ALBERT

Description: This paper describes the crossing of faults 44 and 49 when constructing the 57 km Gotthard base tunnel of the Alp Transit project. Fault 44 is a permeable fault that triggered significant surface deformations 1,400 m above the tunnel when it was reached by the advancing excavation. The fault runs parallel to the downstream face of the Nalps arch dam. Significant deformations were measured at the dam crown. Fault 49 is sub-vertical and permeable, and runs parallel at the upstream face of the dam. It was necessary to assess the risk when crossing fault 49, as a limit was put on the acceptable dam deformation for structural safety. The simulation model, forecasts and action decided when crossing over the faults are presented, with a brief description of the tunnel, the dam, and the monitoring system.

Description: The purpose of this review is to discuss the development and the state of the art in dynamic testing techniques and dynamic mechanical behaviour of rock materials. The review begins by briefly introducing the history of rock dynamics and explaining the significance of studying these issues. Loading techniques commonly used for both intermediate and high strain rate tests and measurement techniques for dynamic stress and deformation are critically assessed in Sects.  2 and 3 . In Sect.  4 , methods of dynamic testing and estimation to obtain stress–strain curves at high strain rate are summarized, followed by an in-depth description of various dynamic mechanical properties (e.g. uniaxial and triaxial compressive strength, tensile strength, shear strength and fracture toughness) and corresponding fracture behaviour. Some influencing rock structural features (i.e. microstructure, size and shape) and testing conditions (i.e. confining pressure, temperature and water saturation) are considered, ending with some popular semi-empirical rate-dependent equations for the enhancement of dynamic mechanical properties. Section  5 discusses physical mechanisms of strain rate effects. Section  6 describes phenomenological and mechanically based rate-dependent constitutive models established from the knowledge of the stress–strain behaviour and physical mechanisms. Section  7 presents dynamic fracture criteria for quasi-brittle materials. Finally, a brief summary and some aspects of prospective research are presented.

Description: This paper focuses on the comparison of damage induced by smooth blasting and presplit blasting based on the excavation of high rock slope. The whole damage process of the smooth blasting and presplit blasting excavation method is studied by using a cumulative blasting damage numerical simulation technology based on the secondary development of the dynamic finite element code LS-DYNA. The results demonstrate that, in the case of contour blasting with the method of smooth blasting, the total damage of rock slope is a result of cumulated damage induced by the production hole, buffering hole, and smooth hole. Among the total damage, the blasting of the production hole is the main resource, followed by the smooth and buffering holes. For the presplit blasting, the final damage of rock slope is mainly induced by presplit blasting itself. The spatial distribution characteristics of the final damage zone of two methods are compared. Two classes of damage zone could be found in smooth blasting excavation; one of them is the columnar high-degree damage zone around the slope surface and the other is the low-degree damage zone located in the middle of the slope. But in the case of presplit blasting, there is only the columnar high-degree damage zone around the slope surface. Finally, a damage control suggestion for two blasting excavation methods is proposed and verified based on the excavation of the temporary shiplock slopes of the Three Gorges Project in China.

Description: The stress state of coal surrounding a coalbed methane (CBM) production well is affected by the bottom hole flowing pressure (BHFP). The permeability of coal shows a marked change under compression. The BHFP must be restricted to a specific range to favor higher permeability in the surrounding coal and thus higher productivity of the well. A new method to determine this specific range is proposed in this paper. Coal has a rather low tensile strength, which induces tensile failure and rock disintegration. The deformation of coal samples under compression has four main stages: compaction, elastic deformation, strain hardening, and strain softening. Permeability is optimal when the coal samples are in the strain softening stage. The three critical values of BHFP, namely, p wmin , p wmid , and p wupper , which correspond to the occurrence of tensile failure, the start of strain softening, and the beginning of plastic deformation, respectively, are derived from theoretical principles. The permeability of coal is in an optimal state when the BHFP is between p wmin and p wmid . The BHFP should be confined to this specific range for the efficient drainage of CBM wells. This method was applied to field operations in three wells in the Hancheng CBM field in China. A comprehensive analysis of drainage data and of the BHFP indicates that the new method is effective and offers significant improvement to current practices.

Description: Triaxial creep tests were performed on diabase specimens from the dam foundation of the Dagangshan hydropower station, and the typical characteristics of creep curves were analyzed. Based on the test results under different stress levels, a new nonlinear visco-elasto-plastic creep model with creep threshold and long-term strength was proposed by connecting an instantaneous elastic Hooke body, a visco-elasto-plastic Schiffman body, and a nonlinear visco-plastic body in series mode. By introducing the nonlinear visco-plastic component, this creep model can describe the typical creep behavior, which includes the primary creep stage, the secondary creep stage, and the tertiary creep stage. Three-dimensional creep equations under constant stress conditions were deduced. The yield approach index (YAI) was used as the criterion for the piecewise creep function to resolve the difficulty in determining the creep threshold value and the long-term strength. The expression of the visco-plastic component was derived in detail and the three-dimensional central difference form was given. An example was used to verify the credibility of the model. The creep parameters were identified, and the calculated curves were in good agreement with the experimental curves, indicating that the model is capable of replicating the physical processes.

Description: This study investigates the water weakening effect on the tensile strength, as well as the fracturing behavior, of an artificially molded Hydrocal B-11 gypsum rock. Brazilian disc tests, with the aid of a high-speed video system to monitor and record the cracking processes, are conducted on dry and wet specimens to determine their tensile strengths. The dry specimens are oven-dried, while the wet specimens are prepared by soaking in water for 1, 3, and 10 weeks to achieve different levels of water content. The test results show that the tensile strength drops to nearly half of its dry value after being soaked in water for only 1 week. The tensile strength reduces only slightly further after the specimens have been immersed in water for 3 and 10 weeks. An analysis of the recorded high-speed footage shows that the primary crack initiates at the center as observed from the surface for the majority of the tested specimens. Most importantly, the cracking processes of dry and wet specimens are distinctly different with regard to the speed of crack propagation and the number of cracks developed.

Description: In one approach to predicting the behaviour of rock masses, effort is being devoted to the use of probabilistic methods to model structures interior to a rock mass (sometimes referred to as ‘inferred’ or ‘stochastic’ structures). The physical properties of these structures (e.g. position, orientation, size) are modelled as random parameters, the statistical properties of which are derived from the measurements of a sample of the population (sometimes referred to as ‘deterministic’ structures). Relatively little attention has been devoted to the uncertainty associated with the deterministic structures. Typical geotechnical analyses rely on either an entirely stochastic analysis, or deterministic analyses representing the structures with a fixed shape (i.e. disc), position, size, and orientation. The simplifications assumed for this model introduce both epistemic and stochastic uncertainties. In this paper, it is shown that these uncertainties should be quantified and propagated to the predictions of behaviour derived from subsequent analyses. We demonstrate a methodology which we have termed quasi - stochastic analysis to perform this propagation. It is shown that relatively small levels of uncertainty can have large influence on the uncertainties associated with geotechnical analyses, such as predictions of block size and block stability, and therefore this methodology can provide the practitioner with a method for better interpretation of these results.

Description: Experiments were conducted to study the relationship between the transmission ratio (TR) and normal stress, joint roughness, joint number and frequency of incident waves, respectively, when ultrasonic waves pass across a rock mass with one joint and multiple parallel joints oriented normally. The ultrasonic waves were generated and received by pairs of piezoelectric transducers and recorded by an ultrasonic detector. The specimens were subjected to normal stress by a hydraulic jack and loading frame. The jointed rock mass was produced by superposing rock blocks in the study. Rough joints were produced by grooving notches on the planar joints formed by sawing directly. In the case of multiple parallel joints, the overall thickness of specimens was maintained while the joint number changed. Three pairs of P-wave transducers and one pair of S-wave transducers with different frequencies were, respectively, applied and all transducers emitted signals perpendicular to the joints in the experiment. The results indicate that TR increases with increasing normal stress while the increment rate decreases gradually. This is particularly so when the normal stress is high enough that TR will approximate 1 even if the rock mass has many joints. In addition, the experiments indicate that the higher the wave’s frequency, the lower its TR, and this phenomenon is gradually reduced as the normal stress increases. In response to S-waves, TR increases with increase in joint roughness; however, in response to P-waves, TR decreases gradually with increase in joint roughness. For multiple parallel joints in a fixed thickness rock mass with normally incident P-waves, TR does not always decrease with increase in the number of joints, and there is a threshold joint spacing for a certain incident wave: when the joint spacing is smaller than the threshold value, TR will increase with a decrease in joint spacing. The experimental results support similar conclusions based on analytical results drawn by Cai and Zhao (Int J Rock Mech Min Sci 37(4):661–682, 2000 ), Zhao et al. (Int J Rock Mech Min Sci 43(5):776–788, 2006b ) and Zhu et al. (J Appl Geophys 73:283–288, 2011a ).

Description: Discrete fracture network representations of discontinuities in rock masses have been shown to be useful in capturing heterogeneity in rock mass properties. Providing computational efficiency in the resulting simulations and analyses is attained, these fracture representations can be combined with structural modelling and sampling algorithms. Multiple fracture network realisations can be generated and the resulting rock mass properties interrogated. Statistical analyses based on fracture connectivity, block size distribution and slope stability can be performed and provide results defined in terms of confidence intervals. For sedimentary geology consisting of dense bedding, equivalent medium continuum methods have traditionally been used in preference to discrete fracture representations due to the large numbers of structures involved and resulting computational complexity. In this paper, it is shown that stochastic representation of these layers can be employed. An analytical solution to accommodate bedding given an assumed block size distribution has been derived. Using this formulation, polyhedral modelling has been used to investigate the influence of bedding on block formation and block size distributions using field data. It is shown that the analysis is both computationally efficient and can capture truncation of size distribution by such layers without numerical methods.

Description: A porosity change influences the transport properties and the elastic moduli of rock while circulating water in a geothermal reservoir. The static and dynamic elastic moduli can be derived from the slope of stress–strain curves and velocity measurements, respectively. Consequently, the acoustic velocities were measured while performing hydrostatic drained tests. The effect of temperature on static and dynamic elastic moduli and porosity variations of Flechtinger sandstone was investigated in a wide range of confining pressure from 2 to 55 MPa. The experiments were carried out in a conventional triaxial system whereas the pore pressure remained constant, confining pressure was cycled, and temperature was increased step wise (25, 60, 90, 120, and 140 °C). The porosity variation was calculated by employing two different theories: poroelasticity and crack closure. The porosity variation and crack porosity were determined by the first derivative of stress–strain curves and the integral of the second derivative of stress–strain curves, respectively. The crack porosity analysis confirms the creation of new cracks at high temperatures. The porosity variation was increasing with an increase in temperature at low effective pressures and was decreasing with a rise in temperature at high effective pressures. Both compressional and shear wave velocities were increasing with increasing pressure due to progressive crack closure. Furthermore, the thermomechanical behavior of Flechtinger sandstone was characterized by an inversion effect where the sign of the temperature derivative of the drained bulk modulus changes.

Description: A proper evaluation of the perturbations of the host rock induced by the excavation and the emplacement of exothermic wastes is essential for the assessment of the long-term safety of high-level radioactive waste disposals in clay formations. The impact of the thermal transient on the evolution of the damaged zone (DZ) has been explored in the European Commission project TIMODAZ (thermal impact on the damaged zone around a radioactive waste disposal in clay host rocks, 2006–2010). This paper integrates the scientific results of the TIMODAZ project from a performance assessment (PA) point of view, showing how these results support and justify key PA assumptions and the values of PA model parameters. This paper also contextualises the significance of the thermal impact on the DZ from a safety case perspective, highlighting how the project outcomes result into an improved understanding of the thermo–hydro–mechanical behaviour of the clay host rocks. The results obtained in the TIMODAZ project strengthen the assessment basis of the safety evaluation of the current repository designs. There was no evidence throughout the TIMODAZ experimental observations of a temperature-induced additional opening of fractures nor of a significant permeability increase of the DZ. Instead, thermally induced plasticity, swelling and creep seem to be beneficial to the sealing of fractures and to the recovery of a very low permeability in the DZ, close to that of an undisturbed clay host rock. Results from the TIMODAZ project indicate that the favourable properties of the clay host rock, which guarantee the effectiveness of the safety functions of the repository system, are expected to be maintained after the heating–cooling cycle. Hence, the basic assumptions usually made in PA calculations so far are expected to remain valid, and the performance of the system should not be affected in a negative way by the thermal evolution of the DZ around a radioactive waste repository in clay host rock.

Description: Determining anisotropic deformation surrounding underground excavations for tunnels is an intuitional task that involves many difficulties due to the inherent anisotropies in the strength and deformability of natural rocks. This study investigates joint-induced anisotropic deformation surrounding a tunnel via a numerical simulation that accounts for the mechanical behavior of intact rock, the orientations of joint sets, and the mechanical behavior of joint planes; this numerical simulation can model the complete stress–strain relationship with anisotropic rock mass characteristics. Simulation results demonstrate that the well-known excavation-induced stress variation–decrease in the radial component and increase in the tangential component–decrease shear strength and increase shear stress for the joint plane tangential to the tunnel wall, resulting in joint sliding failure and considerable shear deformation. This joint sliding failure and significant shear deformation account for the joint-induced anisotropic deformation surrounding a tunnel. When a rock mass has two joint sets with unfavorable joint orientations, the area with joint sliding failure can deteriorate mutually, resulting in large anisotropic deformation. Additionally, for a rock mass containing three joint sets with well-distributed orientations, joint sliding in various joint sets and associated stress variations can counter balance each other, resulting in less anisotropic deformation than those of rock masses containing one or two joint sets.

Description: The behavior of rock damage evolution under unloading conditions is of utmost importance for the analysis of the stress-induced failure of overstressed rock masses. In this paper, a new experimental approach, the incrementally cyclic loading–unloading pressure test (ICLUP test), is designed to quantify stress-induced micro-fracturing and fracturing under the condition of confining pressure reduction. The experimental results demonstrate that the pre-peak damage and deformation characteristics of marble specimens may be easily quantified by irreversible strains, and two damage stages, namely, the linear steady stage and the nonlinear unsteady stage, which are, respectively, represented as a linear steady rate and a nonlinear unsteady rate of damage evolution, occur along with the increase of unloading damage. The new model is proposed to describe the features of pre-peak unloading damage evolution, and the physical meanings and ranges of its material parameters are explained and analyzed. Furthermore, the evolution of volumetric dilation and elastic parameters which occurs along with the increase of unloading damage is revealed. Also discussed in this paper are the inhomogeneity and initial damage of specimens, as well as related research planned to be performed in the future.

Description: This paper presents simulation results related to coupled thermal–hydraulic–mechanical (THM) processes in engineered barrier systems (EBS) and clay host rock, in one case considering a possible link to geochemistry. This study is part of the US DOE Office of Nuclear Energy’s used fuel disposition campaign, to investigate current modeling capabilities and to identify issues and knowledge gaps associated with coupled THMC processes and EBS–rock interactions associated with repositories hosted in clay rock. In this study, we simulated a generic repository case assuming an EBS design with waste emplacement in horizontal tunnels that are back-filled with bentonite-based swelling clay as a protective buffer and heat load, derived for one type of US reactor spent fuel. We adopted the Barcelona basic model (BBM) for modeling of the geomechanical behavior of the bentonite, using properties corresponding to the FEBEX bentonite, and we used clay host rock properties derived from the Opalinus clay at Mont Terri, Switzerland. We present results related to EBS host–rock interactions and geomechanical performance in general, as well as studies related to peak temperature, buffer resaturation and thermally induced pressurization of host rock pore water, and swelling pressure change owing to variation of chemical composition in the EBS. Our initial THM modeling results show strong THM-driven interactions between the bentonite buffer and the low-permeability host rock. The resaturation of the buffer is delayed as a result of the low rock permeability, and the fluid pressure in the host rock is strongly coupled with the temperature changes, which under certain circumstances could result in a significant increase in pore pressure. Moreover, using the BBM, the bentonite buffer was found to have a rather complex geomechanical behavior that eventually leads to a slightly nonuniform density distribution. Nevertheless, the simulation shows that the swelling of the buffer is functioning to provide an adequate increase in confining stress on the tunnel wall, leading to a stabilization of any failure that may occur during the tunnel excavation. Finally, we describe the application of a possible approach for linking THM processes with chemistry, focusing on the evolution of primary and secondary swelling, in which the secondary swelling is caused by changes in ionic concentration, which in turn is evaluated using a transport simulation model.

Description: Evaluation of rock abrasivity is important when utilizing mechanized excavation in various mining and civil projects in hard rock. This is due to the need for proper selection of the rock cutting tools, estimation of the tool wear, machine downtime for cutter change, and costs. The Cerchar Abrasion Index (CAI) test is one of the simplest and most widely used methods for evaluating rock abrasivity. In this study, a new device for the determination of frictional forces and depth of pin penetration into the rock surface during a Cerchar test is discussed. The measured parameters were used to develop an analytical model for calculation of the size of the wear flat (and hence a continuous measure of CAI as the pin moves over the sample) and pin tip penetration into the rock during the test. Based on this model, continuous curves of CAI changes and pin tip penetration into the rock were plotted. Results of the model were used for introduction of a new parameter describing rock–pin interaction and classification of rock abrasion.

Description: A discrete element model is proposed to examine rock strength and failure. The model is implemented by UDEC which is developed for this purpose. The material is represented as a collection of irregular-sized deformable particles interacting at their cohesive boundaries. The interface between two adjacent particles is viewed as a flexible contact whose stress–displacement law is assumed to control the material fracture and fragmentation process. To reproduce rock anisotropy, an innovative orthotropic cohesive law is developed for contact which allows the interfacial shear and tensile behaviours to be different from each other. The model is applied to a crystallized igneous rock and the individual and interactional effects of the microstructural parameters on the material compressive and tensile failure response are examined. A new methodical calibration process is also established. It is shown that the model successfully reproduces the rock mechanical behaviour quantitatively and qualitatively. Ultimately, the model is used to understand how and under what circumstances micro-tensile and micro-shear cracking mechanisms control the material failure at different loading paths.

Description: An apparatus and experimental setup were developed to carry out a series of extremely slow and long-lasting swelling, creep or chemo-mechanics tests simultaneously. The equipment was designed specifically for investigating the behaviour of sulphatic claystones. The tests will take at least 10–15 years to complete and will provide unprecedented information about the so-called swelling law, i.e. the relationship between swelling strain and swelling stress. The swelling law is very important for designing tunnels in swelling rock. Our knowledge of the swelling law, however, is only sufficiently reliable with respect to claystones without anhydrite (e.g. marls, opalinus clay). The swelling law for sulphatic claystones remains unknown, even in qualitative terms. This is due to the underlying physico-chemical mechanisms, which are fundamentally different from those of purely argillaceous rocks. Another reason is the extremely long duration of the swelling process of clay-sulphate rocks, which makes systematic field or laboratory investigations very difficult. In order to close this knowledge gap, a series of 25 long-term simultaneous swelling tests has been started.

Description: Hollow cylinder specimens of cemented sand, i.e., artificial sandstone, were produced and tested by applying normal stresses and shear stresses such as to form closed stress loops to investigate the elastic behavior of the artificial sandstone. The entire process of fabrication of the hollow cylinder specimens, measurement techniques, measurements and analyses are presented to show that the artificial sandstone can be characterized as an elastic material inside the initial cementation yield surface. By applying closed stress loops in which the points of initiation of loading and the final points are identical, it has been shown that the artificially cemented sandstone behaves as a truly elastic material inside the initial cementation yield surface, because no residual energy was generated or dissipated. Furthermore, the isotropic elastic parameters have been determined for the artificially cemented sandstone.

Description: Due to the low permeability of many shale reservoirs, multi-stage hydraulic fracturing in horizontal wells is used to increase the productive, stimulated reservoir volume. However, each created hydraulic fracture alters the stress field around it, and subsequent fractures are affected by the stress field from previous fractures. The results of a numerical evaluation of the effect of stress field changes (stress shadowing), as a function of natural fracture and geomechanical properties, are presented, including a detailed evaluation of natural fracture shear failure (and, by analogy, the generated microseismicity) due to a created hydraulic fracture. The numerical simulations were performed using continuum and discrete element modeling approaches in both mechanical-only and fully coupled, hydro-mechanical modes. The results show the critical impacts that the stress field changes from a created hydraulic fracture have on the shear of the natural fracture system, which in-turn, significantly affects the success of the hydraulic fracture stimulation. Furthermore, the results provide important insight into: the role of completion design (stage spacing) and operational parameters (rate, viscosity, etc.) on the possibility of enhancing the stimulation of the natural fracture network (‘complexity’); the mechanisms that generate the microseismicity that occurs during a hydraulic fracture stimulation; and the interpretation of the generated microseismicity in relation to the volume of stimulated reservoir formation.

Description: Carbon dioxide (CO 2 ) sequestration in depleted sandstone hydrocarbon reservoirs could be complicated by a number of geomechanical problems associated with well drilling, completions, and CO 2 injection. The initial production of hydrocarbons (gas or oil) and the resulting pressure depletion as well as associated reduction in horizontal stresses (e.g., fracture gradient) narrow the operational drilling mud weight window, which could exacerbate wellbore instabilities while infill drilling. Well completions (casing, liners, etc.) may experience solids flowback to the injector wells when injection is interrupted due to CO 2 supply or during required system maintenance. CO 2 injection alters the pressure and temperature in the near wellbore region, which could cause fault reactivation or thermal fracturing. In addition, the injection pressure may exceed the maximum sustainable storage pressure, and cause fracturing and fault reactivation within the reservoirs or bounding formations. A systematic approach has been developed for geomechanical assessments for CO 2 storage in depleted reservoirs. The approach requires a robust field geomechanical model with its components derived from drilling and production data as well as from wireline logs of historical wells. This approach is described in detail in this paper together with a recent study on a depleted gas field in the North Sea considered for CO 2 sequestration. The particular case study shows that there is a limitation on maximum allowable well inclinations, 45° if aligning with the maximum horizontal stress direction and 65° if aligning with the minimum horizontal stress direction, beyond which wellbore failure would become critical while drilling. Evaluation of sanding risks indicates no sand control installations would be needed for injector wells. Fracturing and faulting assessments confirm that the fracturing pressure of caprock is significantly higher than the planned CO 2 injection and storage pressures for an ideal case, in which the total field horizontal stresses increase with the reservoir re-pressurization in a manner opposite to their reduction with the reservoir depletion. However, as the most pessimistic case of assuming the total horizontal stresses staying the same over the CO 2 injection, faulting could be reactivated on a fault with the least favorable geometry once the reservoir pressure reaches approximately 7.7 MPa. In addition, the initial CO 2 injection could lead to a high risk that a fault with a cohesion of less than 5.1 MPa could be activated due to the significant effect of reduced temperature on the field stresses around the injection site.

Description: Shale mechanical properties are evaluated from laboratory tests after a complex workflow that covers tasks from sampling to testing. Due to the heterogeneous nature of shale, it is common to obtain inconsistent test results when evaluating the mechanical properties. In practice, this variation creates errors in numerical modeling when test results differ significantly, even when samples are from a similar core specimen. This is because the fundamental models are based on the supplied test data and a gap is, therefore, always observed during calibration. Thus, the overall goal of this study was to provide additional insight regarding the organization of the non-linear model input parameters in borehole simulations and to assist other researchers involved in the rock physics-related research fields. To achieve this goal, the following parallel activities were carried out: (1) perform triaxial testing with different sample orientations, i.e., 0°, 45°, 60°, and 90°, including the Brazilian test and CT scans, to obtain a reasonably accurate description of the anisotropic properties of shale; (2) apply an accurate interpretative method to evaluate the elastic moduli of shale; (3) evaluate and quantify the mechanical properties of shale by accounting for the beddings plane, variable confinement pressures, drained and undrained test mechanisms, and cyclic versus monotonic test effects. The experimental results indicate that shale has a significant level of heterogeneity. Postfailure analysis confirmed that the failure plane coincides nicely with the weak bedding plane. The drained Poisson’s ratios were, on average, 40 % or lower than the undrained rates. The drained Young’s modulus was approximately 48 % that of the undrained value. These mechanical properties were significantly impacted by the bedding plane orientation. Based on the Brazilian test, the predicted tensile strength perpendicular to the bedding plane was 12 % lower than the value obtained using the standard isotropic correlation test. The cyclic tests provided approximately 6 % higher rock strength than those predicted by the monotonic tests.

Description: It has been widely reported that coal permeability can change from reduction to enhancement due to gas adsorption even under the constant effective stress condition, which is apparently inconsistent with the classic theoretical solutions. This study addresses this inconsistency through explicit simulations of the dynamic interactions between coal matrix swelling/shrinking induced damage and fracture aperture alteration, and translations of these interactions to permeability evolution under the constant effective stress condition. We develop a coupled coal–gas interaction model that incorporates the material heterogeneity and damage evolution of coal, which allows us to couple the progressive development of damage zone with gas adsorption processes within the coal matrix. For the case of constant effective stress, coal permeability changes from reduction to enhancement while the damage zone within the coal matrix develops from the fracture wall to further inside the matrix. As the peak Langmuir strain is approached, the decrease of permeability halts and permeability increases with pressure. The transition of permeability reduction to permeability enhancement during gas adsorption, which may be closely related to the damage zone development in coal matrix, is controlled by coal heterogeneity, external boundary condition, and adsorption-induced swelling.

Description: The prime objective of this work is to improve our understanding of the shear behavior of rock joints. Attempts are made to relate the peak shear strength of rock joints with its three-dimensional surface morphology parameters. Three groups of tensile joint replicas with different surface morphology are tested with direct shear tests under constant normal load (CNL) conditions. Firstly, the three-dimensional surface characterization of these joints is evaluated by an improved roughness parameter before being tested. Then, a new empirical criterion is proposed for these joints expressed by three-dimensional quantified surface roughness parameters without any averaging variables in such a way that a rational dilatancy angle function is used instead of ${\text{JRC}} \cdot \log_{10} \left( {{{\text{JCS}} \mathord{\left/ {\vphantom {{\text{JCS}} {\sigma_{\text{n}} }}} \right. \kern-0em} {\sigma_{\text{n}} }}} \right)$ by satisfying the new peak dilatancy angle boundary conditions under zero and critical-state normal stress (not physical infinite normal stress). The proposed criterion has the capability of estimating the peak shear strength at the laboratory scale and the required roughness parameters can be easily measured. Finally, a comparison among the proposed criterion, Grasselli’s criterion, and Barton’s criterion are made from the perspective of both the rationality of the formula and the prediction accuracy for the three groups of joints. The limitations of Grasselli’s criterion are analyzed in detail. Another 37 experimental data points of fresh rock joints by Grasselli are used to further verify the proposed criterion. Although both the proposed criterion and Grasselli’s criterion have almost equal accuracy of predicting the peak shear strength of rock joints, the proposed criterion is easier and more intuitive from an engineering point of view because of its Mohr–Coulomb type of formulation.

Description: In this paper, the effects of shear velocity on the shearing behavior of artificial joints have been studied at different normal stress levels. Here, artificial joints with planar and rough surfaces were prepared with the plaster (simulating soft rock joints) and concrete (medium-hard rock joints) materials. The rough joints had triangular shaped asperities with 10° and 20° inclination angles. Direct shear tests were performed on these joints under various shear velocities in the range of 0.3–30 mm/min. The planar plaster–plaster and planer concrete–concrete joints were sheared at three levels of normal stress under constant normal load boundary condition. Also, the rough plaster–plaster and concrete–concrete joints were sheared at one level of normal stress under constant normal stiffness boundary condition. The results of the shear tests show that the shearing parameters of joints, such as shear strength, shear stiffness and friction angle, are related to the shear velocity. Shear strength of planar and rough plaster–plaster joints were decreased when the shear velocity was increased. Shear strength of concrete joints, except for rough joints with 10° inclination, increased with increasing shear velocity. Regardless of the normal stress level, shear stiffness of both planar plaster–plaster and concrete–concrete joints were decreased when the shear velocity was increased.

Description: As the use of mechanical excavation in tunneling and mining activities expands, so has the use of disc cutters in various ground conditions. The impact of moisture content on rock behavior and rock excavation by disc cutters is examined in this study. This was done through a series of full-scale cutting tests using a 292 mm (11.5 in.) disc cutter in a moderate strength sandstone. The muck from the cutting tests was used to determine coarseness index (CI) and absolute size constant ( x′ ). x′ was calculated using Rosin–Rammler distribution (or Weibull) curve, a statistical technique to look at the fragmented rock products. This approach is very popular among the mineral-processing professionals for the evaluation of the particle sizes of the comminution products. x′ and CI show a reasonable correlation with the specific energy of cutting and production rate. Both of these indicators were found to be dependent on the cut spacing and the interaction between the adjacent cuts, as anticipated. This paper explains the background information on this topic, reviews the laboratory testing, and offers analysis of the results of grain size distribution and its relationship with specific energy and cutting geometry. It should be noted that the experimental program was limited to only one rock type and additional testing on the other rock types is required to expand the results of the current study.

Description: Roadway instability has always been a major concern in deep underground coal mines where the surrounding rock strata and coal seams are weak and the in situ stresses are high. Under the high overburden and tectonic stresses, roadways could collapse or experience excessive deformation, which not only endangers mining personnel but could also reduce the functionality of the roadway and halt production. This paper describes a case study on the stability of roadways in an underground coal mine in Shanxi Province, China. The mine was using a longwall method to extract coal at a depth of approximately 350 m. Both the coal seam and surrounding rock strata were extremely weak and vulnerable to weathering. Large roadway deformation and severe roadway instabilities had been experienced in the past, hence, an investigation of the roadway failure mechanism and new support designs were needed. This study started with an in situ stress measurement programme to determine the stress orientation and magnitude in the mine. It was found that the major horizontal stress was more than twice the vertical stress in the East–West direction, perpendicular to the gateroads of the longwall panel. The high horizontal stresses and low strength of coal and surrounding rock strata were the main causes of roadway instabilities. Detailed numerical modeling was conducted to evaluate the roadway stability and deformation under different roof support scenarios. Based on the modeling results, a new roadway support design was proposed, which included an optimal cable/bolt arrangement, full length grouting, and high pre-tensioning of bolts and cables. It was expected the new design could reduce the roadway deformation by 50 %. A field experiment using the new support design was carried out by the mine in a 100 m long roadway section. Detailed extensometry and stress monitorings were conducted in the experimental roadway section as well as sections using the old support design. The experimental section produced a much better roadway profile than the previous roadway sections. The monitoring data indicated that the roadway deformation in the experimental section was at least 40–50 % less than the previous sections. This case study demonstrated that through careful investigation and optimal support design, roadway stability in soft rock conditions can be significantly improved.

Description: This paper presents a composite element algorithm of coupled normal stress and fluid flow process for fractured rock mass, developed from the composite element method (CEM). The coupled relation between the fracture flow and normal stress makes use of the “filled model”, which examines the asperities in the fracture as a layer of granular medium having high porosity and being clipped by the two parallel plates. The existence of fractures is not considered in the mesh generation, but it will be considered explicitly in the mapped composite element. The coupled normal stress and fluid flow process has been simulated by applying a cross iterative algorithm between the two fields. The proposed algorithm considers not only the flow through the fractures, but also the flow exchange between fractures and the surrounding rock blocks. In addition, it can be used for both the filled and non-filled fractures. The verification of the proposed algorithm has been conducted through the illustration of three examples by comparison with the conventional finite element method (FEM), from which the advantages and reliability of the proposed algorithm have been shown clearly.

Description: Based on the radiated energy of 133 rock bursts monitored by a microseismic technique at the Jinping II hydropower station, in Sichuan province, China, we analyzed the advantages and disadvantages of qualitative classification methods for the rock burst intensity. Then, we investigated the characteristics, magnitude, and laws of the radiated energy, as well as the relationship between the rock burst radiated energy and intensity. Then, we selected the energy as an evaluation index for the rock burst intensity classification, and proposed a new rock burst intensity quantitative classification method, which utilized the hierarchical clustering analysis technique with the complete-linkage method. Next, we created a new set of criteria for the quantitative classification of the rock burst intensity based on radiated energy and surrounding rock damage severity. The new criteria classified the rock burst intensity into five levels: extremely intense, intense, moderate, weak, and none, and the common logarithms of the radiated energy of each level were 〉7 lg( E /J), 〉4 lg( E /J) and 〈7 lg( E /J), 〉2 lg( E /J) and 〈4 lg( E /J), 〉1 lg( E /J) and 〈2 lg( E /J), and 〈1 lg( E /J), respectively. Finally, we investigated the factors influencing the classification, and verified its feasibility and applicability via several practical rock burst examples.

Description: The experimental determination of anelastic strain recovery (ASR) compliances for three types of rocks (granite, marble, and sandstone) was performed in the laboratory. Preloading of specimens for uniaxial compression creep tests was at 50 % of the uniaxial compressive strength (UCS) for each rock type. We obtained the shear mode Jas( t ) and volumetric mode Jav( t ) ASR compliances and calculated the ratio of Jas( t ) to Jav( t ). The Kelvin model for rock rheology was then applied in numerical simulations and the results were in good agreement with the measured data for Jas( t ) and Jav( t ). These results showed that both the magnitude and rate of increase of the ASR compliances are strongly dependent on the rock type, and the values of the Jas( t )/Jav( t ) ratio for a loading of 50 % of the UCS showed a trend leading to different constants for each of the three rock types. Further experimental and numerical analyses showed approximate power-law relationships between the ASR compliances at 50 % of UCS, and both the UCS and the tangential Young’s modulus at 50 % of UCS ( E t50 ). These relationships may be useful for the preliminary estimation of ASR compliances.

Description: When rock samples are loaded until macroscopic fractures develop, the failure process can be divided into several stages based on axial and lateral strain responses or the acoustic emission sequence during uniaxial compression tests. Several stress thresholds may be identified: the crack closure stress σ cc , crack initiation stress σ ci , crack damage stress σ cd , and uniaxial compressive strength σ ucs ; these may be used as a warning indicator for rock rupture. We investigated the crack damage stress σ cd , its threshold, and a possible relationship between σ cd and the uniaxial compressive strength. The σ cd of different rock types were compiled from previous studies based on uniaxial compression tests. The results showed that the overall averages and standard deviations of σ cd /σ ucs for igneous, metamorphic, and sedimentary rocks were ~0.78 (±0.11), ~0.85 (±0.11), and ~0.73 (±0.18), respectively. There were no significant differences in σ cd /σ ucs between the different rock types, except that the sedimentary rock had a slightly larger standard deviation attributed to the variation of porosity in the samples, while the metamorphic rock had higher average σ cd /σ ucs resulting from the small statistical sample size. By excluding the higher-porosity (〉10 %) rock samples, the averages and standard deviations of σ cd /σ ucs for igneous, metamorphic, and sedimentary rocks were ~0.78 (±0.09), ~0.85 (±0.09), and ~0.78 (±0.11), respectively. The results imply that the rock origin process (i.e., igneous, metamorphic, and sedimentary) has a minimal effect on σ cd /σ ucs . The ratio σ cd / σ ucs could be an essential intrinsic property for low-porosity rocks, which could be used in rock engineering for predicting the failure process.

Description: A split Hopkinson pressure bar (SHPB) system with a special shape striker has been suggested as the test method by the International Society for Rock Mechanics (ISRM) to determine the dynamic characteristics of rock materials. In order to further verify this testing technique and microscopically reveal the dynamic responses of specimens in SHPB tests, a numerical SHPB test system was established based on particle flow code (PFC). Numerical dynamic tests under different impact velocities were conducted. Investigation of the stresses at the ends of a specimen showed that the specimen could reach stress equilibrium after several wave reverberations, and this balance could be maintained well for a certain time period after the peak stress. In addition, analyses of the reflected waves showed that there was a clear relationship between the variation of the reflected wave and the stress equilibrium state in the specimen, and the turning point of the reflected wave corresponded well with the peak stress in the specimen. Furthermore, the reflected waves can be classified into three types according to their patterns. Under certain impact velocities, the specimen deforms at a constant strain rate during the whole loading process. Finally, the influence of the micro-strength ratio ( ${{\tau_{\text{c}} } \mathord{\left/ {\vphantom {{\tau_{\text{c}} } {\sigma_{\text{c}} }}} \right. \kern-0pt} {\sigma_{\text{c}} }}$ τ c τ c σ c σ c ) and distribution pattern on the dynamic increase factor (DIF) of the strength DIF were studied, and the lateral inertia confinement and heterogeneity were found to be two important factors causing the strain rate effect for rock materials.

Description: The Cerchar abrasion index is commonly used to represent rock abrasion for estimation of bit life and wear in various mining and tunneling applications. Although the test is simple and fast, there are some discrepancies in the test results related to the equipment used, condition of the rock surface, operator skills, and procedures used in conducting and measuring the wear surface. This paper focuses on the background of the test and examines the influence of various parameters on Cerchar testing including pin hardness, surface condition of specimens, petrographical and geomechanical properties, test speed, applied load, and method of measuring wear surface. Results of Cerchar tests on a set of rock specimens performed at different laboratories are presented to examine repeatability of the tests. In addition, the preliminary results of testing with a new device as a potential alternative testing system for rock abrasivity measurement are discussed.

Description: Effective stress laws and their application are not new, but are often overlooked or applied inappropriately. The complexity of using a proper effective stress law increases when analyzing stress variation in coal as a result of gas production or mining. In this paper, an effective stress law is derived analytically for coalbed methane reservoirs, combining the concepts of matrix shrinkage/swelling and external stress by including the effect of sorbing gas pressure on the elastic response of the reservoir. The proposed law reduces to that of Terzaghi when the compressibility of bulk material is sufficiently greater than the compressibility of the solid grain, and without the strain associated with matrix shrinkage/swelling effect. Moreover, it is shown that the Biot coefficient ( α ) can have a value larger than unity for self-swelling/dilation materials, such as coal. The proposed stress–strain relationship was validated using experimental results. Overall, the effective stress law for deformation was extended for sorptive materials, providing a new and unique technique to analyze the elastic behavior of coal by reducing three variables, namely, external stress, pore pressure and matrix shrinkage/swelling along with the associated stress, down to one variable, “effective stress”.

Description: Planes of weakness like schistosity and foliation affect the strength and deformational behaviors of rocks. In this paper, an attempt has been made to study the elastic and strength behavior of slate rocks obtained from foundation of Sardasht dam site in Iran. Wet and dry specimens with different orientation of foliation were evaluated under uniaxial, triaxial, and Brazilian tests. According to the results obtained, slate mechanically pronounced U-shaped anisotropy in uniaxial and triaxial compression tests. In addition, the degree of anisotropy for the slates tested in current study was relatively high, showing the effect of foliation plane on strength and elastic parameters. It was concluded that stiffness of the samples decrease as the angle of anisotropy reaches 30–40°. This change was more pronounced for wet comparing to dry samples. However, the tensile strength obtained during Brazilian tests indicated that there is no apparent relationship between angle of anisotropy and tensile strength. However, increasing the water saturation decreased the tensile strength of the samples. The calculated elastic moduli referring to different anisotropy angles could be valuable for the design of various engineering structures in planar textured rock masses.

Description: This paper presents a study on the initiation and progress of anisotropic damage and its impact on the permeability variation of crystalline rocks of low porosity. This work was based on an existing micromechanical model considering the frictional sliding and dilatancy behaviors of microcracks and the recovery of degraded stiffness when the microcracks are closed. By virtue of an analytical ellipsoidal inclusion solution, lower bound estimates were formulated through a rigorous homogenization procedure for the damage-induced effective permeability of the microcracks-matrix system, and their predictive limitations were discussed with superconducting penny-shaped microcracks, in which the greatest lower bounds were obtained for each homogenization scheme. On this basis, an empirical upper bound estimation model was suggested to account for the influences of anisotropic damage growth, connectivity, frictional sliding, dilatancy, and normal stiffness recovery of closed microcracks, as well as tensile stress-induced microcrack opening on the permeability variation, with a small number of material parameters. The developed model was calibrated and validated by a series of existing laboratory triaxial compression tests with permeability measurements on crystalline rocks, and applied for characterizing the excavation-induced damage zone and permeability variation in the surrounding granitic rock of the TSX tunnel at the Atomic Energy of Canada Limited’s (AECL) Underground Research Laboratory (URL) in Canada, with an acceptable agreement between the predicted and measured data.

Description: This paper presents a case study of aspects of the design of large slope cuttings in complex geological conditions in South Korea. During the original cutting of the slopes to geometries prescribed by Korean standards for rock slopes, several translational slides occurred on daylighting bedding planes. From observation it was evident that away from the areas of distress, some parts of the slopes appeared to be relatively stable, apparently because of the strengthening influence of relatively strong and massive igneous dykes and sills through the sedimentary rock. This paper describes how the geological conditions were assessed and stability analysed, section-by-section, along the road. This was achieved using a form of the method of slices to account for those sections of potential failure surfaces where sliding could occur along bedding and others where failure would necessitate shear through the intrusive igneous rock. Results were checked using UDEC models of critical sections. Following these analyses, recommendations were made for localized additional preventive measures including anchors and drainage.

Description: The viscoelastic deformation behavior of a sedimentary rock under different loading rates is numerically modeled and investigated by the numerical manifold method (NMM). By incorporating a modified 3-element viscoelastic constitutive mode in the NMM, crack initiation and propagation criteria, and crack identification and evolution techniques, the effects of the loading rates on the cracking behavior of a sedimentary rock, such as crack open displacement, crack sliding displacement, crack initiation, crack propagation and final failure mode, are successfully modeled. The numerical results reveal that under a high loading rate (〉1,000 MPa/s), due to the viscoelastic property of the sedimentary rock, not only the structural behavior deviates from that of elastic model, but also different cracking processes and final failure modes are obtained.

Description: A new constitutive model to describe the shear behavior of rock joints under constant normal stiffness (CNS) and constant normal load (CNL) conditions is proposed. The model was developed using an empirical approach based on the results of a total of 362 direct shear tests on tensile fractured rock joints and replicas of tensile joints and on a new quantitative roughness parameter. This parameter, the active roughness coefficient C r , is derived from the features of the effective roughness mobilized at the contact areas during shearing. The model involves a shear strength criterion and the relations between stresses and displacements in the normal and shear directions, where the effects of the boundary conditions and joint properties are considered by the shape indices C d and C f . The model can be used to predict the shear behavior under CNS as well as CNL conditions. The shear behavior obtained from the experimental results is generally in good agreement with that estimated by the proposed model, and the effects of joint roughness, initial normal stress, and normal stiffness are reasonably reflected in the model.

Description: The construction of underground tunnels is a time-dependent process. The states of stress and strain in the ground vary with time due to the construction process. Stress and strain variations are heavily dependent on the rheological behavior of the hosting rock mass. In this paper, analytical closed-form solutions are developed for the excavation of a circular tunnel supported by the construction of two elastic liners in a viscoelastic surrounding rock under a hydrostatic stress field. In the solutions, the stiffness and installation times of the liners are accounted for. To simulate realistically the process of tunnel excavation, a time-dependent excavation process is considered in the development of the solutions, assuming that the radius of the tunnel grows from zero until its final value according to a time-dependent function to be specified by the designers. The integral equations for the supporting pressures between rock and first liner are derived according to the boundary conditions for linear viscoelastic rocks (unified model). Then, explicit analytical expressions are obtained by considering either the Maxwell or the Boltzmann viscoelastic model for the rheology of the rock mass. Applications of the obtained solutions are illustrated using two examples, where the response in terms of displacements and stresses caused by various combinations of excavation rate, first and second liner installation times, and the rheological properties of the rock is illustrated.

Description: The evolution of gouge materials in rock fractures or faults undergoing shear can change fracture properties in terms of shear strength and dilation, fluid transmissivity and retardation for contaminants. In order to conceptually understand gouge mechanical behaviors including movement, microcracking, abrasion and redistribution, particle mechanics models were used to simulate single- and multi-gouge particles in a rough fracture segment undergoing shear. The results show that gouge particles behave in two different ways under low and high normal stresses, respectively. Under low normal stress, gouge particles mainly roll with the moving fracture walls, with little surface damage and small dilation during the shear process. Under high normal stress, gouge particles can be crushed into a few major pieces and a large number of minor comminuted particles, accompanied by more severe damage (abrasion and microcracking) in fracture walls and continuous fracture closure. The modeling results were also compared with published experiments and used to explain the observed macroscopic behaviors of rock fracture undergoing shear. The effects of microparameters used in the particle mechanics models on the simulation of gouge behaviors were also investigated through sensitivity analysis.

Description: The study of heat transfer, water flow, and swelling pressure development in engineered clay barriers and the evaluation of the influence of these phenomena on the barrier properties are important issues for predicting the performance of nuclear waste repository facilities. In this work, an experimental setup is presented especially meant to assess the response of the sand–bentonite mixture under conditions close to that of the buffer in a radioactive waste repository. A newly developed column device for laboratory testing of coupled thermo-hydro-mechanical (THM) behaviour of clay-buffer materials is introduced and its calibration, verification and the first experimental data are presented and discussed. The main features of the column device are: hydraulic and thermal gradients are possible to be applied; water content, suction and temperature development can be measured continuously at three locations along the sample height; swelling stress can be measured at top and the bottom of the sample. Measuring transient temperature, water content and suction simultaneously at the same height levels and with special care to minimise the sample disturbance is one of the advantages of the column device proposed here when compared to that previously reported in the literature. The main objectives of this paper are: (1) to describe the experimental device, (2) to introduce the sensors implemented and their calibration, and (3) to present and discuss the first experimental results obtained with the new equipment. The first experimental results show promise in the ability of the newly developed column device to provide reliable data for assessing the THM behaviour of expansive materials that are foreseen as buffer material in high level waste repositories.

Description: This study investigates the wear performance of diamond circular saw blades in cutting of granitic rocks. An alternative wear measuring method is developed to measure the reduced blade radius without taking the blade off the machine. The effect on and contribution to the specific wear rate (SWR) of each operating variable are determined, and the SWR is correlated with rock properties. Morphologies of wearing surfaces of segments and rocks are also evaluated using scanning electron microscopy (SEM). Depending on both operating variables and rock properties, prediction models are developed for estimation of the SWR. Results show that the SWR increases with an increase in the peripheral speed and the traverse speed, while it decreases with an increase in the cutting depth and the flow rate of the cooling fluid. The peripheral speed, and the microhardness and proportions of minerals such as quartz, plagioclase, and feldspar are statistically determined as the significant variables affecting the SWR. Finally, it is disclosed that models developed for estimation of SWR have great potential for practical applications.

Description: The Cappadocia Region is one of the seven sites in Turkey included in the World Heritage List in 1985 by the United Nations Educational, Scientific and Cultural Organization (UNESCO) and includes structures carved into thick and soft tuffs that have survived and kept their integrity for a number of centuries. Besides existing ancient structures, new underground constructions have been undertaken. Considering the historical characteristics of the region, the construction of an underground structure was planned by the Municipality of Avanos Town to utilize a hill remnant from an abandoned quarry as an underground congress centre in the 1980s, but its construction has not yet been completed due to financial problems. In this study, integrated experimental, analytical and numerical analyses and an in situ monitoring program were undertaken to assess the stability conditions of the congress centre, which was carved into a soft tuff susceptible to long-term degradation processes. The experimental results indicate that the surrounding rock is quite vulnerable to cyclic freezing–thawing and wetting–drying processes. The strength of the rock is drastically reduced under saturated conditions, and the processes of freezing and thawing further accelerate the rock degradation under such conditions. Simple short-term stability analyses clearly show that some tensile cracking may take place and that the opening may suffer from some cracking problems 28–30 years after excavation. The analyses carried out for the long-term safety of the structure indicate that the most critical condition exists for the widest opening and that some supports at the middle of the widest opening may be necessary. Nevertheless, further studies on the long-term characteristics of this tuff are necessary to check this conclusion. The in situ monitoring clearly showed that some further crack propagation will occur, especially after rainy and freezing–thawing periods.

Description: The creep deformation behavior of the northern slope of an open-pit mine is introduced. Direct shear creep tests are then conducted for the samples taken from the northern slope to study the rheological characteristics of the rock mass. The experimental results are analyzed afterwards using an empirical method to develop a rheological model for the rock mass. The proposed rheological model is finally applied to understand the creep behavior of the northern slope, predict the long-term stability, and guide appropriate measures to be taken at suitable times to increase the factor of safety to ensure stability. Through this study, a failure criterion is proposed to predict the long-term stability of the slope based on the rheological characteristics of the rock mass and a critical deformation rate is adopted to determine when appropriate measures should be taken to ensure slope stability. The method has been successfully applied for stability analysis and engineering management of the toppling and slippage of the northern slope of the open-pit mine. This success in application indicates that it is theoretically accurate, practically feasible, and highly cost-effective.

Description: Fracture network modelling plays an important role in many application areas in which the behaviour of a rock mass is of interest. These areas include mining, civil, petroleum, water and environmental engineering and geothermal systems modelling. The aim is to model the fractured rock to assess fluid flow or the stability of rock blocks. One important step in fracture network modelling is to estimate the number of fractures and the properties of individual fractures such as their size and orientation. Due to the lack of data and the complexity of the problem, there are significant uncertainties associated with fracture network modelling in practice. Our primary interest is the modelling of fracture networks in geothermal systems and, in this paper, we propose a general stochastic approach to fracture network modelling for this application. We focus on using the seismic point cloud detected during the fracture stimulation of a hot dry rock reservoir to create an enhanced geothermal system; these seismic points are the conditioning data in the modelling process. The seismic points can be used to estimate the geographical extent of the reservoir, the amount of fracturing and the detailed geometries of fractures within the reservoir. The objective is to determine a fracture model from the conditioning data by minimizing the sum of the distances of the points from the fitted fracture model. Fractures are represented as line segments connecting two points in two-dimensional applications or as ellipses in three-dimensional (3D) cases. The novelty of our model is twofold: (1) it comprises a comprehensive fracture modification scheme based on simulated annealing and (2) it introduces new spatial approaches, a goodness-of-fit measure for the fitted fracture model, a measure for fracture similarity and a clustering technique for proposing a locally optimal solution for  fracture parameters. We use a simulated dataset to demonstrate the application of the proposed approach followed by a real 3D case study of the Habanero reservoir in the Cooper Basin, Australia.

Description: Several high-altitude slope instability phenomena, involving rock blocks of different volumes, have been observed in recent years. The increase in these phenomena could be correlated to climatic variations and to a general increase in temperature that has induced both ice melting with consequent water seepage and glacial lowering, with a consequent loss of support of the rock face. The degradation of the high-altitude thermal layer, which is known as “permafrost”, can determine the formation of highly fractured rock slopes where instabilities can concentrate. The present research has developed a methodology to improve the understanding and assessment of rock slope stability conditions in high mountain environments where access is difficult. The observed instabilities are controlled by the presence of discontinuities that can determine block detachments. Consequently, a detailed survey of the rock faces is necessary, both in terms of topography and geological structure, and in order to locate the discontinuities on the slope to obtain a better geometric reconstruction and subsequent stability analysis of the blocky rock mass. Photogrammetric surveys performed at different times allow the geostructure of the rock mass to be determined and the rock block volumes and detachment mechanisms to be estimated, in order to assess the stability conditions and potential triggering mechanisms. Photogrammetric surveys facilitate both the characterisation of the rock mass and the monitoring of slope instabilities over time. The methodology has been applied in a case study pertaining to the North Face of Aiguilles Marbrées in the Mont Blanc massif, which suffers from frequent instability phenomena. A slope failure that occurred in 2007 has been back-analysed using both the limit equilibrium method (LEM) and 3D distinct element modelling (DEM). The method has been supported and validated with traditional in situ surveys and measurements of the discontinuity orientation and other rock mass features.

Description: Geophysical site investigation techniques based on elastic waves have been widely used to characterize rock masses. However, characterizing jointed rock masses by using such techniques remains challenging because of a lack of knowledge about elastic wave propagation in multi-jointed rock masses. In this paper, the roughness of naturally fractured rock joint surfaces is estimated by using a three-dimensional (3D) image-processing technique. The classification of the joint roughness coefficient (JRC) is enhanced by introducing the scan line technique. The peak-to-valley height is selected as a key indicator for JRC classification. Long-wavelength P-wave and torsional S-wave propagation across rock masses containing naturally fractured joints are simulated through the quasi-static resonant column (QSRC) test. In general, as the JRC increases, the S-wave velocity increases within the range of stress levels considered in this paper, whereas the P-wave velocity and the damping ratio of the shear wave decrease. In particular, the two-dimensional joint specimen underestimates the S-wave velocity while overestimating the P-wave velocity. This suggests that 3D joint surfaces should be implicated to obtain the reliable elastic wave velocity in jointed rock masses. The contact characteristic and degree of roughness and waviness of the joint surface are identified as a factor influencing P-wave and S-wave propagation in multi-jointed rock masses. The results indicate a need for a better understanding of the sensitivity of contact area alterations to the elastic wave velocity induced by changes in normal stress. This paper’s framework can be a reference for future research on elastic wave propagation in naturally multi-jointed rock masses.

Description: Crack initiation in uniaxial compressive loading of rocks occurs well before the peak strength is reached. The factors that may influence the onset of cracking and possible initiating mechanisms were explored using a discrete element numerical approach. The numerical approach was based on grain-based model that utilized the Voronoi tessellation scheme to represent low porosity crystalline rocks such as granite. The effect of grain size distribution (sorting coefficient ranging from 1.5 to 1.03), grain size (average grain size ranging from 0.75 to 2.25 mm), and the heterogeneities of different mineral grains (quartz, K-feldspar, plagioclase) on the onset of cracking were examined. The modelling revealed that crack initiation appears to be a tensile mechanism in low porosity rocks, and that shear cracking along grain boundaries is only a prominent mechanism near the peak strength. It was also shown that the heterogeneity introduced by the grain size distribution had the most significant effect on peak strength and crack initiation stress. The peak strength ranges from 140 to 208 MPa as the grain size distribution varies from heterogeneous to uniform, respectively. However, the ratio of crack initiation to peak stress showed only minor variation, as the heterogeneity decreases. The other factors investigated had only minor effects on crack initiation and peak strength, and crack initiation ratio.

Description: The variation of the shear strength of infilled rock joints under cyclic loading and constant normal stiffness conditions is studied. To simulate the joints, triangular asperities inclined at angles of 9.5° and 18.5° to the shear movement were cast using high-strength gypsum plaster and infilled with clayey sand. These joints were sheared cyclically under constant normal stiffness conditions. It was found that, for a particular normal stiffness, the shear strength is a function of the initial normal stress, initial asperity angle, joint surface friction angle, infill thickness, infill friction angle, loading direction and number of loading cycles. Based on the experimental results, a mathematical model is proposed to evaluate the shear strength of infilled rock joints in cyclic loading conditions. The proposed model takes into consideration different initial asperity angles, initial normal stresses and ratios of infill thickness to asperity height.

Description: Underground excavation normally causes instability of the mother rock due to the release and redistribution of stress within the affected zone. For gaining deep insight into the characteristics and mechanism of rock crack evolution during underground excavation, laboratory tests are carried out on 36 man-made rock specimens with single or double cracks under two different unloading conditions. The results show that the strength of rock and the evolution of cracks are clearly influenced by both the inclination angle of individual cracks with reference to the unloading direction and the combination geometry of cracks. The peak strength of rock with a single crack becomes smaller with the inclination angle. Crack propagation progresses intermittently, as evidenced by a sudden increase in deformation and repeated fluctuation of measured stress. The rock with a single crack is found to fail in three modes, i.e., shear, tension–shear, and splitting, while the rock bridge between two cracks is normally failed in shear, tension–shear, and tension. The failure mode in which a crack rock or rock bridge behaves is found to be determined by the inclination angle of the original crack, initial stress state, and unloading condition. Another observation is that the secondary cracks are relatively easily created under high initial stress and quick unloading.

Description: Rockburst is a sudden and violent failure of rocks and it often occurs in hard rocks in highly stressed ground. Strainburst is classified as one type of rockburst and it often occurs in rocks near or at the excavation boundary. Deep insight into the strainburst phenomenon is essential for safe underground construction at depth. In this paper, an experimental laboratory study on the strainburst behavior of Beishan granite is presented. Based on in-situ stress measurement data from the Beishan area in China, a series of tests under different unloading rates were performed to investigate the strainburst process using a true-triaxial strainburst test system which was equipped with an acoustic emission (AE) monitoring system. In addition, a high-speed video camera was used to record and visualize the initiation and ejection of rock fragments as well as the sudden dynamic failure (strainburst) of the test samples. AE characteristics associated with the cumulative energy and frequency–amplitude distributions were analyzed. Characteristics of the microscopic structure of a fragment generated from one test were observed using a scanning electron microscope. The experimental results indicate that the degree of violence during failure and the associated AE energy release in the strainburst process are dependent on the unloading rate. When the unloading rate is high, the rock is prone to strainburst. On the other hand, as the unloading rate decreases, the failure mode changes from strainburst to spalling. In addition, the cumulative AE energy is not sensitive to unloading rates greater than 0.05 MPa/s. When the unloading rate is less than 0.05 MPa/s, the cumulative AE energy shows a marked decreasing trend during rock failure.

Description: Backbreak is an undesirable side effect of bench blasting operations in open pit mines. A large number of parameters affect backbreak, including controllable parameters (such as blast design parameters and explosive characteristics) and uncontrollable parameters (such as rock and discontinuities properties). The complexity of the backbreak phenomenon and the uncertainty in terms of the impact of various parameters makes its prediction very difficult. The aim of this paper is to determine the suitability of the stochastic modeling approach for the prediction of backbreak and to assess the influence of controllable parameters on the phenomenon. To achieve this, a database containing actual measured backbreak occurrences and the major effective controllable parameters on backbreak (i.e., burden, spacing, stemming length, powder factor, and geometric stiffness ratio) was created from 175 blasting events in the Sungun copper mine, Iran. From this database, first, a new site-specific empirical equation for predicting backbreak was developed using multiple regression analysis. Then, the backbreak phenomenon was simulated by the Monte Carlo (MC) method. The results reveal that stochastic modeling is a good means of modeling and evaluating the effects of the variability of blasting parameters on backbreak. Thus, the developed model is suitable for practical use in the Sungun copper mine. Finally, a sensitivity analysis showed that stemming length is the most important parameter in controlling backbreak.

Description: Altered rock, as the abutment materials of Xiaowan Hydropower Station in China, is a kind of geological defective rock mass. It is loosely structured and its strength is low, with some development of pores and cavities. Research on the hydro-mechanical coupling of the altered rock are of important significance to hydropower projects. In this study, the advanced fully automatic triaxial fluid flow-rheological test servo system is employed to study the hydro-mechanical coupling characteristics of the altered rock, and the water pressures and confining pressures in the laboratory tests are set to simulate the conditions of excavation and impoundment of Xiaowan Hydropower Station. Based on the test results, the stress–strain laws of the rock specimens under the effect of complete hydro-mechanical coupling, as well as the lateral strain and volumetric strain characteristics, are studied. The fluid flow laws of the rock specimens and the effects of the confining pressures on the fluid flow are analyzed. The fluid flow failure characteristic under the effect of the complete hydro-mechanical coupling is discussed. The research achievements show that with the change of the stress states, the permeability of the rock also changes, and the permeability evolution shows the phase characteristic during the process of stress and strain. The impacts of the confining pressures on the strength and deformation and permeability of the altered rock are obvious. The failure behaviours of the rock specimens under the effect of coupling relates to the confining pressures, including two kinds of splitting failure and shear failure. The fluid flow failure characteristic of the rock specimens depend upon the initiation, growth and coalesce of micro-cracks, heterogeneity, confining pressures and properties of the rock.

Description: This paper reports some results of a large experimental program on Boom Clay conducted in Grenoble in the framework of the European project SELFRAC. The program included isotropic compression up to relatively high stress, drained triaxial compression tests at different cell pressures, as well as permeability measurements under isotropic and deviatoric stress. Local measurement of axial and radial displacements allowed the detection of strain localization during deviatoric loading. The permeability of Boom Clay is found to depend on the mean effective stress. The response of Boom Clay during deviatoric loading appears to be strongly affected by the swelling experienced during the isotropic stage preceding triaxial compression. The rate of swelling decreases with isotropic stress. The longer the swelling before shear, more the response under shear becomes ductile and the lower the initial stiffness. Permeability depends on the mean effective stress and it is found to decrease of about two orders of magnitude when the mean stress increases from 1 to 32 MPa. Permeability during shear loading is essentially constant and does not seem to be affected by strain localization. These results are complemented by a few observations obtained using X-ray microtomography in the framework of the more recent European project TIMODAZ. These findings illustrate the impact of pre-existing inclusions and fissures on specimen deformation upon deviatoric loading in the laboratory.

Description: The Mohr–Coulomb (M–C) failure criterion is one of the most widely used failure criteria in rock mechanics, although it has a number of shortcomings such as neglecting the nonlinear strength observed in rock or the effect of the intermediate principal stress σ 2 . Other failure criteria have been proposed to effectively include in the predictions of failure the non-linear response of rock to confinement or the effects of the intermediate principal stress. The M–C criterion is still widely used, and it is arguably the criterion most used in practice. For example, stability evaluations of shallow rock structures such as slopes and foundations are routinely carried out by estimating a friction angle and a cohesion of the rock mass. To include the dependency of cohesion and friction angle on stresses, efforts are being made to estimate equivalent values of the M–C parameters for the range of stresses applicable to a particular design. The paper suggests a new and convenient approach to find the equivalent friction angle and cohesion from any failure criterion that can be expressed in terms of the Nayak and Zienkiewicz’s stress invariants. To demonstrate the capabilities and application of the methodology, the new approach is applied to two failure criteria: the Hoek–Brown (H–B) criterion and the Hoek–Brown and Willam–Warnke (HB–WW) criterion, 2-D and 3-D failure criteria, respectively. Results from the new method, in terms of equivalent friction and cohesion for the H–B criterion, are exactly the same as the results obtained from Balmer’s theory, which confirms the validity of the new method. The predicted equivalent friction and cohesion for the HB–WW criterion show a dependency on σ 2 , which does not occur for a 2-D failure criterion.

Description: In this paper, the two computer codes TOUGH2 and RDCA (for “rock discontinuous cellular automaton”) are integrated for coupled hydromechanical analysis of multiphase fluid flow and discontinuous mechanical behavior in heterogeneous rock. TOUGH2 is a well-established code for geohydrological analysis involving multiphase, multicomponent fluid flow and heat transport; RDCA is a numerical model developed for simulating the nonlinear and discontinuous geomechanical behavior of rock. The RDCA incorporates the discontinuity of a fracture independently of the mesh, such that the fracture can be arbitrarily located within an element, while the fluid pressure calculated by TOUGH2 can be conveniently applied to fracture surfaces. We verify and demonstrate the coupled TOUGH–RDCA simulator by modeling a number of simulation examples related to coupled multiphase flow and geomechanical processes associated with the deep geological storage of carbon dioxide—including modeling of ground surface uplift, stress-dependent permeability, and the coupled multiphase flow and geomechanical behavior of fractures intersecting the caprock.

Description: The rock cutting performance of an abrasive waterjet is affected by various parameters. In this study, rock cutting tests are conducted with different energy (i.e., water pressure, traverse speed, and abrasive feed rate), geometry (i.e., standoff distance), and material parameters [i.e., uniaxial compressive strength (UCS)]. In particular, experimental tests are carried out at a long standoff distance (up to 60 cm) to consider field application. The effective parameters of the rock cutting process are identified based on the relationships between the cutting performance indices (depth, width, and volume) and parameters. In addition, the cutting efficiency is analyzed with effective parameters as well as different pump types and the number of cutting passes considering the concept of kinetic jet energy. Efficiency analysis reveals that the cutting depth efficiency tends to increase with an increase in the water pressure and traverse speed and with a decrease in the standoff distance and UCS. Cutting volume efficiency strongly depends on standoff distance. High efficiency of cutting volume is obtained at a long standoff distance regardless of the pump type. The efficiency analysis provides a realistic way to optimize parameters for abrasive waterjet rock excavation.

Description: It is widely accepted that the in-situ strength of massive rocks is approximately 0.4 ± 0.1 UCS, where UCS is the uniaxial compressive strength obtained from unconfined tests using diamond drilling core samples with a diameter around 50 mm. In addition, it has been suggested that the in-situ rock spalling strength, i.e., the strength of the wall of an excavation when spalling initiates, can be set to the crack initiation stress determined from laboratory tests or field microseismic monitoring. These findings were supported by back-analysis of case histories where failure had been carefully documented, using either Kirsch’s solution (with approximated circular tunnel geometry and hence σ max  =  3σ 1 −σ 3 ) or simplified numerical stress modeling (with a smooth tunnel wall boundary) to approximate the maximum tangential stress σ max at the excavation boundary. The ratio of σ max / UCS is related to the observed depth of failure and failure initiation occurs when σ max is roughly equal to 0.4 ± 0.1 UCS. In this article, it is suggested that these approaches ignore one of the most important factors, the irregularity of the excavation boundary, when interpreting the in-situ rock strength. It is demonstrated that the “actual” in-situ spalling strength of massive rocks is not equal to 0.4 ± 0.1 UCS, but can be as high as 0.8 ± 0.05 UCS when surface irregularities are considered. It is demonstrated using the Mine-by tunnel notch breakout example that when the realistic “as-built” excavation boundary condition is honored, the “actual” in-situ rock strength, given by 0.8 UCS, can be applied to simulate progressive brittle rock failure process satisfactorily. The interpreted, reduced in-situ rock strength of 0.4 ± 0.1 UCS without considering geometry irregularity is therefore only an “apparent” rock strength.

Description: This article deals with the problem of assisting disc cutters by means of high-velocity jets of water, with the aim of increasing the excavation rate while improving the working conditions, with particular reference to wear. The results of an experimental research undertaken at the Waterjet Laboratory of the University of Cagliari on a medium–hard abrasive rock clearly show that a higher removal rate is achieved owing to the weakening action of a jet directed on one side of the disc, causing deeper penetration. This outcome is interpreted on the basis of the scale formation model, which explains why smaller scales are obtained on the water jet’s side of the groove. Accordingly, it is suggested that the results can be further improved if the jet is directed ahead of the tool along the same path, since, in this way, larger scales can be produced on both sides.

Description: Most of the recent research on rockfall and the development of protective systems, such as flexible rockfall barriers, have been focused on medium to high levels of impacting energy. However, in many regions of the world, the rockfall hazard involves low levels of energy. This is particularly the case in New South Wales, Australia, because of the nature of the geological environments. The state Road and Traffic Authority (RTA) has designed various types of rockfall barriers, including some of low capacity, i.e. 35 kJ. The latter were tested indoors using a pendulum equipped with an automatic block release mechanism triggered by an optical beam. Another three systems were also tested, including two products designed by rockfall specialised companies and one modification of the initial design of the RTA. The research focused on the influence of the system’s stiffness on the transmission of load to components of the barrier such as posts and cables. Not surprisingly, the more compliant the system, the less loaded the cables and posts. It was also found that removing the intermediate cables and placing the mesh downslope could reduce the stiffness of the system designed by the RTA. The paper concludes with some multi-scale considerations on the capacity of a barrier to absorb the energy based on experimental evidence.

Description: The erosion of schist due to impinging quartz and glass particles is investigated. The experimental set-up used to perform the erosion tests is briefly described. The erosion process is characterized by two failure modes, namely intercrystalline fracture and lateral cracking. The power exponent for particle velocity effects is 2.44. The schist shows the features of plastic deformation, and a model proposed by Ruff and Wiederhorn (Erosion. Academic Press, New York, pp 69–126, 1979 ) can be applied to the erosion process. The formation of glassy microfibers is noted, and it is attributed to localized heating due to the particle impact process.

Description: The fraction of swelling-type clay minerals (smectites) is a first measure to estimate the swelling potential of natural rocks. This swelling potential is only activated if water can be incorporated in the sheet silicates. The samples studied in this work are volcanic rocks from the Southern Andes. According to the petrographic and mineralogical studies, the rocks are characterized as vulcanite with high content of smectite. Undisturbed samples showed minor swelling behaviour in conventional swelling tests, although X-ray diffraction and differential thermal analysis revealed swelling clay contents of more than 70 % in some samples. Powder swelling tests were performed on ground and recompacted rock samples. The results of these tests show a better agreement with those expected according to the mineralogical composition of the samples. The reason for the non-activation of the swelling potential in undisturbed samples was suspected to be the influence of the iron-oxide/hydroxide content, which was on average around 11 %. Scanning electron microscope images showed that the edge terminations of the clay particles are coated with hematite. The hydrophobic properties of hematite impede the access of water between the interlayers of the clay minerals and, in addition, hematite cementation prevents the expansion. In a further series of swelling tests water was replaced by a reducing solution (0.1 molar hydroxylammonium chloride) which removes the Fe-oxide coating. It can be shown that thus the swelling potential of the clay minerals in such undisturbed rocks can be activated.

Description: Most blasting operations are associated with various forms of energy loss, emerging as environmental side effects of rock blasting, such as flyrock, vibration, airblast, and backbreak. Backbreak is an adverse phenomenon in rock blasting operations, which imposes risk and increases operation expenses because of safety reduction due to the instability of walls, poor fragmentation, and uneven burden in subsequent blasts. In this paper, based on the basic concepts of a rock engineering systems (RES) approach, a new model for the prediction of backbreak and the risk associated with a blast is presented. The newly suggested model involves 16 effective parameters on backbreak due to blasting, while retaining simplicity as well. The data for 30 blasts, carried out at Sungun copper mine, western Iran, were used to predict backbreak and the level of risk corresponding to each blast by the RES-based model. The results obtained were compared with the backbreak measured for each blast, which showed that the level of risk achieved is in consistence with the backbreak measured. The maximum level of risk [vulnerability index (VI) = 60] was associated with blast No. 2, for which the corresponding average backbreak was the highest achieved (9.25 m). Also, for blasts with levels of risk under 40, the minimum average backbreaks (〈4 m) were observed. Furthermore, to evaluate the model performance for backbreak prediction, the coefficient of correlation ( R 2 ) and root mean square error (RMSE) of the model were calculated ( R 2  = 0.8; RMSE = 1.07), indicating the good performance of the model.

Description: In the context of carbon capture and storage, deep underground injection of CO 2 induces the geomechanical changes within and around the injection zone and their impact on CO 2 storage security should be evaluated. In this study, we conduct coupled multiphase fluid flow and geomechanical modeling to investigate such geomechanical changes, focusing on probabilistic analysis of injection-induced fracture reactivation (such as shear slip) that could lead to enhanced permeability and CO 2 migration across otherwise low-permeability caprock formations. Fracture reactivation in terms of shear slip was analyzed by implicitly considering the fracture orientations generated using the Latin hypercube sampling method, in one case using published fracture statistics from a CO 2 storage site. The analysis was conducted by a coupled multiphase fluid flow and geomechanical simulation to first calculate the three-dimensional stress evolution during a hypothetical CO 2 injection operation and then evaluate the probability of shear slip considering the statistical fracture distribution and a Coulomb failure analysis. We evaluate the probability of shear slip at different points within the injection zone and in the caprock just above the injection zone and relate this to the potential for opening of new flow paths through the caprock. Our analysis showed that a reverse faulting stress field would be most favorable for avoiding fracture shear reactivation, but site-specific analyses will be required because of strong dependency of the local stress field and fracture orientations.

Description: This paper presents a practical procedure for assessing the system reliability of a rock tunnel. Three failure modes, namely, inadequate support capacity, excessive tunnel convergence, and insufficient rockbolt length, are considered and investigated using a deterministic model of ground-support interaction analysis based on the convergence–confinement method (CCM). The failure probability of each failure mode is evaluated from the first-order reliability method (FORM) and the response surface method (RSM) via an iterative procedure. The system failure probability bounds are estimated using the bimodal bounds approach suggested by Ditlevsen ( 1979 ), based on the reliability index and design point inferred from the FORM. The proposed approach is illustrated with an example of a circular rock tunnel. The computed system failure probability bounds compare favorably with those generated from Monte Carlo simulations. The results show that the relative importance of different failure modes to the system reliability of the tunnel mainly depends on the timing of support installation relative to the advancing tunnel face. It is also shown that reliability indices based on the second-order reliability method (SORM) can be used to achieve more accurate bounds on the system failure probability for nonlinear limit state surfaces. The system reliability-based design for shotcrete thickness is also demonstrated.

Description: This research work studied the influence of texture on the mechanical properties of crystalline rocks at the scale of the laboratory sample. The experiments were performed on a marble varying in texture, so that the study was conducted on homogeneous (entirely xenoblastic or totally granoblastic) and heterogeneous (mix of the two textures) specimens. The mechanical behavior of the homogeneous and heterogeneous samples was investigated with static and dynamic, destructive and nondestructive tests, in natural conditions, at the laboratory temperature, and at higher temperatures. The specimens were heated to 100, 200, and 300 °C, in order to examine the effect of heating temperature on the elastic modulus and P-waves velocity. As a result, the pure granoblastic marble exhibits values of the elastic modulus, P-waves velocity, and strengths, both in natural conditions and on heated specimens, lower than xenoblastic samples. Such different behavior can be explained by a higher grain boundaries porosity of the granoblastic marble. On heterogeneous samples, only the Rock Impact Hardness Number (RIHN) appears able to highlight the dependence of the mechanical properties on the rock texture. In particular, the impact strength improves with increasing the percentage of xenoblastic texture inside the specimen.

Description: Different failure modes during fracture shearing have been introduced including dilation, sliding, asperity cut-off and degradation. Several laboratory studies have reported the complexity of these failure modes during shear tests performed under either constant normal load (CNL) or constant normal stiffness (CNS) conditions. This paper is concerned with the mechanical behaviour of synthetic fractures during direct shear tests using a modified shear cell and related numerical simulation studies. The modifications made to an existing true triaxial stress cell (TTSC) in order to use it for performing shear tests under CNL conditions are presented. The large loading capacity and the use of accurate hydraulic pumps capable of applying a constant shear velocity are the main elements of this cell. Synthetic mortar specimens with different fracture surface geometries are tested to study the failure modes, including fracture sliding, asperity degradation, and to understand failure during shearing. A bonded particle model of the direct shear test with the PFC2D particle flow code is used to mimic the tests performed. The results of a number of tests are presented and compared with PFC2D simulations. The satisfactory results obtained both qualitatively and quantitatively are discussed.

Description: This paper presents an experimental study on the sawing of granitic rocks by circular diamond sawblades. The influence of the operating variables and rock properties on the specific energy were initially investigated and analyzed. To determine the most significant operating variables and rock properties influencing the specific energy, statistical analyses were then employed and the models were built for the estimation of specific energy depending on the operating variables and the rock properties. Moreover, the derived models were validated through statistical tests such as the determination coefficient, t -test, F -test, and residuals. The results indicated that the specific energy decreased with the decreasing of peripheral speed and the increasing of traverse speed, cutting depth, and flow rate of cooling fluid, respectively. It was concluded that, rather than the physico-mechanical properties, the mineralogical properties were the dominant rock properties affecting the specific energy. Additionally, the peripheral speed was statistically determined as the most significant operating variable affecting the specific energy. The peripheral speed was followed by the cutting depth, traverse speed, and flow rate of cooling fluid with respect to their level of significance on the specific energy. Furthermore, the model results revealed that the developed models have high potentials as a guidance for practical applications.

Description: Shallow slope failure in mountainous regions is a common and emergent hazard in terms of its damage to important traffic routes and local communities. The impact of dry granular flows consisting of rock fragments and other particles resulting from shallow slope failures on retaining structures has yet to be systematically researched and is not covered by current design codes. As a preliminary study of the impact caused by dry granular flows, a series of dry granular impact experiments were carried out for one model of a retaining wall. It was indirectly verified that the total normal force exerted on a retaining wall consists of a drag force ( F d ), a gravitational and frictional force ( F gf ), and a passive earth force ( F p ), and that the calculation of F d can be based on the empirical formula defined in NF EN Eurocode 1990 ( Eurocode structuraux. Base de calcul des structures , AFNOR La plaine Saint Denis, 2003 ). It was also indirectly verified that, for flow with Froude number from 6 to 11, the drag coefficient ( C d ) can be estimated using the previously proposed empirical parameters.

Description: Derinkuyu Underground City, located in the Cappadocia Region of Turkey, is an important structure not only for its antique and archaeological characteristics, but also as a structure in terms of the long-term stability of underground rock structures excavated by mankind. The authors carried out some observational, experimental and theoretical rock mechanics studies in the region from 1996 in the context of a research project supported by the Ministry of Education, Culture, Sports, Science and Technology of Japan for the assessment of the long-term behaviour of Derinkuyu Underground City, and these studies are still continuing. In addition to the monitoring of the environmental conditions such as temperature, moisture and air pressure, they also installed acoustic emission (AE) and electrical potential (EP) measurement systems to monitor the behaviour and response of the surrounding rock at the fifth and seventh floors of the underground city. In this article, the geology, seismicity and state of stress of the Cappadocia Region, climatic conditions in the underground city and its vicinity, short- and long-term behaviours of the surrounding rock, its index and mechanical properties, and effects of water content and freezing–thawing processes were investigated. The stability of Derinkuyu Underground City was also evaluated using theoretical and numerical methods, and the results were presented. Furthermore, its implications in modern geoengineering are also discussed.

Description: During deep rock mass excavation with the method of drill and blast, accompanying the secession of rock fragments and the formation of a new free surface, in situ stress on this boundary is suddenly released within several milliseconds, which is termed the transient release of in situ stress. In this process, enormous strain energy around the excavation face is instantly released in the form of kinetic energy and it inevitably induces microseismic events in surrounding rock masses. Thus, blasting excavation-induced microseismic vibrations in high-stress rock masses are attributed to the combined action of explosion and the transient release of in situ stress. The intensity of stress release-induced microseisms, which depends mainly on the magnitude of the in situ stress and the dimension of the excavation face, is comparable to that of explosion-induced vibrations. With the methods of time–energy density analysis, amplitude spectrum analysis, and finite impulse response (FIR) digital filter, microseismic vibrations induced by the transient release of in situ stress were identified and separated from recorded microseismic signals during a blast of deep rock masses in the Pubugou Hydropower Station. The results show that the low-frequency component in the microseismic records results mainly from the transient release of in situ stress, while the high-frequency component originates primarily from explosion. In addition, a numerical simulation was conducted to demonstrate the occurrence of microseismic events by the transient release of in situ stress, and the results seem to have confirmed fairly well the separated vibrations from microseismic records.

Description: The influence of roughness on the hydro-mechanical behavior of rock discontinuities has long been recognized. As a result, several definitions and measures of roughness have been developed. According to the ISRM (Int J Rock Mech Min Sci Geomech Abstr 15(6):319–368, 1978 ), discontinuity roughness comprises large-scale (waviness) and small-scale (unevenness) components. However, the division between these scales is not clear and most investigations of surface roughness have been restricted to small fracture surfaces (〈1 m 2 ). Hence, the large-scale components of roughness are often neglected. Furthermore, these investigations typically define roughness using two-dimensional profiles rather than three-dimensional surfaces, which can lead to biased estimates of roughness. These limitations have led to some contradictory findings regarding roughness scale dependency (scale effects). This paper aims to provide some explanation of these contradictory findings. Through the in situ digitization and analysis of two adjacent large-scale (~2 × 3 m 2 and ~2 × 2 m 2 ) migmatitic-gneiss fracture surfaces, the influence of sample size on roughness estimates are investigated. In addition, the influence of measurement resolution on roughness estimates is investigated by digitizing small-scale (100 × 100 mm 2 ) samples from the same fracture with varying resolution. The findings show roughness to increase as a function of the sampling window size, in contrast to what is commonly assumed. That is, the combined waviness and unevenness of a discontinuity relative to its mean plane increases with scale. Compared to the sampling window size, the resolution of surface measurements is shown to have a far greater influence on roughness estimates. This influence of measurement resolution may explain some of the contradictory roughness scale relationships that have been published previously. It is important to note that the observed decrease in shear strength with increasing scale, as observed in many prior studies, is not being questioned; rather, a clarification of the role of roughness in this phenomenon is sought.

Description: This paper investigates the application of a double Fourier series technique to the construction of an elastic stress field in a cylindrical bar subject to lateral boundary loads. The lateral loads, including the constant load boundary conditions, are represented by two Fourier series: one on the perimeter of the circular section ( r 0 , θ ) and the other on the longitudinal curved surface parallel to the bar axis ( z ). The technique invokes acceptable potential functions of the Papkovich–Neuber displacement field, satisfying the governing partial differential equations, to assign appropriate odd and even trigonometric Fourier terms in cylindrical coordinates ( r ,  θ ,  z ). The generic solution decomposes the problem of interest to a state of stress caused by two independent boundary conditions along the z axis and θ -polar angle, both superimposed on a solution for which these potentials are the product of the trigonometric terms of the independent variables ( θ ,  z ). Constants appearing in the resultant second-order partial differential equations are determined from the generally mixed (tractions and/or displacements) boundary conditions. While the solutions are satisfied exactly at the ends of an infinite bar, they are satisfied weakly on average, in the light of Saint Venant’s approximation at the two ends of a finite bar. The application of the proposed analysis is verified against available elastic solutions for axisymmetric and non-axisymmetric engineering problems such as the indirect Brazilian Tensile Strength and Point Load Strength tests.

Description: The axial, crack and total volumetric strains, porosity, elastic constants, crack damage stresses, uniaxial compressive strengths, as well as fracture apertures and number of fracture traces in rock samples surface after compression were defined for different chalk, basalt, dolomite, granite, limestone and sandstone samples exhibiting negative total volumetric strain at failure. It is established that the total (summed) aperture of vertical fractures obtained on the lateral surface of rock sample is related to three characteristic strain parameters: axial strain at the onset of negative total volumetric strain, axial failure strain and negative total volumetric strain at failure. The relation is based on Descartes folium equation, where the length of the loop of folium is equal to axial strain coordinate at the onset of negative total volumetric strain. This relation shows that the total aperture increases according to power law with increasing difference between axial failure strain and axial strain at the onset of negative total volumetric strain. Simultaneously, an increase in this difference leads to an increase in the value of negative total volumetric strain at failure. It is found that a direct correlation between total aperture of fractures and negative total volumetric strain at failure is relatively weak. Nevertheless, total aperture of fractures tends to increase with increasing absolute value of negative total volumetric strain at failure. It is revealed that there is no connection between the number of fracture traces and negative total volumetric strain at failure.

Description: The construction of the Aica-Mules tunnel, completed in 2010, provides a relevant case history for improving the knowledge of hydrogeological issues related to the excavation of deep tunnels in granitic massifs. The Aica-Mules tunnel is a 10 km-long structure, forming part of the high-speed railway connection between Austria and Italy across the Alpine chain, located at an average depth of 500–1,000 m below the surface. Prior to and during the construction, intense hydrogeological monitoring was set up, allowing the collection of abundant data concerning: (1) the evolution of water inflows into the tunnel; (2) the chemistry and temperature of drained groundwater; and (3) the influence of tunnel drainage on springs. Based on detailed analysis of geological/hydrogeological data, this article provides an insight into the permeability distribution in granitic rocks affected by relevant brittle tectonic deformation, and the consequences of water inflow during excavation. The available time series from the principal water discharges in the tunnel have been used in order to test the reliability of some of the most commonly applied analytical methods for the forecast of water inflows into tunnels.

Description: Sub-angular-shaped aggregates are used as rail foundation ballasts and must remain sub-angular during their service life time to maintain particle–particle interlocking, in order to ensure the stability of the rail line and prevent accidents by derailment. Here, the screening of dolerite quarry aggregates for use as railway foundation ballasts was investigated by employing simple digital image and chart methods. The average particle size ( d 50 ), flakiness index (FI), Los Angeles abrasion index (LAAI), sphericity (SPH) and roundness (RND) were determined for two batches of dolerite ballasts from the Rooikraal quarry in Johannesburg and Ngagane quarry in Newcastle. Thirty samples from each of the two batches of ballast were analysed. The ballasts were progressively abraded using a Los Angeles abrasion device and were analysed after each cycle of abrasion. A decrease in d 50 and an increase in FI with increased number of abrasion cycles were observed for both batches of dolerite ballast. The difference in the chart and digital image values of RND and SPH were marginal before abrasion; however, these differences increased with each abrasion cycle. The LAAI, d 50 , mean RND and mean SPH correlated significantly and were found to have high regression coefficients. Thus, statistical models are proposed for the non-destructive routine screening of in-place ballasts in order to track marginal changes in aggregate shapes, facilitate ballast replacement programmes and avoid rail line instability.

Description: In this paper a numerical method for the simulation of the steady-state fluid flow in discrete fracture networks is described. It is based on the use of non-conforming meshes, enrichment functions and an optimization procedure. The meshing process is performed on each fracture independently of the other fractures, i.e. without geometrical conformity at the intersections (traces). The slope discontinuities due to the flux exchange at the traces are then captured with the enrichment functions of the extended finite elements, and finally a functional is minimized by resorting to an optimization procedure. The method can be easily implemented for parallel computers being based on many small independent problems. In order to show the effectiveness of the method and the quality of the results, simulations of fluid flow in simple networks are illustrated.

Description: Experiments on man-made flawed rock-like materials are applied extensively to study the mechanical behaviour of rock masses as well as crack initiation modes and crack coalescence types. A large number of experiments on specimens containing two or three pre-existing flaws were previously conducted. In the present work, experiments on rock-like materials (formed from a mixture of sand, plaster, limestone and water at mass ratio of 126:9:9:16) containing multiple flaws subjected to uniaxial compression were conducted to further research the effects of the layout of pre-existing flaws on mechanical properties, crack initiation modes and crack coalescence types. Compared with previous experiments in which only three types of cracks were found, the present experiments on specimens containing multiple flaws under uniaxial compression revealed five types of cracks, including wing cracks, quasi-coplanar secondary cracks, oblique secondary cracks, out-of-plane tensile cracks and out-of-plane shear cracks. Ten types of crack coalescence occurred through linkage among wing cracks, quasi-coplanar secondary cracks, oblique secondary cracks, out-of-plane shear cracks and out-of-plane tensile cracks. Moreover, the effects of the non-overlapping length and flaw angle on the complete stress–strain curves, the stress of crack initiation, the peak strength, the peak strain and the elastic modulus were also investigated in detail.

Description: In oil and gas drilling or geothermal well drilling, the temperature difference between the drilling fluid and formation will lead to an apparent temperature change around the borehole, which will influence the stress state around the borehole and tend to cause borehole instability in high geothermal gradient formations. The thermal effect is usually not considered as a factor in most of the conventional borehole stability models. In this research, in order to solve the borehole instability in high-temperature formations, a calculation model of the temperature field around the borehole during drilling is established. The effects of drilling fluid circulation, drilling fluid density, and mud displacement on the temperature field are analyzed. Besides these effects, the effect of temperature change on the stress around the borehole is analyzed based on thermoelasticity theory. In addition, the relationships between temperature and strength of four types of rocks are respectively established based on experimental results, and thermal expansion coefficients are also tested. On this basis, a borehole stability model is established considering thermal effects and the effect of temperature change on borehole stability is also analyzed. The results show that the fracture pressure and collapse pressure will both increase as the temperature of borehole rises, and vice versa. The fracture pressure is more sensitive to temperature. Temperature has different effects on collapse pressures due to different lithological characters; however, the variation of fracture pressure is unrelated to lithology. The research results can provide a reference for the design of drilling fluid density in high-temperature wells.