress is established. • Borehole diametral deformation gage (BDD gage) is developed for measuring cross-sectional borehole deformation. • A novel geo-stress determining method incorporating borehole diametral deformation measurement and overcoring is proposed. In deep energy engineering such as coal mining, oil & gas extraction, underground hydropower projects and deep buried water tunnels, rock failure and large deformation hazards often occur due to the complicated geological conditions and high geo-stress. Knowledge of the geo-stress state is an essential content in deep rock engineering. The complex environment of high temperature and high hydraulic pressure in deep rock mass makes geo-stress measurement more difficult than shallow strata. In this study, a novel method for geo-stress measurement based on the cross-sectional borehole diametral deformation is proposed. A borehole diametral deformation gage (BDD gage) is developed based on the measurement method. The scheme design of multi-contacts and micro-optical imaging avoid the influence of the high temperature and high hydraulic pressure environment. The results of the calibration experiment show that the measurement accuracy of BDD gage reaches 3 μm. Incorporating with the wireline coring tool for rapid overcoring, geo-stress measurement in deep vertical borehole via stress relief is realized. Field test procedure is summarized and application is carried out in a deep borehole of limestone stratum in Guizhou Province, China. Measurement process is verified to be rapid and simple. Measurement results including magnitude and direction of principal stresses are consistent with the results from hydraulic fracturing method. Special discussions were conducted on the measurement error of BDD gage and the improvement design of wireline coring tools. This article provides new ideas for geo-stress measurement in deep borehole. [ABSTRACT FROM AUTHOR]

This paper presents a back analysis procedure for identification of the elastic parameters of transversely isotropic rock cores, containing an overcoring triaxial strain probe, from the strains measured during a biaxial test. A three-dimensional finite element model was developed to simulate the biaxial test on the overcored rock specimen and to compute the strains at the location of the strain gauges. Different optimisation algorithms were tested and the most suitable one was selected. The back analysis procedure was tested for identification of the five elastic parameters and the two orientation angles that characterise a transversely isotropic rock core. Despite that, with the developed methodology, convergence was reached and all those parameters could be identified, sensitivity analyses demonstrated that the results obtained were not stable, and therefore, they were not reliable. By introducing constrains based on common practice and previous experience, a stable and robust methodology was achieved: the three elastic parameters, E , E and ν , are reliably identified using the value of G calculated with Saint-Venant expression and a fixed value of ν , while the orientation parameters are obtained from observation of overcored rock. Analysis of the results shows that application of this methodology represents an enormous step forward when compared with the traditional use of isotropy. Besides, the methodology is general and can also be used with other types of overcoring equipment. The five elastic parameters and the two orientation angles obtained can then be used, together with the overcoring strains, to compute the complete in situ state of stress. [ABSTRACT FROM AUTHOR]

In this work, we present the results of a new in situ experiment to complete the existing scientific dataset on well integrity in the context of CO 2 storage. This experimentation has been designed to evaluate the sealing behaviour of a monitored well after mechanical and chemical stresses due to pressure and temperature changes ( stage A ) and due to the exposure to carbonated brine ( stage B ), before a final overcoring stage for retrieving the well system and the surrounding clay. The stage A has been the subject of a first publication (Manceau et al., 2015; Water Resour. Res., 51, 6093–6109) and the two latter stages are described in this paper. Multidisciplinary methods (hydraulic tests and modelling, fluid sampling and modelling, analysis of cement and clay samples on the overcore) are used to get better insight, in a realistic wellbore context, on the interplay between the geochemical questions, and the operational and construction issues. In particular, this study shows that when good integrity pre-exists before a well is in contact with carbonated water, the exposure to dissolved CO 2 does not seem to lead to a degradation of the well hydraulic properties but rather to their improvement. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Abstract: An anisotropy in the elastic moduli values of intact rock with a ratio of more than 1.3–1.5 has been reported to have an effect on the calculated magnitudes and orientations of the in situ principal stress components as measured by the overcoring method. Work related to the on-going site investigation for a deep radioactive waste repository at the Olkiluoto site in Western Finland has shown that the migmatic mica gneiss is anisotropic which could therefore affect the interpretation of overcoring stress measurement results. This paper includes a summary of the theory of anisotropy concerning the intact rock moduli via the strain compliance matrix, a description of the core sample testing methods, and interpretation of results for the migmatic mica gneiss from two site investigation boreholes. In this case study, 19 specimens were tested and the results showed a modulus anisotropy of about 1.4. Because such anisotropy is high enough to produce significant errors in the estimation of the in situ principal stresses, it is recommended to take this into account in the interpretation of the stress measurement results, both in the context of the current work in Finland and in other projects where similar anisotropy is encountered. [Copyright &y& Elsevier]

Abstract: This paper is the second dealing with measurement-related uncertainties of overcoring data undertaken at the Äspö Hard Rock Laboratory and focuses on the biaxial test data from CSIRO HI overcore samples. The first paper dealt with measurement-related uncertainties in connection with the overcoring phase [1]. The uncertainties identified in connection to biaxial tests of CSIRO HI overcore samples include too large applied pressures and poor sampling frequency. At the Äspö HRL, the results yield that most overcore samples fractured during biaxial testing, meaning that a significant part, 56%, of available strain gauge combinations were removed from calculations of the elastic parameters. Remaining strain gauge combinations indicate average values of 62±5GPa for Young''s modulus and 0.25±0.01 for Poisson''s ratio, which are considerably lower than previously published values [2–5], and are in good agreement with results from biaxial tests on Borre Probe overcore samples [6–10]. The stress calculations were obtained from re-analyzed elastic parameters and strains, and show primarily a reduction in stress magnitudes. Overall, the stress field obtained with different stress measurement methods and its variation with depth is now quite well resolved. The overcoring data suggest that the principal stresses are inclined with a vertical component dipping about 65° from the horizontal over the investigated rock volume (140–420m depth). This is interpreted as a result of influence from the sub-vertical NE-2 Fracture Zone that divides the stress data into two stress domains [11], although it may also be an artefact because the - and -magnitudes are of the same order of magnitude. [Copyright &y& Elsevier]

Abstract: The Integrated Stress Determination Method (ISDM) is a powerful tool for estimating the regional stress tensor from in-situ measurements of local stress tensors using a wide variety of stress measuring techniques. This study presents new developments of the ISDM: The stress field may be described with up to 12 model parameters; and is applicable to data from CSIR- and CSIRO HI-type of overcoring devices, hydraulic fracturing, hydraulic tests of pre-existing fractures (HTPF), as well as to combined data sets. Furthermore, in combined data sets, the hydraulic fracturing and/or HTPF data may be used to constrain the average elastic parameters, Young''s modulus and Poisson''s ratio. The new ISDM developments were applied to the extensive and recently re-analysed rock stress data at the Äspö Hard Rock Laboratory. The results reveal a good fit of the re-analysed data. Overall, the re-analysis indicates that the stress field at Äspö HRL is relatively well constrained and consistent with depth. The NE-2 Fracture Zone influences the stresses, and dividing the regional stress field into a NW and a SE stress domain. When the hydraulic fracturing data were used to constrain the average elastic parameters, Young''s modulus, E, and Poisson''s ratio, ν, quite similar results were obtained ( and ) compared with results from biaxial tests of overcore samples ( and ). [Copyright &y& Elsevier]

Abstract: This paper presents the results of seismic and geotechnical studies carried out after a fatal accident that occurred during mining of the Frieda5 coal seam at Merlebach mine of HBL (Houillères du Bassin Lorrain, East France). On June 21, 2001, a violent rockburst (local magnitude of 3.6) affected the Frieda5 seam at depth of approximately 1250 m. The zone of the accident is located in the main gate 200 m ahead of the longwall face. Subsequently, an investigation was carried out to determine the causes of the accident and to understand the induced phenomenon. The program included an analysis of a comprehensive set of seismic data and the collection of in-situ stress measurements. The objectives were twofold: (1) to provide input data for the back analysis of the rockburst and (2) to identify other potential high-risk zones in the next panels to be mined. The joint analysis of the seismic, geological and geotechnical investigations suggests that the rockburst is largely due to a specific geological phenomenon, a sandstone channel in the floor of the coal seam characterized by high horizontal stresses. [Copyright &y& Elsevier]

A re-evaluation of overcoring results from the Borre Probe and the CSIRO HI cell was carried out at the A¨spo¨ HRL. The re-analysis revealed a number of measurement-related uncertainties for the two cells, e.g. bonding, temperature effects and identification of elastic parameters. These uncertainties were corrected and the re-analyzed strains were then used to determine the in situ stress field using standard least-squares technique. The results indicate lower stress magnitudes compared to the original interpretations and the results are in good agreement with existing hydraulic fracturing data as well as with theoretical vertical stress. Overall, the re-analysis indicates that the stress field at A¨spo¨ HRL is well constrained and relatively consistent with depth. [Copyright &y& Elsevier]

The fracture network in the excavation disturbed zone (EDZ) of an argillaceous formation, the Opalinus Clay in the Mont Terri Rock Laboratory in northwest Switzerland, is characterised using structural and hydrogeological techniques. Sampling of unloading fractures is done by analysing resin-injected overcores taken from the EDZ, resulting in a structural data set of fracture orientations, frequencies and extent of the EDZ. These parameters are derived in tunnel sections, which were excavated by blasting, pneumatic hammering and road header techniques. Pneumatic and hydrogeological tests are subsequently carried out to derive hydraulic parameters such as permeability and transmissivity distributions of the fracture network.Synthesis of the structural and hydrogeological data led to a conceptual model of the EDZ in the Mont Terri Rock Laboratory, which is defined by a 2-m plastically deformed zone around the tunnel. This plastic zone is further divided into an inner 1 m zone consisting of an air-filled fracture network and an outer zone with mostly saturated and isolated fractures. Transmissivities in the inner zone are orders of magnitude higher than those of the undisturbed Opalinus Clay.Finally, a methodology is proposed for the structural and hydrogeological characterisation of the EDZ. This methodology is not site-specific and is thus applicable at any argillaceous underground facility. [Copyright &y& Elsevier]