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The effect of in-situ reservoir conditions (high temperature and pressure) on the fracture characteristics (aperture, asperities) and behaviour of fluid through the fractures is a very niche study area for varied energy fields like (i) enhanced geothermal energy systems (ii) nuclear energy storage (iii) CO2 sequestration, (iv) petroleum engineering and (v) storage of liquified natural gas. Keeping this in view many researchers have conducted studies to understand the heat and mass transfer processes occurring in the fractures. Along the same lines Tan et al., 2023 published the paper Thermal Effects Promotes Non-Darcian Flow in Heated Rock Fractures to understand the effect of fluid flow through rock fractures at high temperatures. However, the data presented in the paper by Tan et al., Rock Mech Rock Eng 2023 related to (i) the fracture closure, (ii) the time required for the changes along the fracture, (iii) the thermal expansion effect, and (iv) changes to the phase of water while heating requires further explanation and will otherwise result in the dissemination of incomplete information if not taken care of. The same has been further elaborated in the discussion that follows. [ABSTRACT FROM AUTHOR]

We admit, however, that fracture mechanical principles and rock physics elements behind rock fracturing and fault activation may be similar, and that the joint discussion of fracturing and faulting, therefore, has a long tradition in the rock mechanics and rock engineering community. 1 Participants of the '13th EURO-Conference on Rock Physics and Geomechanics - The Guéguen Conference' during the social event at the Winzerberg Potsdam This special issue consists of 19 scientific papers which are based on these conference contributions and all of which address different important aspects relevant to the variety of industrial applications outlined above. This special issue is dedicated to our dear colleague and friend Prof. Ove Stephansson, who was not only one of Sweden's most influential geologists, but also an enthusiastic supporter of the 13th EURO-Conference on Rock Physics and Geomechanics. Special Issue on Rock Fracturing and Fault Activation: Experiments and Models: Selected Papers Presented at the 13th EURO-Conference on Rock Physics and Geomechanics - The Guéguen Conference Held on 2-6 September 2019 in Potsdam, Germany. [Extracted from the article]

This editorial discusses the journal's policy on publishing papers related to coal mining. While papers that address rock mechanics problems using coal as a material of interest may be considered, the journal recognizes that some studies focused solely on coal mining may not have broader significance for understanding other rocks or rock engineering scenarios. Therefore, studies specifically focused on coal mining without broader significance, including studies on coal mining technology and coal mechanics in the context of specific coal mining methods, will not be considered within the scope of the journal. The publisher remains neutral on jurisdictional claims and institutional affiliations. [Extracted from the article]

The recent rise of methods using Artificial Intelligence (AI) and Machine Learning (ML) has led to similar problems. Although the title was relatively narrow, the editorial addressed the general issue of papers that concentrated on the numerical method development without much relation to rock mechanics or rock engineering. [Extracted from the article]

A response from the author of the article "An Empirical Failure Criterion for Intact Rocks" in the 2013 issue is presented.

A response by G. Anagnostou and L. Canteini to a discussion on their article "On a Paradox of Elasto-Plastic Tunnel Analysis," is presented. The article focused on the counter-intuitive feature of behaviors exhibited under certain conditions. The authors explain the nature of simplifying assumption of zero gravity and use a simplified model to show that the elasto-plastic tunnel paradox persists in the presence of body forces.

Special thanks are due to the 2020 ARMA Symposium technical committee and the Rock Mechanics and Rock Engineering editorial team. Invited authors expanded their original conference papers prior to submission to Rock Mechanics and Rock Engineering. This special issue contains papers based on contributions submitted to the 54th US Rock Mechanics/Geomechanics Symposium. [Extracted from the article]

The article discusses the issue related to evaluation of basic friction angle ɸ in context to the saw-cut rock surfaces. It mentions S.R. Hencher, scientist in context to the contribution of useful references related to the basic friction angle with low values, and highlights the work of Hencher regarding the laboratory estimatation of the angle for the input of the formulation of peak shear strength criterion.

Based on the DP (Drucker–Prager) yield criterion, a new semi-analytical method for hydraulic–mechanical coupling in circular tunnels is proposed which takes into account the evolution of the permeability coefficient by incorporating it into the seepage equation. The definite condition is supplemented through the method of equal flow at the elastic–plastic junction. Constitutive equation and yield criterion are expressed by effective stress. When the pore water pressure drops to zero, the solution presented in this paper can degenerate into the classical Lamé solution in elastic region, and the solution obtained after yielding is consistent with the Drucker–Prage's solution. The numerical simulation method is used to verify the proposed solution, and then, the sensitivity of the strength parameters is discussed. The results show that the radius of the plastic zone decreases with increasing cohesion C and internal friction angle φ°, and the decreasing tendency is more sensitive at lower cohesion and internal friction angle. Higher initial yield stresses are easily obtained by increasing the values of these two parameters appropriately. The solution presented in this paper is not applicable when the internal friction angle is greater than 40°. Considering the intermediate principal stress, the solution presented in this paper shows a higher initial yield in-situ stress than that of MC (Mohr–Coulomb) and Hoek–Brown (HB) materials. Compared with the solution of DP criterion, the solution in the current work has a larger range of plastic region, and is more obvious in high in-situ stress area. In addition, the initial yield stress also increases linearly with the head loss. Highlights: A new closed semi-analytical solution of hydraulic–mechanical coupling in circular tunnel is proposed using equivalent permeability coefficient. The hydraulic–mechanical coupling equation takes into account the action of intermediate principal stresses. The solution proposed in this paper can degenerate into the classical Lamé solution when pore water pressure is equal to zero. [ABSTRACT FROM AUTHOR]

An introduction is presented in which the editor discusses papers on rock dynamics and applications on topics including dynamic response of rock masses in deep mines, rock bursting in discontinuous deep rock masses, and numerical testing and dynamic fracture modelling.

This paper investigates potential differences between creep and consolidation in mechanised tunnelling through squeezing ground, placing focus on the practical question of using experiences gained from existing tunnels about the required thrust force as a reference for tunnels of different diameter or adjacent tunnels. The investigations focus on two aspects. First, the effect of the tunnel diameter on the risk of shield jamming is examined. The paper demonstrates that larger-diameter tunnels are more favourable in poor-quality ground, while the opposite holds in better-quality ground, as well as in the case of pronouncedly time-dependent ground behaviour due to consolidation or creep. Second, the effect of a tunnel on the required thrust force in a neighbouring tunnel built later is examined. The paper shows that this interaction effect is particularly important in water-bearing ground of low permeability, where the drainage action of the first tunnel induces pore pressure relief and ground consolidation in an extensive area, leading to a remarkable reduction of the thrust force in the second tunnel. Conversely, in the case of creep the interaction is negligible even under extremely squeezing conditions, due to the fundamentally different nature of the purely mechanical rheological processes from coupled hydromechanical processes. The presented investigations into the transferability of experiences are valuable for tunnelling practice, in cases of twin tunnels as well as in situations where a smaller-diameter tunnel is constructed prior to the main tunnel (e.g. a pilot tunnel for exploration, advance drainage or ground improvement), or also the opposite (e.g. upgrade of a road tunnel by later construction of a safety tunnel with a smaller diameter). Highlights: The rate of ground deformations is independent of diameter in the case of creep, but extremely increases with decreasing diameter in the case of consolidation. The risk of shield jamming is lower in larger-diameter tunnels crossing poor-quality ground without pronounced time-dependent behaviour. The risk of shield jamming is lower in smaller-diameter tunnels crossing ground of higher quality or exhibiting pronounced time-dependent behaviour. The effect of adjacent tunnel on shield jamming risk is generally minor in the case of creep, even for extreme squeezing conditions and large overcuts. The effect of adjacent tunnel on shield jamming risk is remarkable in the case of consolidation, leading to a substantial reduction of the thrust force. [ABSTRACT FROM AUTHOR]

Burden is one of the most important parameter which plays an immense role in rock breakage using explosive energy. It is very difficult to select the optimum burden under different geotechnical and geomining condition. The selection becomes more critical when the excavation method is underground as very few researches or studies have been carried out in such cases. Therefore, in this paper, numerical simulation-based approach has been employed to estimate the optimum burden for ring blasting. The numerical models were created under different scenario including blasthole diameter and strength of the rock. The RHT concrete constitutive model has been used in this paper to study the damage contour under different circumstances. The model output has been used to calculate the volume of rock damage along the free face. This damage volume is further correlated with various parameters. This correlation shows that the damage volume along the free face initially rises with the increase in burden and then starts decreasing after a certain value. The burden value which recorded the maximum volume of rock excavated along the free face has been considered as the optimum burden in this paper. The trial blast has been also carried out at the experimental site for twenty-four rings. The optimum burden achieved from the experimental trial for 102 mm blasthole diameter and rock having uniaxial compressive strength as 92 MPa lies between 2.4 and 2.65 m. However, the optimum burden predicted through developed empirical equation from the numerical model is 2.6 m. The study shows that the simulation-based prediction exhibits a minor deviation within 15% with the experimental result. Hence, to assess the optimum burden under various geotechnical and geomining circumstances, numerical simulation-based approach may be carried out using the proposed methodology. Highlights: Burden for ring blasting is optimised using different numerical simulation models. Volume of rock excavated along free face (EV) has been calculated in each case. The optimum burden from experimental trial and empirical data is 2.4–2.65 m. The optimum burden predicted from developed model is 2.6 m. [ABSTRACT FROM AUTHOR]

Electric impulse drilling (EID) technology has great industrialization potential since it has unique advantages in crushing hard rocks of deep formation. To study the mechanism of dielectric breakdown is of great significance for accelerating the practical application of EID technology. In this paper, the dielectric breakdown damage model (DBDM) is established based on the heterogeneous granite model. On the basis of DBDM, the whole process of absolute dielectric breakdown (ADB) is studied, including the formation of the plasma channel and the variation of the electric field. Then, the mechanism of regional dielectric breakdown (RDB) development to ADB is studied, and the influence of peak voltage, electrode spacing and the parameters of a high-voltage electric pulse (HVEP) is discussed. Besides, the efficiency of two dielectric breakdown modes is evaluated. It is found that dielectric breakdown will only occur in the domain where the strength of the local electric field formed by HVEP exceeds the critical breakdown field strength of rock. RDB will occur when the pulse duration is too short or the peak voltage is too low. It is worth noting that RDB can be developed into ADB under multi-HVEPs. Moreover, under the same voltage, there is almost no difference between the failure volume caused by RDB under multi-HVEPs and ADB under a single HVEP. However, the energy consumption of the RDB under multi-HVEPs decreases exponentially with increasing voltage and is always higher than that of the ADB under a single HVEP. When designing the supporting tools and bits of EID technology, the life of insulation, the characteristics of formation, and energy utilization should be fully considered. The related research in this paper aims to provide reference and guidance for the application of EID technology in drilling engineering. Highlights: The variation of the electric field, the change of minerals and the generation of plasma channel in granite during dielectric breakdown is studied. The effect of the characteristics of high-voltage electric pulse (HVEP) on the dielectric breakdown in granite is studied. The mechanism and rock-breaking efficiency of absolute dielectric breakdown (ADB) and regional dielectric breakdown (RDB) are explored. [ABSTRACT FROM AUTHOR]

Abandoned underground mines appear to be very sensitive to the variations in their environment. Due to seasonal environmental changes often related to climate and resulting in changes in the water table and hygrometry, the pillars are often subjected to cyclic variations of saturation. This could affect the short- and long-term stability of these abandoned structures. In this paper, a preliminary study on the behaviour of Château-Landon chalk has been carried out. Samples were extracted from pillars of the abandoned Royer chalk mine (Château-Landon, France). The mechanical behaviour of this chalk was then investigated by performing conventional hydrostatic, triaxial compression, and creep tests under drained conditions at different water saturation degrees and confining pressures. The obtained results show up fundamental aspects of this chalk behaviour. Its high sensitivity to water and confining pressure are described, and a water saturation-induced plastic deformation is analysed. The results presented in this paper provide some key mechanisms that can be used in the framework of back analysis and expertise in underground chalk mines, as well as the data required for the development of a specific constitutive model. Highlights: Compression tests were performed on the Château-Landon chalk at different water saturation degrees (10%, 40%, 60%, 90%, 100%) and confining pressures (0.5, 1, 1.5, 2, 3 Mpa). An analysis of the creep behaviour of this material at a partial and total saturated state was shown. The variation of mechanical properties with water saturation degree and confining pressure was analysed. [ABSTRACT FROM AUTHOR]

Coal pillar is a crucial element formed during underground coal mining, playing a significant role in ensuring coal mining operations are conducted safely. Historically, research on coal pillars has primarily focused on regular-shaped pillars, with limited attention given to special-shaped pillars. Therefore, this paper aims to investigate the special-shaped coal pillar formed after the excavation of the newly constructed 3307 headgate in Dayang Coal Mine, China. The primary object of this paper is to elucidate the mechanism behind the instability failure of special-shaped coal pillars and verify the influence of various geological conditions on their stability. To accomplish these goals, numerical simulation techniques were employed to analyze the stress distribution patterns and plastic zone failure characteristics of special-shaped coal pillar. The investigation revealed the presence of stress concentration phenomena within special-shaped coal pillar, where the stress peak values were directly correlated with the width of the pillar. Moreover, it was observed that as the width of coal pillar decreased, the stress curve gradually transitioned from a bimodal shape to a unimodal shape. Additionally, the ratio of the plastic zone within coal pillar increased as the width decreased, with pronounced tensile failures occurring at sharp corners. Building upon theoretical analysis and numerical simulation results, the interior of special-shaped coal pillar was classified into four distinct zones: damaged zone, progressive zone, stable zone, and special zone. Based on this zoning approach, a stability control technology was developed, employing a combination of "pier pillar support + flexible formwork concrete + grouting reinforcement + high-strength cable and bolt + U-steel." Subsequently, an industrial experiment was conducted to validate the efficacy of this control technology. Field monitoring results demonstrated that the newly excavated roadway exhibited minimal deformation and damage, thereby satisfying the requirements for safe and efficient mine production over the anticipated service life. Highlights: The failure characteristics and bearing mechanism of special-shaped coal pillars were analyzed, and a mechanical model was established, allowing exploration of the influence of different geological conditions on pillar stability. The stress evolution and plastic zone development characteristics of special-shaped coal pillar during the whole cycle were studied. A zoning criterion for special-shaped coal pillar was proposed, along with corresponding control measures, which were successfully applied to the site, effectively controlled the deformation of roadway. [ABSTRACT FROM AUTHOR]

Although squeezing ground may undergo rapid convergences following tunnel excavation, its behaviour is often markedly time dependent due to creep or consolidation. The effects of creep and consolidation on shield tunnelling are comparatively evaluated in two companion papers, with the aim of demonstrating their qualitative similarities and distinctive features. The present, first paper investigates the basic time effects, placing focus on the time development of ground deformations and the complex interaction between ground, tunnel boring machine (TBM) and tunnel support during excavation and during construction standstills. The presented numerical simulations indicate several qualitative similarities between the two mechanisms of time dependency, in respect of the time development of ground deformations, the counter-intuitive behaviour of increasing shield loading with increasing rate of advance under certain conditions, as well as the thoroughly adverse effect of the additional time-dependent deformations taking place during construction standstills on the shield loading. However, they also underscore two prominent differences resulting from the fundamentally different nature of creep (a purely mechanical rheological process) and consolidation (a coupled hydromechanical process): first, the consistently more extensive plastic yielding in consolidating ground, which is partially associated with the seepage forces exerted by the pore water on the solid rock constituents. Second, the role of seepage forces as a potential destabilising agent, particularly for the tunnel face, which does not happen in the case of creep and may be critical for shield and cutterhead jamming. Building upon these investigations, the companion paper compares creep and consolidation with respect to the transferability of experiences about the required thrust force to tunnels of different diameter or to adjacent tunnels. Highlights: Analysis of similarities and differences between the effects of creep and consolidation on mechanised shield tunnelling through squeezing ground. Similar time development of rock deformations and shield loading during advance and during standstills in creep and consolidation. Similar counter-intuitive behaviour of increasing shield loading with increasing advance rate under certain conditions in creep and consolidation. More extensive ground plastification in consolidation than in creep, partially due to the effect of seepage forces on the solid rock constituents. Potential instability due to seepage forces may induce excessive convergences or face extrusion and render shield or cutterhead jamming critical. [ABSTRACT FROM AUTHOR]

Fluid leakage caused by debonding at the cement interfaces driven by low-temperature fluids is recognized as a significant challenge to wellbore integrity. Previous studies have primarily focused on casing/cement interface debonding, without thoroughly analyzing the simultaneous propagation of debonding fractures at the casing/cement interface and cement/formation interface. Furthermore, prior investigations have been conducted based on hydro-mechanical coupling analysis, neglecting the influence of injection fluid temperature on interface debonding. Therefore, this paper aims to reveal the debonding mechanism of the two cement interfaces using a coupled thermal–hydrological–mechanical (THM) modeling approach. Considering the effect of injected fluid temperature, a 3D numerical model of casing-cement-formation is developed for the simulation of the fluid-driven debonding based on the coupled pore pressure cohesive zone method. The influences of various factors including injection fluid temperature, flow rate, viscosity, casing pressure and cement Young's modulus on the cement sheath debonding are studied, and grey relational analysis is carried out on the influence degree of different factors. The results indicate that the fracture propagation pressure (FPP) is higher, and the fracture height is lower at the casing/cement interface compared to the cement/formation interface. When the fluid temperature is considered by the model, the FPP at the cement sheath interface is lower, and fracture initiation and propagation are more likely to occur. The results also demonstrate that higher fluid temperature, flow rate, viscosity and casing pressure increase the FPP and reduce the risk of cement sheath interface debonding. Casing pressure and fluid temperature exhibit a more prominent influence on FPP. The method can effectively predict the propagation of debonding failures at the cement sheath interface, and provide a foundation for the optimization of injection parameters. Highlights: This paper investigates the simultaneous propagation of the fluid-driven debonding of two cement interfaces considering the injected fluid temperature. A novel numerical model considering the fully coupled thermal–hydrological–mechanical processes is developed. Effects of injection fluid temperature, flow rate, viscosity, casing pressure and cement Young's modulus on the interface debonding are examined. A grey relational analysis is conducted to rank the influence degree of different factors. [ABSTRACT FROM AUTHOR]

Rock images play a vital role in providing data for engineering geological studies. However, low-light (a.k.a. dark) rock images are often obtained, and the existing methods for image enhancement, whether they are software-based, algorithm-based, or data-driven-based, are incapable of addressing the issue. This paper establishes a novel unsupervised deep learning (DL) model that caters to low-light rock image enhancement. The light enhancement process is carried out automatically and pixel-wise by utilizing the deep curve estimation (DCE) algorithm and a convolutional neural network (CNN). This study establishes a rock image dataset, encompassing diverse (1) rock types (tuff, siltstone, & granite groups), (2) light levels (2400 lx, 1200 lx, &100 lx), (3) color temperatures (5500 K, 4200 K, & 3000 K), and (4) surface conditions (wet & dry). The novel DL model is developed based on (1) high-order curves of the DCE algorithm, (2) an elaborate CNN architecture with step-wise convolutions and a squeeze and excitation module, (3) three non-reference loss functions, and (4) an alerting level for better generalization. The DL model possesses several advantageous features: (1) automatic light enhancement of rock images without subjective inputs, (2) independence from paired data, which requires manual retouching, (3) pixel-wise adjustment of intensities across color channels, (4) preservation of rock details without distortion by the attention mechanism, (5) high inference speeds, e.g., ~ 3 ms per rock image, and (6) most importantly, state-of-the-art performance when compared to 10 other widely-used enhancement methods, both visually and quantitatively, by three evaluation indices. Despite being designed and trained for rock images, the DL model also demonstrates impressive performance on non-rock images. Highlights: This study establishes a rock image dataset, encompassing diverse rock types, light levels, color temperatures, and surface conditions. This paper establishes a novel unsupervised deep learning (DL) model that caters to low-light rock image enhancement, automatically and pixel-wise. The DL model achieves state-of-the-art performance in enhancing low-light rock images when compared to 10 other widely-used light enhancement methods, both visually and quantitatively. [ABSTRACT FROM AUTHOR]

In-situ stress measurement is an important prerequisite for underground engineering excavation design and surrounding rock stability analysis. However, it is still a difficult problem to complete in-situ stress measurement quickly and accurately, especially in deep vertical borehole with more complex environment. Aiming at the existing problems, a new in-situ stress measurement method is proposed in this study. The principle of in-situ stress calculation based on the borehole diametrical deformation analysis is established, and a borehole diametrical deformation measurement equipment which can realize the directional measurement of multi-directional borehole wall displacement is developed according to this principle. The traditional single action double-tube drilling tool is optimized, and the corresponding in-situ stress measurement process is formulated, forming a rapid in-situ stress measurement method with the cooperation of drilling tools. The measurement method has been applied to the in-situ stress measurement at − 410 m and − 500 m levels in Zhangfushan deposit of Jinshandian Iron Mine, and effective diametrical deformation data of 6 measuring points were obtained. The measurement results show that the optimized single action double-tube drilling tool has the ability of accurate drilling and complete coring, and can assist in the rapid measurement of in-situ stress. The principal stress at the 6 measuring points basically increases with the increase of depth, and the direction of the maximum horizontal principal stress is about SN, which is basically consistent with the existing in-situ stress data. The results show that the method proposed in this paper can realize the rapid and accurate measurement of in-situ stress in vertical borehole. Highlights: A principle for calculating in-situ stress based on the analysis of diametrical deformation characteristics of boreholes has been established. A device has been developed that can measure diametrical displacement of borehole in multiple directions. The method was used for in-situ measurement and the accuracy of the measurement results was verified through historical data. [ABSTRACT FROM AUTHOR]

Flysch formations are widespread all around the Mediterranean basin and in many other parts of the world. Their special fabric, characterized by a strong heterogeneity due to alternations of often deformed lapideous and fine-grained layers and by a network of discontinuities, brings to light the strong limitations of the geo-engineering sciences when dealing with such complex materials. Based on the results of an investigation carried out in a long section of the Southern Apennines, the paper examines this problem focusing in particular on the role played by material fabric at different scales on the hydraulic and mechanical behavior of the deposit. Highlights: The mechanical behaviour of the Red Flysch formation is carefully analysed with a particular attention to the influence of the soil fabric at different scales. The Red Flysch formation is described within the frame of the structurally complex formations The results of a wide in situ and laboratory investigation on a Red Fysch deposit are collected and interpreted The role played by material fabric at different scales on the hydraulic and mechanical behavior of the deposit is evidenced. The strong limitations of the geo-engineering sciences when dealing with such complex materials are highlighted [ABSTRACT FROM AUTHOR]

The existence of confining pressure is an important factor that can't be ignored in rock cutting process. Rocks show high strength and strong plasticity under confining pressure condition (CPC), leading to the low rock-breaking efficiency of drilling bit, which is more prominent in deep formation drilling. However, the understanding of rock-breaking mechanism of Polycrystalline Diamond Compact (PDC) cutter under CPC is still unclear. Given that fact, in this paper, a series of rock cutting experiments on sandstone and granite under different confining pressures and atmospheric pressure conditions were carried out using a self-made true triaxial rock cutting device. The cutting force, mechanical specific energy (MSE) and cuttings mass fraction under confining pressure and atmospheric pressure conditions were studied, and the cutting grooves were analyzed. The results show that the existence of confining pressure greatly increases the difficulty of rock cutting, which is embodied by the increase of cutting force and MSE. In addition, the size of cuttings produced under CPC is smaller, and the proportion of powdery cuttings increases with increasing confining pressure. Meanwhile, the cutting grooves obtained under CPCs are smoother. Microscopically, the existence of confining pressure greatly enhances the contact stress between PDC cutter and rock, resulting in a larger area of dense core when PDC cutter contacts with rock. Confining pressure leads to the difficulty of cuttings discharge to a certain extent, which easily results in the secondary and repeated destruction of cuttings and increases the energy dissipation. In general, confining pressure leads to the lower damage degree of PDC cutter to the rock, which makes the rock difficult to be drilled. The research results can provide some special guidance for rock breaking in deep formation, and provide practical thoughts and enlightenment for rock breaking, bits arrangement and optimization under CPC. Highlights: The cutting experiments under confining pressure condition are carried out by using the self-made rock cutting device that exerts three-dimensional stress on rock, which truly simulates the rock breaking process at the bottom of the well. The rock-breaking performances under atmospheric pressure condition and confining pressure conditions are comprehensively analyzed from the aspects of cuttings mass fraction, cuttings size, cutting groove and coarseness index. The rock-breaking mechanism of PDC cutter under confining pressure condition is investigated and the rock-breaking behavior of PDC cutter under confining pressure condition is studied. Three kinds of changing trends of mechanical specific energy with confining pressure are put forward, and the concept of confining pressure threshold is also proposed. [ABSTRACT FROM AUTHOR]

In this paper, a simplified method for predicting rock mass resistance against tensile forces from rock anchors (anchor) is presented. The interaction of anchors and rock mass was investigated using three-dimensional discontinuous numerical modelling. Several patterns of rock discontinuities were assumed in the numerical modelling while a single anchor is embedded in it. The numerical results show that the existence of a discontinuity set sub-parallel to the anchor significantly improves the rock mass resistance against the tensile force from the anchors. This phenomenon is due to the rock block interlocking at the sub-parallel discontinuity set. Rock block interlocking generates a zone of stress concentration inside the rock mass which has an arch shape (i.e., a pressure arch), resisting against the anchor's tensile force. The load-bearing capacity of the pressure arch plays a significant role in the resistance of the rock mass against the forces from the rock anchor. A voussoir beam analogy was utilised to study the load-bearing capacity of the pressure arch. A simplified analytical approach was developed to assess the load-bearing capacity of the voussoir beams. Then, it was used in combination with the weight of the mobilised rock mass by the anchor to assess the maximum anchoring resistance of the rock mass (anchoring capacity). The suggested method was calibrated by numerical modelling and relevant published pull-out test results. The technique developed in this paper shows the significance of rock block size, shear behaviour of rock discontinuities, Young's modulus of the rock mass, and uniaxial compressive and tensile strength of the intact rock in anchoring capacity of rock masses. Highlights: A theoretical framework is presented on how rock mass and rock anchors interact based on numerical modelling. An analytical method is developed to calculate the anchoring capacity of a rock mass which has at least one joint set sub-parallel to the anchor. The importance of the rock block size and the shear behaviour of rock discontinuities in determining the anchoring capacity of the rock mass is discussed. [ABSTRACT FROM AUTHOR]

The shale oil reservoirs of the Lucaogou Formation in the Jimsar Sag developed a large number of natural fractures and thin interlayers. The expansion of hydraulic fractures is full of randomness and unknowingness due to this complex geological feature. Since the multi-cluster fracturing process with small well spacing is widely used, the growth of hydraulic fractures requires a combined consideration of competition between perforation clusters and interference between wells. From the perspective of engineering application, this paper summarizes the fracture propagation law under single-fracture, multi-cluster fracturing, and adjacent well interference from simple to complex based on three-dimensional discrete element methods and a large number of field data. Then the key controlling factors of fracture group morphology were quantitatively analyzed. Comparative analysis reveals that under complex geological conditions, a single crack can still be described by existing theories, but the expansion law of the fracture group is not a simple superposition of single fractures. The competitive expansion of multiple fractures is reflected in the Fracture Surface Area (FSA) differences between clusters, asymmetry, and irregularity. Moreover, under inter-well stress interference, the hydraulic fractures of the posterior-construction well will tend to expand in the opposite direction of the first-construction well. The optimal well spacing of the Jimsar shale oil reservoir is 200–300 m. Highlights: In this paper, there are five highlights worthy of attention: This paper discussed the fracture propagation law under actual multi-cluster fracturing conditions through three aspects: 1. Clarifying the difference between the expansion law of multi-fracture competition and traditional cognition by comparing the morphology of fracture group and single-fracture; 2. Competitive expansion modes were quantitatively analyzed through FSA curves and inter-cluster stress interference. 3. Based on inter-well stress interference, a more complex simulation of actual working conditions was carried out to elaborate further on the influence of inter-well stress interference on fracture expansion. This paper compared the penetration expansion ability of hydraulic fractures under the condition of single fractures and fracture group. We quantitatively analyzed the dynamic characteristics of multi-fracture competitive expansion through Fracture Surface Area (FSA) and evaluated the morphological attributes of fracture groups under inter-cluster and inter-well stress interference. The stress interference range and the increment of the minimum horizontal in situ stress caused by fracturing adjacent well under different well spacings were quantitatively analyzed. The research was based entirely on the actual working conditions of the oil field, and the microseismic data and simulation results were thoroughly compared and demonstrated, which ensured the reliability of the conclusions. [ABSTRACT FROM AUTHOR]

This paper presents a comprehensive investigation of the swelling behaviour of a compacted bentonite–sand mixture subjected to hydration under constant volume conditions. Contrary to previous studies, the tested sample was isotropically compacted before being hydrated under constant volume conditions until full saturation was reached. The total axial pressure, total radial pressures at four different heights of the sample, and injected water volume were recorded over time. The experimental data reveal a complex and non-uniform evolution of the axial and radial stresses over time, as well as anisotropy of the total stresses, which persist at the saturated equilibrated state. To gain further insights, a numerical analysis was performed using an advanced hydromechanical framework for partially saturated porous media, accounting for the evolving microstructure of the material. The complex evolution of the total axial and radial pressures with time is attributed to the advancing hydration and swelling front in the sample, along with the development of irreversible strains. The good agreement between the numerical results and the experimental data enables validation of the developed framework. Implications for engineered barriers in deep geological disposal of radioactive waste are discussed. Highlights: The swelling behaviour of an isotropically compacted bentonite-based material under constant-volume conditions is investigated. Hydration of the sample generates stress heterogeneity and anisotropy, which persist at the saturated equilibrated state. Numerical modelling shows that the complex evolution of the total axial and radial pressures can be attributed to the advancing hydration and swelling front in the sample, along with the development of irreversible strains. The relationship between the local dry density and the radial stress seems to follow the global dry density-swelling pressure trend determined on small-scale samples. [ABSTRACT FROM AUTHOR]

This paper presents several series of multi-stage permeability tests in laboratory with both steady-state and pulse decay methods on core samples of the low permeable Callovo-Oxfordian claystone from Meuse/Haute-Marne (France). The focus is to explain why water permeability values obtained in laboratory with the steady-state method are lower than those obtained with the pulse decay method and those obtained in situ in the Underground Research Laboratory designed for a feasibility of a deep geological repository of radioactive waste. The focus is also to assess the influence of the percolating water chemistry and the duration of resaturation and steady flow stages on the permeability values, and the impact of these permeability measurements on the mechanical properties (elastic coefficients, peak strength). The water permeability values measured with the pulse decay method are homogeneous and around 10−20 m2. Those obtained with the steady flow method are significantly lower (up to one order of magnitude) when using the synthetic water of ANDRA. This difference can be explained by an additional hydration of the clay minerals during steady-state flow which induces a swelling and then a decrease of the interparticle porosity. These physicochemical mechanisms slow down the water flow during the steady flow tests. When using a chemically balanced water and reducing the test duration, the difference is only about half an order of magnitude. Sample resaturation and steady-state flow induce a significant damage of the material which is mainly due to the opening of the bedding planes of swelling clay minerals. Highlights: Laboratory permeability tests with steady state and pulse decay methods were performed on core samples of Callovo-Oxfordian claystone from Meuse/Haute-Marne (France) Permeability values measured with the steady flow method are significantly lower than those obtained from the pulse decay method when using the synthetic water of ANDRA This difference could be explained by an additional hydration of the clay minerals during steady state flow, which induces swelling and slows down the water flow A more accurate assessment of the permeability with the steady state method was obtained using a chemically balanced water and experiment with shorter run time Initial resaturation and steady state flow induce a significant damage of the material which is mainly due to the opening of the bedding planes of swelling clay minerals [ABSTRACT FROM AUTHOR]

Numerical simulation of radon migration in fractured rock masses is crucial for environmental radon pollution protection, radon tracing technology and the co-exploitation of coal and uranium resources. This paper proposed a novel model based on a fractal discrete fracture network (DFN) to simulate radon migration in fractured rock masses. The paper provided a detailed method for determining the locations of three-dimensional fractures using the multiplicative cascade process. The governing equations for radon migration in fractures and the rock matrix were established and numerically solved using the finite element method. The study developed open-source software DFNRn (Discrete Fracture Network model for Radon migration), which was publicly available on GitHub. The software simulated radon migration in a fractured rock mass with a domain size of 100 m. The results demonstrate that fluid convection in fractures drives radon migration, which is dominant compared to the diffusion process. The accuracy of our proposed model was verified by comparing it with the measured data of the fractured rock. Furthermore, the study investigated the relationship between the percolation threshold of the fracture density P 32 ′ and the key parameters of the DFN. The results show that P 32 ′ decreased with the fractal dimension Df (1.4 ≤ Df ≤ 1.9) and normalization constant α (1.4 ≤ α ≤ 1.8) and is not correlated with the length exponent a (1.2 ≤ a ≤ 2.8). P 32 ′ decreased with the mean orientation angle and has no correlation with the Fisher constant. The radon source term of the rock matrix has the greatest impact on radon migration in fractured rock masses compared to the radon diffusion coefficient and permeability. Highlights: A new approach is proposed to model radon migration in fractured rock masses. An open-source software DFNRn was developed and is publicly available on GitHub. The procedure of the multiplicative cascade method in 3D space is given in detail. [ABSTRACT FROM AUTHOR]

The no-pillar longwall mining method (NPLM) by roof cutting technology (RCT) is an innovative longwall mining method proposed in China. Its essential technology is the employment of RCT to replace the chain pillar to safeguard the roadway. In this paper, a novel physical model test method is designed to investigate the protective mechanism of RCT for roadways in greater detail. Furthermore, the physical model test of RCT is conducted in consideration of the effect of broken roof rock bulking. Combined with numerical simulation, the results indicate that RCT can increase the degree of gangue bulking in goaf, hence providing effective support to the overlying strata and maintaining their integrity. Moreover, RCT can significantly reduce the abutment stress of the roadway roof and solid coal side, thereby ensuring the stability of roadway surrounding rock. This paper proposes three protection mechanisms of RCT for roadway based on the findings of the conducted research: I. Reducing the dimensions of roadway roof; II. Utilizing bulking gangue to support the overlying strata; III. Eliminating the cavities in the goaf adjacent to the roadway. The in situ application provides evidence that RCT is effective in controlling roadway deformation. This paper can serve as a reference for no-pillar mining and analogous geotechnical engineering physical model tests. Highlights: A novel two-dimensional physical model test method is developed that can achieve bulking after failure. Three protection mechanisms of roof cutting technology for the gob-side roadway are elucidated. The effect of roof cutting technology on the bulking of broken roof rock is analyzed by experiment. In conjunction with the in-situ application, the key to no-pillar longwall mining by roof cutting technology is proposed. [ABSTRACT FROM AUTHOR]

The phenomena of freezing damage in the cold regional tunnels have happened frequently. Therefore, in order to precisely calculate frost heaving force ( σ 1 ), based on the displacement method, a new analytical solution was proposed. The analytical solution presented in this paper was verified by the results of the model test and field measurement of σ 1 , the imperfect contact effect between frozen and unfrozen zones of surrounding rock was considered and the influence of the interaction between any two factors on frost heaving force was studied by means of orthogonal experiment and analysis of variance. The results show that: (1) the inner radius of lining ( r a ), excavation radius ( r 1 ) and freezing radius ( r f ) are highly significant for σ 1 and the interaction between the inner and outer radiuses of lining (i.e., changes of the size in inner and outer radius of lining) is also highly significant for frost heaving force ( σ 1 ); (2) the analytical solution presented in this paper can avoid the frost heaving force ( σ 1 ) with tensional property under the condition of the circular tunnel under hydrostatic pressure; and (3) sensitivity function shows that the larger Young's modulus of lining ( E I ), inner radius of lining ( r a ), freezing radius ( r f ), linear frost heaving rate ( β 0 ) and Young's modulus of frozen surrounding rock ( E II ) are, the more sensitive the factors E I , r a , r f , β 0 and E II are to σ 1 . It is expected that the results of this paper can give some novel insights for the anti-freezing design of tunnels. Highlights: Inner radius of lining is the more sensitive to frost heaving force. The interaction between the inner and outer radiuses of lining is highly significant. The ra, r1 and rf are highly significant for frost heaving force. The analytical solution can avoid frost heaving force ( σ 1 ) with tensional property. [ABSTRACT FROM AUTHOR]

This paper introduces the development of a dynamic seepage network method (DSN) within a two-dimensional discontinuous deformation analysis (2D-DDA) framework. The primary goal is to identify actual fractures and seepage spaces, establish a seepage network, and automatically compute hydraulic water pressure within blocky systems. In addition, a new coupled dynamic hydromechanical model (DDA–DSN) is proposed to investigate seepage behavior in time-varying fractured rock masses. The DSN method tracks fracture propagation between blocks during seepage flow, while the DDA component characterizes the translation, rotation, and deformation of arbitrarily formed blocks. The paper covers fundamental theories, the determination of neighboring non-contact block pairs, the creation of seepage networks, hydraulic pressure calculations, and the integration of DDA and DSN in detail. Furthermore, the validity of the coupled DDA–DSN hydromechanical model is confirmed by comparing numerical results with experimental measurements. These results highlight the model's effectiveness in capturing fluid flow behavior within fractured rock masses, emphasizing the exceptional capabilities of DSN in identifying seepage spaces, constructing seepage networks, and automatically computing hydraulic pressure. Overall, this proposed model holds significant promise for addressing engineering challenges related to seepage flow behavior inner the rock masses. Highlights: Developed a new algorithm for identifying the real fractures, forming the seepage pathways, and calculating the hydraulic water pressure on blocks automatically in 2D-DDA. Proposed a new coupled DDA–DSN model to investigate the seepage behavior in time-varying fractured rock mass systems. Checked validity of the DDA–DSN model by contrasting several numerical findings with experimental measurements. [ABSTRACT FROM AUTHOR]

The rock mechanics parameters are important indicators for risk assessment in tunnel construction. However, rock mechanical parameters estimated by traditional mechanical testing in the field are subject to considerable uncertainties due to limitations testing technology, which can directly lead to unreliable risk assessment results. Deformation data for parametric back analysis is a reliable and efficient method for estimating mechanical parameters of rock masses. Deformation monitoring data used for parameter back analysis is an efficient and reliable method for estimating rock mechanical parameters, but the current research generally suffers from inaccurate constitutive models, confusing selection of input data, and failure to consider the intrinsic correlation of mechanical parameters. In this paper, a deformation-based back analysis method for rock mechanical parameters was proposed by XGBoost with RC. Taking the Changning Tunnel in Yunnan Province, China, as an example, the construction process of the back analysis model was described in detail. To improve the reliability of the sample data, the Ubiquitous-Joint constitutive model was used and a more characteristic deformed data selection method was proposed. Furthermore, the performance of the back analysis model was improved through Bayesian Optimization of hyperparameters and order filtering of Regressor Chain, and the intrinsic correlation of the parameters was effectively considered through the Regressor Chain. The results showed that applying the rock mechanics parameters obtained from the back analysis model to numerical simulation can effectively predict deformation during tunnel construction, and the error rate of deformation prediction was within 10%, which demonstrates the reliability of the method in this paper. This method can be extended to other heavily fractured tunnels. Highlights: Summarized the technical process of back analysis of mechanics parameters based on machine learning, analyzed the current research status, and put forward the existing problems. Through the spatial–temporal effect analysis of deformation of rock mass, a reasonable selection method of the surrounding rock deformation index in the sample data is proposed. The performance of the back analysis model of rock mechanics parameters is effectively improved by hyperparameter optimization of Bayesian and order filtering of Regressor Chains. [ABSTRACT FROM AUTHOR]

Shale anisotropy characteristics have great effects on the mechanical behaviour of the rock. Understanding shale anisotropic behaviour is one of the key interests to several geo-engineering fields, including tunnel, nuclear waste disposal and hydraulic fracturing. This research adopted the finite discrete element method (FDEM) to create anisotropic shale models in ABAQUS. The FDEM models were calibrated using the mechanical values obtained from published laboratory tests on Longmaxi shale. The results show that the anisotropic features of shale significantly affect the brittleness and fracturing mechanism at the micro-crack level. The total fracture number in shale under the Uniaxial Compressive Strength (UCS) test is not only related to the brittleness of shale. It is also strongly dependent on the structure of the shale, which is sensitive to shale anisotropy. Two new brittleness indices, BIf and BICD, have been proposed in this paper. The expression for BIf directly incorporates the number of fractures formed inside of the rock, which provides a more accurate frac-ability using this brittleness index. It can be used to calculate the frac-ability of rocks in projects where there are concerns about fractures after excavation. Meanwhile, BICD links brittleness to the CD/UCS ratio in shale for the first time. BICD is easy to obtain in comparison to other brittleness indices because it is based on the Uniaxial Compressive Strength test only. In addition, it has been shown there is a relationship between tensile strength and the crack damage strength in shale. Based on this, an empirical relationship has been proposed to predict the tensile strength based on the Uniaxial Compressive Strength test. Highlights: Through the use of FDEM simulations, the fracturing mechanics of shale, fracturing behaviour, and brittleness of shale have been investigated from a micro-scopic viewpoint taking shale anisotropy into account. Two new brittleness indices are proposed. One relies on the number of fractures in the shale to reflect the shale's frac-ability, and the other uses strength parameters that can be easily obtained from UCS tests. This paper proposes an empirical relationship to predict the tensile strength of rocks based on UCS tests. [ABSTRACT FROM AUTHOR]

A discussion on the article "On a Paradox of Elasto-Plastic Tunnel Analysis," by L. Cantieni and G. Anagnostou, published in a previous issue, is presented. It cites that the article focuses on the paradox that persists in elasto-plastic analyses of tunnels. It argues that counter-intuitive behavior is a stress relief that occurs in the ground prior of the face.

An introduction to the journal is presented in which the editors discuss the 48th US Rock Mechanics/Geomechanics Symposium held at the University of Minnesota from June 1-4, 2014.

Bimslopes are slopes underlain by block-in-matrix rocks/soils (bimrocks/bimsoils), which are heterogeneous geomaterials with complex behavior. Abundant uncertainties in the mechanical, spatial, and geological nature of bimslopes make it difficult to estimate stability conditions of bimslopes. In this paper, some factors affecting the stability of bimslopes are investigated using the finite-element method. The main contributory factors include Volumetric Block Proportion (VBP), the inclination of blocks, and the strength of matrix/block interfaces. By performance of sensitivity analysis on these factors, a statistically significant equation was obtained to estimate the safety factor of bimslopes using the multivariate regression method. The influence of block configurations and groundwater levels on the stability of bimslopes was then investigated to more confidently use the multivariate equation. Finally, to check the validity of procedures presented in this paper, a real failed bimslope in the Sungun mine (Iran) was investigated. The acceptable results from the Sungun analysis demonstrated the safety factor of bimslopes can be estimated using the practical solution presented in this paper. Highlights: Development of more than 1300 numerical models to analyze the key factors affecting the stability of block-in-matrix slopes (bimslopes). Investigation of the influence of volumetric block proportion, the inclination of blocks, and strength of matrix/block interface on the safety factor of bimslopes. Consideration of the substantial uncertainties affecting the analysis of bimslopes. Presentation of the results of research in the form of applied schemes and tables. Provision of a practical procedure and equation for estimating the safety factor of bimslopes. [ABSTRACT FROM AUTHOR]

For many years, the relationship between permeability and temperature of rocks after high temperature was described by a classical exponential model based on thermos-elastic theory. However, recent experiments have shown that the thermal damage incurred by rocks after high temperatures is irreversible plastic damage. This paper aims to propose a new approach for creating a temperature-dependent permeability model that considers this irreversibility of thermal damage. To achieve this, energy dissipation theory and fractal theory were used to develop a model that describes the irreversible thermal damage evolution and temperature-dependent permeability of rocks after high temperature. The new model was verified through CT and gas permeability experiments. The results indicate that: 1. in the temperature range of 200 ~ 800 ℃, thermal damage develops into an irreversible plastic state due to energy dissipation; 2. the thermal damage of granite exhibits a porous structure at 50 ~ 600 ℃, which turns into a mixed pore-fracture structure at 600 ~ 800 ℃; 3. the granite matrix followed fractal distribution patterns after thermal treatment, demonstrating a quadratic correlation with temperature; 4. based on the fractal evolution of granite after high temperature, the temperature permeability model is derived. The new model is compared to the two commonly used models. The new model showed smaller fitting relative errors, and produced a better representation of the causes of plastic thermal damage, offering a promising solution for similar problems. Highlights: The classical exponential model between temperature and permeability is based on thermos-elastic theory. But in this paper, the development of plastic irreversible thermal damage is explained by energy dissipation Fractal theory is used to describe the evolution of thermal damage quantitatively Based on the fractal expression of thermal damage, a new temperature-dependent permeability model is deduced The fractal expression of thermal damage and the new temperature-dependent permeability model are verified by meso-CT test and gas permeability test [ABSTRACT FROM AUTHOR]

Reliable shear strength determination of large in situ discontinuities is still a challenge faced by the rock mechanics field. This is principally due to the limited availability of surface roughness and morphology information of in situ discontinuities and the unresolved management of the 'scale effect' phenomenon. Recently, a stochastic approach for predicting the shear strength of large-scale discontinuities was established, encompassing random field theory, a semi-analytical shear strength model, and a stochastic analysis framework. A key aspect of the new approach is the application at field scale, thereby minimising or bypassing the scale effect. The approach has been validated at laboratory scale and an initial large-scale deterministic-based validation showed promising results. However, to date, no large-scale experimental-based validation has been undertaken. This paper presents the first rigorous application of the employed semi-analytical shear strength model and the stochastic approach on a 2 m-by-2 m discontinuity surface, with comparison of prediction to experimental shear strength data. The shear strength model was found to generally produce peak and residual predictions within a ± 10% relative error range, with good agreement between predicted and observed damage areas. It was observed that, applying the stochastic approach to seed traces with gradient statistics equivalent to that of the surface, produced predictions that closely resemble the experimental results. Whereas, predicting shear strength from different seed traces results in more variability of predictions, with many falling within ± 20% of the experimental data. The predictions of residual shear strength tended to be more accurate than peak shear strength. Highlights: The stochastic approach for discontinuity shear strength was applied to a 2 m per 2 m rock surface replica made of mortar. A sensitivity analysis conducted on the analytical shear strength model highlights the parameter influencing the most the shear strength prediction. The predictions of peak and residual strengths fall within 20% of the experimental data. The accuracy of prediction depends on the trace used to define the input statistics. [ABSTRACT FROM AUTHOR]

An accurate measurement method of crack fabric is proposed. Considering the density, geometry and spatial distribution in cracks, a novel method suggests the formula of high-order fabric tensor in the orthogonal space to describe cracks quantitatively. The crack is automatically identified by morphology and determined by stereo scans of image processing techniques. The fabric tensor is defined based on the idea of probability and statistics. The zero-order tensor describes the average effect of the density of rectangular cracks, the second-order tensor describes the two dominant directions of spatial cracks, and the higher-order tensor more accurately describes the anisotropy of cracks. With the increase of the tensor order, the more accurate direction and degree of the anisotropy can be described, and the morphological automatic identification of crack and its physical meaning of quantitative determination are clear. The plane description of the crack can be described by the invariants of the plane tensor. One invariant describes the degree of the anisotropy, and the other invariant describes its direction. The scan line is rotated and scanned on the rock plane image to obtain the accurate distribution of plane cracks, and the distribution law can be accurately described by the proposed high-order fabric tensor. In this paper, the rationality and effectiveness of the proposed measurement method are well verified by CT image analyses. Highlights: An accurate measurement method of crack fabric is proposed. The expression of crack fabric tensor is redefined by the normalized idea. The plane crack fabric is automatically identified by morphology, determined by stereo scans, and described by the its invariants equivalently. On the basis of stereology, the rationality and effectiveness of the proposed measurement method are well verified by SEM and CT image analyses. [ABSTRACT FROM AUTHOR]

The paper applies the observation cycle to rock fracturing processes. Specifically, it repeats the observation cycle by going from the large (field) scale to increasingly detailed laboratory scales. With this, it is possible to determine that what appear to be tensile or shear fractures on the large scale are produced through a combination of tensile and shear mechanisms on the smaller scale. This involves observations with a variety of techniques that are briefly described. What is also emphasized is that observation has to go hand in hand with abstraction, i.e., modeling to help understand the observed phenomena. The paper, therefore, makes two contributions—a better understanding of rock fracturing processes and showing the importance of the observation cycle in a scientific process. [ABSTRACT FROM AUTHOR]

Depending on the type of failure mechanism, the strength of intact rock may be susceptible to the effects of scale. The influence of scale is apparent in the spalling of rock around underground openings. To reconcile field-scale observations, an understanding of the unconfined rock strength has to be appreciated from the standpoint of energy instability, particularly for instances in which the mechanism of failure involves propagation of fractures. Fracture energy principles were used to create a link between laboratory-measured uniaxial compressive strength (UCS) and rock failure as observed in spalling around circular openings. An Instability Line (IL) is proposed to predict the stresses above which spalling could occur with different opening diameters. Good comparisons were obtained for a range of dimensions from small boreholes to large tunnels. Based on field observations and analytical study, the results suggest that two conditions are required for spalling to occur. The first is that the crack initiation stress has to be exceeded, and the second is that the stress needs to be above the IL. The findings suggest that the effects of scale are limited by the crack initiation stress. For large-diameter openings, instability occurs immediately at crack initiation stress, whereas for smaller-diameter openings instability occurs at higher stresses post-crack initiation stress. Highlights: This paper aims to explain the discrepancies in the rock mechanics literature to predict spalling for small-diameter boreholes as against large-diameter tunnels. The failure mechanism of spalling is studied based on the framework of energy stability. An Instability Line is proposed based on the analytical study. The results of the analytical study compared well with published laboratory and field observations. The results suggest that two conditions are required for spalling to occur. The first is that the crack initiation stress has to be exceeded, and the second is that the stress needs to be above the Instability Line. The findings suggest that the effects of scale are limited by the crack initiation stress. [ABSTRACT FROM AUTHOR]

In this article the author responds to the comments on their article "Characteristic Curves for Deep Circular Tunnels in Poroplastic Rock," which appeared in the Volume 43, Number 2 issue in 2010. He addresses issues including pore pressure and effective stress jump at the elastic-plastic interface, unnecessary parameters and equations, and inapplicability of the long-term solution. He also clarifies the intent and scope of their paper and asserts that the paper complements current literature.

The microscopic parameters in the numerical method have an important influence on the results and computational efficiency of the models. The existing calibration methods of microscopic parameters in the finite-discrete element method (FDEM) are based on two-dimensional (2D) FDEM model, and there is a lack of calibration methods for microscopic parameters suitable for three-dimensional (3D) FDEM model. Therefore, this paper proposes a fast and convenient calibration method for the microscopic parameters of the 3D FDEM model. First, a tangential contact force calculation algorithm based on the total normal force is proposed and verified by the slider model. Subsequently, assuming that the penalty parameter of the solid element is equal to the penalty parameter of the joint element, the relationship between the normal penalty parameter and the tangential penalty parameter, and the relationship between the normal penalty parameter and the Young's modulus are studied using several numerical models. Moreover, the tensile strength and crack propagation path of the Brazilian disc model under different fracture energy parameter combinations are studied, and the fracture energy parameters are calibrated. Then, the triaxial compression model under different confining pressures is used to calibrate the cohesion c and the internal friction angle φ. In addition, the effects of loading rate, mesh size and time step on the model strength and calculation time are considered, respectively. Finally, the numerical simulation of dynamic Brazilian splitting test is carried out to verify the reliability of the parameter calibration method proposed in this paper. Although the calibration method of microscopic parameters is not unique, the microscopic parameter calibration method has important reference significance and is conducive to the application of 3D FDEM model. Highlights: A tangential contact force calculation algorithm based on the total normal contact force is proposed and verified by the slider model. The tangential penalty parameter in the 3D FDEM model needs to be greater than the normal penalty parameter, and the threshold of the normal penalty parameter is 50 times Young 's modulus. The tensile strength of the Brazilian disc model always begins to converge when the fracture energy GII is equal to 10GI, and the crack propagation path is along the central diameter direction. [ABSTRACT FROM AUTHOR]

It is very important to predict the blasting fragmentation size of fractured rock mass. Based on the geological conditions of practical engineering rock mass, a plane analysis model of bench blasting in layered rock mass considering microcrack structure was established. And the distribution law of stress in the bench blasting of layered rock mass with different occurrence and layer thickness is analyzed by using the CDEM numerical simulation method. Combined with the theory of rock fracture mechanics and relevant test results, this paper quantitatively studied the change laws of the angle for the cracks to propagate, speed and crack initiation length at different particle points in bench blasting and realized the time history analysis of the crack development and rock fragmentation in bench blasting. And then, this paper derived the quantitative relationship between the post-expansion cranny ratio and the blasting fragmentation from a theoretical point of view, and proposed the prediction method of the median fragmentation in bench blasting of layered rock mass. Combined with the bench blasting productive test of Changjiu (Shenshan) limestone mine, the largest sand aggregate production mine in the world, the correctness of the theoretical results is verified. The test results demonstrate that in a vertically inclined rock bench, the degree of rock fragmentation intensifies in the horizontal direction, and the overall fragmentation effect can be optimized by adjusting the blasting hole spacing; the blasting fragmentation prediction method proposed in the paper can accurately predict the median fragmentation of layered rock, which provides a novel method for predicting the effects of bench blasting. Highlights: The prediction method for the median fragmentation of layered bench rock mass blasting is obtained The quantitative relationship between the post-expansion cranny ratio and the median fragmentation of bench blasting is theoretically derived The distribution law of microcracks in bench rock mass is simulated by discrete fracture network model. The internal force distribution characteristics are analyzed by continuous medium discrete element method The propagation law and rock breaking process of microcracks and layered cracks in bench blasting are studied At the same time, the correctness of the prediction method is verified by the field test and production of Changjiu (Shenshan) limestone mine [ABSTRACT FROM AUTHOR]

Burden is one of the major influencing blast design parameters which should be optimised for the extraction of narrow vein ore bodies. Numerical simulation-based approach has been used for the optimisation of burden in this paper. Different models were prepared under varying parametric conditions including width of the vein, blasthole diameter and burden. The RHT concrete constitutive model was used for the analysis of damage contour. The volume of the excavated rock along the free face (EV) was quantified from the model output in all the scenario. The study showed that EV initially increases with the increase in burden for a particular width of vein. Furthermore, EV starts decreasing after a point termed as pivot point. The value of burden at this pivot point was considered as optimal in this study. The best fit curve between EV and burden for three blasthole diameter and seven different width of vein were plotted. In all instances, the outputs of the model followed the second-degree polynomial equation. The study also suggests that the optimum burden reduces with the increase in the width of the vein. It has also been found that the optimum burden follows power trend with the increase in width of the ore body irrespective of blasthole diameter. Based on the optimum burden obtained from the output of simulation along with different blasthole diameter and width of the ore body, an empirical relationship has been established. The developed empirical relationship has a good agreement with the experimental trial data. Highlights: Numerical simulation technique has been used in this paper for the optimisation of burden while extraction of narrow vein stopes. The study suggest that the relation of volume of the excavated rock along the free face and burden is second-degree polynomial in nature. It has been found that the optimum burden for narrow vein is higher as compared to wider width ore body. Study also suggest that optimum burden follows power trend with the increase in width of ore body. The influence of width of the ore body in the selection of burden vanishes after certain width of ore body as per the observation of model output. [ABSTRACT FROM AUTHOR]

The Norwegian University of Science and Technology (NTNU) has since the early 1970s published prediction models for estimated production rates, time consumption, and costs for rock works. Since the early development of the NTNU models the tunnelling industry has continuously improved and the models have been updated on several occasions. The NTNU models are presently being updated to better fit the modern tunnelling trends in Norway. The current prediction model for D&B blast design is in this research paper compared with new data from thirteen recent Norwegian tunnel projects. The paper demonstrates that the empirical methodology behind the prediction model is still highly relevant, but some further improvements and additional empirical relations are suggested to enable the model to better cope with the recent 16 years of development in technology, equipment, contractual-issues and blast design. The new datasets show that modern D&B tunnelling employ more drill holes and specific charging per blast round than before. The current NTNU model thus under-predicts the actual tunnelling performance in the blast design. Some suggestions are made for further improving the model, where the updated drill length, the contour requirements, and the tendency to including the longitudinal ditches into the main blast are suggested as the main reasons for the disparity in drill hole numbers. Yet, the current trend in specific charging seem to have doubled since the early 2000s and cannot be explained by the increase in drill holes alone. Further development ought to be included in the blastability index (SPR) so that geological conditions that are on the extreme side of poor blastability can be properly accounted for in the blast design. Highlights: Validation of prognosis model for drill and blast tunnelling Collection and treatment of tunnelling production data Evaluation of blastability for tunnelling applications The use of explosives in tunnelling has dramatically increased The use of drill holes in conventional tunnelling has increased [ABSTRACT FROM AUTHOR]

A three-dimensional representation of the random distribution of fractures in rock masses, known as the discrete fracture network (DFN), is widely used to analyze the stability of jointed rock slopes. In this paper, a new framework for constructing three-dimensional DFN models in rock masses has been proposed to overcome the limitations of conventional circular or polygon-based models. The framework utilizes the universal elliptical disc (UED) model and integrates it with the discrete element method in 3DEC for the stability evaluation of jointed rock slopes. This paper starts by introducing the basic principles of the UED model. The procedures for constructing a three-dimensional DFN using the UED model is then outlined. In the present study, a case study of a rock slope in Zhejiang Province, China is used to demonstrate the implementation of the proposed framework. A comprehensive comparative study is conducted to investigate the impacts of several UED model parameters, including the ratio of major axis to minor axis and rotation angle, and discontinuity density, on the stability of rock slopes and compared to the conventional Baecher disc model. The results show that the framework can effectively integrate the UED model into 3DEC and provide a realistic representation of the three-dimensional DFN, leading to improved accuracy and efficiency in the stability evaluation of jointed rock slopes. The framework also shed light on the interactive effects of the UED model parameters and discontinuity density on the rock slope stability, providing a strong reference for using the UED model in constructing DFN models for rock slopes. Highlights: Three-dimensional discrete fracture networks (DFNs) of rock masses are successfully constructed using the universal elliptical disc (UED) model. Python subroutines are developed in 3DEC to construct 3D DFNs using the UED model for improved discrete element analyses of jointed rock slopes. The effects of several characteristic parameters of the UED model on the stability of jointed rock slopes are investigated. Based on a key block model, the mechanisms underlying the relationship between jointed rock slope stability and fracture discontinuity are rigorously explained. The advantages of the UED model over the conventional Baecher disc model are comprehensively discussed based on a rock slope case study. [ABSTRACT FROM AUTHOR]

Salt caverns are internationally recognized as excellent facilities for underground energy storage. Creep shrinkage deformation will occur in deep salt caverns under the action of high-ground stress, and it is a key factor to evaluate the safety of salt caverns. However, there has been no salt cavern creep shrinkage mechanism research on ultra-deep salt caverns. In this paper, the creep shrinkage mechanism of an ultra-deep salt cavern is established from the theory, numerical simulation, and field application. First, the actual creep field experiments (pressure testing and sonar testing) of the salt cavern (depth ≥ 2000 m) are carried out. Second, theoretical models of salt cavern creep shrinkage are established from four influence aspects (creep shrinkage, heat conduction, salt dissolution, and brine permeability). Third, a 3D geological model is built to analyze the creep stability of deep salt caverns based on their field conditions. The novelty of this paper is analyzing the creep shrinkage of the ultra-salt cavern by the theoretical model, numerical model, and field experimental data systematically. The results show that the shrinkage of the salt cavern and brine thermal expansion is key factors leading to pressure lifting of the salt cavern, which accounts for 0.6121 and 0.2147 in the four influence aspects. Three creep phases are obtained: rapid rising stage, steady rising stage and decelerating rising stage. The study provides a reference for the creep shrinkage and field application of salt caverns. Highlights: Creep field experiments of the salt cavern are carried out. Theoretical models of ultra-deep salt cavern creep mechanisms are built. An ultra-deep 3D salt cavern creep numerical model is built to analyze the creep stability of salt caverns. A mathematical model is proposed to analyze the factors that influence salt cavern creep. [ABSTRACT FROM AUTHOR]