The Gotschna Tunnel is currently being reconstructed. For approximately 10 years, various measurements have been carried out in the tunnel to monitor the swelling behaviour. In addition, laboratory tests have been performed to investigate the rock mass. The findings from these on-site measurements and laboratory tests have been analysed and regularities have been identified, which help to better understand the current behaviour of the tunnel in conjunction with its geology, and thus to better extrapolate the future behaviour. Furthermore, the observations are compared with those from other tunnels to determine similarities and differences.

Abstract In material sciences, it is a well-known fact that linear or linearized theory based on Hooke's law does not offer a satisfactory description of solids in special regimes, which include e.g. too high strains under large uniaxial stresses. Therefore, in general, the response to biaxial or triaxial loading cannot be obtained as superposition of uniaxial load responses. Striking paper book example of material demonstrating such behavior is rubber subjected to uniaxial or isotropic compression. Despite this fact, linear mechanical moduli, being secant or differential, determined through standard rock-mechanics tests, mostly from the uniaxial compression, are still widely used for description of deformational behavior of rocks. Without doubt, an appropriate interpretation of these effective quasi-elastic or stiffness moduli can give useful information about mechanical properties of the rocks, especially in comparative sense. However, for more reliable constitutive modeling of any solid materials, paricularly rocks, an experimental investigation of deformational responses to uniaxial, biaxial and triaxial loading or unloading regimes is very useful. This contribution presents the results of an experimental case study on homogeneous sandstone exposed to isotropic triaxial, and equi-biaxial or uniaxial loading regimes. The measured deformational response of this rock is compared with behavior of elastic solid materials. Finally, benefit of the experimental testing for constitutive modeling based on phenomenological description is briefly discussed. 1. Introduction 1.1. Triaxial experiment for study of mechanical behavior of rocks in variable stress conditions Investigation of mechanical behavior of rocks under applied external stress fields constitutes necessary base for understanding many natural geophysical processes in the Earth's crust, or for solutions of engineering problems connected with a wide spectrum of human activities. For these tasks, simple or very sophisticated numerical models can be adopted. However, for a satisfactory prediction of mechanical evolutions, these models need experimental data in the form of basic mechanical parameters. Rocks usually represent the complicated aggregates of mineral grains of variable size, chemical compositions, or magnitude of binding forces. Such microscopically heterogeneous materials, when subjected to increasing external stress, from the start of loading, undergo reversible elastic deformation, as well as permanent deformation originating preferably from microfractures. Standard material engineering can successfully adopt generalized Hook's law, describing elastic and mostly linear response through constant elastic moduli, for majority of construction materials at sufficiently low strains. However, in rock mechanics, the mechanical moduli determined from laboratory experiments play the role of linearized effective coefficients rather than constant intrinsic material parameters. For ideal material described through linear relations between strain and stress, a deformational response to general loading conditions can be determined through superposition of uniaxial loads. Therefore, the simplest uniaxial compression/extension laboratory test may be often sufficient to estimate the deformational response in other loading regimes including general triaxial stress state. This consideration is widely used also in geomechanical modeling, i.e. Young moduli and Poisson ratios determined from uniaxial tests on rocks as secant or linearized coefficient are accepted as inputs. However, such attempt is not fully satisfied, and laboratory investigation of the deformational behavior of rocks under alternative triaxial loading regimes is necessary.

Abstract Based on laboratory testing of bedded specimens made of artificial geomaterial, meta-siltstone and schist subjected to uniaxial compression and triaxial compression by the Karman scheme, the deformation and strength characteristics of the materials are determined as functions of the bedding angle. The elasticity properties and strength are estimated in accordance with the theoretical model by Salamon and Tien and the Hoek–Brown criterion. For the layered artificial geomaterial, the theoretical model by Salamon and Tien ensures a well-contended estimate of elastic properties. The strength parameter obtained based on the Hoek–Brown criterion is independent of the level of the lateral pressure applied. 1. Introduction Predictions of rock mass stability due to mining activities, including deep-level excavations, or estimations of load-bearing capacity of soil and rocks in foundation engineering and building construction as well as high quality modeling of rock mass behavior under various type loading should take into account the complicated structure, non-uniformity and anisotropic properties of rocks. The review of the recent literature shows that researchers focus on the effect of structure and texture of rocks, in particular with bedding, on their deformation [1–5]. It is technically difficult to produce sufficient number of specimens with the required undisturbed structure for testing and studying. This issue makes one of the main obstacles to development of adequate relations and strength criteria for anisotropic rock masses. In this respect, equivalent artificial geomaterials are advantageous in terms of repeatability of experiments and predictability of structure. This study addresses mechanisms of change in strength and deformation characteristics of specimens made of a layered-structure artificial geomaterial and bedded rocks (meta-siltstone and schist) under uniaxial and triaxial compression. 2. Experimental tests and results In the tests on INSTRON 8802 Servohydraulic Fatigue Testing System, the strain rate was 0.1 mm/min. The strength and deformation characteristics of the test materials were determined in accordance with ASTM. The artificial geomaterial specimens were made of two alternating layers of different compositions, namely, layer 1: sand 30 g; cement 10 g, Neolit glue 4 g and water 2.5 g; layer 2: sand 30 g, cement 5 g, Neolit glue 3.5 g and water 2.5 g. The cylindrical specimens were 60 mm long, with a diameter of 30 mm. The angle of bedding (angle between the cylinder specimen axis and the normal to the layer plane (isotropy)) was Ψ = 0°, 15°, 30°, 45°, 60°, 75° and 90°. The specimens made of the artificial geomaterial, meta-siltstone and schist are shown in Figs. 1–3, respectively.

Abstract The first hydraulic stimulation for enhanced geothermal system (EGS) development in Korea had been conducted in the PX-2 well of 4,348 m depth in Pohang EGS site from January 29 th to February 20 th , 2016. Treatment histories of injection rate, wellhead pressure and corresponding induced microseismicity data were obtained from the stimulation test upon 140 m long open hole section at the well bottom. Wellhead pressure was up to 89 MPa and considerable level of flow rate was attempted up to 47 L/sec. Microseismicity observation showed a trend of lager and more frequent seismicity occurrence in shut-in phase than in injection phase. The injectivity index during the stimulation periods had increased as 2.7 times in January 30 th at the wellhead pressure of 73 MPa. Postulating the existence of a major fracture zone intersecting the open hole section, the transmissivity and the corresponding equivalent aperture of the fracture were evaluated. Required breakdown pressures by hydrofracturing and hydroshearing mechanisms were estimated based on the various scenarios on the in-situ stress condition, major fracture zone orientation and shear failure criteria. 1. Introduction 1.1. Pohang EGS development site The first enhanced geothermal system (EGS) development project in Korea was launched at the end of 2010 in Pohang. Five boreholes are located within 5 km from the site (Fig. 1): BH-1 of 1.1 km depth, BH-2 of 1.5 km depth, BH-3 of 0.9 km depth, BH-4 of 2.4 km depth, and EXP-1 of 1 km depth. The Pohang EGS site is owned and operated by NexGeo Inc., and it is located at 129°22'46.08″E, 36°06'23.34″N. Drilling of PX-1 and PX-2 wells were finished with final depths of 4,127 m and 4,348 m, respectively, and it is planned to be expanded to a triplet system, i.e., a fluid circulation system with three wells in the target reservoir, after stimulations in PX-2 and PX-1.

Abstract Rockburst and methane prevention are methods of rock mass destruction. The use of explosives can reduce these hazards in the immediate bore hole. Currently in the Central Mining Institute intensive research in creating new charges explosives and blasting methods which weaken rock mass is underway. One condition of effective rock mass destruction is intensive rock cracking. The effectiveness of such preventions methods dictates activity destruction and penetrating explosive gases. A complex testing device was designed and constructed for the assessment of dynamic pressure increase of explosive gases. This allows confining pressure in the explosive material's surroundings to be changed. This article presents the complex testing device and the results of confining pressure influence on explosive dynamics. According to the authors the results open new possibilities for the use of explosives, creating new charge explosives and their methods of use. 1. Introduction In mines in which rock bursts and methane hazards were found, drilling technology and explosives are used for local destruction of rock mass in order to obtain a thick web of cracks. The radius of the cracks, i.e. permanent consequences of the explosives, explotion, mainly depends on rock resistance, the kind of explosives used, the construction of the charges, and the concentration of explosive per meter hole blasting [4]. Very strong rock destruction is carried out only in the immediate vicinity of the blasting hole. This is proven by the results of in situ work that indicates that this destruction is limited to a few diameters of blasting hole. Respectively, a wider range of permanent causes of detonation is gained using very strong explosives. But, still in this case the widest distance of a gap is limited to about 3–4 meters from the shot-hole [2, 4, 5, 10, 11]. Gaps of such distances determine the effectiveness of mining hazard prevention that require rock destruction. The use of mining explosives guarantees the production of a vast network of subsurface cracks only in the case of a dense network of blast holes. It is necessary for methane drainage of the deck before exploitation. However, due to the power of explosives approved for use in methane hazardous conditions it is difficult to implement. Effective methane drainage deck requires its destruction at a level similar to the form of excavated material from the wall. Such a strong fragmentation deck blasting technique would require large expenditures for blasting and due to its current cost, this is economically unjustified. With this in mind, and its common usage in underground mining blasting techniques, the Central Mining Institute has begun research to develop new techniques for stimulating seam for methane drainage whilst cutting the exploitation field. This is possible as a result of producing vast and targeted zones of intense fracture. As a result, the priority of this research is to develop a new construction of explosives dedicated to specially prepared blasting holes [6]. The preparation of blasting holes is performed by a series of disk-shaped slots. At the moment of explosion, a slot which has a much wider range than gained by traditional blasting methods is created. In the case of the sequenced detonation of charges in the hole in which the disk-shaped slots have been inserted, the range and intensity of the slots will be greater in comparison to those used with an instant charge. The sequenced detonation of charges in a hole is connected with use in conditions of growing explosive gas pressure. Developing such an explosive technique requires tests which give opportunity to evaluate detonators' and highly energetic materials' working in the conditions of growing confining pressures.

Abstract It is essential to understand the mode I fracture toughness of rock under in situ conditions found underground, such as confining pressure, for assessment of the stability of underground rock structures. Some researchers have performed several types of fracture toughness tests of rocks under confining pressure condition. However, the ordinary fracture toughness tests do not consider the effect of confining pressure in their estimation equations. A modified estimation equation for the mode I fracture toughness of rock under confining pressure condition was suggested in this study. This study was intended for the semi-circular bend (SCB) test, which is one of the ISRM-suggested methods for determining the mode I fracture toughness of rock. The SCB test under various confining pressures from 0 to 10 MPa was performed using Kimachi sandstone. A finite element analysis of the SCB test considering several confining pressure conditions was performed to calculate the stress intensity factor. Based on the numerical simulation results, the modification of the fracture toughness estimation was suggested. Applicability of the suggested estimation was discussed with comparing the mode I fracture toughness of the rock obtained in this study and previous research. 1. Introduction Fracture toughness K c is one of the most important mechanical properties in fracture mechanics. It is considered to be the critical value of stress intensity factor K and indicates the resistance to fracture initiation. In rock engineering, K c has been applied as a parameter for classification of rock materials, and interpretation and modeling of rock fracturing [1, 2]. It is essential to understand K Ic of rocks under in situ conditions found underground, such as water content, temperature and confining pressure, for its application. Mode I (opening mode) is the most important mode of fracture initiation because it is the predominant loading condition over the fracture of rocks [1]. Many researchers have been conducting the fracture toughness tests under confining pressure [3–6]. It was found that K c tends to increase with increasing confining pressure. However, some results show that K c is not changed under several confining pressure values. In the experiments under confining pressure, the specimen is immersed in an incompressible fluid such as oil, and the fluid pressure is often used as confining pressure. The specimen requires jacketing to prevent contamination by the oil. The specimen in fracture toughness tests has a notch, and the previous results may show the different trends due to the different conditions of the pressure inside the notch as well as pore pressure. It also should be pointed out that the general fracture toughness tests [7–9] have not been theorized for the mechanical condition of the specimen under confining pressure. Some researchers have attempted modifications considering the effect of pressure inside the notch [4, 6]. Numerical simulation may support the estimation of the fracture toughness of rocks under confining pressure.

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 Strong mining seismic events are the source of very high values of peak particle velocity (PPV) recorded in near wave field. The high value of PPV and in low frequency band can exert high dynamic load on the support and underground excavation. As a result of significant increase in dynamic load on the support there is instability and the rockburst phenomena in excavations are observed. The stress concentration zones linked to edges or remnants of the old exploitation are the places where rockbursts have occurred most often. The performed numerical modeling showed, that under the some level of PPV, an important increase in the calculated load on the arch steel support were observed in the stress concentration zones. These results are in good correlation with numerous cases of rockbursts occurred in the Polish mines. In our opinion, continuous measurement and controlling the level of PPV and numerical modeling of dynamic loads in the stress concentration zones allow us for better forecasting of potential rockbursts hazard in mines. 1. Introduction The statistics of rockbursts in the Upper Silesia Coal Basin (USCB) in Polish part shows that the sources of mine seismic events leading to destruction of a mine excavations were usually located no farther than 300 meters away [10]. Such data indicate that the vibrations originate from the near-field wave and are characterized by high values of the amplitudes of peak particle velocity (PPV), [9]. The mechanisms of the rockbursts indicate that they occurred as a result of the summation of static stresses and dynamic stresses and in case of exceeding level of the critical stress in the zone around the tunnel [4]. In Chinese coal mines there are similar rockbursts mechanisms [8, 5]. The article [11] presents a system for monitoring, recording and processing the results of velocity vibration and the calculation of dynamic loads in the arch profiles support at the field study in gateroad 4/510 in Bobrek mine. In the article was proved that only the PPV recorded in low frequency band (mostly below 30 Hz and named PPV W ) has influence on the increase of load exerted on arch support. Applying data on geological and mining conditions in the area of the field study and the record of the tremor on 5 November, 2013 and seismic energy E=3E5 J, numerical calculations were conducted using FLAC software. Maximum value PPVw = 0.110 m/s (in situ measurement) was analysed; in addition, increased values of PPVw=0.250, 0.500, 0.750 and 1.0 m/s were analysed too. The vibrations in each case caused a specific increase in reduced stress in the profile of an arch support. Maximum increase for PPV W =1.0 m/s was nearly 25% compared with the static phase. When the values of PPV W were just one fourth (0.25 m/s), the increase in reduced stress was approximately 5% compared with the static phase, while for the PPV W =0.11 m/s the increase in reduced stress reached 2.5%. The results presented in the article [11] indicated that for analyzed values of PPV W , absolute values of the reduced stresses ranged from 250 to 450 MPa. They were below permissible stress for steel arch support, amounting to σ max = 623 MPa. The vibrations registered for seismic events at Bobrek field site were safe to maintain the stability of the excavations.

Abstract An experimental study was carried out on grout samples prepared from both Stratabinder and BU100 cementitious products. Samples were prepared with various water to grout ratios and tested for uniaxial compressive and shear strength. Triaxial tests were performed on cylindrical samples to determine values for internal friction angle, cohesion and tensile strength. It was found that the water to cement ratio affects the uniaxial compressive and shear strength of grout. The triaxial test indicated that both internal friction angle and cohesion of Stratabinder do not differ significantly from BU100. 1. Introduction Prior to the late 1940's, a large proportion of roof supports in underground mines in Australia consisted solely of timber deployed along roadways. The fragile nature of the timber was the cause of a considerable roof failures and rib collapses prior to the introduction of roof bolting. The early roof bolts consisted predominantly of a mechanical anchor positioned at the base of the drill hole. Subsequently, the fully encapsulated rock bolts were developed to bind the bolt and surrounding rock after installation by means of resin or grout. The capability of load transfer of an encapsulated rock bolt is influenced by the resin or grout mechanical properties. Aziz et al. [1–3] carried out a detailed research study with the aim of establishing a general practice standard for determination of mechanical properties of resin. The study included; determination of the Uniaxial Compressive Strength (UCS), the Elastic modulus (E) value in compression, shear strength and rheological properties. These mechanical properties were examined at the University of Wollongong laboratory in relation to resin sample shape, size, height to width or diameter ratio, resin type, resin age and cure time. The following conclusions were reported: The UCS values determined from various shaped samples differed with respect to the sample shape and size and height to diameter ratio, Typically, the UCS values were highest for 40 mm cubes and 40 mm diameter cylindrical sample with height to diameter ratio of two, The ratio between cube strength and cylinder strength varied from El to 1.3, The E value increased as the resin sample curing time increased from 7 to 21 days, The cube samples exhibited higher E values in comparison with cylindrical specimens at various curing time, Similar to UCS values, the average shear strength of grout samples increased with increasing curing time, Cube samples were suggested as a universal shape for testing resin products as they can be easily prepared and tested.

ABSTRACT: An enriched finite element procedure is proposed for formulating interaction of steel rock bolt along with multiple joints in a rock mass. This numerical method involves analysis of bolt behavior with varying joint dip angle, bolt angle, joint-bolt-grout properties in extended finite element platform. This paper presents the procedures and derivation of stiffness matrices of the bolt crossing multiple joints (BCMJ) at any arbitrary orientation. Multiple joints and a bolt can intersect an element enriching nodal degrees of freedom. In this case, constitutive model of rock, rock-joint rock-bolt and joint-bolt are incorporated into the formulation of stiffness matrix. The proposed method is implemented using 3-noded triangular elements and can be upgraded for higher order elements. This procedure is then applied to evaluate the behavior of a bolted joint sample under direct shear test. The method is also applied to analyze reinforced circular tunnel made in rock mass having multiple intersecting joints at different orientations. The results in terms of displacements and stresses are compared with unbolted rock mass model. The proposed method shows the efficacy of enriched finite element method for incorporating discrete discontinuities and rock bolts in finite element platform. 1 INTRODUCTION Rock mass in general contains large number of joint sets, strength of which is characterized by the joint spacing, aperture, joint fill strength and intact rock strength. In underground excavations and sometimes open pit slopes, rock bolt is widely used primary support system to stabilize the rock mass. Accurately modelling the rock bolt in rock mass is complex if interaction behaviour between rock, joint and bolt are to be considered. In general, rock bolt reinforces rockmass through restraining deformation within rock mass (Li & Stillborg 1999) and reduces the yield region around the excavation boundary. For a fully grouted passive rock bolt installed in a deformable rock mass, the axial load distribution along the bolt length indicates a neutral point where the shear stress at the interface between the bolt and grout material vanishes. Closed form elasto-plastic analytical solution of grouted bolt around a circular tunnel have been presented by many researchers (Stille et al. 1989, Cai et al. 2004, Carranza-Torres 2009, Osgoui & Oreste 2010). Modelling of rock bolt in jointed rock mass becomes a challenging task if complex mechanisms of rock bolt behaviour and dowel effects due to joint crossing bolt are taken into consideration. A three dimensional elasto-viscoplastic constitutive model has been derived to take special attention of bolt behaviour at a joint (Chen & Egger 1999).

ABSTRACT: In this study, a new apparatus which have new jaw clutch mechanism is designed to determine the direct tensile strength of rocks. The determination of the direct (with new apparatus) and indirect tensile strength as Brazilian test method of travertine rock was carried out in compliance with related standard test methods. In order to analyse rock tensile strength between direct and indirect test methods was conducted and obtained results were compared between them. Dumbbell shape samples were used in direct tensile tests and cylindrical specimens were used in Brazilian test. Stress distribution and the stress intensity within the sample were analysed with Finite Elements Method and numerical modelling techniques using ANSYS R.14 software. The direct tensile test yielded lower strength values than the indirect test. Direct tensile test is affected to a greater degree by the presence of microfractures and microfissures in the rock. For the specimens tested in direct tensile test, failure occurred near the radius of the dumbbell shape sample due to stress intensity factor. Therefore, direct tensile stress should be re-calculated by considering stress intensity factor. Since Brazilian disc test strain value in the diametrical line of the specimen has been found higher than nominal strain value according to FEM, tensile strength value which should be re-calculated by using proposed the coefficient. 1 INTRODUCTION Characterizing the tensile strength of rocks thus is of great importance in many engineering and geophysical applications (Dai et al. 2010). The tensile strength is a key parameter for determining the load bearing capacity of rocks, their deformation, damage and fracturing, crushing, drilling, tunnel boring, and blasting etc., and is used to analyze the stability and serviceability of rock structures (Danet al. 2013, Daiet al. 2013). Rocks are less resistant to tension than to compression or shear. Tension cracks often develop before compression or shear failure (Liao 1997). Rock is a brittle material and because all brittle materials are weak in tension, the tensile strength of rock becomes one of the most important parameters influencing its deformability and fracture toughness (Erarslan&Williams 2012). These phenomena provide evidence of direct tensile failure in rock structures. Thus, an understanding of the rock behavior under tension can be beneficial in the analysis that involves either intact rock or rock masses (Liao 1997). Tensile strength is defined as the failure stress of a rock element in pure uniaxial tensile loading. Thus, the direct pull test appears to be best suited for tensile strength measurement (Dai et al. 2010). However, direct tensile strength and deformability of rocks are difficult in the laboratory, and field. Because of the difficulties associated with experimentation in direct tensile tests, various of indirect methods have been suggested for determining the tensile strength of materials such as the Brazilian disc test, ring tests, and bending tests. These indirect methods aim at generating tensile stress in the sample by far-field compression, which is much easier, cheaper and common method for determining the tensile strength of rock in instrumentation than direct pull tests (Dai et al. 2010).

ABSTRACT: Test methods for evaluating the dynamic mechanical properties of a discontinuous rock mass have never previously been established. Therefore, we developed an in-situ triaxial test method for dynamic (cyclic) triaxial tests. Then, in-situ cyclic triaxial tests were carried out on discontinuous rock made of weathered sandstone. The strain level dependencies of the Young's modulus and damping ratio were evaluated based on the results of these in-situ tests. 1 INTRODUCTION In Japan, the design earthquake motions for nuclear power plant increased than ever before since the Great East Japan Earthquake. Therefore, a dynamic analysis in the stability evaluation of a rock foundation and surrounding rock slope at a plant has become increasingly important. To improve the credibility of the analysis results, it is necessary to adequately evaluate the dynamic (cyclic) deformation characteristics of the rock used for the dynamic analysis. Sedimentary soft rocks, in which discontinuities have less influence, do not have a significant scale effect (Ikemi et al. 1998). Therefore, dynamic deformation characteristics (strain-level dependencies ofYoung's modulus and hysteresis damping ratio), that is used for the dynamic analysis, are examined by elastic wave velocity logging in the field and cyclic triaxial tests in the laboratory, as well as the soil ground. On the other hand, the mechanical properties of a discontinuous rock mass are evaluated using in-situ rock tests of large-scale specimens. These tests include a plate loading test (PLT) and rock shear test (RST). In the past, several cyclic loading PLTs and RSTs for in-situ rock masses were attempted to evaluate the cyclic deformation and strength characteristics (JSCE 1989, Fujiwaraet al. 1984). In recent years, these tests have not been put into practical use because they are not element tests, and the interpretation of the test results is difficult. Therefore, we developed an in-situ cyclic loading triaxial test method that uses a large scale specimen, which is equivalent to an in-situ rock test. Then, two in-situ cyclic triaxial tests were carried out on a discontinuous rock mass of weathered sandstone.

ABSTRACT: The underground facilities are used for a wide range of civil and military applications (railways, highways and material storage). These underground structures are constructed in different possible ground conditions ranging from soft ground to massive jointed rock. Generally, the loads on tunnels consist of both static (overburden) and dynamic (Blast, earthquake and traffic) loads. Therefore, in the present work an attempt is made to simulate the in-situ condition through the physical modeling, to understand the tunnel deformation behavior under different loading conditions in soft rocks. The dimensions (30 cm × 20 cm × 20 cm) of the model are considered on the basis of all boundary conditions. The engineering properties of the different materials are evaluated experimentally (UCS, elastic modulus and Poisson's ratio) and the Plaster of Paris (PoP) mixed with sand and some percent of clay is selected for physical modeling, as the PoP alone shows brittle behavior. It is interpreted that the height of overburden is a critical factor in correlating degree of fracture and damage to the tunnel - lining. Series of experiments are carried out with lining materials and litho-static pressures under static loading conditions. Further, the deformation and degree of damage is determined for lined and unlined conditions. Also fracture thresholds are computed for different overburden thicknesses for safety of underground structures in weak soft rocks. 1 INTRODUCTION Civil engineering tunnels are constructed in the full spectrum of possible ground conditions ranging from soft ground to massive unjointed rock, coupled with various states of in situ stress and hydrogeology. Dams and hydroelectric power plants look for a better geological condition; tunnels and highways look for a better alignment escaping from weak zones, whenever possible. Even though the shape of the opening mainly depends on the purpose for which it is to be used, the safe design and construction of an underground opening requires the knowledge of the stress distribution and the displacements that occurs in and around the openings. When an opening is excavated in a medium, the in situ stress field is disturbed and new set of stresses are induced around the surrounding of the opening. Before the tunnel is excavated, the in situ stresses σ ν , σ h 1 , and σ h 2 are uniformly distributed in the slice of rock under consideration. After removal of the rock from within the tunnel, the stresses in the immediate vicinity of the tunnel are changed and new stresses are induced. Three principal stresses σ 1 , σ 2 and σ 3 acting on a typical element of rock are shown and explained well in Figure 1. Most of the theories, existed compute the vertical stress at a point as the stress due to the lithostatic pressure, and the horizontal stress as that stress required to completely restrain lateral deformation of an elastic body acted upon by the vertical stress. The theory predicts that the horizontal stress will be some fraction of the vertical or overburden stress dependent upon the value of Poisson's ratio for the rock. The stability of underground excavations is affected by its shape, size of opening, in situ stress, soil/rock conditions etc. Knowledge of the magnitudes and directions of these in-situ and induced stresses is an essential component of underground excavation (Goodman 1989). The horizontal stresses acting on an element of rock at a depth ‘z’ belowthe surface is much more difficult to estimate than the vertical stresses. (equation) Terzaghi & Richart (1952) suggested that, for a gravitationally loaded rock mass in which no lateral strain was permitted during formation of the overlying strata, the value of k is independent of depth and is given by k = v /(1 — ν), where ν is the Poisson's ratio of the rock mass. Measurements of horizontal stresses at civil and mining sites around the world show that the ratio k tends to be high at shallow depth and that it decreases at depth (Brown & Hoek 1978, Herget 1988).

ABSTRACT: The mechanical properties of conglomeratic rocks are difficult to study by conventional laboratory tests. On the other hand, in-situ tests are more representative to establish the mechanical properties of these materials but, in general, take a long time to execute and represent a high cost for projects. This paper presents a methodology to obtain the mechanical properties of a conglomerate, using the results of a numerical modeling study carried out in virtual rock samples. A conglomeratic rock from Costa Rica was used as a study case. To determine the validity of the estimation, the results from the numerical modeling were compared with data from in-situ tests performed in the same conglomerate. 1 INTRODUCTION Many engineering works in Costa Rica are developed in conglomerates. The engineering design of these works requires a comprehensive protocol for characterization and for evaluation of the mechanical properties of these materials. The Costa Rican Electricity Institute (ICE) is currently studying the largest hydroelectric project in Central America, El Diquís Hydroelectric Project (PHED). For this project, it is expected to have open cut sections up to 110 m high, underground excavations with a span between 6 m and 32 m, and a foundation for the 170 m high concrete face rockfill dam. The conglomeratic rocks found in the area of study are classified as unwelded bimrocks (Sonmez et al. 2009). These rocks, in essence, are a mixture of good-quality blocks, embedded in a less competent matrix, whose shear strength and deformability are mainly ruled by the volumetric block proportion (VBP) (Lindquist 1994). This paper presents some of the in situ investigations carried out to describe the conglomerate of the Paso Real Formation. Samples representative of the matrix were obtained and tested in laboratory scale for both physical and mechanical properties. Field tests were carried out in order to obtain deformability and shear strength. Jiménez (2015) presents a full description of the laboratory tests and field works. The paper also presents a methodology to incorporate blocks within a fine-grain matrix and generate the rock properties through numerical experiments. Synthetic samples are generated in order to represent the conglomerate in the field. The numerical experiments results compare well with the field test results.

ABSTRACT: The effective pressure law for the permeability of sandstone is experimentally studied by conducting Darcy's law permeability test under various confining pressure (from 14 to 58 MPa) and pore pressure (from 8 to 23 MPa). Supercritical CO 2 as a pore fluid has been adopted instead of water for carbon capture and storage (CCS) study. The effective pressure law for permeability is associated with curved iso-permeability lines. We can express the effective pressure as P eff = P c — χ(P c ,P p )P p , where the coefficient χ corresponds to the slope of iso-permeability line according to the confining(P c ) and pore(P p ) pressure. It was confirmed that the coefficient is not a constant because of the nonlinearity of iso-permeability line. Cycling test of the permeability has been conducted for analyzing the phenomenon using a modified factorial method. The influence factor and interaction between the pore and confining pressure were analyzed. 1 INTRODUCTION In CO 2 storage project, CO 2 permeability of reservoir rock for a given in-situ stress and pore pressure is very important to monitor and predict diffusion range of the CO 2 . The permeability of gas is different from the permeability of liquid such as distilled water or brine. The permeability of supercritical CO 2 (hereafter scCO 2 ) should be examined considering the mixed phases and therefore mixed properties of gas and liquid. For the laboratory test, pore pressure of rock specimens generated by CO 2 injection must be 7.3 MPa or higher to keep CO 2 in supercritical state. Confining pressure on the specimens should be greater than the pore pressure. The influence of external (confining) and internal (pore) pressures on rock permeability has been reported by many researchers. There are, however, no cases where scCO 2 , has been used as injection fluid. It is well known that the permeability of porous media is a pressure-dependent property. Therefore, an effective pressure law affects the permeability. Because K (permeability) depends on P c (confining pressure) and P p (pore pressure), we can replace the two-variable function by a single variable one, P eff (P c , P p ), according to the effective pressure law: (equation)(1)

ABSTRACT: Carbon dioxide (CO 2 ) storage in deep coal seams significantly alters the petro-physical properties of those coal masses, hence precisely evaluation of such alternations is difficult due to the highly heterogeneous nature of coal. This urges the important of testing reconstituted coal samples (homogeneous) in this regards. The main objective of this study is therefore to investigate the effect of different fluid saturations on the mechanical properties of coal using reconstituted low-rank coal samples. These reconstituted coal samples were made using the powdered Australian brown coal, which were first molded and then subjected to an axial load until the specimens achieve the mechanical properties of natural specimens. The prepared samples were then saturated in water, N 2 and CO 2 to check the various saturation influences on coal mechanical properties. Findings indicate that water and CO 2 saturation causes significant strength reduction in low-rank coal compared to unsaturated samples while N 2 exhibited a strength gain. 1 INTRODUCTION The process of Enhanced Coal Bed Methane (ECBM) recovery is being implemented and tested as a viable option to store and reduce the amount of carbon dioxide (CO 2 ) in the Earth's atmosphere, as well as for the recovery of useful methane (CH 4 ) gas (Perera et al. 2013, Ranathunga et al. 2014). Overall, the ECBM process involves introducing CO2 through injecting wells into deep coal seams. This CO 2 then acts as a displacing gas, which allows the already adsorbed CH4 to be desorbed from the coal matrix. The methane is finally obtained through a recovery well and used to produce energy in a cost-effective and environmental friendly way. However, the ECBM recovery technique has its associated advantages and disadvantages. One of the major issues involved in injection of CO 2 , is the significant alternations of coal seams petro-physical properties. Perera et al. (2013) and Ranathunga et al. (2016) have found that the strength and the Young's modulus of coal vary with the adsorption of CO 2 into coal matrix. Moreover, since coal is a highly heterogeneous material, it is difficult to determine the properties of the entire coal seam in the field. Therefore, it is essential to establish an acceptable method of manufacturing reconstituted coal samples to stimulate the field properties. Several approaches have been trialed to produce a more homogeneous representation of natural coal that would be more amenable to experimental investigation of mechanical and flow behavior.

Abstract This paper addresses the issue of the tensile strength of incipient discontinuities in rock and presents preliminary results from a series of laboratory studies. In most rock masses rock discontinuities, as veins or incipient fractures, often retain some tensile strength that may approach that of the parent rock. This fact is of high importance to rock mass strength but is generally ignored, neglected or underestimated. Samples of incipient rock discontinuities including joints, bedding and mineral veins have been tested in direct tension. It has been confirmed that incipient, visible and discrete discontinuities, that might be recorded as ‘joints’ in a rock mass characterisation programme can indeed have high tensile strength, approaching that of parent rock. Others are of course far weaker. The factors contributing to tensile strength have been examined. It is concluded that the degree of incipiency of rock discontinuities needs to be differentiated in the process of rock mass classification and engineering design and this can best be done with reference to the tensile strength relative to that of the parent rock. 1 Introduction Rock discontinuity is a collective term which encompasses joints, beddings, faults, layers and fissures, etc. Generally these fractures develop from an original incipient state with high tensile strength to fully mechanical discontinuities with zero tensile strength (Hencher & Knipe 2007). Incipient discontinuities have a controlling influence on many projects in terms of strength, deformability and permeability. For engineering simplification purpose, incipiency of rock discontinuities is usually neglected or underestimated. ISRM standard defines discontinuities have very low or zero tensile strength (ISRM 1978). Rock mass classification schemes such as Q system, RMR and RQD are defined on the basis of this engineering assumption which means that they all fail to resolve the issue of incipiency of discontinuities as well as varying degrees of tensile strength. Degree of incipiency of rock discontinuities should not be neglected as it influences the overall strength of rock. Figure 1a presents an example of joint incipiency provided by Hencher when shearing a rock core. Light rock bridge segment is revealed after shearing test. Another incipient joint is shown in Figure 1b, intact rock bridges indicated by red arrows are shown after breaking along this fracture with geological hammer.

Abstract The long-term deformation behavior of the Callovo-Oxfordian and Opalinus clay rocks has been extensively investigated with laboratory experiments under various thermo-hydro-mechanical conditions. For the highly consolidated clay rocks, the effective stress is both theoretically and experimentally examined by involving the bound porewater between clay particles. The long-term deformation of the clay rocks is determined with triaxial creep tests on water-saturated samples at ambient temperature. Reponses of the clay rocks to humidity changes are examined by drying and wetting the samples under different load conditions. Thermal effects are studied by heating and cooling the stressed samples in drained and undrained conditions. Major findings are presented and discussed in this paper. 1 Introduction Clay formations are being globally investigated as host medium for deep disposal of high-level radioactive waste. The long-term deformation behavior of the clay host rock under varying repository conditions is crucial for the long-term performance and safety of a repository. This key issue has been extensively studied by GRS for the last decade with laboratory experiments on the Callovo-Oxfordian and Opalinus claystones (Zhang et al. 2004, 2007, 2010, 2013). Relevant thermo-hydro-mechanical conditions expected in the potential repositories were simulated in the experiments: stress ranging from the initial lithostatic state to redistribution after excavation, humidity variations during the excavation ventilation and water migration from the farfield to the nearfield, and heating from ambient temperature to the designed maximum of 90°C and subsequently cooling down again. Most tests lasted over long time durations up to several years. Major findings are summarized in this paper. 2 Characteristics of Claystones Core samples were taken from the Callovo-Oxfordian argillite (COX) at the Meuse/Haute-Marne- URL in France and the Opalinus clay (OPA) at the Mont-Terri-URL in Switzerland. Both argillaceous formations are results of a specific geological history that lasted hundreds of millions of years, beginning with deposition and aggregation of fine-grained particles in sea water, followed by sedimentation and consolidation with a concurrent expelling of porewater, development of diagenetic bonds between mineral particles, and other processes. They have been highly consolidated to low porosities of 14–18%. The pore sizes mainly range from nanoscale in between the parallel platelets of the clay particles to micro- and mesoscale between solid particles. The fraction of pores smaller than 20 nm amounts to about 60–80% for the clay rocks (Bock et al. 2010). On average, the COX claystone contains 25–55% clay minerals, 20–38% carbonates and 20–30% quartz, while the OPA clay-schist has higher clay contents of 58–76%, less carbonates of 6–24% and quartz of 5–28%. The clayey matrix of the clay rocks is embedding other mineral particles. The clayey matrix consists of particles with strongly adsorbed interlayer water and strongly to weakly adsorbed water at the external surfaces. Water in large pores is freely movable.

Abstract The paper discusses the reasons for the introduction of new mining system. Details of investigations to determine in situ rock mass strength are presented. Back analysis of existing mine tunnels based on visual tunnel observations, diamond drill core analysis and numerical simulations and the RFC-system for estimating tunnel conditions in deep gold mines yielded similar rock strength values. Hoek-Brown method of rock mass strength determination resulted in unrealistically low uniaxial compressive strength and relatively low global compressive strength values. Behavior of 7 m wide stope pillars during a full mining cycle comprising of development of 5 m high top and bottom headings, removal of 11 m high sill pillar between these headings, backfilling of stope and mining of adjacent stope has been monitored using multiple rod extensometers. Results of a full stoping cycle are presented. The beneficial effects of early backfilling of the high stopes have been demonstrated. 1 Introduction At the Breitenau mine a string of magnesite lenses of irregular shape having strike and dip extensions of up to 200 m and a height of up to 100 m are mined using post pillar mining method. Pillar and room dimensions are about 7.5 m. The height of individual lifts is 3.5 m. After completing a lift it is backfilled using gravity fill. Up to 35 lifts have been mined in some of the larger magnesite lenses. With increasing mining depth rock pressure problems have been encountered. For this reason it was decided to change the stoping method in the deeper areas of the mine from post pillar mining to a cut and fill method where 70 m to 80 m long 6 m to 7 m wide and 21 m high stopes are excavated and subsequently back filled using paste fill. The stopes are separated by 7 m wide pillars. Between mining panels of four to six stopes wide barrier pillars are left in place to ensure regional stability. In contrast to the post pillar mining method where the pillar workings extend over the full areal extent of the mineral body with the new method only one stope is excavated in a mining panel at any moment in time. After backfilling a stope the adjacent stope is excavated. Mining sequence in a stope is such that at the top and bottom of the stope 5 m to 6 m high headings with a width corresponding to the stope width are developed. The sill pillar between the two headings is then mined by bench blasting. The advantages of new stoping method are that the stope does not have to be entered by mining personnel and that only one stope is mined in a panel at any moment in time. This paper describes the rock mechanics investigations that have been carried out in connection with the introduction of the new stoping method.

Abstract At shallow depth, in situ stress can affect significantly the behavior of the rock mass around excavations, but stress measurements are usually subjected to high levels of uncertainty. This is the case at the Odenplan railway station in Stockholm where unexpected large values of displacements were recorded during the excavation. A sensitivity analysis of in situ stress in this area is thus required. As the rock mass is highly discontinuous, discrete modeling using 3DEC software is performed with three different in situ stress cases estimated from measurements. The results show that joint slip and rock mass dilatancy explain the large displacements and heaving of the ground and that a continuum approach is not reliable for this study. A comparison between the displacements from numerical modeling and those measured in situ provides indication on the range of in situ stress to be used in the future for this area. 1 Introduction Underground constructions in urban areas constitute an important part of the infrastructure needed for society. Being at shallow depth, their potential impact on the environment and surrounding structures can be significant. Particularly in Scandinavia, where horizontal stresses are high, performing excavations at shallow depth in brittle hard rock, combined with irregular surface topology and open fractures, can lead to large deformations and upheaving of the ground. The estimation of in situ stress is required in order to study the behavior of the rock mass around such excavations and limit its potential deformation. However, data concerning in situ stress field is usually associated with high uncertainties particularly at shallow depth where it is affected by the topography of the rock surface, near-surface weathering, exfoliation joints… Additionally, stress measurements are performed at specific points and are usually subjected to high scatter. It is therefore important to complement these measurements with back analysis at a larger scale (Kaiser et al. 2000), in order to gain insight into the in situ stress field in a specific area. This study deals with a preliminary back analysis of in situ stress in the Stockholm area using displacement measurements performed during the excavation of Odenplan station, which is part of the Citybanan, a railway tunnel under construction by Trafikverket. Around this station, in situ overcoring stress measurements have been performed (Perman & Sjöberg 2007) and present a considerable scatter. It is also important to note that the rock mass surrounding Odenplan station is highly discontinuous. Available data from mapping shows that open fractures are unusually persistent especially those with a dip angle from 50 to 70 degrees. During the excavation of the railway tunnel, large displacements were recorded in the area where the tunnel crosses under the surface subway track. Measurements in extensometers installed between the surface and the tunnel (up to 10 mm of extension) suggest heaving of the surface rock at some locations. Such amplitudes of displacements were not accounted for during the design. Thus, a precise analysis is required to explain the phenomena observed. The access at the Odenplan station to a wide set of data (rock mass and joint properties, joint geometry, deformation and stress measurements) makes it a unique opportunity to perform a sensitivity analysis of in situ stress for the first time in this area.