Abstract In-situ stress is a natural stress existing in rock mass. As an important basic parameter, accurate acquisition of in-situ stress is of great significance to the theoretical research and engineering application of rock mechanics. In the in-situ stress testing under the deep-borehole environment of high temperature and high pressure, the testing process is complex and the measuring elements are easy to fail, which restricts the development of in-situ stress deep-borehole testing technology. In this paper, an in-situ stress measurement method based on borehole cross-sectional shape analysis is proposed, a borehole crosssectional shape measuring device based on micro-optical imaging measurement is developed, the whole process test auxiliary equipment based on wireline coring drilling tool is designed, formed a complete in-situ stress testing technology. The in-situ stress testing technology has been successfully applied in a deep borehole of 703 m. The testing results are as follows: (1) At 636 m, the whole process of in-situ stress testing operation, such as "drilling operation of test hole", "equipment installation operation", "stress relief operation", "equipment salvage operation" has been completed, which only takes about 3 hours; (2) Through the calculation of the elastic mechanics based on the cross-sectional shape of the borehole at the testing point, the maximum horizontal principal stress is 23.14 MPa, the minimum horizontal principal stress is 16.59 MPa, and the maximum horizontal principal stress direction is 181.3 °. By comparing with the pumping pressure in the process of hydraulic fracturing and the geological structural phenomena, the results of in-situ stress measurement are consistent. In a word, the in-situ stress measurement technology based on borehole shape analysis is feasible, reliable and operable, and makes in-situ stress test possible to rapidly carry out in the deep borehole with complex environment. 1 Introduction In the field of geological engineering, in-situ stress is the basic data for the design of oil and gas field development schemes, the study of hydraulic fracturing crack propagation law, and the prediction of stratum fracture pressure. Obtaining accurate in-situ stress data is of great significance for oil and gas field exploration and development.

Abstract The nuclear disposal concept is based on compacted bentonite barrier placed into a borehole in the host rock. In crystalline rock, one of the issues is a non-uniform saturation of bentonite caused by inhomogeneity of the rock, typically the flow being concentrated to fractures. We solve a hydro-mechanical model problem in bentonite in the general engineering multiphysics simulation software ANSYS. The elasto-plastic transition as result of saturation is represented by a special non-linear stress- strain curve of elastic model (saturation-dependent weakening). The model is further extended to a contact-problem formulation, where the swelling is possible either as free swelling until filling the gap or as confined state after. It is a synthetic case of one horizontal fracture crossing a vertical borehole. We demonstrated that for large time period, the saturation is limited to a part of the volume, producing a non-uniform stress field in the whole volume. The stress is also controlled by the gap width. 1. Introduction Numerical modelling is one of the tools for analysis of the processes in barriers of the spent nuclear fuel disposal concept. Our work is motivated by a potential damage of disposal barriers due to non-uniform swelling of bentonite caused by inhomogeneous hydration. In the repository, the interaction of the bentonite and the host rock (fractured granite considered) is equally important, requiring the joining of the two disciplines of soil and rock mechanics. Recent studies deal with modelling of thermo-hydro-mechanical processes and also compare results from full scale experiments, e.g. [8, 11]. Additionally, we developed a non-linear hydro-mechanical (HM) method for solution of bentonite hydration with relatively simple implementation which can be easily-defined in a number of commonly used simulation tools. It offers an alternative to complex models of hydraulic or mechanic behaviour of bentonite, e.g. [1, 9]. It extends the previous work [10] on hydraulic-only models of non-uniform saturation process based on data from field experiments.

Abstract To examine the change in in-situ stress between before and after the 2011 Tohoku-oki earthquake, we performed stress measurements after the earthquake in the Kamaishi mine, located near the northern termination of the mainshock rupture. The in-situ stress measurement period was from 1991 to 2016. The results showed that the magnitudes of the three-dimensional principal stresses and the vertical stress drastically increased during the mainshock and, at one year after the earthquake, were more than double those before the earthquake. The principal stress magnitudes then decreased with time, and returned almost to the pre-earthquake levels at about five years after the earthquake. The increasing and decreasing trends in stress in the Kamaishi mine can be interpreted in terms of the effects of coseismic rupture behavior of the Tohoku-oki earthquake mainshock and the occurrence of aftershocks in the Sanriku-oki low-seismicity region (SLSR), where the Kamaishi mine is located. 1. Introduction The Tohoku-oki earthquake that occurred on March 11, 2011 was in the largest class of earthquakes. The Tohoku region experienced crustal movement of up to 5.3 m in the horizontal direction and up to 1.2 m in the vertical direction (subsidence) within a short period [1]. This crustal disturbance with a large displacement rate is likely to have had a major impact on the crustal stress field at a relatively shallow depth. Therefore, it is important that we clarify the change in in-situ stress between before and after the earthquake. 2. Overview of the in-situ stress measurement The measurement location is the Kamaishi mine in northeast Japan (Fig. 1). The Kamaishi mine is located about 170 km northwest of the earthquake epicenter. In this study, we selected a measurement station that is about 5 km from the mine opening of the 550 mL site at the Kamaishi mine. The overburden at this measurement station is about 290 m. This measurement station is not affected by galleries or the goaf cavern in this mine. In addition, several stress measurements have been performed at the 550 mL site before the Tohoku-oki earthquake [2–6]. In-situ stress measurements were performed using the Compact Conical-ended Borehole Overcoring (CCBO) technique [7]. The CCBO technique is the one of the suggested methods of the ISRM [5].

Abstract To reduce the risk of accidents in underground deposits of water-soluble ores, a flexible system of geomechanical monitoring of mine safety is required. Many years of experience in studying the physical and mechanical properties and stress-strain state of saliferous rocks when mining sylvinite layers in the mines of the Upper Kama potash salt deposit has made it possible to develop the principles of experimental and theoretical evaluation related to the load-carrying elements of the mining system. The essence of this method lies in the meaningful interpretation of the results of in situ studies using mathematical modelling. 1. Introduction The increase in demand for potash-magnesia ores, which are used in many industries, requires a substantial increase in the production of potash salts. This is achieved both by increasing the output of the existing mines and by building new plants. At the same time, as a rule, areas with very complex geological conditions are also used for mining, thus increasing the risk of technogenic accidents. In many cases, safe underground mining of potash and magnesia is ensured by the use of room and pillar system when the rib pillars are left in the mines. The overlying rocks should preserve their continuity for the entire life of the mine, serving as an impermeable pillar. Worldwide, in practice, there are more than 80 known cases of potash mines flooding as a result of fresh water inflow into the worked-out area. In most cases, the main cause of accidents is non-compliance of the room and pillar system parameters with the mining and geological conditions of the specific mined areas [1, 2]. The analysis of survey results related to the state of the developed areas in the mines of the Upper Kama potash salt deposit shows that in some areas, the "hard" pillars which are used when developing contiguous sylvinite layers (KrII and AB) get significantly destroyed after 20–30 years, although they are supposed to have "infinite" service life. The depth of scaling-off of rocks from the walls of the pillars can be up to 1.5–2.5 m and there are failures of cap pillars and partings at the room top. This situation may become worse due to a decrease in the strength properties in some areas of geological anomalies. All of these factors lead to a gradual destruction of rib pillars and intensify the displacement process of undermined rocks. The convergence rate can be as high as 500 mm/year, and the final deformation of the earth surface can reach 3.0–4.5 m. The mentioned factors are especially typical for the southern part of the deposit, where there are strong clay interlayers in the roof of the lower sylvinite layer [3].

Abstract A near field network with 11 acoustic emission (AE) sensors was installed for the in situ underground experiment (Nova project 54–14–1) that took place 410 m below surface in the Äspö Hard Rock Laboratory, Sweden. The acquisition system for the piezoelectrical sensors has been improved to record signals with 1 MHz sampling rate, to detect signals produced by weaker sources and enhance the microseismic catalogue. The acquisition system was capable to operate in trigger and continuous mode. The basic idea of the experiment was to compare hydraulic fracturing growth and induced seismicity under controlled conditions for different loading scenarios as conventional versus progressive, and pulse-like water injections. In this work, we consider continuous recordings and apply recently developed automated full waveform detection and location algorithms which are based on the stacking of characteristic functions calculated from squared amplitudes. Waveform stacking and coherence techniques are adapted to detect and locate AE signals for massive datasets with extremely high sampling. We significantly increase the detection rate in comparison to trigger mode routines. Most detection concentrated during the fluid injection occurred around the fracking stages. Frequency-magnitude distribution characteristics are investigated using a relative magnitude scale estimated from the amplitude recorded at AE sensors. We demonstrate that the stacking of characteristic functions yields to a significant improvement of the detection and location also in presence of noisy records, supporting the adoption of similar techniques for other induced and natural seismic activity monitoring systems. 1. Introduction An in situ hydraulic fracturing experiment was performed at Äspö Hard Rock Laboratory (Sweden) aiming at optimizing geothermal heat exchange in crystalline rock mass [1]. A near field network with 11 acoustic emission (AE) sensors was installed 410 m below surface to map the seismic response of hydraulic fractures for different fluid injection scenarios (Figure 1). The location of the experimental tunnel TASN is seen from which four long boreholes were drilled sub-parallel to orientation of minimum horizontal compressive stress. The middle borehole (Figure 1, blue line) serves as hydraulic testing borehole and was drilled to a total length of 28.40 meter, down dipping −4°. The rest of monitoring boreholes were drilled with inclination upwards to allow water outflow from AE sensor chains. This geometry in the predetermined stress state allows propagating hydraulic fractures perpendicular to the hydraulic testing borehole, in the direction of maximum horizontal compressive stress, and in a direction towards the monitoring boreholes.

Abstract The stress-strain state of the rock mass in the neighbourhood of the mining galleries is absolutely critical considering their behaviour particularly load and deformation. The stress-strain state can be affected by numerous geological and mining factors i.e. depth, mechanical parameters of the rock mass, the presence of faults, previous mining operations in case of multiple seam extraction and by the influence of the current longwall mining process. Considering this fact the advanced measurement plan was developed. The measurements were conducted in one longwall maingate of the Czech coal mines. The holistic investigation program consisted identification of initial rock mass stress tensor and monitoring of its changing during the approach of the longwall face. Two CCBO (modified overcoring) probes were installed to obtain pre-mining full stress tensor and afterwards three CCBM probes were installed to continuous monitoring stress state in rock mass ahead of advancing longwall. The triple height telltales and endoscopic methods were applied to monitor the behaviour of rockmass in close proximity of the gateroad. Hydraulic dynamometers and strain gauged rockbolts were used to determine the support load. Also the gateroad deformation observations were performed. The results of aforementioned measurements allowed to determine the potential relation between stress-strain rock mass state and behaviour of the investigated gateroad in coarse of longwall face approaching. 1. Introduction When applying a longwall extraction system in hard coal seams, it is essential to ensure that the gateroads are stable and of proper size during the whole mining process. In majority of European hard coal mining industry, there is a single gateroad system that prevails, consisting of steel arch yielding support. To ensure the proper maintenance of the roadway the mine managers have to efficiently cope with the rock mass movements of various intensity (e.g. roof sag, floor heave, and horizontal convergence) as a result of the extraction pressure impact [1].

Abstract The exploitation of geo-energies often leads to difficulties that rock mechanics helps to overcome. These essentially consist of instability problems caused by changes in the state of stress, associated pore pressures, and temperature. Besides the classical near field problem of wellbore stability, the changes can impact a larger scale, triggering earthquakes and inducing seismicity. In this paper the current approaches envisaged to tackle these problems are briefly reviewed and discussed. Some extensions that have been made possible thanks to the combination of new numerical techniques will be outlined. 1. Introduction The term geo-energy could be defined as the set of energy sources that the Earth produces. As in physical science where all is Energy or Matter, this would include fossil fuels such as coal, oil, gas or organic matter (wood) which is nowadays classified as unconventional geo-resources. In fact, the neologism "geo-energy" is mostly devoted to the energy that has been created from the temperature of the Earth and which is used to heat or to produce electricity. This is the case for geothermal energy [1]; the exploitation of geothermal energy was considered a long time ago [2], but it has only taken off in the last twenty years. The main reason is the urgent need to limit emissions of greenhouse gases and the lure of a renewable resource [3]. Despite the fact that many other issues could be related to Geo-energies, such as energy storage by compressed air for instance, this paper will be limited to rock mechanics problems associated with Enhanced Geothermal Systems. 2. Enhanced Geothermal System (EGS) 2.1. Principle and brief history The principle of a geothermal installation can be schematically represented by two boreholes. From the first borehole the injected cold water circulates into the rock mass and is heated up in contact with the hot rock. This hot water is then extracted in the second borehole and treated to remove energy. Later, this cooled water is re-injected in the first borehole. The effectiveness and the efficiency of the system depends on the existence or the formation of a sufficient exchange surface in the hot dry rock formation, between the two boreholes.

Abstract Crystalline rocks are being considered as potential host rocks for the construction of deep geological repositories (DGR) for radioactive waste in a number of European countries including the Czech Republic. Advection is considered as a main process within such a type of test. Tracer tests, using various substances, are used for a number of different purposes; however, they are generally employed particularly for the characterisation of flow systems. A system consisting of a flow field intersecting a water bearing fracture was first identified at the Josef Underground Laboratory, whereupon two boreholes were drilled and fitted with instrumentation so as to determine and characterise an ideal location for the potential injection of a radioactive tracer into the fractured rock massif. A number of methods were employed in order to describe the rock system and the hydraulic flow field in detail prior to tracer injection. This was followed by the conducting of safety calculations aimed at providing a description of any potentially hazardous impacts on humans and the biosphere as required by the site licensing procedure. 1. Introduction Crystalline rocks are being considered as potential host rocks for the construction of deep geological repositories (DGR) for radioactive waste in a number of European countries including the Czech Republic. The main transport mechanism in crystalline rocks consists of advection; however, migration processes from fractures through fracture coatings into unaltered rock will also have to be studied. The conceptual model is based on the presumption that non-advective migration is driven by diffusion into both altered mineral layers and the undisturbed rock matrix adjacent to water-bearing fissures. The diffusion process depends on a large number of factors: molecule size and charge, sorption, effective porosity, pore construction, tortuosity and groundwater composition. It is intended that radionuclide migration in fractured crystalline rock environments will be studied using different scale rock samples and in-situ testing as part of the PAMIRE project (TA04020986).

Abstract Fractures have a significant impact on rock mass mechanical and hydraulic properties, which is a concern for rock engineering applications like excavation or repository design, support design, slope stability and caving in mines. To address this issue, a sound description of the fracturing pattern is required. DFN models are statistical models which define the density of fractures having given geometrical properties (size and orientation) and which include an intrinsic variability term. One of the main challenging task is to combine all available data. Data remain sparse and scarce and are acquired at different scales and from different support shapes and dimension (1D, 2D). We present a 3D modelling approach combining data from borehole logs, outcrop trace maps and tunnel walls mapping. It is applied to the Äspö site in Sweden, for which a large database is available, containing tens of thousands of records. Using stereological rules and assumptions about the underlying DFN scaling model, we are able to integrate all data to define the fracturing properties from the borehole scale (ten centimeters) to the repository scale (several kilometers). An advanced DFN modeling framework is applied, accounting for fractures mechanical interactions. This model has proved to be almost universal in crystalline rocks and reproduces, with very few parameters, the scaling properties of fractures. We show that this modelling framework better reproduces observations at all available scales and yields DFN, which structure and associated properties have a better consistency with natural cases than for simple DFN approaches. 1. Introduction Studying fractured systems is a requirement for many industrial applications including nuclear waste deep repositories, geothermal energy exploitation in crystalline hard rocks and oil and shale gas extraction. In these fields, fractures are key factors for rock masses flow and mechanical properties. Fracture networks are complex systems arising from the physics of fracture development and from complex interactions between fractures. Because of this intrinsic complexity, fracture systems present the classical characteristics of complex systems with power-law scaling relationships [1]. This is now widely recognized from geological studies [2–4]. The major difficulty for defining Discrete Fracture Network (DFN) models is the limited amount and the nature of available data. Despite constant technical improvements, high-resolution measurements of fracture patterns are mostly limited to borehole and surface mapping, thus raising both under-sampling and stereological issues. The fracture system is defined at best from statistical distributions, which are the basic ingredients for interpolating local measurements at the site scale. The precise knowledge of these distributions, including scaling, is a critical issue for site modeling.

Abstract 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. 1. Introduction After the 2011 Tohoku-oki earthquake, in-situ rock stress was measured repeatedly at a drift in the Kamaishi mine at a depth of 290m [1]. Stress was measured conducted by the Compact Conical-ended Borehole Overcoring (CCBO) technique [2–4]. The magnitude of the rock stress measured one year after the earthquake was more than twice the values measured at nearby locations in the same drift before the earthquake. To confirm the reliability of the values measured after the earthquake, we tried to check the vertical stress using the Hydraulic Fracturing (HF) method. In this case, the σ1/σ3 ratio in the plane perpendicular to the borehole axis is about 5, which was considered to be beyond the scope of the HF application. If HF tests are performed under this stress condition, they should not be used to measure the reopening pressure of artificial fractures. However, since we were interested in the relationship between pressure and flow rate in the re-opening test, we performed this test anyway. While the reopening pressure was not used, the shut-in pressure was compared to the stress component measured by the CCBO. As a result, we found that the rock stress measured by the CCBO test at five years after the earthquake was highly reliable.

Abstract The hydraulic fracturing technique has applications in the development of solid mineral deposits. The particularities of the method performed in a mine condition include confined space and high requirements for the safety of the equipment used in underground workings. One of the most relevant tasks in in-mine hydraulic fracturing is creating parallel cracks oriented across the axis of the well. In-seam boreholes with this type of fracture system can be used for coal bed methane production, sealing of degassing boreholes, controlled collapse of the roof and other applications. In this paper we consider transverse crack formation on borehole wall in a given plane when the fracture initiation occurs due to an indenter impression located between two inflatable packers. During indentation fracturing, fluid under high pressure is supplied and this contributes to crack propagation in the desired direction. On the basis of the calculation of the stress intensity factor for the semi-infinite plane with the edge crack affected by the forces applied at points on the plane surface the length of the formed crack is obtained. The results show that the force acting on the wall of the borehole with a 76–05 mm diameter will be sufficient to create an initiation fracture in a predetermined direction. To implement this approach an experimental device is developed. It is implemented as an additional module of the hydraulic fracturing tool and equipped with a system of three indenters. The design of the device allows to obtain a total acting force in one plane and this will increase the effectiveness of the impact. 1. Introduction Hydraulic fracturing is a widely used method in oil and gas production to increase hydrocarbon inflow to the wellbore. Under the influence of the high fluid pressure delivered into an isolated interval of the well, an artificial crack forms in the reservoir rock. Subsequently, this method has been adopted in the mining of solid minerals. The in-mine hydraulic fracturing, for which the method is implemented, in wells drilled in underground conditions possesses distinctive limitations, such as limited space and strict safety requirements for the operated equipment. Usually hydraulic fracturing solves the following problems:

Abstract With increasing pressure to reduce the fraction of energy coming from fossil fuels, there is an increased need for research into feasible and sustainable energy sources, such as solar energy. The problem with solar energy is the mismatch between supply and demand, and so the energy needs to be stored. This research paper is a part of the project titled "Tackling the Challenges of a Solar-Community Concept in High Latitudes", and aims in helping to design a thermal energy storage system for southern Finland that is economically feasible and has a high performance. For this purpose, a back-calculation of the underground thermal energy storage (UTES) of the Kerava Solar Village was performed. The primary objective was to calibrate the numerical models to be used in an optimisation by quantifying the thermal properties of the surrounding granite and soil. The UTES of the Kerava Solar Village consisted of a rock pit filled with water and two surrounding rings of boreholes. COMSOL Multiphysics 5.2 ® was used to create a model in which the temperature of the rock pit was used as the heat source and the heat propagation through the surrounding rock as the output to which the measured data was compared. The best replication of the temperature inside the rock near the surface was achieved with a conductivity of 2.8 and 1.0 W/(m•K) for granite and soil respectively. When looking at the deeper sections, the best fit was obtained for a conductivity of 5.5 and 1.0 W/(m•K) for granite and soil respectively. These results are conflicting, and outside the realistic range for granite. The strange behaviour of the measured isotherms could be explained by a presence of an additional heat source in the ground originating from a leakage of hot water from the tank into surrounding rocks. The critical issue identified in this study was the lack of data. All the parameters of the system, such as the geology, hydrology, and detailed technical drawings, but also the temperature distribution inside the heat source, and heat storage medium need to be known for back-calculation study to be successful.

Abstract Relatively accurate data regarding the temperature distribution in a rock mass can be obtained from measurements carried out in short boreholes drilled directly into mine workings. Such thermal field measurements were undertaken in selected mine workings in all active mines of the Ostrava-Karviná District (OKR), situated in the Czech part of the Upper Silesian Coal Basin. A temperature field survey of the rock mass at high depths was performed in the years 2011–2013, with a total of 204 valid in situ measurements of initial temperature in the carboniferous rock mass recorded, ranging from 27.0°C to 48.9°C. All measurements were registered in a newly created database. During the project, many archival sources of individual temperature measurements made in the OKR were found, complementing and extending the existing knowledge base regarding the distribution of the temperature field. A new spatial distribution model of primary temperature values for the carboniferous massif in the Czech part of the Upper Silesian Coal Basin was the final output of the presented project. The main purpose of this spatial modelling and data visualisation was to create an image of the area's complex topological arrangement based on the available data. Following the generated thermal model, a 3D map of OKR temperature fields at around 1000 m depth was generated. In order to determine the temperature at a particular position in the thermal model, an executable plugin was created. It is also possible to import the final thermal model of the carboniferous massif in the Czech part of the Upper Silesian Coal Basin into commercial computational software for the determination of appropriate mine air conditions. 1. Introduction Mining activities involved in the extraction of energy resources in the Czech Republic are reaching ever more significant depths, a phenomenon that has led to increased consideration of mine worker safety, as well as the indirect safety of the general public. The project "Mine workings design and direction at depths of 800 metres and greater" focuses on the safe direction and operation of mine workings at depths exceeding 800 metres, due to the fact that at such depths, increased rock stresses and temperatures may lead to potentially dangerous anomalous stress phenomena or disturbances in ventilation and air quality. The latter in particular require specific procedures to be undertaken by the national Mines Rescue Service when dealing with such emergency situations.

Abstract To solve various technological problems of mining, such as, prevention of air inflow into coal-seam degassing holes and steam entry into underground galleries when thermoshaft production of heavy oil is performed. There are developed the chemical compositions to form the membranes' impermeable and drainage elements. Also, there is provided the stand's description and laboratory studies' results which are the compositions' influence on permeability of porous medium. A method of creating membranes by hydraulic fracturing of the rocks, in which to get a transverse fracture, shear loading of the walls along the wellbore axis is used. A comparative analysis of this method with directional hydraulic fracturing on the basis of slot initiator is performed. In the article calculations of influence of the fracture pressure and fracture propagation from the wellbore and fracture radii are presented. The technological advantages of the developed method of hydraulic fracturing for the creation of impermeable membranes are represented. 1. Introduction In mining the impervious membranes are used to protect a mine working from water and gas inflow and to create edges of the metals leaching zone and etc. Among these purposes there is prevention from air filtering from the mine working into degassing boreholes. The sealing quality estimates the efficiency of the preliminary coal beds degassing. There is a problem of the air inflow into methane withdrawal point through the rock mass. A well-known solution addressing the problem is to apply insulating compound onto the mine working surface. Its uneven surface causes great specific consumption of expensive chemical reagents. However, isolation of a surface part around the degassing borehole head does not interfere with the air filtering through the disturbed rock bed around the mine working made out of its non-isolated parts. More effective solution is impervious membranes created inside of the rock mass with the help of one or several hydraulic fracturing fractures parallel to each other and located across an uncased borehole. This paper suggests several schemes for the membranes made of drainage and impermeable elements. In order to obtain the elements a fill fluid and insulating compound have been created. The work presents results of the laboratory research on the fill fluid and insulating compound's influence on porous medium permeability.

Abstract In this article, the concept of Fatigue Hydraulic Fracturing (FHF) is described, and its geothermal application is discussed. The basic idea behind fatigue fracturing is to vary the effective stress magnitudes at the fracture tip to optimize fracture initiation and growth. The optimization process can include lowering seismic radiated energy and/or generating fracture networks with various geometry and permeability. Historically, we start referring to results from mechanical laboratory core testing, discrete element simulation of fluid-induced seismicity, and application of cyclic water-fracs at the enhanced geothermal system site Gnβ-Schönebeck, Germany. Then, an in situ experiment at Äspö Hard Rock Laboratory is summarized to bridge the gap between laboratory core testing and wellbore-size hydraulic fracture treatments in hard rock. Three different fluid injection schemes (continuous, progressive and pulse injection) are tested underground in naturally fractured, crystalline rock mass in terms of associated induced seismicity and permeability performance. Under controlled conditions, hydraulic fractures are extended to about 20–40 m 2 in size from a 28 m long, horizontal borehole drilled from a tunnel at 410 m depth. The facture process is mapped by an extensive array of acoustic emission and micro-seismic monitoring instruments. Results from three water-injection tests in Ävrö granodiorite indicate that the fracture breakdown pressure in tendency becomes lower and the number of fluid-induced seismic events becomes less when continuous, conventional fluid injection is replaced by progressive fluid-injection with several phases of depressurization simulating the fatigue treatment. One reason for this may be that in the dynamic, fatigue treatment a larger fracture process zone is generated compared to the size of the fluid pressurized zone developing during the injection phases into crystalline rock. We see mine-scale tests with hybrid sensor arrays of importance to identify and understand the actual hydraulic fracture mechanisms in hard rock. In addition, the mesoscale data obtained underground allow downscaling to laboratory core results, and upscaling to borehole reservoir stimulation results. 1. Introduction Hydraulic fracture growth through naturally fractured rock is an area of interest and current research for petroleum, mining and geothermal applications. In particular in the development of enhanced geothermal systems (EGS), hydraulic fracturing is used to form fracture networks connecting injection and production well for heat exchange purposes. However, hydraulic fracturing imposes environmental risks, one of which is induced seismicity associated with the permeability enhancement process [1–3]. Of particular interest are, therefore, methods that limit the number and magnitude of seismic events while the required fracture permeability is obtained. In this context, the fatigue hydraulic fracturing (FHF) concept and the multi-stage hydraulic stimulation concept have been proposed. The key point in fatigue hydraulic fracturing is the frequent lowering of the injection pressure to allow stress relaxation at the fracture tip [4]. Reducing the maximum injection pressure by alternative injection schemes will affect the damage zone surrounding the fracture and also the radiation pattern of seismic events associated with fracture growth [5]. For multi-stage stimulation, instead of massive stimulation, injection rate and pressure are controlled, and the reservoir is formed stage by stage [6, 7].

Abstract Extensive rock grouting has been carried out during the construction phase of the Cityringen Metro Project. The main goal of the rock grouting has been to reduce the permeability in the rock by filling joints, cracks and fissures, in order to minimize the groundwater inflow during excavation of caverns, pump sumps and TBM break in/out through the stations/shafts. Both horizontal and vertical grouting has been performed above and below the water table, with unique challenges and advantages/disadvantages. The grouting has been performed as up-stage, where the grout hole is drilled to full depth and then grouted from the bottom and up in stages using a pneumatic packer. The split-spacing method has been used to expand the grouting pattern if needed after primary grouting campaign has been finished and tested. The article describes, through case histories, the achieved results, challenges and limits of the applied methods, including future recommendations. The rock characteristics of the Copenhagen limestone, in regards to the performed rock grouting, are also discussed. 1. Introduction The Cityringen Metro is expected to be running in 2019 and will consist of two single tracks in twin tunnels, each approximately 16.5 km in length. A total of 17 underground stations, 4 cross-over facilities and 3 construction and ventilation shafts have been made when the project is finished. The Cityringen is built entirely in the urban area of Copenhagen; most of the stations are located in congested areas, surrounded by historical building. One of the most sensitive activities during construction is the break-in/-out of the station/shafts. It is mandatory to ensure a dry or at least a controllable groundwater inflow during the break-in/out phases. Between various technical solutions, the contractor developed a grouting plug to reduce the inflow of water, which has proved efficient in the Copenhagen limestone. 2. Geology The Geology of the Copenhagen area consists of Quaternary Deposit overlying the Paleogene Deposits, primarily consisting of Danian Limestone and Locally of Selandian Greensand [1]. The Copenhagen Limestone Formation is divided in 3 stratigraphically units: Upper Copenhagen Limestone (UCL), Middle Copenhagen Limestone (MCL) and Lower Copenhagen Limestone (LCL). The tunneling and excavation activities are entirely in UCL and MCL.

Abstract 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. 1. Introduction Knowledge, which should be as accurate as possible, of the stress–strain state in rock mass is the determining factor for the proper planning of roadway supports [1]. That is why stress monitoring, primarily of the changes induced by longwall mining, is considered within this research project. The locality of longwall No. 371 202 in a mine of the Ostrava-Karvina Coalfields (Upper Silesian Coal Basin – USCB) was chosen for the research.

ABSTRACT: In the preliminary investigations of mountain tunneling, a seismic refraction survey from the ground surface is conventionally conducted, and a tunnel support pattern is designed based on the seismic wave velocity of the ground. The resolution of the velocity profile by this technique, however, is poor as the tunnel overburden becomes large. In response to this problem, the authors developed a new survey method in which drilling vibration data were used, called tunnel seismic probe drilling (T-SPD), to estimate the seismic wave velocity distribution ahead of the tunnel face so that it could be used in the design of tunnel support patterns. We conducted field demonstration tests of the T-SPD at three tunneling projects. In this study, we will report the results of the additional verification for the remaining section of a tunnel project previously reported. 1 INTRODUCTION In conventional planning of mountain tunnels, seismic refraction surveys from the ground surface are performed as preliminary surveys in accordance with the Japanese Standard (JSCE 2006). Tunnel support patterns are then designed based on the seismic wave velocity of the ground, as obtained by the seismic refraction survey. The resolution of the velocity profile estimated by this technique, however, is poor as the tunnel overburden becomes large. In such conditions, a survey ahead of the face during tunnel construction is considered to be more effective than the seismic survey from the ground surface. Two methods are generally used in the surveys ahead of the tunnel face: seismic reflection method known as tunnel seismic prediction (TSP) or horizontal seismic profiling (HSP), and borehole logging known as drilling survey system (DRISS). Though we can obtain information on the position and direction of the discontinuous surface using TSP or HSP, it is not possible to obtain the width of the fracture zone, which is important for designing tunnel support patterns. The drill machine data provides the distribution of specific energy, which is related to the strength of the rocks, and is defined as the energy consumed to drill the rock of a unit volume. Although probe drilling provides logging data and drainage for groundwater at the same time, its disadvantages are that the drilled cross-sectional area is small relative to that of the tunnel face, and that it is difficult to correlate the drilling specific energy with the quality (integrity) of the rock.

ABSTRACT: The stability of mining excavations in the Bushveld Complex is influenced by the structural geology of the surrounding rock mass. Structural complexities exposed in underground excavations are the cause for potentially unstable ground conditions. A fall of ground at Lonmin's Rowland Shaft identified abnormalities with reference to the hangingwall conditions of the stope area as well as pillar and support behaviour. This resulted in premature loading of the grout pack support and fracturing of the pillar edges. The support design was adjusted to cater for the behaviour identified and prevent future instabilities. This paper presents the investigations and interpretations of the structural geology, the impact on the stope stability, pillar behaviour and support. 1 INTRODUCTION Rowland Shaft is one of Lonmin's first generation vertical shafts, extracting both the Merensky and Upper Group 2 (UG2) Chromitite Reefs. A structurally related fall of ground (13.5 m × 3.75 m) occurred at 31 Chrome East (UG2 Stope), parting from one of the chromitite layers of the UG2A Markers (triplets) situated approximately 1.25 m above the reef. The fall of ground (FOG) occurred at a depth of approximately 1003 metres below surface (mbs). Conventional mining is applied in an up-dip split panel configuration at a stoping width of approximately 1.2 m. During the fall of ground site investigation, hangingwall alterations were identified as well as abnormal behaviour of the pillars and support units. This identified the necessity for an in depth study in the prevailing geology and associated failure mechanisms resulting. This paper provides an overview of the investigations and findings in the study area and changes in the support strategies to ensure stope stability. 1.1 Geographic location Lonmin's Marikana Operations are located in the western limb of the Bushveld Complex of South Africa (Fig. 1). Large-scale fault structures compartmentalize the Marikana Operations, with the Rowland Shaft mining area being bounded by the Marikana Fault and Elandsdrift Fault Zone to the west and east respectively (Fig. 2). Both reef horizons occur stratigraphically in the Upper Critical Zone of the Rustenburg Layered Suite (Kruger 2005), striking in an E-W direction at the Marikana Operations with an average dip of 10° to the north. Mining is currently taking place between 16 Level (467 mbs) and 32 Level (1038 mbs). Access to the reef horizon is through a vertical shaft to 26 Level, whereafter a decline shaft provides access to the deeper levels. Strike haulages, located approximately 20 m in the footwall of the respective reef horizon provide access to the stopes by means of short travelling ways. The planar continuity of the orebody is disrupted by geological features, including faults, dykes, potholes, rolling reef and iron-rich ultramafic pegmatites (IRUPs).

ABSTRACT: Japanese island arcs are located at the western Pacific arc-trench system, the fractured characteristic is frequently observed in rock and the discrete fracture network exists between the water curtain and the storage cavern is concerned an impact factor on the air-tightness of the hydraulic containment type LPG storage cavern. In order to avoid the high pressure LPG gas leaked out the water curtain system, a whole covering water curtain boreholes system and the homogenization of the storage cavern vicinity by grouting works are considered as countermeasures. In this study, the authors originally designed the vertical/horizontal boreholes to enclose the storage caverns and developed an independent water supply pipe sys-tem for the water curtain boreholes. Furthermore, the authors developed the efficiency test method to evaluate each water curtain borehole individually and to ensure the air-tightness of storage caverns. The completed water curtain system had been verified its hydraulic containment ability by the severe air-tightness test and had been launched into operation for three years. All the hydraulic potential, injection rate of water curtain boreholes, water curtain tunnel and operation shafts are continuously monitored and the monitored data indicates the stability. 1 INTRODUCTION 1.1 LPG underground storage cavern in Japan Underground energy storage in unlined cavern is initially recommended by Hagerman (1938) and has been widely implemented for liquefied petroleum gas (LPG) storage in Europe, United State, Korea, China and India etc. In Japan, Japan Oil, Gas and Metals National Corporation (JOG-MEC) has constructed two LPG underground storage caverns in Namikata (450,000 tons, Ehime prefecture) and Kurashiki (400,000 tons, Okayama prefecture). The constructions were completed and the operations were launched in March of 2013. In order to preserve propane in liquefied phase at room temperature (boiling point 22°C at 0.8 MPa), refer-ring to the conventional construction cases of LPG storage caverns, the water curtain tunnels and water curtain boreholes were designed to ensure the air-tightness of the storage caverns. However, comparing to the European sites which are mainly located in old (Precambrian) and stable plate form, Japanese island arcs are located at the western Pacific arc-trench system, the geological age is relatively young (Mesozoic) and is characterized by fractured rock mass. Hence, the heterogeneous characteristics of fractured medium and their effects have to be considered in design and construction of the underground LPG storage cavern.