ALBERT

Description: In this paper, fracture stiffness in rock samples is determined by means of hydromechanical laboratory testing. The aim is three-fold: to develop a procedure for sampling, to update testing equipment and to relate fracture stiffness to the geological history (e.g., stress history and fracture infillings). The hydraulic properties of twenty rock cores (diameter 190 mm, c. 100 mm high) from the Äspö Hard Rock Laboratory were tested in a permeameter cell under different isotropic pressures up to 2.5 MPa. The flow rate through individual fracture samples was recorded. Four of the samples were re-tested in the permeameter cell using an updated hydromechanical procedure with deformation measurement across the fracture. Four load cycles of gradually increasing cell pressure were applied, resulting in a clearly observed hysteresis effect in the first and second cycles. Hydraulic aperture changes calculated using the cubic law were compared with their mechanical equivalents. The aperture changes followed similar trends, although these differed between the samples. Fracture stiffness was determined from the tests, and the stiffness to hydraulic aperture relationship was found to follow previously published patterns linked to the storativity of fractures. Differences in stiffness are explained in the context of the geological history of individual samples, particularly their stress history. The paper presents a conceptualisation of the stiffness behaviour, which includes flow properties, geometric properties and the geological stress history of the tested samples.

Description: Methane extraction from in-seam boreholes is the main approach for recovering methane in China. However, the methane concentration for this method is generally lower than 30 %, which incurs a risk of methane outbursts during pipeline transportation. To increase the methane concentration, we first conducted permeability experiments to investigate the relationships between the permeability and the effective stress at different stages in the complete effective stress–strain process. We then adopted FLAC3D software to calculate the stress distributions around roadways and boreholes after their consecutive excavations and thereby divided the coal mass around the roadway and borehole according to different effective stress stages to understand the gas flow characteristics. The results show that the coal mass along the radial direction of the roadway and borehole can be sequentially divided into four zones, including the full flow zone (FFZ), the transitive flow zone (TFZ), the flow-shielding zone (FSZ), and the in situ rock flow zone (IRFZ), which have been proven correct by field experiments. The methane in the IRFZ was difficult to extract because of the low permeability of coal mass in this zone. The permeability of the FSZ was lower than that of the IRFZ. The permeability along the interface between the FSZ and TFZ was nearly one time as low as that of the IRFZ, while the permeability of the FFZ was two orders of magnitude higher than that of the IRFZ. This four-zone division demonstrates the decaying mechanism of methane extraction concentration and flow in the in-seam borehole and can provide theoretical guidance for improvement of methane extraction.

Description: This paper presents a new, fast and economical method for digitizing a rock surface profile to determine its joint roughness coefficient (JRC) value (Barton and Choubey, Rock Mech Rock Eng 10:1–54, 1977 ) using a set of existing parameters, the appropriateness of which has been tested. The new procedure was applied to four rock road cuts in the Alpujarra area of Granada province (Andalusia, Spain), supported by 117 digitized profiles. The method avoids any personal subjectivity bias. The usual subjectivity in the visualization and assessment of the JRC value is estimated here by a survey test answered by 90 trained users. The survey test consisted of a set of JRC visual comparisons of 12 roughness profiles, selected from the 117 profiles surveyed during the field research in the Alpujarra region, with the Barton typical profiles. The results of the survey clearly show systematic inaccuracies in the traditional procedure that are reduced if the advice presented in the conclusions of this paper is taken in account when performing the visual estimation.

Description: This study focuses on the characterisation of thermal conductivity for three potential host rocks for radioactive waste disposal. First, the heat conduction process is reviewed on the basis of an analytical solution and key aspects related to anisotropic conduction are discussed. Then the existing information on the three rocks is summarised and a broad uncertainty range of thermal conductivity is estimated based on the mineralogical composition. Procedures to backanalyse the thermal conductivity on the basis of in situ heating tests are assessed and a methodology is put forward. Finally, this methodology is used to estimate the impact of experimental uncertainties and applied to the four in situ heating tests. In the three potential host rocks, a clear influence of the bedding planes was identified and anisotropic heat conduction was shown to be necessary to interpret the observed temperature field. Experimental uncertainties were also shown to induce a larger uncertainty on the anisotropy ratio than on the equivalent thermal conductivity defined as the geometric mean of the thermal conductivity in the three principal directions.

Description: The mechanical behavior of bedded and jointed rock was examined in the context of the design of slopes, tunnels and tunnel portals along a planned new rail line. To obtain representative properties for describing the mechanical behavior of this material, large scale triaxial tests were performed. Specimens with a size of 60 cm diameter and 120 cm height composed of sandstone and claystone beds were tested in triaxial compression using a multi-stage technique. The specimens showed a ductile behavior for the stress regime tested. The strength parameters were calculated according to the Mohr–Coulomb failure criterion. The axial and volumetric deformations were measured and the viscosity of the composite rock material was assessed by deformation rate alterations. For some specimens, direct shear tests were conducted in samples containing joints. The results of the tests show that the failure of the bedded specimens was given by the combined failure of both materials. This was confirmed by a numerical model including rock discontinuities.

Description: Hydraulic fracturing is a complex operation which is influenced by several factors including the formation properties, state of stresses in the field, injecting fluid and pumping rate. Before carrying out the expensive fracturing operation in the field, it would be useful to understand the effect of various parameters by conducting physical experiments in the laboratory. Also, laboratory experiments are valuable for validating numerical simulations. For this purpose, laboratory experiments may be conducted on synthetically made samples to study the effect of various parameters before using real rock samples, which may not be readily available. To simulate the real stress conditions in the field, experiments need to be conducted on cube-shaped samples on which three independent stresses can be applied. The hydro-mechanical properties of a sample required for modelling purposes and the design of a scaled hydraulic fracturing test in the laboratory can be estimated by performing various laboratory experiments on cylindrical plugs. The results of laboratory experiments are scaled to field operation by applying scaling laws. In this paper, the steps to prepare a cube-shaped mortar sample are explained. This follows a review of the sample set-up procedure in a true tri-axial stress cell for hydraulic fracturing experiments. Also, the minimum tests on cylindrical plugs required to estimate the hydro-mechanical properties of the rock sample are explained. To simulate the interaction mode when a hydraulic fracture approaches an interface in the laboratory, the procedure for producing samples with parallel artificial fracture planes is explained in this paper. The in-fill material and the angle of fracture planes were changed in different samples to investigate the effect of interface cohesion and the angle of approach on the interaction mechanism.

Description: Long-term stability of the waterway system of the hydropower plants is crucial and should not be underestimated. The compromise may result in severe economic consequences related to revenue loss caused by the plant closedown for needed repair, extra resources and time required for repair work, and third party loss related to industries and societies at large. In addition, possible contractual disputes between the clients and the contractors may arise in some occasions. Serious accidents may happen during repair and construction work with loss of life, since engineering geological environment (conditions) in the rock mass changed once under water for long period. This article focuses on one of the recent shaft collapse that happened in Norway in 2008. The article discusses and analyses the dynamics of burst debris flood that took place on 9 May 2009, while removing the slide rock mass deposited in the 45° inclined shaft of the Svandalsflona hydropower plant located at the Southern Norway. Careful review on the geological conditions inside the shaft, evaluation on the course of events, investigations on the inspections and inspections reports, assessment on the temperature and precipitation conditions have been carried out to come to the conclusion on what might have triggered the sudden burst flood.

Description: Water inflows are a major challenge in tunnelling and particularly difficult to predict in geological settings consisting of heterogeneous sedimentary rock formations with complex tectonic structure. For a high-speed railway line between Bologna and Florence (Italy), a series of seven railway tunnels was drilled through turbiditic formations, ranging from pelitic rocks with thin arenitic layers over sequences including thick-bedded sandstone to calcareous rocks showing chemical dissolution phenomena (karstification). The tunnels were built as draining tunnels and caused significant impacts, such as drying of springs and base-flow losses at mountain streams. A comprehensive hydrological monitoring programme and four multi-tracer test were done, focusing on four sections of the tunnel system. The tracer tests delivered unprecedented data on groundwater flow and transport in turbiditic aquifers and made it possible to better characterize the differential impacts of tunnel drainage along a geological gradient. The impact radius is 200 m in the thin-bedded sequences but reaches 2.3–4.0 km in calcareous and thick-bedded arenitic turbidites. Linear flow velocities, as determined from the peaks of the tracer breakthrough curves, range from 3.6 m/day in the thin-bedded turbidites to 39 m/day in the calcareous rocks (average values from the four test sites). At several places, discrete fault zones were identified as main hydraulic pathways between impacted streams and draining tunnels. This case shows that ignoring the hydrogeological conditions in construction projects can cause terrible damage, and the study presents an approach to better predict hydraulic impacts of draining tunnels in complex sedimentary rock settings.

Description: Water ingress represents one of the main challenges in subsea tunnelling, particularly when this occurs in sections with poor rock mass quality. This paper is discussing the main characteristics of water ingress in subsea hard rock tunnels based on the experience from almost 50 such tunnels that have been built in Norway. Following a brief description of the geological conditions and the basic design of the subsea tunnels, pre-construction investigations and investigations during excavation are discussed with particular emphasis on prediction of water ingress. Two cases with particularly difficult conditions; the Bjorøy tunnel and the Atlantic Ocean tunnel, are discussed in detail. In these cases, large water inflow with pressure of up to 2.4 MPa was encountered at major faults/weakness zones during excavation, and special procedures were required to cope with the problems. Based on the experience from the Norwegian projects, it is concluded that continuous follow-up by experienced engineering geologists, probe drilling with the drilling jumbo and pre-grouting where required are the most important factors for coping with water ingress and ensuring stability.

Description: The Løren road tunnel is a part of a major project at Ring road 3 in Oslo, Norway. The rock part of the tunnel is 915 m long and has two tubes with three lanes and breakdown lanes. Strict water ingress restriction was specified and continuous rock mass grouting was, therefore, carried out for the entire tunnel, which was excavated in folded Cambro-Silurian shales intruded by numerous dykes. This paper describes the rock mass grouting that was carried out for the Løren tunnel. Particular emphasis is placed on discussing grout consumption and the challenges that were encountered when passing under a distinct rock depression. Measurement while drilling (MWD) technology was used for this project, and, in this paper, the relationships between the drill parameter interpretation (DPI) factors water and fracturing are examined in relation to grout volumes. A lowering of the groundwater table was experienced during excavation under the rock depression, but the groundwater was nearly re-established after completion of the main construction work. A planned 80-m watertight concrete lining was not required to be built due to the excellent results from grouting in the rock depression area. A relationship was found between leakages mapped in the tunnel and the DPI water factor, indicating that water is actually present where the DPI water factor shows water in the rock. It is concluded that, for the Løren tunnel, careful planning and high-quality execution of the rock mass grouting made the measured water ingress meet the restrictions. For future projects, the DPI water factor may be used to give a better understanding of the material in which the rock mass grouting is performed and may also be used to reduce the time spent and volumes used when grouting.

Description: One of the most challenging aspects of tunnelling is prognostication of water inflows. More reliable prediction of groundwater inflow may give considerable economical saving for future tunnel projects and may also prevent damage of environment and installations on the surface. This paper is discussing the significance of eight hypotheses regarding geological parameters for predicting water inflow in tunnels. The respective hypotheses have been tested as part of a recent research project in Norway. Six Norwegian tunnels with different geological conditions were selected for the research; the Romeriksporten, Frøya, T-baneringen, Lunner, Skaugum, and Storsand tunnels. Based on detailed study of these tunnels, the hypotheses are tested by comparing water inflow with geological parameters and factors such as Q value, faulting, rock stress orientation, rock cover, thickness of permeable soil or depth of lake/sea above the tunnel, rock type, and width of weakness zones. It is found that four out of the eight tested hypotheses are supported, two have low to medium support and two are not supported. One unexpected result is that for the tunnels covered by this study, the water inflow was found to increase with rock cover.

Description: Interception of aquifers by tunnel excavation results in water inflow and leads to drawdown of the water table which may induce ground settlement. In this work, analytical and numerical models are presented which specifically address these groundwater related processes in tunnel excavation. These developed models are compared and their performance as predictive tools is evaluated. Firstly, the water inflow in deep tunnels is treated. It is shown that introducing a reduction factor accounting for the effect of effective stress on hydrodynamic parameters avoids overestimation. This effect can be considered in numerical models using effective stress-dependent parameters. Then, quantification of ground settlement is addressed by a transient analytical solution. These solutions are then successfully applied to the data obtained during the excavation of the La Praz exploratory tunnel in the Western Alps (France), validating their usefulness as predictive tools.

Description: Since the 1960s, there has been an increasing interest in the understanding of the hydraulic flow inside a hard rock mass, since water inflow into deep tunnels constitute a hazard, in addition to being an important factor in controlling the advancement of excavation. The characterisation of fluid flow through hard rock masses is still one of the most challenging problems faced by geologists and engineers. A rock mass is characterised by networks of discrete and ubiquitous discontinuities that strongly affect its hydraulic properties, but detailed knowledge of the discontinuity properties allows for the evaluation of the hydraulic flow in the rock mass affected by the excavation of a tunnel. A geostructural field survey is fundamental in order to correctly define the discontinuity types, settings and networks. Numerous approaches have been proposed to estimate the water inflow based on empirical relations supported by field experience and case studies, as well as analytical solutions. Often, however, these approaches are not easily applicable in standard practice and in complex scenarios. The most appropriate approach to characterising the hydraulic flow of the rock mass and to predicting in the most effective way the expected water inflow during the excavation of a tunnel is based on a detailed geological model and geostructural analysis as described in this paper.

Description: Groundwater has a negative impact not only in construction activity, but also in stability of a tunnel. Severity increases particularly in tunnels passing through fault gouge and breccia, where rock material is completely crushed and extremely weak. Instantaneous collapse and excessive plastic deformation is most likely in tunnels passing through such zones. Often, ‘flowing’ conditions may prevail if groundwater is mixed in the rock mass. This paper presents one such tunnel case in the Nepal Himalaya; i.e. the Modi pressure tunnel. This pressure tunnel passes through a tectonic fault consisting of gouge material. High deformation in the tunnel was observed while excavating the tunnel through the fault. Based on the tunnel deformation that was actually measured, the paper first back-calculates the rock mass strength by analytical approach. Then, the extent of in-situ stress condition in the area is determined by numerical modeling for the rock mass with no ground water in consideration. The ground water effect is then analyzed. We found that the effect of ground water with a static head 〈1.5 bar pressure may increase the deformation by up to a maximum of 30 %. Finally, we briefly discuss uncertainties related to the input parameter study and used methodologies.

Description: In this paper, we describe the hydrogeological problems related to the closure of the mine at Gavorrano (Tuscany, Italy). The geological and geo-structural settings of the Gavorrano area affect the groundwater flow systems and their chemical composition; hence, the settings also affect the chance and modalities to rehabilitate and re-utilize these water resources. This paper reports the results of studies, analyses of existing data and the consequent implementation of preliminary numerical models with particular reference to the effects of controlled water recovery and the complex measures required for stopping dewatering. The study incorporates recent data and the available historical records in an analysis of the hydrodynamic impact of water recovery and an assessment of the consequences of water recovery for water resources restoration. The applied numerical simulations have been demonstrated to be a promising and effective tool for planning and managing the future applications of groundwater recovery in the Gavorrano mining area. Using these applications, it will be possible to implement mitigation measures and types of re-use that consider thermal, chemical and discharge features.

Description: The aim of this paper is to evaluate and compare the durability, life expectancy and maintenance needs of traditional Norwegian waterproofing concepts to the generally more rigid waterproofing concepts seen in other European countries. The focus will be on solutions for future Norwegian tunnel boring machine railway tunnels. Experiences from operation of newer and older tunnels with different waterproofing concepts have been gathered and analyzed. In the light of functional requirements for Norwegian rail tunnels, some preliminary conclusions about suitable concepts are drawn. Norwegian concepts such as polyethylene panels and lightweight concrete segments with membrane are ruled out. European concepts involving double shell draining systems (inner shell of cast concrete with membrane) and single shell undrained systems (waterproof concrete segments) are generally evaluated as favorable. Sprayable membranes and waterproof/insulating shotcrete are welcomed innovations, but more research is needed to verify their reliability and cost effectiveness compared to the typical European concepts. Increasing traffic and reliance on public transport systems in Norway result in high demand for durable and cost effective solutions.

Description: Indentation testing has been developed as an unconventional method to determine intact rock strength using small rock specimens within the size of drill cuttings. In previous investigations involving indentation testing, researchers have used different indenter stylus geometries, penetration rate (PR) and specimen sizes. These dissimilarities can restrict applications of this method for strength measurement and lead to non-comparable results. This paper investigates the influence of indenter diameter (ID) and PR on indentation indices for carbonate rocks to provide objective comparison and application of the existing correlations. As part of this research, several indentation tests were conducted using different IDs and PRs. The laboratory test results showed that indentation indices can be affected by ID while PR has only minor effect on the indentation indices. Thus, a normalizing function was presented to reduce the dependency of test results to ID. Verification of the findings with independent data confirms the suitability of the suggested normalizing function in determining the rock uniaxial compressive strength using testing data obtained from various IDs and PRs.

Description: The drilling of galleries induces damage propagation in the surrounding medium and creates, around them, the excavation damaged zone (EDZ). The prediction of the extension and fracture structure of this zone remains a major issue, especially in the context of underground nuclear waste storage. Experimental studies on geomaterials indicate that localised deformation in shear band mode usually appears prior to fractures. Thus, the excavation damaged zone can be modelled by considering the development of shear strain localisation bands. In the classical finite element framework, strain localisation suffers a mesh-dependency problem. Therefore, an enhanced model with a regularisation method is required to correctly model the strain localisation behaviour. Among the existing methods, we choose the coupled local second gradient model. We extend it to unsaturated conditions and we include the solid grain compressibility. Furthermore, air ventilation inside underground galleries engenders a rock–atmosphere interaction that could influence the damaged zone. This interaction has to be investigated in order to predict the damaged zone behaviour. Finally, a hydro-mechanical modelling of a gallery excavation in claystone is presented and leads to a fairly good representation of the EDZ. The main objectives of this study are to model the fractures by considering shear strain localisation bands, and to investigate if an isotropic model accurately reproduces the in situ measurements. The numerical results provide information about the damaged zone extension, structure and behaviour that are in very good agreement with in situ measurements and observations. For instance, the strain localisation bands that develop in chevron pattern during the excavation and rock desaturation, due to air ventilation, are observed close to the gallery.

Description: The Xiaowan super-high arch dam has faced challenging construction problems. Here, we provide a scientifically-based reference for applying geomechanical model testing to support the nonlinear design of super-high arch dams. We applied experimental similarity theory and techniques. Based on four 3D geomechanical model tests, the dam stress characteristics, deformation distribution, and the safety factors of the dam foundation were identified and compared. We also analyzed cracking characteristics of the up- and downstream dam surfaces and induced joints in the dam heel, the rock mass failure process of the dam-foundation interface, and the abutments. We propose foundation reinforcement measures for weak rock masses, alteration zones, and other faults in the abutments based on the 3D and plane tests each at a different elevation. The results show that all dam deformations remained normal with no yielding or tensile cracking under a normal water load. The reinforced rock mass increased the crack initial safety in the dam heel and toe by ~20 %. The minimum crack initial safety factor ( K 1 ) of the dam heel was 1.4. The induced joint in the dam heel contributed to a reduction in tensile stress at the upstream dam heel, improving K 1 . Compared with similar projects following reinforcement measures, the abutment stiffness and overall stability of the Xiaowan arch dam satisfy operational requirements. Four years of monitoring operations show that key areas near the dam remained normal and the dam foundation is functioning well. Our results may also be applicable to the design and construction of similar projects worldwide.

Description: Reconstruction of geological structures has the potential to provide additional insight into the effect of the depositional history on the current-day geomechanical and hydro-geologic state. Accurate modeling of the reconstruction process is, however, complex, necessitating advanced procedures for the prediction of fault formation and evolution within fully coupled geomechanical, fluid flow and temperature fields. In this paper, a 3-D computational approach is presented that is able to forward model complex structural evolution with multiple intersecting faults that exhibit large relative movement within a coupled geomechanical/flow environment. The approach adopts the Lagrangian method, complemented by robust and efficient automated adaptive meshing techniques, an elasto-plastic constitutive model based on critical state concepts, and global energy dissipation regularized by inclusion of fracture energy in the equations governing state variable evolution. The proposed model is validated by comparison of 2-D plane strain and 3-D thin-slice predictions of a bench-scale experiment, and then applied to two conceptual coupled geomechanical/fluid flow field-scale benchmarks.

Description: Under compression, gathering data related to the post-failure part of the stress–strain curve requires stiff servo-controlled testing systems. In unconfined conditions, data related to the post-peak region of the intact rock parameters are not common as pre-peak and peak state parameters of stress–strain behavior. For problems involving rock in the failed state around structures, proper choice of plastic constitutive laws and post-failure parameters is important for the modeling of the failed state. The aim is to relate commonly used intact rock parameters of pre-failure (tangent modulus E i and secant modulus E s ) and peak strength ( σ ci ) states to parameters of the post-failure state under unconfined compression. Post-failure parameters are the drop modulus ( D pf ), representing the slope of the falling portion in brittle state, residual strength ( σ cr ), and dilatancy angle ( ψ °). Complete stress–strain curves were generated for various intact rock of different origin. Seventy-three post-failure tests were conducted. Samples included in the testing program were chosen to represent rocks of different origin. Specimens of granite, rhyodacite, dunite, quartzite series, glauberite, argillite, marl, and lignite were used in the tests. The results from the pre-failure and peak state testing parts were processed and compared to the post-failure stress–strain parameters. For the estimation of post-failure parameters in terms of the pre-peak and peak states, the functional relations were assessed. It was found that the drop modulus D pf increases with rock strength σ ci , following a power function with an approximate power of two. With an exponential trend, the D pf / E s ratio increases with decreasing E i / σ ci ratio. Relations estimating the residual strength and dilatancy from the pre-peak and peak state parameters are in logarithmic and exponential functional forms, respectively.

Description: The field measurement of permeability within the strata affected by mining is a challenging and expensive task, thus such tests may not be carried out in large numbers to cover all the overburden strata and coal seams being affected by mining. However, numerical modelling in conjunction with a limited number of targeted field measurements can be used efficiently in assessing the impact of mining on a regional scale. This paper presents the results of underground packer testing undertaken at a mine site in New South Wales in Australia and numerical simulations conducted to assess the mining-induced strata permeability change. The underground packer test results indicated that the drivage of main headings (roadways) had induced a significant change in permeability into the solid coal barrier. Permeability increased by more than 50 times at a distance of 11.2–11.5 m from the roadway rib into the solid coal barrier. The tests conducted in the roof strata above the longwall goaf indicated more than 1,000-fold increase in permeability. The measured permeability values varied widely and strangely on a number of occasions; for example the test conducted from the main headings at the 8.2–8.5 m test section in the solid coal barrier showed a decline in permeability value as compared to that at the 11.2–11.5 m section contrary to the expectations. It is envisaged that a number of factors during the tests might have had affected the measured values of permeability: (a) swelling and smearing of the borehole, possibly lowering the permeability values; (b) packer bypass by larger fractures; (c) test section lying in small but intact (without fractures) rock segment, possibly resulting in lower permeability values; and (d) test section lying right at the extensive fractures, possibly measuring higher permeability values. Once the anomalous measurement data were discarded, the numerical model results could be seen to match the remaining field permeability measurement data reasonably well.

Description: Rock bolts are widely used in coal mines throughout China. Approximately 8,000 km of roadways are excavated in coal mines every year in China, 80 % of which are supported by rock bolts. At present, the design of rock bolt support schemes is mainly based on analogies and experience from previous projects. In the present study, in order to evaluate the design of rock bolt support in roadways, several cross sections of a roadway were monitored for rock bolt stress during the roadway excavation and mining. The study results show that the stress in the rock bolts varied in the areas 20 m behind the excavating face and 30 m ahead of the mining face. For the rock bolts observed in this study, the max axial force was within the design limit of the bolts, thus the support design was shown to be acceptable. Then, numerical simulation was performed using FLAC 3D to investigate the stress evolution in the rock bolts during the mining of the fully mechanized longwall face. The simulation results show an overall agreement with the in situ measurements. Finally, parametric study pertaining to length, anchorage length, and rock bolt spacing was carried out with the numerical model, and several suggestions for the support design were proposed.

Description: This paper examines a new method for evaluating the stability of rock blocks on slopes using a remotely positioned Laser Doppler Vibrometer (LDV). A series of experiments using physical models were conducted to evaluate the validity of this new method. Based on the experimental studies, the applicability of LDV was examined by comparing results with a conventional seismometer measurement. To examine the quantitative correlations between vibration properties and the stability of a rock block, the effects on the vibration properties of the size of the rock block, the initial block position, the slope incline, and the type of ground surface were studied. The experimental results showed that LDV measurements agreed with conventional seismometer measurements. There was also a good correlation between vibration properties and rock-block stability. On the other hand, it was found that for a boulder on tightly compacted ground, the application of block stability assessment by tonometry was difficult when measuring microtremors or sloppy vibration due to nearby vehicle traffic. Furthermore, numerical analysis of the slope model was carried out to examine the validity of the model experiment and application of the suggested technique. The results of the analysis demonstrated that the suggested technique was effective for application to stability monitoring of a block and evaluation of the effect of stability measures.

Description: Mechanical responses induced by temperature and air pressure significantly affect the stability and durability of underground compressed air energy storage (CAES) in a lined rock cavern. An analytical solution for evaluating such responses is, thus, proposed in this paper. The lined cavern of interest consists of three layers, namely, a sealing layer, a concrete lining and the host rock. Governing equations for cavern temperature and air pressure, which involve heat transfer between the air and surrounding layers, are established first. Then, Laplace transform and superposition principle are applied to obtain the temperature around the lined cavern and the air pressure during the operational period. Afterwards, a thermo-elastic axisymmetrical model is used to analytically determine the stress and displacement variations induced by temperature and air pressure. The developments of temperature, displacement and stress during a typical operational cycle are discussed on the basis of the proposed approach. The approach is subsequently verified with a coupled compressed air and thermo-mechanical numerical simulation and by a previous study on temperature. Finally, the influence of temperature on total stress and displacement and the impact of the heat transfer coefficient are discussed. This paper shows that the temperature sharply fluctuates only on the sealing layer and the concrete lining. The resulting tensile hoop stresses on the sealing layer and concrete lining are considerably large in comparison with the initial air pressure. Moreover, temperature has a non-negligible effect on the lined cavern for underground compressed air storage. Meanwhile, temperature has a greater effect on hoop and longitudinal stress than on radial stress and displacement. In addition, the heat transfer coefficient affects the cavern stress to a higher degree than the displacement.

Description: Based on experimental investigations, we propose a coupled elastoplastic damage model to simulate the mechanical behavior of granite under compressive stress conditions. The granite is taken from the Beishan area, a preferable region for China’s high-level radioactive waste repository. Using a 3D acoustic emission monitoring system in mechanical tests, we focus on the cracking process and its influence on the macroscopic mechanical behavior of the granite samples. It is verified that the crack propagation coupled with fractional sliding along the cracks is the principal mechanism controlling the failure process and nonlinear mechanical behavior of granite under compressive stress conditions. Based on this understanding, the coupled elastoplastic damage model is formulated in the framework of the thermodynamics theory. In the model, the coupling between damage and plastic deformation is simulated by introducing the independent damage variable in the plastic yield surface. As a preliminary validation of the model, a series of numerical simulations are performed for compressive tests conducted under different confining pressures. Comparisons between the numerical and simulated results show that the proposed model can reproduce the main features of the mechanical behavior of Beishan granite, particularly the damage evolution under compressive stress conditions.

Description: In this paper, two analytical procedures which are independent from the existence of empirical data are presented for the calculation of (1) the size distribution of potentially unstable rock masses that expresses the potential rockfall size distribution, including big volumes corresponding to potential rare events with low susceptibility of failure and (2) the in situ block distribution on the slope face. Two approaches are, respectively, used. The first one involves the detection of kinematically unstable surfaces on a digital elevation model (DEM) and on orthophotos and the calculation of the volumes resting on them. For the second one the in situ block volumes formed by the intersection of the existing discontinuity sets are calculated using a high-resolution DEM. The procedures are presented through an application example at the country of Andorra and in particular at the chute of Forat Negre. The results from the first procedure indicate that it is kinematically possible to have mobilized volumes of some thousands of cubic meters; however, these are considered rare events with low susceptibility of failure. The size distribution of potentially unstable rock masses for big volume events was well fitted by a power law with an exponent of −0.5. The in situ block distribution on the slope face from the second procedure, assuming three types of intersection between the joints of the existing discontinuity sets and two extreme cases of discontinuity persistence, was also found to follow a power law, but with an exponent of −1.3. The comparison with the observed in the field block volume distribution on the slope face indicates that in reality discontinuities have a very high persistence and that considering only their visible trace length overestimates volumes, which is conservative.

Description: The thin-layer interface model for filled joints is extended to analyze shear wave propagation across filled rock joints when the interfacial shear strength between the filling material and the rocks is taken into account. During the wave propagation process, the two sides of the filled joint are welded with the adjacent rocks first and slide on each other when the shear stress on the joint is greater than the interfacial shear strength. By back analysis, the relation between the shear stress and the relative tangential deformation of the filled joints is obtained from the present approach, which is shown as a cycle parallelogram. Comparison between the present approach and the existing method based on the zero-thickness interface model indicates that the present approach is efficient to analyze shear wave propagation across rock joints with slippery behavior. The calculation results show that the slippery behavior of joints is related to the interfacial failure. In addition, the interaction between the shear stress wave and the two sides of the filling joint influences not only the wave propagation process but also the dynamic response of the filled joint.

Description: Replicas were produced of 20 natural rock joints with different roughness. Factors affecting shear strength were examined and direct shear tests were performed using the replica joints to determine their quantitative shear strength characteristics. Results from the shear tests were best fitted by the power law equation, τ  =  Aσ n B , where τ is the shear strength and σ n is the normal stress, and regression coefficients A and B were determined. The coefficient A (equal to τ when σ n is 1 MPa) is defined as the friction angle, and B , which determines the curvature of the plot of shear versus normal strength, is a factor that reduces the shear strength. The physical and mechanical properties of the coefficients A and B were defined, and the relationship between these coefficients and the factors affecting shear strength, such as roughness and joint wall strength, were analyzed quantitatively. A new equation, τ  =  σ n B tan[ ϕ b +  ϕ J +  s n ], was suggested to measure and predict shear strength accurately based on results from these analyses, where ϕ b is the basic friction angle, ϕ J is the joint roughness angle, and s n is the shear component. Although the new shear strength equation is nonlinear, it is as simple to use as a linear equation and the shear strength can be estimated using only three easily measurable parameters ( ϕ b , ϕ J , and σ j , the joint wall compressive strength). The failure envelope estimated using the new shear strength equation not only closely matches the measured shear strength, but also reflects the nonlinear relationship between the normal stress and shear strength.

Description: Specimens of coarse-grained Äspö diorite were axially compressed to observe stress-induced spalling. The specimens had a novel design characterized by two manufactured large radius notches on opposite sides. The tangential stress occurring in the notches aimed to represent the tangential loading around a circular opening. Fracture stages were monitored by acoustic emission measurements. Rock chips were formed similar to those found in situ, which indicates a similar fracture process. Slabs were cut out from the specimens and impregnated using a fluorescent material to visualize the cracks. The cracks were subsequently examined by the naked eye and by means of microscopy images, from which fracture paths could be identified and related to different minerals and their crystallographic orientations. The microscopy analyses showed how the stress field and the microstructure interact. Parallel cracks were formed 2–4 mm below the surface, sub-parallel to the direction of the maximum principal stress. The crack initiation, the roles of minerals such as feldspar, biotite and quartz and their grain boundaries and crystallographic directions are thoroughly studied and discussed in this paper. Scale effects, which relate to the stress gradient and microstructure, are discussed.

Description: Analyses of rock failure mechanisms beneath disc cutters are presented. Full-scale cutting tests are conducted to assess the global energy input in comparison with rock chips and excavated volume. Small-scale cutting tests are subsequently used for macro- and microscopic analyses of rupture modes and crack propagation. A high spatial resolution allows to obtain pictures of crack networks in different rock types. It is shown that all specimens develop lateral cracks in sufficiently confined areas whereas median cracks typically develop in boundary regions. Regarding cutting forces, a hypothesis is proposed that associates sudden force drops accompanied by sudden sound emission with grain crushing in the proximity of the cutter tip.

Description: In order to investigate the influence of water vapor pressure in the surrounding environment on mode I fracture toughness ( K Ic ) of rocks, semi-circular bend (SCB) tests under various water vapor pressures were conducted. Water vapor is one of the most effective agents which promote stress corrosion of rocks. The range of water vapor pressure used was 10 −2 to 10 3 Pa, and two anisotropic rock types, African granodiorite and Korean granite, were used in this work. The measurement of elastic wave velocity and observation of thin sections of these rocks were performed to investigate the microstructures of the rocks. It was found that the distribution of inherent microcracks and grains have a preferred orientation. Two types of specimens in different orientations, namely Type-1 and Type-3, were prepared based on the anisotropy identified by the differences in the elastic wave velocity. K Ic of both rock types was dependent on the water vapor pressure in the surrounding environment and decreased with increasing water vapor pressure. It was found that the degree of the dependence is influenced by the orientation and density of inherent microcracks. The experimental results also showed that K Ic depended on the material anisotropy. A fracture process was discussed on the basis of the geometry of fractures within fractured specimens visualized by the X-ray computed tomography (CT) method. It was concluded that the dominant factor causing the anisotropy of K Ic is the distribution of grains rather than inherent microcracks in these rocks.

Description: The effects of the wor king angle on pneumatic down-the-hole (DTH) hammer drilling was investigated since these hammers were developed for vertical drilling and their performances in inclined positions need to be tested. The investigation begins by establishing a calculation model with which to simulate the percussive drilling of the hammer. The model consists of two parts. The first gives the kinetic energy of the piston at impact by simulating the thermodynamic cycle of the DTH hammer, and the second simulates the percussive drilling process according to the analyses of stress-wave propagation. In the meantime, a laboratory-scale test device was made for multi-angle hammer drilling tests. This device is based on a small pneumatic DTH hammer used in deep well drilling. By analyzing the force acting on the rock during impact at different angles, the percussive drilling process is carefully examined. Theoretical predictions and experimental results appear in fairly good agreement. The hammer’s working performance in inclined positions can be obtained directly using the proposed model. Furthermore, when detailed evaluation is required, the hammer can be tested using the method proposed in this paper.

Description: Combining environmental scanning electron microscopy (ESEM) and digital image correlation techniques, the mechanical behaviour of mudstones is studied at the scale of their composite microstructure (that is, grains of carbonate and quartz embedded in a clay matrix). A specially designed apparatus is developed to allow in-situ uniaxial compression tests on samples with controlled humidity states in the ESEM chamber. As the mechanical behavior of mudstones is sensitive to water content, two tests on samples with contrasting water contents (3.8 and 7.4 %) are performed to identify the unified mechanisms of deformation and damage. We illustrate heterogeneous local strain fields that well correlate with the microstructure of mudstones. Three types of deformation bands involving different mechanisms have been classified: orthogonal (compaction of macro-pores and closure of pre-existing cracks), parallel (micro-cracking) and inclined (shear deformation) to the uniaxial compression direction. These deformation modes are activated at different stress levels, and they strongly interact: for instance, a high-strained shear band may result in tensile micro-cracks at its tip. We also illustrate damage phenomena, particularly at the inclusion-matrix interface, which is found to be a hazardous position for nucleation of micro-cracks.

Description: The damage evolution mechanism of rocks is one of the most important aspects in studying of rock fatigue behavior. Fatigue damage evolution of three rock types (onyx marble, sandstone and soft limestone) with different brittleness were considered in the present study. Intensive experimental tests were conducted on the chosen rock samples and acoustic emission (AE) sensors were used in some of them to monitor the fracturing process. Experimental tests indicated that brittleness strongly influences damage evolution of rocks in the course of static and dynamic loading. AE monitoring revealed that micro-crack density induced by the applied loads during different stages of the failure processes increases as rock brittleness increases. Also, results of fatigue tests on the three rock types indicated that the rock with the most induced micro-cracks during loading cycles has the least fatigue life. Furthermore, the condition of failure surfaces of the studied rocks samples, subjected to dynamic and static loading, were evaluated and it was concluded that the roughness of failure surfaces is influenced by loading types and rock brittleness. Dynamic failure surfaces were rougher than static ones and low brittle rock demonstrate a smoother failure surface compared to high brittle rock.

Description: We conducted hydraulic fracturing (HF) in situ stress measurements in Seokmo Island, South Korea, to understand the stress state necessary to characterize a potential geothermal reservoir. The minimum horizontal principal stress was determined from shut-in pressures. In order to calculate the maximum horizontal principal stress ( S Hmax ) using the classical Hubbert–Willis equation, we carried out hollow cylinder tensile strength tests and Brazilian tests in recovered cores at depths of HF tests. Both tests show a strong pressure rate dependency in tensile strengths, from which we derived a general empirical equation that can be used to convert laboratory determined tensile strength to that suitable for in situ. The determined stress regime (reverse-faulting) and S Hmax direction (ENE–WSW) at depths below ~300 m agrees with the first order tectonic stress. However the stress direction above ~300 m (NE–SW) appears to be interfered by topography effect due to a nearby ridge. The state of stress in Seokmo Island is in frictional equilibrium constrained by optimally oriented natural fractures and faults. However, a severe fluctuation in determined S Hmax values suggests that natural fractures with different frictional coefficients seem to control stress condition quite locally, such that S Hmax is relatively low at depths where natural fractures with low frictional coefficients are abundant, while S Hmax is relatively high at depths where natural fractures with low frictional coefficients are scarce.

Description: In this paper, the embedded discontinuity approach is applied in finite element modeling of rock in compression and tension. For this end, a rate-dependent constitutive model based on (strong) embedded displacement discontinuity model is developed to describe the mode I, mode II and mixed mode fracture of rock. The constitutive model describes the bulk material as linear elastic until reaching the elastic limit. Beyond the elastic limit, the rate-dependent exponential softening law governs the evolution of the displacement jump. Rock heterogeneity is incorporated in the present approach by random description of the mineral texture of rock. Moreover, initial microcrack population always present in natural rocks is accounted for as randomly-oriented embedded discontinuities. In the numerical examples, the model properties are extensively studied in uniaxial compression. The effect of loading rate and confining pressure is also tested in the 2D (plane strain) numerical simulations. These simulations demonstrate that the model captures the salient features of rock in confined compression and uniaxial tension. The developed method has the computational efficiency of continuum plasticity models. However, it also has the advantage, over these models, of accounting for the orientation of introduced microcracks. This feature is crucial with respect to the fracture behavior of rock in compression as shown in this paper.

Description: This paper aims to study the shear behaviour of rock joints in a direct shear test using the particle flow code PFC2D. In this numerical approach, the intact rock is simulated by densely packed circular particles that are bonded together at their contact points; joint surfaces can be explicitly simulated using the modified smooth joint (SJ) model. In the modified SJ model for simulation of direct shear test, micro-scale slip surfaces (smooth joint contacts) are applied at contacts between the particles of the upper and lower blocks of the shear box and the mechanical behaviour of the joints is controlled by the micro-scale properties of the smooth joint contacts. Two joint profiles of standard JRC 10–12 and a sawtooth triangular joint with a base angle of 15° were selected for testing. The results of direct shear tests under different normal stresses on these two profiles show that for the sawtooth triangular joints under a normal stress of 1 MPa, the shearing mechanism is purely sliding, and for the JRC 10–12 profile under a normal stress of 4 MPa, the shearing of first-order asperities controls the shearing mechanism. A parametric study of the micro-properties of the smooth joints under these two different shearing mechanisms was undertaken. The results of this study show that the SJ normal stiffness and the SJ shear stiffness have insubstantial effect on the peak shear strength in sliding mode, but that the SJ normal stiffness has a significant effect on the dilation rate in both sliding and shearing modes.

Description: The paper presents a methodology in the SPH framework to analyze physical phenomena those occur in detonation process of an explosive. It mainly investigates the dynamic failure mechanism in surrounding brittle rock media under blast-induced stress wave and expansion of high pressure product gases. A program burn model is implemented along with JWL equation of state to simulate the reaction zone in between unreacted explosive and product gas. Numerical examples of detonation of one- and two-dimensional explosive slab have been carried out to investigate the effect of reaction zone in detonation process and outward dispersion of gaseous product. The results are compared with those obtained from existing solutions. A procedure is also developed in SPH framework to apply continuity conditions between gas and rock interface boundaries. The modified Grady–Kipp damage model for the onset of tensile yielding and Drucker–Prager model for shear failure are implemented for elasto-plastic analysis of rock medium. The results show that high compressive stress causes high crack density in the vicinity of blast hole. The major principal stress (tensile) is responsible for forming radial cracks from the blast hole. Spalling zones are also developed due to stress waves reflected from the free surfaces.

Description: The Niagara Tunnel Project (NTP) is a 10.1 km long water-diversion tunnel in Niagara Falls, Ontario, which was excavated by a 7.2 m radius tunnel boring machine. Approximately half the tunnel length was excavated through the Queenston Formation, which locally is a shale to mudstone. Typical overbreak depths ranged between 2 and 4 m with a maximum of 6 m observed. Three modelling approaches were used to back analyse the brittle failure process at the NTP: damage initiation and spalling limit, laminated anisotropy modelling, and ubiquitous joint approaches. Analyses were conducted for three tunnel chainages: 3 + 000, 3 + 250, and 3 + 500 m because the overbreak depth increased from 2 to 4 m. All approaches produced similar geometries to those measured. The laminated anisotropy modelling approach was able to produced chord closures closest to those measured, using a joint normal to shear stiffness ratio between 1 and 2. This understanding was applied to a shaft excavation model in the Queenston Formation at the proposed Deep Geological Repository (DGR) site for low and intermediate level nuclear waste storage in Canada. The maximum damage depth was 1.9 m; with an average of 1.0 m. Important differences are discussed between the tunnel and shaft orientation with respect to bedding. The models show that the observed normalized depth of failure at the NTP would over-predict the depth of damage expected in the Queenston Formation at the DGR.

Description: The stability of wells drilled into bedded formations, e.g., shales, depends on the orientation between the bedding and the borehole axis. If the borehole is drilled sub-parallel to bedding, the risk of borehole instabilities increases significantly. In this study, we examined the formation of stress-induced borehole breakouts in Posidonia shale by performing a series of thick-walled hollow cylinder experiments with varying orientations of the bedding plane with respect to the borehole axis. The thick-walled hollow cylinders (40 mm in diameter and 80 mm in length containing an 8 mm diameter borehole) were loaded isostatically until formation of breakouts. The onset of borehole breakout development was determined by means of acoustic emission activity, strain measurements, ultrasonic velocities and amplitudes. The critical pressure for breakout initiation decreased from 151 MPa by approximately 65 % as the bedding plane inclination changed from normal to parallel to the borehole axis. The finely bedded structure in the shale resulted in an anisotropy in elasticity and strength from which the variation in strength dominated the integrity of the thick-walled hollow cylinders.

Description: Squeezing failure is a common failure mechanism experienced in underground coal mine roadways due mainly to mining-induced stresses, which are much higher than the strength of rock mass surrounding an entry. In this study, numerical simulation was carried out to investigate the mechanisms of roadway squeezing using a novel UDEC Trigon approach. A numerical roadway model was created based on a case study at the Zhangcun coal mine in China. Coal extraction using the longwall mining method was simulated in the model with calculation of the mining-induced stresses. The process of roadway squeezing under severe mining-induced stresses was realistically captured in the model. Deformation phenomena observed in field, including roof sag, wall convexity and failed rock bolts are realistically produced in the UDEC Trigon model.

Description: The tensile strength of rock subjected to dynamic loading constitutes many engineering applications such as rock drilling and blasting. The dynamic Brazilian test of rock specimens was conducted with the split Hopkinson pressure bar (SHPB) driven by pendulum hammer, in order to determine the indirect tensile strength of rock under an intermediate strain rate ranging from 5.2 to 12.9 s −1 , which is achieved when the incident bar is impacted by pendulum hammer with different velocities. The incident wave excited by pendulum hammer is triangular in shape, featuring a long rising time, and it is considered to be helpful for achieving a constant strain rate in the rock specimen. The dynamic indirect tensile strength of rock increases with strain rate. Then, the numerical simulator RFPA-Dynamics, a well-recognized software for simulating the rock failure under dynamic loading, is validated by reproducing the Brazilian test of rock when the incident stress wave retrieved at the incident bar is input as the boundary condition, and then it is employed to study the Brazilian test of rock under the higher strain rate. Based on the numerical simulation, the strain-rate dependency of tensile strength and failure pattern of the Brazilian disc specimen under the intermediate strain rate are numerically simulated, and the associated failure mechanism is clarified. It is deemed that the material heterogeneity should be a reason for the strain-rate dependency of rock.

Description: In this paper, the authors report on a series of laboratory and numerical modelling experiments aiming to quantify the behaviour of intact veined rock. Five controlled triaxial compression experiments were conducted on intact veined specimens of mafic intrusive complex andesite (CMET) from the El Teniente mine. The experiments demonstrated that veins controlled the fracturing and resulting peak strengths of the specimens. High-quality experimental data made possible the development of numerical experiments using a 3D Particle Flow Code and the synthetic rock mass (SRM) methodology. The numerical experiments demonstrated that the SRM approach could be successful for modelling the behaviour of veined rock. Greater value was gained by coupling the results of high-quality laboratory tests to comprehensive numerical models.

Description: During the operation of nuclear waste disposal facilities, some sprayed concrete reinforced underground spaces will be in use for approximately 100 years. During this time of use, the local stress regime will be altered by the radioactive decay heat. The change in the stress state will impose high demands on sprayed concrete, as it may suffer stress damage or lose its adhesion to the rock surface. It is also unclear what kind of support pressure the sprayed concrete layer will apply to the rock. To investigate this, an in situ experiment is planned in the ONKALO underground rock characterization facility at Olkiluoto, Finland. A vertical experimental hole will be concreted, and the surrounding rock mass will be instrumented with heat sources, in order to simulate an increase in the surrounding stress field. The experiment is instrumented with an acoustic emission system for the observation of rock failure and temperature, as well as strain gauges to observe the thermo-mechanical interactive behaviour of the concrete and rock at several levels, in both rock and concrete. A thermo-mechanical fracture mechanics study is necessary for the prediction of the damage before the experiment, in order to plan the experiment and instrumentation, and for generating a proper prediction/outcome study due to the special nature of the in situ experiment. The prediction of acoustic emission patterns is made by Fracod 2D and the model later compared to the actual observed acoustic emissions. The fracture mechanics model will be compared to a COMSOL Multiphysics 3D model to study the geometrical effects along the hole axis.

Description: Shielded tunnel boring machines (TBMs) can get stuck in squeezing ground due to excessive tunnel convergence under high in situ stress. This typically coincides with extended machine stoppages, when the ground has sufficient time to undergo substantial displacements. Excessive convergence of the ground beyond the designated overboring means ground pressure against the shield and high shield frictional resistance that, in some cases, cannot be overcome by the TBM thrust system. This leads to machine entrapment in the ground, which causes significant delays and requires labor-intensive and risky operations of manual excavation to release the machine. To evaluate the impact of the time factor on the possibility of machine entrapment, a comprehensive 3D finite difference simulation of a double-shielded TBM in squeezing ground was performed. The modeling allowed for observation of the impact of the tunnel advance rate on the possibility of machine entrapment in squeezing ground. For this purpose, the model included rock mass properties related to creep in severe squeezing conditions. This paper offers an overview of the modeling results for a given set of rock mass and TBM parameters, as well as lining characteristics, including the magnitude of displacement and contact forces on shields and ground pressure on segmental lining versus time for different advance rates.

Description: Infill materials in rock joints usually cause a reduction in the joint shear strength. The shear behavior of rock discontinuities depends upon whether they are clean and unfilled or filled, so this concern invites accurate understanding of the shear behavior and strength of infilled joints. A series of constant normal load direct shear tests was performed to investigate the shear strength of artificial samples with infilled rough joint surfaces having different asperity and infill characteristics. The current study focuses on the effects of factors that influence the shear strength of infilled rock joints samples, with emphasis on forward and reverse shearing. In the forward cycle, the front joint wall is compressed and possibly sheared, and the back side fill is unbonded from the joint surface and slightly disturbed. In the reverse cycle, the disturbed and weakened back side fill is under shearing. The effect of the normal stress on the joint is studied, as this factor plays an important role on the shear behavior of infilled rock joint samples. The results show that joints with low asperity angle exhibit higher shear strength during the forward shearing cycle than the reverse cycle, but in joints with steeper asperity angle, the reverse cycle exhibits greater shear strength. In the reverse cycle, the joint infill has less influence compared to the effect of the rougher surface and higher asperity inclination, even in higher normal stress.

Description: In this paper, we have investigated the quantitative relationship between the axial strain threshold at the volumetric strain reversal point ( ε 1cd ) and the strain threshold at the peak stress ( ε 1ucs ) during the uniaxial compression tests. We found that normalized values of ε 1ucs / ε 1cd for different rock types are typically ~1.34(±0.24). This is especially true for igneous and metamorphic rocks, or low-porosity sedimentary rocks. To verify this finding, we investigated seven granitic gneiss samples using the uniaxial compression test. Our results were consistent with the above statistics. We inferred that ε 1ucs / ε 1cd may be an intrinsic property of low-porosity rocks, and may be a potential indicator for predicting failure strains in laboratory-scale rock samples.

Description: This paper investigated the theoretical relationship between the axial strain thresholds at the volumetric strain reversal point ( ε 1cd ) and the strain threshold at the peak stress ( ε 1ucs ) during a uniaxial compression test. Firstly, we used renormalization group theory with the stress transfer mechanism to reveal the essence of the phase transition phenomenon at the axial strain threshold ( ε 1cd ). Then, we constructed mathematical expressions for ε 1ucs and ε 1cd , respectively. Finally, a theoretical expression for the normalized quantity ε 1ucs / ε 1cd was deduced. Results indicate that the theoretical solution of ε 1ucs / ε 1cd is a function of the shape parameter m . We determined m values in the granitic gneiss experiments presented in Part I of this paper, and then compared the experimental results with theoretical solutions for ε 1ucs / ε 1cd . It was observed that, for a given m , the experimental results were approximately consistent with the theoretical values for the two- and three-dimensional renormalization group models. This indicates that the two- and three-dimensional renormalization group models of rock fracture processes can be used to describe the mechanical behavior of laboratory-scale rock samples. Therefore, this study has thrown some light on failure prediction for laboratory-scale rock samples. Furthermore, the results show that ε 1ucs / ε 1cd can be regarded as a potential indicator for predicting the failure strain of laboratory-scale rock samples.

Description: Nordland in northern Norway is characterized by enhanced seismicity and uplift that makes it the most tectonically active area in Norway. This study is part of a project entitled Neotectonics in Norway—Implications for Petroleum Exploration, which aims at enhancing the understanding of regional-scale stress and strain dynamics in Nordland, and to impact risk and hazard assessment and petroleum exploration. This paper attempts to constrain the orientation of in situ horizontal stress using high-resolution acoustic televiewer logging data. The Geological Survey of Norway has drilled two 0.8 km deep near-vertical boreholes on opposite sides of the Vestfjord in Nordland, the open bight of sea that separates the Lofoten archipelago from the Norwegian mainland. Both boreholes are drilled just North of 68°N, with borehole Leknes Bh located near the geographic center of the Lofoten archipelago, and borehole Drag Bh located on approximate equal distance from the shore, on the Norwegian mainland. The results of this study are in most practical aspects inconclusive, mainly due to poor data quality. The data analysis has revealed erroneously high-borehole diameter, and several artifacts such as eccentric logging tool, rugose borehole wall, spiral hole, tool sticking and missing data. Four intervals with passive in situ stress indicators (borehole breakout and drilling-induced fractures) were found in travel time and amplitude images of the Drag Bh, suggesting approximately N–S orientation of maximum horizontal stress. However, these intervals are not found in cross-plots. Either result yields the lowest World Stress Map ranking quality (E).

Description: A simple three-dimensional (3D) failure criterion for rocks is proposed in this study. This new failure criterion inherits all the features of the Hoek–Brown (HB) criterion in characterizing the rock strength in triaxial compression, and also accounts for the influence of the intermediate principal stress. The failure envelope surface has non-circular convex sections in the deviatoric stress plane, and is smooth, except in triaxial compression. In particular, the failure function of the proposed criterion has a similar simple expression as that of the HB criterion in terms of the principal stresses. The material parameters can be calibrated from tests in conventional triaxial compression, and predictions using this new criterion generally compare well with polyaxial testing data for a variety of rocks. Comparison of the new 3D failure criterion and two existing criteria demonstrates that the new failure criterion performs better in characterizing the rock strength. A unified expression for the 3D failure criteria is further provided, retaining the features of the classical criteria and recovering several existing ones as specific cases.

Description: Many abandoned room and pillar mines have been excavated not far from the surface of large areas of important European cities. In Rome, these excavations took place at shallow depths (3–15 m below the ground surface) in weak pyroclastic soft rocks. Many of these cavities have collapsed; others appear to be in a stable condition, although an appreciable percentage of their structural components (pillars, roofs, etc.) have shown increasing signs of distress from both the morphological and mechanical points of view. In this study, the stress–strain behaviour of soft rock pillars sustaining systems of cavities under vertical loads was numerically simulated, starting from the in situ initial conditions due to excavation of the cavities. The mechanical behaviour of the constituent material of the pillar was modelled according to the Modified Cam-Clay constitutive law (elasto-plastic with strain hardening). The influence of the pillar geometry (cross-section area, shape, and height) and mechanical parameters of the soft rock on the ultimate compressive strength of the pillar as a whole was parametrically investigated first. Based on the numerical results, an original relationship for pillar strength assessment was developed. Finally, the estimated pillar strengths according to the proposed formula and well-known formulations in the literature were compared.

Description: Brittleness is an important characteristic of rocks, for it has a strong influence on the failure process no matter from perspective of facilitating rock breakage or controlling rock failure when rocks are being loaded. Various brittleness criteria have been proposed to describe rock brittleness. In this paper, the existing brittle indices are summarised and then analysed in terms of their applicability to describe rock brittleness. The analysis demonstrates that the widely used strength ratio or product ( σ c / σ t , σ c · σ t ) of rocks cannot describe rock brittleness properly and that most of the indices neglect the impact of the rock’s stress state on its brittleness. A new evaluation method that includes the degree of brittleness ( B d ) and brittle failure intensity ( B f ) is proposed based on the magnitude and velocity of the post-peak stress drop, which can be easily obtained from the conventional uniaxial and triaxial compression tests. The two indices can accurately account for the influence of the confining pressure on brittleness, and the applicability of the new evaluation method is verified by different experiments. The relationship between B d and B f is also discussed.

Description: Early warning of rockbursts remains a worldwide challenge in rock engineering. In this work, a microseismicity-based method of rockburst warning in tunnels is proposed to warn of and reduce the risk of rockburst. The method uses real-time microseismic data and an established rockburst warning formula to provide dynamic warning of rockburst risk during excavation of a tunnel. The establishment of the rockburst warning formula involves several key parts. These include a rockburst database, selection of typical rockburst cases, functional relationships between microseismicity and rockbursts, optimal weighting coefficients, and dynamic updating. By using the proposed method, the probability of strain and strain-structure slip rockbursts of different intensity (extremely intense, intense, moderate, slight, and none) can be warned of in real time. The method has been successfully applied to rockburst warning in deeply buried tunnels at the Jinping II hydropower project (about 11.6 km in total for D & B tunnels). This success illustrates the applicability of the proposed method. In addition, it is found that during the rockburst development process, the microseismic eigenvalues for strainbursts are bigger than in strain-structure slip rockbursts of the same intensity.

Description: In low permeability shale reservoirs, multi-stage hydraulic fracturing is largely used to increase the productivity by enlarging the stimulated rock volume. Hydraulic fracture created alters the stress field around it, and affects the subsequent fractures by the change of the stress field, in particular, mostly increased minimum principal stress at the area of subsequent fracturing. This is called stress shadow which accumulates as the fracturing stages advance from toe to heel. Hydraulic fractures generated in such altered stress field are shorter and compact with orientation deviating significantly from the far-field maximum horizontal stress orientation. This paper presents 2D discrete element-based numerical modeling of multi-stage hydraulic fracturing in a naturally fractured reservoir and investigates stress shadowing. The stress shadowing is tested with two different injection scenarios: constant and cyclic rate injections. The results show that cyclic injection tends to lower the effect of stress shadow as well as mitigates the magnitude of the induced seismicity. Another modeling case is presented to show how the stress shadow can be utilized to optimize a hydraulic fracture network in application to Groß Schönebeck geothermal reservoir, rather than being mitigated. The modeling demonstrated that the stress shadow is successfully utilized for optimizing the geothermal heat exchanger by altering the initial in situ stress field from highly anisotropic to less or even to isotropic.

Description: Time dependency in tunnel excavation is mainly due to the rheological properties of rock and sequential excavation. In this paper, analytical solutions for deeply buried tunnels with elliptical cross-section excavated in linear viscoelastic media are derived accounting for the process of sequential excavation. For this purpose, an extension of the principle of correspondence to solid media with time varying boundaries is formulated for the first time. An initial anisotropic stress field is assumed. To simulate realistically the process of tunnel excavation, solutions are developed for a time-dependent excavation process with the major and minor axes of the elliptical tunnel changing from zero until a final value according to time-dependent functions specified by the designers. In the paper, analytical expressions in integral form are obtained assuming the incompressible generalized Kelvin viscoelastic model for the rheology of the rock mass, with Maxwell and Kelvin models solved as particular cases. An extensive parametric analysis is then performed to investigate the effects of various excavation methods and excavation rates. Also the distribution of displacements and stresses in space at different times is illustrated. Several dimensionless charts for ease of use of practitioners are provided.

Description: Thermal stress within rock subjected to thermal load is induced due to the different expansion rates of mineral grains, resulting in the initiation of new inter-granular cracking and failure at elevated temperatures. The heterogeneity resulting from each constituent of rock should be taken into account in the study of rock thermal cracking, which may aid the better understanding of the thermal cracking mechanisms in rock. In this paper, a mesostructure-based numerical model for the analysis of rock thermal cracking is proposed on the basis of elastic damage mechanics and thermal–elastic theory. In the proposed model, digital image processing (DIP) techniques are employed to characterize the morphology of the minerals in the actual rock structure to build a numerical specimen for the rock. In addition, the damage accumulation induced by thermal (T) and mechanical (M) loads is considered to modify the elastic modulus, strength and thermal properties of individual elements with the intensity of damage. The proposed model is implemented in the well-established rock failure process analysis (RFPA) code, and a DIP-based RFPA for the analysis of thermally induced stress and cracking of rock (abbreviated as RFPA-DTM) is developed. The model is then validated by comparing the simulated results with the well-known analytical solutions. Finally, taking an image from a granite specimen as an example, the proposed model is used to study the thermal cracking process of the granite at elevated temperatures and the effects of temperature on the physical–mechanical behaviors of the granite are discussed. It is found that thermal cracks mostly initiate at the location of mineral grain boundaries and propagate along them to form locally closed polygons at the elevated temperatures. Moreover, the effects of temperature on the uniaxial compressive strength and elastic modulus of the granite are quite different. The uniaxial compressive strength decreases consistently with increasing temperature, but there exists a threshold temperature for elastic modulus which starts to decrease as the temperature increases after it exceeds the threshold.

Description: Presence of an excavation damage zone (EDZ) around a tunnel perimeter is of significant concern with regard to safety, stability, costs and overall performance of the tunnel. For deep-buried tunnel excavation by drill and blast, it is generally accepted that a combination of effects of stress redistribution and blasting is mainly responsible for development of the EDZ. However, few open literatures can be found to use numerical methods to investigate the behavior of rock damage induced by the combined effects, and it is still far from full understanding how, when and to what degree the blasting affects the behavior of the EDZ during excavation. By implementing a statistical damage evolution law based on stress criterion into the commercial software LS-DYNA through its user-subroutines, this paper presents a 3D numerical simulation of the rock damage evolution of a deep-buried tunnel excavation, with a special emphasis on the combined effects of the stress redistribution of surrounding rock masses and the blasting-induced damage. Influence of repeated blast loadings on the damage extension for practical millisecond delay blasting is investigated in the present analysis. Accompanying explosive detonation and secession of rock fragments from their initial locations, in situ stress in the immediate vicinity of the excavation face is suddenly released. The transient characteristics of the in situ stress release and induced dynamic responses in the surrounding rock masses are also highlighted. From the simulation results, some instructive conclusions are drawn with respect to the rock damage mechanism and evolution during deep-buried tunnel excavation by drill and blast.

Description: Effects of fracture toughness on the impingement of geomaterials (rocks and cementitious composites) by quartz particles at velocities between 40 and 140 m/s are investigated experimentally and analytically. If schist is excluded, relative erosion (in g/g) reduces according to a reverse power function if fracture toughness increases. The power exponent depends on impingement velocity, and it varies between −0.64 and −1.33. Lateral cracking erosion models, developed for brittle materials, deliver too high values for relative material erosion. This discrepancy is partly attributed to stress rate effects. Effects of R -curve behavior seem to be marginal. An integral approach E R  =  K 1  ·  E R P  + (1 −  K 1 ) ·  E R L is introduced, which considers erosion due to plastic deformation and lateral cracking. A transition function \(K_{1} = f\left( {K_{\text{Ic}}^{12/4} /\sigma_{\text{C}}^{23/4} } \right)\) is suggested in order to classify geomaterials according to their response against solid particle impingement.

Description: The influence of gas diffusion behavior on gas flow and permeability evolution in coal seams is evaluated in this paper. Coalbed methane (CBM) reservoirs differ from conventional porous media and fractured gas reservoirs due to certain unique features, which lead to two distinct gas pressures: one in fractures and the other in the coal matrix. The latter pressure, also known as the sorption pressure, will be used in calculating sorption-based volume changes. The effective stress laws for single-porosity media is not suitable for CBM reservoirs, and the effective stress laws for multi-porosity media need to be applied. The realization of the above two points is based on the study of the two-phase state of gas migration (involving Fickian diffusion and Darcy flow) in a coal seam. Then, a general porosity and permeability model based on the P-M model is proposed to fit this phenomenon. Moreover, the Klinkenberg effect has been taken into account and set as a reference object. Finally, a coupled gas flow and coal deformation model is proposed and solved by using a finite element method. The numerical results indicate that the effects of gas diffusion behavior and Klinkenberg behavior can have a critical influence on the gas pressure, residual gas content, and permeability evolution during the entire methane degasification period, and the impacts of the two effects are of the same order of magnitude. Without considering the gas diffusion effect, the gas pressure and residual gas content will be underestimated, and the permeability will be overestimated.

Description: Theoretical and experimental studies have revealed that energy dissipation and release play an important role in the deformation and failure of coal rocks. To determine the relationship between energy transformation and coal failure, the mechanical behaviors of coal specimens taken from a 600-m deep mine were investigated by conventional triaxial compression tests using five different confining pressures. Each coal specimen was scanned by microfocus computed tomography before and after testing to examine the crack patterns. Sieve analysis was used to measure the post-failure coal fragments, and a fractal model was developed for describing the size distribution of the fragments. Based on the test results, a damage evolution model of the rigidity degeneration of coal before the peak strength was also developed and used to determine the initial damage and critical damage variables. It was found that the peak strength increased with increasing confining pressure, but the critical damage variable was almost invariant. More new cracks were initiated in the coal specimens when there was no confining pressure or the pressure was too high. The parameters of failure energy ratio β and stress drop coefficient α are further proposed to describe the failure mode of coal under different confining pressures. The test results revealed that β was approximately linearly related to the fractal dimension of the coal fragments and that a higher failure energy ratio corresponded to a larger fractal dimension and more severe failure. The stress drop coefficient α decreased approximately exponentially with increasing confining pressure, and could be used to appropriately describe the evolution of the coal failure mode from brittle to ductile with increasing confining pressure. A large β and small α under a high confining pressure were noticed during the tests, which implied that the failure of the coal was a kind of pseudo-ductile failure. Brittle failure occurred when the confining pressure was unloaded—an observation that is important for the safety assessment of deep mines, where a high in situ stress might result in brittle failure of the coal seam, or sudden outburst.

Description: In this study, hydraulic fracturing tests were conducted on 10 and 15 cm synthetically manufactured cubic tight mortar samples. The use of cube samples allowed application of three independent stresses to mimic real far field stress conditions. A true triaxial stress cell was used for this purpose. The lab test parameters were scaled to simulate the operations at field scale. The hole and perforations were made into the sample after casting and curing were completed. Various scenarios of vertical and horizontal wells and in situ stress regimes were modeled. These factors are believed to play a significant role in fracture initiation and near-wellbore propagation behavior; however, they are not independent parameters, hence should be analyzed simultaneously. In addition to experimental studies, analytical solutions were developed to simulate the mechanism of fracture initiation in perforated boreholes in tight formations. Good agreements were observed between the experimental and analytical results. The results of this study showed that a lower initiation pressure is observed when the minimum stress component is perpendicular to the axis of the perforations. It was also seen that, even when the cement sheath behind the casing fails, the orientation of the perforations may affect the initiation of the induced fracture noticeably. Furthermore, it was found that stress anisotropy influences the fracturing mechanism in a perforated borehole, and affects the geometry of the initiated near-wellbore fracture.

Description: We develop a new analytical model, called OpenT, that solves the elasticity problem of a hydraulic fracture (HF) contact with a pre-existing discontinuity natural fracture (NF) and the condition for HF re-initiation at the NF. The model also accounts for fluid penetration into the permeable NFs. For any angle of fracture intersection, the elastic problem of a blunted dislocation discontinuity is solved for the opening and sliding generated at the discontinuity. The sites and orientations of a new tensile crack nucleation are determined based on a mixed stress- and energy-criterion. In the case of tilted fracture intersection, the finite offset of the new crack initiation point along the discontinuity is computed. We show that aside from known controlling parameters such stress contrast, cohesional and frictional properties of the NFs and angle of intersection, the fluid injection parameters such as the injection rate and the fluid viscosity are of first-order in the crossing behavior. The model is compared to three independent laboratory experiments, analytical criteria of Blanton, extended Renshaw−Pollard, as well as fully coupled numerical simulations. The relative computational efficiency of OpenT model (compared to the numerical models) makes the model attractive for implementation in modern engineering tools simulating hydraulic fracture propagation in naturally fractured environments.

Description: It is known that rigid circular particle models proposed in the literature do not properly reproduce the rock friction angle and the rock tensile strength to compressive strength ratio. A 2D rigid particle model is here presented which tries to overcome these issues while keeping the simplicity and the reduced computational costs characteristic of circular particle models. A particle generation algorithm is adopted which generates polygonal shape particles based on the Laguerre–Voronoi diagrams of the circular particle gravity centres. Several parametric studies are presented to show the influence of the micromechanical properties on both the macroscopic elastic and strength properties. It is shown that a good agreement with the known rock direct tensile to indirect tensile test ratio requires the incorporation of bilinear softening contact laws under tension and shear. Finally, the proposed model is validated against known triaxial and Brazilian tests of a granite rock.

Description: This work is devoted to characterization of the deformation and strength properties of cataclastic sandstones. Before conducting mechanical tests, the physical properties were first examined. These sandstones are characterized by a loose damaged microstructure and poorly cemented contacts. Then, a series of mechanical tests including hydrostatic, uniaxial, and triaxial compression tests were performed to study the mechanical strength and deformation of the sandstones. The results obtained show nonlinear stress–strain responses. The initial microcracks are closed at hydrostatic stress of 2.6 MPa, and the uniaxial compressive strength is about 0.98 MPa. Under triaxial compression, there is a clear transition from volumetric compressibility to dilatancy and a strong dependency on confining pressure. Based on the experimental evidence, an elastoplastic model is proposed using a linear yield function and a nonassociated plastic potential. There is good agreement between numerical results and experimental data.

Description: A novel meshless numerical method, called general particle dynamics (GPD), is proposed to simulate samples of rock-like brittle heterogeneous material containing four preexisting flaws under uniaxial compressive loads. Numerical simulations are conducted to investigate the initiation, growth, and coalescence of cracks using a GPD code. An elasto-brittle damage model based on an extension of the Hoek–Brown strength criterion is applied to reflect crack initiation, growth, and coalescence and the macrofailure of the rock-like material. The preexisting flaws are simulated by empty particles. The particle is killed when its stresses satisfy the Hoek–Brown strength criterion, and the growth path of cracks is captured through the sequence of such damaged particles. A statistical approach is applied to model the heterogeneity of the rock-like material. It is found from the numerical results that samples containing four preexisting flaws may produce five types of cracks at or near the tips of preexisting flaws including wing, coplanar or quasi-coplanar secondary, oblique secondary, out-of-plane tensile, and out-of-plane shear cracks. Four coalescence modes are observed from the numerical results: tensile (T), compression (C), shear (S), and mixed tension/shear (TS). A higher load is required to induce crack coalescence in the shear mode (S) than the tensile (T) or mixed (TS) mode. It is concluded from the numerical results that crack coalescence occurs following the weakest coalescence path among all possible paths between any two flaws. The numerical results are in good agreement with reported experimental observations.

Description: The experimental device previously used to study the hydromechanical behaviour of individual fractures on a laboratory scale, was adapted to make it possible to measure flow through porous rock mass samples in addition to fracture flows. A first series of tests was performed to characterize the hydromechanical behaviour of the fracture individually as well as the porous matrix (sandstone) comprising the fracture walls. A third test in this series was used to validate the experimental approach. These tests showed non-linear evolution of the contact area on the fracture walls with respect to effective normal stress. Consequently, a non-linear relationship was noted between the hydraulic aperture on the one hand, and the effective normal stress and mechanical opening on the other hand. The results of the three tests were then analysed by numerical modelling. The VIPLEF/HYDREF numerical codes used take into account the dual-porosity of the sample (fracture + rock matrix) and can be used to reproduce hydromechanical loading accurately. The analyses show that the relationship between the hydraulic aperture of the fracture and the mechanical closure has a significant effect on fracture flow rate predictions. By taking simultaneous measurements of flow in both fracture and rock matrix, we were able to carry out a global evaluation of the conceptual approach used.

Description: The purpose of this research was to evaluate the production rate (PR) and cutting performance of surface miners (SM) based on rock properties and specific energy (SE). We use data from equipment manufacturers and experimental data in this study and propose a new method and equations to determine both the PR and the cutting speed of SM. The unconfined compressive strength (UCS) of the rock, its abrasivity, and the machine’s engine power are the three most important factors influencing the PR. Moreover, the cutting depth, UCS, and engine power have a significant impact on the cutting speed. We propose a new method and equations to determine the energy required to cut a volume unit and a surface unit, i.e., specific energy, and establish the relationship between SE, UCS, and PR. The results of this study can be used by surface miner operators to evaluate the applicability of the machines to a specific mine site.

Description: The parameter m i is an important rock property parameter required for use of the Hoek–Brown failure criterion. The conventional method for determining m i is to fit a series of triaxial compression test data. In the absence of laboratory test data, guideline charts have been provided by Hoek to estimate the m i value. In the conventional Hoek–Brown failure criterion, the m i value is a constant for a given rock. It is observed that using a constant m i may not fit the triaxial compression test data well for some rocks. In this paper, a negative exponent empirical model is proposed to express m i as a function of confinement, and this exercise leads us to a new empirical failure criterion for intact rocks. Triaxial compression test data of various rocks are used to fit parameters of this model. It is seen that the new empirical failure criterion fits the test data better than the conventional Hoek–Brown failure criterion for intact rocks. The conventional Hoek–Brown criterion fits the test data well in the high-confinement region but fails to match data well in the low-confinement and tension regions. In particular, it overestimates the uniaxial compressive strength (UCS) and the uniaxial tensile strength of rocks. On the other hand, curves fitted by the proposed empirical failure criterion match test data very well, and the estimated UCS and tensile strength agree well with test data.

Description: The fracture toughness of a rock often varies depending on the specimen shape and the loading type used to measure it. To investigate the mode I fracture toughness using semi-circular bend (SCB) specimens, we experimentally studied the fracture toughness using SCB and chevron bend (CB) specimens, the latter being one of the specimens used extensively as an International Society for Rock Mechanics (ISRM) suggested method, for comparison. The mode I fracture toughness measured using SCB specimens is lower than both the level I and level II fracture toughness values measured using CB specimens. A numerical study based on discontinuum mechanics was conducted using a two-dimensional distinct element method (DEM) for evaluating crack propagation in the SCB specimen during loading. The numerical results indicate subcritical crack growth as well as sudden crack propagation when the load reaches the maximum. A K -resistance curve is drawn using the crack extension and the load at the point of evaluation. The fracture toughness evaluated by the K -resistance curve is in agreement with the level II fracture toughness measured using CB specimens. Therefore, the SCB specimen yields an improved value for fracture toughness when the increase of K -resistance with stable crack propagation is considered.

Description: The connection between fluid flow and seismic stiffness in single fractures is governed by the geometry of the fracture through the size and spatial distributions of the void and contact areas. Flow and stiffness each exhibit scaling behavior as the scale of observation shifts from local to global sample sizes. The purpose of this study was to explore the joint scaling of both properties using numerical models. Finite-size scaling methods are used to extract critical thresholds and power laws for fluid flow through weakly correlated fractures under increasing load. An important element in the numerical fracture deformation is the use of extended boundary conditions that simulate differences between laboratory cores relative to in situ field studies. The simulated field conditions enable joint scaling of flow and stiffness to emerge with the potential to extrapolate from small laboratory samples to behavior on the field scale.

Description: Recent discrete fracture network (DFN) related analysis of a number of block caving projects has demonstrated the role that the 3D volumetric fracture intensity measure (P32) plays on controlling a number of rock mass properties critical to caving operations. P32 represents the fracture area per unit volume and as such represents a non-directional intrinsic measure of the degree of rock mass fracturing, incorporating both a frequency measure and a fracture size component. Preliminary results suggest that the P32 intensity of a DFN model would strongly control the overall fragmentation of the rock mass. The implication would be that by taking the overall distribution of P32, the in situ fragmentation of a large rock mass volume could be determined in a computationally efficient way. With P32 also being shown to be one of the dominant controls on DFN derived directional stiffness measures, increasingly these DFN related work flows are being shown to be central to an improved rock mass characterisation process and ultimately the more accurate capturing of the caving process.

Description: The purpose of this study is to identify the crack initiation and damage stress thresholds of granite from the Korea atomic energy research institute’s Underground Research Tunnel (KURT). From this, a quantitative damage evolution was inferred using various methods, including the crack volumetric strain, b value, the damage parameter from the moment tensor, and the acoustic emission (AE) energy. Uniaxial compression tests were conducted, during which both the stress–strain and AE activity were recorded simultaneously. The crack initiation threshold was found at a stress level of 0.42–0.53  σ c , and the crack damage threshold was identified at 0.62–0.84  σ c . The normalized integrity of KURT granite was inferred at each stress level from the damage parameter by assuming that the damage is accumulated beyond the crack initiation stress threshold. The maximum deviation between the crack volumetric strain and the AE method was 16.0 %, which was noted at a stress level of 0.84  σ c . The damage parameters of KURT granite derived from a mechanically measured stress–strain relationship (crack volumetric strain) were successfully related and compared to those derived from physically detected acoustic emission waves. From a comprehensive comparison of damage identification and quantification methods, it was finally suggested that damage estimations using the AE energy method are preferred from the perspectives of practical field applicability and the reliability of the obtained damage values.

Description: The effect of groundwater on strength and deformation behavior of fractured crystalline rocks is one of the important issues for design, performance and safety assessments of surface and subsurface rock engineering problems. However, practical difficulties make the direct in situ and laboratory measurements of these properties of fractured rocks impossible at present, since effects of complex fracture system hidden inside the rock masses cannot be accurately estimated. Therefore, numerical modeling needs to be applied. The overall objective of this paper is to deepen our understanding on the validity of the effective stress concept, and to evaluate the effects of water pressure on strength and deformation parameters. The approach adopted uses discrete element methods to simulate the coupled stress-deformation-flow processes in a fractured rock mass with model dimensions at a representative elementary volume (REV) size and realistic representation of fracture system geometry. The obtained numerical results demonstrate that water pressure has significant influence on the strength, but with minor effects on elastic deformation parameters, compared with significant influence by the lateral confining pressure. Also, the classical effective stress concept to fractured rock can be quite different with that applied in soil mechanics. Therefore, one should be cautious when applying the classical effective stress concept to fractured rock media.

Description: We investigate why the sulphatic claystones of the Gypsum Keuper contain anhydrite rather than gypsum even at small depths of cover. This question is relevant due to the phenomenon of swelling of anhydritic claystones, which is attributed to the transformation of anhydrite into gypsum and has caused serious damage to a number of tunnels. In tunnelling, the Gypsum Keuper formation is crossed at rather small depths, where simplified thermodynamic considerations indicate that the calcium sulphate should be encountered in its hydrated form, i.e. as gypsum rather than as anhydrite. Understanding why anhydrite can be found at small depths is not only interesting from a fundamental point of view, but also necessary in order to formulate adequate initial conditions for the continuum-mechanical models that simulate the chemo-mechanical and transport processes in swelling anhydritic claystones. The paper quantitatively examines three reasons which, alone or in combination, might explain the occurrence of anhydrite: the small size of the pores in argillaceous rocks; locally high stresses in the vicinity of the sulphate crystals; and the thermodynamic state of the pore water. The computations of the paper take account of the results of porosimetry experiments on samples from two Swiss tunnels in Gypsum Keuper and show that the most probable reason is the thermodynamic state of the pore water, i.e. its ability to participate in chemical reactions. More specifically, the clay minerals reduce the chemical potential of the pore water, thus increasing the solubility of the gypsum and shifting the thermodynamic equilibrium in favour of anhydrite.

Description: Spalling damage can pose significant risks during the construction of underground excavations in brittle rock. While deterministic analyses have traditionally been used in the design of these structures, reliability-based design (RBD) methods provide a more rational approach to quantify spalling risk by directly incorporating input uncertainty into the design process and quantifying variable ground response. This paper presents a new RBD approach to evaluate the excavation response and support performance for a tunnel in brittle ground. Guidance for the selection of appropriate parameters for variable brittle materials is provided using a combination of the damage initiation and spalling limit method and theories of microcrack initiation. System performance is then evaluated using a proposed global response surface method (GRSM) coupled with the first-order reliability method, random sampling and finite element analysis. The proposed GRSM provides a computationally efficient way to evaluate the probability of failure for various limit states, allowing for the selection of appropriate design parameters such as minimum bolt length and required bolt capacity during early stages of design. To demonstrate the usefulness of this approach, a preliminary design option for a proposed deep geologic repository located in Canada was assessed. Numerical analyses were completed using finite element modeling to determine the depth of spalling around the excavation and support loads over the range of possible rock mass and in situ stress conditions. The results of these analyses were then used to assess support performance and make support recommendations.

Description: This paper is devoted to the experimental characterization and constitutive modeling of saturated porous rocks. A typical porous chalk is investigated. Drained hydrostatic and triaxial compression tests are first performed to characterize the basic mechanical behavior of chalk. Drained triaxial tests with constant interstitial pressure are then carried out to study the effects of interstitial pressure on the plastic deformation and failure criterion. Finally, undrained triaxial compression tests are performed to investigate poromechanical coupling in saturated conditions. Based on the experimental data and some relevant micromechanical considerations, a micromechanics-based plastic model is proposed and extended to poroplastic coupling using the effective stress concept. The proposed model is verified through comparisons between the numerical results and experimental data for both drained and undrained tests.

Description: A series of clay shale specimens in equilibrium with various humidity conditions were used to establish the water retention characteristics, the influence of suction on ultrasonic p-wave velocity and rock mechanical properties such as Young’s modulus, Poisson’s ratio, onset of dilatancy, unconfined compressive strength and Brazilian tensile strength. Opalinus Clay, a clay shale considered as host rock for the disposal of nuclear waste in Switzerland was utilized. The results showed that the p-wave velocity normal to bedding ( v p,n ) dropped sharply upon desaturation until suction approached the air-entry value. The sharp decrease was associated with desiccation cracks solely oriented parallel to bedding. For suction in excess of the air-entry value, v p,n was constant, indicating no further desiccation damage. The suction at the shrinkage limit and at the air-entry point is similar in magnitude. The p-wave velocity parallel to bedding ( v p,p ) remained constant in the entire range of suction investigated in this study. The constant v p,p with increasing suction might be associated with the disproportional decrease in the Poisson’s ratio and Young’s modulus and its opposing effect on p-wave velocity. An almost linear increase in unconfined compressive strength, Brazilian tensile strength, stress at the onset of dilatancy and Young’s modulus with increasing suction was observed up to a suction of 56.6 MPa. For suction larger than 56.6 MPa, relatively constant strength and stiffness was observed. The increase is associated with the net contribution of suction to strength/stiffness, which decreases nonlinearly with decreasing volumetric water content. The rate of increase in tensile strength and unconfined compressive strength with increasing suction is different depending on the rock anisotropy. Compared to the strength values (Brazilian tensile and uniaxial compressive strength) obtained from specimens loaded parallel to bedding, the tensile strength parallel to bedding and the unconfined compressive strength obtained from specimens loaded normal to bedding are considerably more affected by increasing suction or decreasing water content. The reasons for the different rates in strength increase are considered to be related to local variations in suction (i.e., local suction) as a consequence of zones of contrasting pore-size distribution. These variations may influence the effect of suction on strength, especially when the load is applied parallel to bedding and crack growth occurs predominately along bedding layers with comparably low suction.

Description: Light Detection and Ranging (LiDAR) has become more widely used in the geotechnical community as its number of applications increases. It has been shown to be useful in tunneling for applications such as rockmass characterization and discontinuity measurement. LiDAR data can also be used to measure deformation in tunnels, but before a comprehensive methodology can be developed, the accuracy issues associated with scanning must be fully understood. Once the accuracy issues associated with LiDAR are well understood, any analysis technique that uses LiDAR data must be tested to ensure that the determined accuracy issues have minimal impact on the results of the analysis. To prove the usefulness of the newly developed elliptical fitting method for the measurement of convergence in tunnels and shafts proposed by Delaloye et al. (Eurock 2012 ), a comprehensive analysis of accuracy issues associated with LiDAR scanning was conducted and then a sensitivity test of the convergence measurement technique was completed. The results of the analysis show that using the statistical techniques built into the elliptical fit analysis and LiDAR profile analysis, levels of real change (convergence), within the nominal level of random and systematic noise included in the data, can be measured with confidence. Furthermore, the new analysis is robust enough to handle large amounts of occlusion or missing perimeter coverage within data sets.

Description: The determination of rock mass dynamic parameters including the influence of joints/discontinuities on wave propagation is of interest for solving problems in geophysics, rock protective engineering, rock dynamics and earthquake engineering. This topic is covered in this paper by means of laboratory tests on intact and jointed rock specimens performed with the resonant column apparatus (RCA). Attention is dedicated to the determination, with this equipment, generally used for testing soils, of the shear modulus at small strain and the damping ratio of intact and jointed rock specimens. A correction procedure based on the RCA tests performed on aluminium specimens is described. The results of the tests performed are analysed in detail. Three-dimensional distinct element method modelling is used to evaluate the applicability of the RCA and the correctness of the laboratory tests performed. A comparison with the results obtained using the scattering matrix method is also presented.

Description: Reinforcement measures are often used in high-arch dams with complicated geological foundations. The geomechanical model test is an effective method to study the global stability of arch dams and to evaluate the reinforcement effects of foundation treatments. The block masonry technique was developed to simulate the jointed rock mass, tectonic discontinuities, and reinforcement measures. A tailor-made low-strength binder and small blocks were developed to simulate the strength and deformation of the jointed rock mass and discontinuities, respectively. We applied this technique to geomechanical model tests of the Dagangshan arch dam with and without foundation reinforcements. A rupture test was conducted, and the stress and displacement distribution of the dam and abutments were recorded; the failure mechanisms and processes were explored. The reinforcement effects of the foundation treatment were evaluated by comparing the test results of the models with and without foundation reinforcements. Our analysis indicates that foundation reinforcements can improve the stress distribution, decrease deformation, prevent slides, reduce fault movement, and improve the global stability of high-arch dams.

Description: Within a research work aimed to better understand frost weathering mechanisms of rocks, laboratory tests have been designed to specifically assess a theoretical model of crack propagation due to ice segregation process in water-saturated and thermally microcracked cubic samples of Arolla gneiss. As the formation and growth of microcracks during freezing tests on rock material is accompanied by a sudden release of stored elastic energy, the propagation of elastic waves can be detected, at the laboratory scale, by acoustic emission (AE) sensors. The AE receiver array geometry is a sensitive factor influencing source location errors, for it can greatly amplify the effect of small measurement errors. Despite the large literature on the AE source location, little attention, to our knowledge, has been paid to the description of the experimental design phase. As a consequence, the criteria for sensor positioning are often not declared and not related to location accuracy. In the present paper, a tool for the identification of the optimal sensor position on a cubic shape rock specimen is presented. The optimal receiver configuration is chosen by studying the condition numbers of each of the kernel matrices, used for inverting the arrival time and finding the source location, and obtained for properly selected combinations between sensors and sources positions.

Description: A systematic approach of using the distinct element method (DEM) to provide useful insights for tunnel support in moderately jointed rock masses is illustrated. This is preceded by a systematic study of common failure patterns for unsupported openings in a rock mass intersected by three independent sets of joints. The results of our simulations show that a qualitative description of the failure patterns using specific descriptors is unattainable. Then, it is shown that DEM analyses can be employed in the preliminary design phase of tunnel supports to determine the main parameters of a support consisting of rock bolts or one lining or a combination of both. A comprehensive parametric analysis investigating the effect of bolt bonded length, bolt spacing, bolt length, bolt pretension, bolt stiffness and lining thickness on the tunnel convergence is illustrated. The highlight of the proposed approach of preliminary support design is the use of a rock bolt and lining interaction diagram to evaluate the relative effectiveness of rock bolts and lining thickness in the design of the tunnel support. The concept of interaction diagram can be used to assist the engineer in making preliminary design decisions given a target maximum allowable convergence. In addition, DEM simulations were validated against available elastic solutions. To the authors’ knowledge, this is the first verification of DEM calculations for supported openings against elastic solutions. The methodologies presented in this article are illustrated through 2-D plane strain analyses for the preliminary design stage. More rigorous analyses incorporating 3-D effects have not been attempted in this article because the longitudinal displacement profile is highly sensitive to the joint orientations with respect to the tunnel axis, and cannot be established accurately in 2-D. The methodologies and concepts discussed in this article, however, have the potential to be extended to 3-D analyses.

Description: The International Society of Rock Mechanics (ISRM) suggested cracked chevron notched Brazilian disc method falls into a major testing category of rock fracture toughness measurement by virtue of chevron notched rock samples. A straight through crack front during the whole fracturing process is assumed in the testing principle but is never assessed. In this study, the progressive rock fracture mechanism of cracked chevron notched Brazilian disc rock specimens is numerically simulated for the first time. Two representative sample types with distinct geometry of notch ligaments are modelled. The assumption of a straight through crack front for chevron notched fracture samples is critically assessed. The results show that not only the notch tip but also the saw-cut chevron notch cracks during the experiments. The straight through crack front assumption is never satisfied in the realistic rock fracture progress of chevron notched disc samples. In addition, the crack features prominent curved front, far from being straight. In contrast to the sample type with narrow notch ligament, the acoustic emission (AE) of the simulation on the sample with wide notch ligament depicts obvious biased fracturing of the prescript fracturing route of the notch. The numerically observed progressive fracture mechanism calls for more attention on how to accurately calibrate the critical dimensionless stress intensity factor for a better measurement of Mode I fracture toughness via chevron notched samples.