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In the field of high-temperature underground excavations, previous scholars often focused on the seepage characteristics of intact host rock after high-temperature treatment. However, relatively little is known about the coupled thermo-hydro-mechanical (THM) responses of the damaged host rock in excavation damage zone under real-time ultra-high temperature and triaxial stress/seepage pressure (RTUHTTSSP). Therefore, using a self-designed triaxial testing system coupled with THM response system, the permeability evolution of sandstone with varying damage degrees under RTUHTTSSP was measured in this study. Then, mercury injection measurements were carried out on these specimens subjected to coupled THM conditions. The results show that (1) under RTUHTTSSP, permeability/porosity does not continue to increase with increasing temperature as expected. They show a strong dependence on the interaction of temperature field, stress field, seepage field, and damage regime. At 300 °C, the permeability/porosity of 0–0.8-damaged specimens is expected to decrease due to the closure of internal pores/fissures. However, at 450 and 300 °C, permeability/porosity of the 0.8–0.9-damaged specimens is expected to increase vertically due to the generation of the through-going macrocracks. Subsequently, with the continuous increase in the temperature, the permeability/porosity decreases slightly due to the fracturing/compression of the propping asperities. (2) The relationship between permeability and seepage pressure is closely related to the damage degree. For 0–0.6-damaged specimens, the permeability first decreases and then increases with the increase in seepage pressure. For 0.7–0.8-damaged specimens, the permeability increases with the increase in seepage pressure. The research results of this study can provide theoretical basis for the evaluation of stability of high-temperature underground excavation. [ABSTRACT FROM AUTHOR]

This study aims to investigate the rockburst characteristics of hard rock during the successive excavation unloading of twin circular tunnels subjected to high active stresses. The entire evolution process of the rockburst phenomena around the tunnels is reproduced. The numerical results indicate that the unloading rates, burial depths, and presence of structural planes between the twin tunnels play important roles in the occurrence and damage degrees of rockbursts. The failure intensity and dynamic responses are aggregated with the increase of the unloading rate of the subsequent adjacent tunnel. The rockburst damage degree is exacerbated with increasing buried depth, and the rock response of the twin tunnels becomes more sensitive to the dynamic disturbance (as compared to a single tunnel at a great depth). The presence of a structural plane between the twin tunnels has both favourable and unfavourable effects on the stability of the surrounding rock. When the structural plane is parallel to the maximum tangential stress, the dynamic disturbance from the adjacent tunnel can be attenuated by the structural plane or rock joints via reflection and scattering, thus reducing the dynamic response between the twin tunnels. However, for those structural planes oblique to the maximum tangential stress, a violent rockburst is more prone to be induced, owing to the integrated response to shearing and sliding along the structural plane, and slabbing from the excavation unloading process. It is also found that the effect of the structural plane on the rockburst response is largely dependent on the burial depth. [ABSTRACT FROM AUTHOR]

During deep excavation in weak rock masses, the time-dependent deformation of the rock and longitudinal advancement process have significant influences on the tunnel performance. Accordingly, a combined support system consisting of rockbolts and linings is commonly used in such tunnels to ensure their stability. However, there are no well-established solutions for quickly and accurately evaluating the safety of those support systems. To address this problem, mechanical model of a circular tunnel with a rockbolt–lining combined system is analytically built in this study. Then mathematical formulas for the stresses and displacements around the tunnel are derived, considering the time-dependent behaviors of the rocks and tunnel face advancement effect. Furthermore, the analytical formula proposed in this study is compared with existing solutions and numerical results obtained using the finite-element software Abaqus, and a good agreement is achieved. Finally, a parametric investigation is conducted regarding the influences on the tunnel's mechanical performance. The results show that the installation time (t0) and distribution density of rockbolts (SθSz) and the installation time of the lining (t1) have a significant influence on the deformation of the surrounding rock. Although the installation times of bolt and lining are positively related to the deformation suppression effect of the surrounding rock, they are negatively related to the stress of the bolt and lining. It is found that the closer the installation time of bolt and lining, the greater the stress of lining; otherwise, the greater the stress of bolts. Furthermore, the initial release coefficient (m) and the ratio of tunnel excavation rate to influence radius can influence the deformation rate of surrounding rock. Finally, the study proposes a new method for quickly evaluating the viscoelastic deformation of a tunnel. [ABSTRACT FROM AUTHOR]

Copyright of Rudarsko-Geolosko-Naftni Zbornik is the property of Faculty of Mining, Geology & Petroleum Engineering and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The weakening effect is one of the most important causes triggering large deformation and failure of soft-rock engineering; however, few studies paid attention to damage evolution and constitutive relationship of rock in tensile damage in the excavation unloading and water-weakening process, not to mention the coupling process of unloading and water-weakening. In this paper, the mechanism and engineering characteristics of unloading softening and water-softening of water-rich soft rock are analyzed and summarized. Then with the aid of the strain equivalent principle, the damage of surrounding rock caused by unloading softening and water-softening is coupled, and the compression shear damage and the tensile damage of surrounding rock under the unloading process are analyzed. A damage constitutive model of rock subjected to excavation unloading and water-weakening is proposed considering the influence of water immersion time, and the proposed model is applied in a newly established finite element simulation method, which is suitable for excavation in the water-rich soft rock. Based on the mechanical-hydraulic-damage coupled method, the progressive failure process of surrounding rock under the dual softening effects can be reflected by the deteriorated parameters of damage elements. Finally, the field monitoring data of a typical section in the Xujiadi tunnel is used to verify the applicability and accuracy of the proposed dual softening model and simulation method. [ABSTRACT FROM AUTHOR]

This study is aimed at better understanding the deformation and failure mechanism of surrounding rock during excavation unloading of a high-stress rock mass and determining the reasonable reinforcement time for the surrounding rock. To fulfill this aim, true triaxial tests were carried out on different loading and unloading paths during the unilateral unloading of a high-stress rock mass. The variational condition for minimization of plastic complementary energy is obtained, the optimal reinforcement time is determined, and the range of the plastic zone in the surrounding rock reinforced by anchor mesh-cable-grouting is compared and analyzed. The results are as follows: (1) Based on the Mohr-Coulomb yield criterion and the deformation reinforcement theory of surrounding rock, the stable state with the minimum reinforcement force is obtained. (2) After the true triaxial tests on the unilateral unloading of the third principal stress were carried out under different confining pressures, loading continued to be performed. Compared with rock failure without confining pressure, in the conventional uniaxial compression test, the failure of samples is dominated by composite splitting-shear failure; the unilateral unloading stress-concentration failure is a progressive failure process of splitting into plates followed by cutting into blocks and then the ejection of blocks and pieces. (3) The relationship between the time steps of the surrounding rock stability and the excavation distance is obtained. The supporting time can be divided into four stages: presupport stage, bolt reinforcement stage, anchor cable reinforcement stage, and grouting reinforcement stage. (4) In the range of within 5 m behind the tunneling face, the plastic zone of the surrounding rock with support is reduced by 7 m as compared with that with no support. In the range of over 5 m behind the tunneling face, the plastic zone of the roadway floor with support is reduced by 2.6 m as compared with that without support, and the deformation is reduced by 90%. These results can serve as a reference for controlling the behavior of surrounding rock during excavation unloading of high-stress rock masses. [ABSTRACT FROM AUTHOR]

Rock excavation has been the hot spot and frontier of scientific research. Rock excavation research is in a period of rapid development. The bibliographies included in ISI Web of Knowledge database from 1999–2020 were used as data samples, and the collected data were analyzed by literature co-citation and cluster analysis using CiteSpace and VOSviewer information visualization techniques and dynamic network analysis tools. A knowledge map of the evolution of bibliometric research development is drawn to reveal the representative literature in the field of bibliometrics. The hot areas of bibliometric research are introduced. The development trend of bibliometrics is proposed. The results of the study show that the amount of the literature on rock excavation is growing rapidly. A large amount of the foreign literature is available in China, the United States, Australia, Canada, France, and other countries. The main included journals are Tunnelling and Underground Space Technology, International Journal of Rock Mechanics and Mining Sciences, "Rock Mechanics and Rock Engineering," "Engineering Geology," and "Bulletin of Engineering Geology and the Environment." Keyword co-occurrence analysis includes the following contents: rock damage constitutive model, excavation damage area, numerical simulation, stability analysis support, and prediction technology of rock. [ABSTRACT FROM AUTHOR]

Predicting and controlling the collapse of surrounding rock (especially broken rock masses) in underground chambers is an important topic in mining and geotechnical engineering. Based on an example, this paper introduces a case study of surrounding rock stability control technology in stope mining around abandoned areas. Based on on-site coring, mechanical properties of rock samples, and on-site grouting reinforcement technology, the TRT6000 advanced geological prediction system was used to predict the stability status of the surrounding rock of the underground chamber. AUTODYNA software was used to build a dynamic coupling model for numerical simulation prediction and optimization of blasting parameters and to reveal the dynamic variation in the surrounding rock. The dynamic failure process of the surrounding rock of the chamber before and after optimization of the blasting parameters is simulated, and the deformation characteristics and damage and acoustic emission characteristics of the surrounding rock are clearly shown. The surrounding rock failure first appeared around the surface of the underground chamber because of the high stress concentration around the surface of the chamber after blasting; with the interaction between the explosive gas and the rock mass, the damaged area further propagated into the external rock, eventually leading to a large damage area. At the same time, there is a large tensile failure in the rock, resulting in expansion and rupture around the underground chamber. Finally, the 3D laser scanning method is used to verify the superiority of the optimized blasting initiation sequence. The new edge hole detonation sequence can effectively improve the blasting vibration and successfully control the further damage of the surrounding rock of the underground chamber, thus proving the edge hole drug pack. Moreover, the initiation mode of the delay stage of the side hole charge is determined. This study provides a useful reference for the stability control of surrounding rock in mining in mining areas. [ABSTRACT FROM AUTHOR]

An experimental study on the strainburst characteristics of Beishan granite under different intermediate principal stress (σ2) loading conditions was conducted using a servo-controlled true-triaxial rockburst test machine equipped with an acoustic emission (AE) monitoring system. The primary objective is to investigate the influence of end effect under loading in the σ2 direction on the strainburst behaviors of the rock. Rectangular prism specimens with dimensions of approximately 200 × 100 × 100 mm3 were prepared and divided into two groups. For the first group, the specimens were in direct contact with platens during loading. For the second group, the specimen's two surfaces loaded by σ2 were daubed with a layer of anti-friction agent to reduce the end friction of the platens. In the test, a loading mode, which kept one specimen surface free and applied loads on another five surfaces of the specimen, was used to simulate the stress state of a rock element on the excavation boundary. A high-speed video camera was used to capture the failure process of the specimens. The experimental results indicate that regardless of the use of the anti-friction agent or not, strainburst characteristics of the specimens depend on the magnitude of σ2. With increasing σ2, the degree of violence during rock failure presents an increasing trend and the dynamic failure process of the free specimen surface changes from local rock ejection to full-face burst. However, the σ2-dependent crack initiation, peak strength, failure mode and energy release characteristics of the lubricated specimens differ significantly from those of the non-lubricated ones. With increasing σ2, rock strengths increase to a peak value and then decrease, but the peak strength of the lubricated specimen is lower than that of the non-lubricated specimen under a given σ2. In addition, the peak strength difference between two groups increases as σ2 increases. Meanwhile, large end effect suppresses crack initiation of the specimens, causing crack initiation of the specimens to occur at higher stress levels, leading to higher peak strengths. Moreover, when σ2 = 1 MPa, the failure modes of the non-lubricated and lubricated specimens are similar. When σ2 is greater than 1 MPa, the number of the fractures in the surfaces loaded by σ2 for the lubricated specimens is larger than that for the non-lubricated specimens. In addition, the weight and the associated kinetic energy of ejected rock fragments from the lubricated specimen are higher than those from the non-lubricated specimen. The difference of the weight and the kinetic energy of the rock fragments between the non-lubricated and lubricated specimens increases with increasing σ2. [ABSTRACT FROM AUTHOR]

Tunnel excavations in rock usually cause initial stress release, which is different from the stress path in classical triaxial compression tests. To analyze the permeability variations in excavation disturbed zone, two different stress paths, e.g. classical triaxial compression and confining pressure unloading, are applied on granite, and the permeability at different damage levels is measured using gas as transport medium. The differences in deformation and failure mechanism and their impacts on gas permeability variation are discussed. The test results show that lateral deformation of samples in confining pressure unloading tests is obviously larger than that in classical triaxial compression tests. Unlike the permeability decrease-increase transition in classical triaxial compression tests, the permeability in confining pressure unloading tests shows an overall progressively increase trend. The permeability in both tests increases significantly near failure and achieves the same order of magnitude. The differences in permeability evolutions under the two stress paths are directly attributed to their different deformation mechanisms. The test results could provide a new understanding of permeability properties during the underground excavation. [ABSTRACT FROM AUTHOR]

The processing and interpretation of microseismic (MS) signals are the basis for obtaining source information of MS events in rock engineering. However, MS signal processing is subjective and time-consuming, in which different strategies have to be empirically adopted to cope with different issues. To facilitate MS signal analysis, this study proposes an entirely data-driven method based on local mean decomposition (LMD), which has been applied in various signal processing fields. Through some numerical experiments, the superiorities (e.g., smaller reconstruction errors and fewer sifting iterations) of the proposed method over other methods are demonstrated, and the proposed method is identified as an effective filtering and frequency analysis tool to process recorded MS signals. Our analysis results show that the proposed method can effectively remove interfering noise existing in MS signals collected from the Lianghekou hydropower station during underground rock excavation. Moreover, the proposed method can be employed to analyze the frequency revolution features of MS signals before and after rock mass failure. In addition, it is found that MS signal frequency migrates to a low-frequency range when approaching rock failure, which can be regarded as an early warning indicator of rock instability. The proposed method is believed to facilitate MS monitoring and offer a new clue to conduct MS signal processing and interpretation. [ABSTRACT FROM AUTHOR]

During deep buried hard-brittle rock tunnel excavation, the surrounding rock experiences a complicated stress path and stress adjustment process. Once the adjusted stress exceeds the ultimate bearing capacity of rockmass, a rock failure mode defined as stress-cracking type will occur. In order to investigate the effect of stress paths on failure mechanism and progressive damage of deep-buried rockmass, the cyclic loading-unloading, loading-unloading, uniaxial, conventional and unloading triaxial compression tests on samples of hard-brittle sandstone were conducted. According to the experimental results, increase in the confining pressure was beneficial to improve the mechanical parameters of rock, but it will reduce the brittle failure features. Compared with conventional triaxial compression, the sandstone under unloading state had more remarkable stress drop and unstable failure characteristics. Meanwhile, it was found that the energy dissipation and energy release in the whole process of rock deformation were the internal power of driven rock progressive damage. With the increase of confining pressure, the energy hardening and energy accumulation features of rock were weakened, while the progressive damage evolution characteristics could be enhanced. In unloading state, more energy could be converted into elastic energy in the energy softening phase (σeb-σP), so that the pre-peak damage rate of rock was lower than that of conventional triaxial compression state. Thus, the energy dissipation rate of rock after peak strength decreased linearly with the increase of confining pressure under conventional triaxial compression state, while in unloading state it showed the opposite law. [ABSTRACT FROM AUTHOR]

The initial stresses have a strong effect on the mechanical behavior of underground rock masses, and the initial stressed rock masses are usually under strong dynamic disturbances such as blasting and earthquakes. The influence mechanism of a blasting excavation on underground rock masses can be revealed by studying the propagation of stress waves in them. In this paper, the improved Mohr-Coulomb elasto-plastic constitutive model of the intact rock considering the initial damage was first established and numerically implemented in Universal Distinct Element Code (UDEC) based on the variation of the experimental stress wave velocity in the initial stressed intact rock, and the feasibility of combining the established rock constitutive model and the BB (Bandis-Barton) model which characterizes the nonlinear deformation of the joints to simulate stress waves across jointed rock masses under initial stress was validated by comparing the numerical and model test results subsequently. Finally, further parameter studies were carried out through the UDEC to investigate the effect of the initial stress, angle, and number of joints on the transmission of the blasting stress wave in the jointed rock mass. The results showed that the initial stress significantly changed the propagation of the stress waves in the jointed rock mass. When the initial stress was small, the transmission coefficients of the stress waves in the jointed rock were vulnerable to be influenced by the variation of the angle and the number of joints, while the effect of the angle and the number of joints on the stress wave propagation gradually weakened as the initial stress increased. [ABSTRACT FROM AUTHOR]

The uniaxial compressive strength (UCS) represents the strength of the rock. It frequently requires during the introductory phase of mining projects such as tunneling, rock excavation, blast hole designs, etc. Usually, the determination of UCS of rocks is carrying out in a concerned laboratory. The main drawback of determining the UCS in a laboratory requires at least five core samples of high-grade quality. Many problems and limitations are associated with removing the core, and also preparing the test specimen for UCS is tedious, time-consuming, and expensive. Therefore an attempt is made to develop an efficient indirect method to estimate the UCS of rocks without using the core samples. In this experimental investigation, the drilling response, such as thrust, is gathered by drill tool dynamometer considering the different drill operating parameters. The prediction model is developed with a regression technique using the measured thrust and calculated torque. The prediction capacity and validation of the model are carried out using the standard procedure. The experimental results show that the model could explain the variance in UCS up to 93.60%. RMSE and MAPE values in terms of percentage are 3.49% and 11.27%, respectively. Besides, the model's validation is checked for sandstone and limestone having the UCS 28 MPa and 35 MPa, respectively, and yielded the best prediction results with an error of 8.51% and 8.01% suggesting that the developed model could predict the UCS of sedimentary rock types within acceptable error limit, and reasonably. The correlation of UCS of rocks and drilling specific energy is also tested and found that linear relationship between them with an R2 value of 92.10%. [ABSTRACT FROM AUTHOR]

Rock excavation has experienced complex stress paths. The development of the original crack under the path of principal stress magnitude and principal stress direction is a key scientific problem that needs to be solved in rock underground engineering. The principal stress magnitude dominates the initiation and propagation of the crack and increases rock damage under the action of principal stress rotation. In this study, the theoretical calculation and numerical analysis method have been combined with the crack propagation conditions to study the stress-driven mechanism of brittle rock crack propagation under principal stress rotation. The results show that the "relative initial angle" of crack angle is being updated in time during the principal stress rotation process; once the stress is rotated, it will become the next initial crack angle; the crack propagation direction is deviated under the applied shear load, and it is always in the direction of minimum shear load, leading to a certain degree of inhibition of crack propagation depth in the initial direction. According to the results of numerical simulation, the effect of principal stress rotation caused by mining excavation is obvious and has a certain range of influence depth, the stress of surrounding rock of roadway is the highest within the depth range of 1∼2 m, and the maximum principal stress is as high as 26.89 MPa. The rotation of principal stress direction on the roadway surrounding rock surface is the strongest, which makes the surrounding rock more fragmented, and the middle principal stress and the maximum principal stress rotate about 90° counterclockwise along the Ox axis. Studying the action mechanism of principal stress rotation on fractured rock masses can provide scientific basis for geotechnical engineering design and rock mass surrounding support. [ABSTRACT FROM AUTHOR]

The excavation of rock significantly changes the initial stress state of rock slopes, which makes rock in complex loading and unloading conditions. However, the failure mechanisms and macro-micromechanical properties of rock under loading and unloading conditions are not very clear. This study investigates the macro-micromechanical properties and failure law of granite under loading and unloading conditions through traditional laboratory tests and particle flow simulations. Granite specimens are taken from Shuichang iron mine, and stress unloading experiments are designed based on the engineering practices. The stress-strain curves and failure modes under different confining pressures and unloading paths are obtained to analyze the granite properties from the viewpoint of the macroscopic mechanism. Moreover, numerical models are established in PFC software. The microcracks developments, failure characteristics, and energy evolutions under loading and unloading conditions are obtained and discussed. Results show that compared with the loading tests, the brittle failure characteristics of specimens under unloading tests are more obvious. When the confining pressure reduces to about 66% of the initial confining pressure, the specimen loses its load-bearing capacity and destroyed due to the lateral expansion. For loading tests, an inclined plane can be produced as the main failure surface. While for confining pressure unloading tests, there are many damage zones parallel to or intersecting with the main failure surface. [ABSTRACT FROM AUTHOR]

The rock mass properties are typically influenced by the excavation technique and the changes in the state of stress due to the rock excavation. The amount and extent of damage introduced in the rock mass depends on the excavation technique and practice quality. The influence of blasting in the rock mass near the tunnel periphery is far more significant due to the energy of waves and redistribution of stresses and the severity of the damage diminishes as the radial distance from the tunnel opening increases. Therefore, it is important to consider the effect of the damaged zone when analysing the stresses and deformations around a tunnel. This study aimed at providing a new numerical solution for the determination of the ground response (reaction) curve with the consideration of the non-uniform damage zone around the tunnel periphery. A deep circular tunnel subjected to hydrostatic stress condition and excavated in rock materials obeying the Hoek-Brown failure criterion is considered. A solution for the determination of stresses, strains, and deformations around the circular deep tunnel is presented in order to correctly assess the attenuation of damaged rock as the distance from the tunnel perimeter increases considering the loads applied to the supporting structure. [ABSTRACT FROM AUTHOR]

Large quantities of excavated rocks are generated during tunnel driving. Recycling possibilities and total percentage of recycling are depending on the rock conditions (rock type and rock properties), as well as the applied driving method. If recycling of the excavated rock mass is not possible or reasonable from a technical or economical point of view, the excavated material needs to be deposited in landfills. Thereby, the consumed landfill volume depends on the material volume of non‐recyclable rock and its compactability as a function of the loosening of the rock by excavation and landfill installation. Both, material volume and compactability of the excavated material, are difficult to evaluate in advance. Apart from unpredictable lithological changes and variation of the driving parameters (e.g. depth of advance in conventional driving, contact pressure of tunnel boring machine), external influences – like afflux of water or weather conditions during installation of the material – significantly affect the compactability of the excavated aggregates. In this study, the actual example of phyllitic rock material excavated by conventional drilling and blasting from the main tunnel tubes of the Brenner Base Tunnel has been chosen to take a closer look at the research question concerning the compactability of phyllitic rock material. Since excavation material is increasingly used in construction industry, detailed knowledge on this matter is necessary for a successful application. [ABSTRACT FROM AUTHOR]

• This study presents new analytical closed-form solutions for an underwater tunnel considering the damaged zone. • The solutions are presented for tunnels in elastic–brittle–plastic rock masses with Mohr-Coulomb failure criterion. • The damaged zone is assumed to have cylindrical shape with finite radius. • Three different paths for plasticity evolution including six different states are considered. • It was concluded that the proposed model covers a wide range of tunnel problems. Rock excavation using drill and blast method is commonly used in tunneling world-wide. Drill and blast method has inherent disadvantage of deteriorating surrounding rock mass due to development of a blast-induced damage zone with reduced strength and stiffness parameters and increased permeability. Traditional tunnel analysis adopts same parameters for the entire rock mass, leading to the underestimation of tunnel stability. The blast damage zone with finite thickness is significant in tunnel stability. Tunneling below the groundwater table affects the hydraulic equilibrium. This will, in turn, cause seepage into the tunnel through the pores and discontinuities in the rock masses. The developed seepage force should be considered as an additional body force acting on both damaged and undamaged rock masses. This study presents a new analytical closed-form solution for the determination of stresses, strains, and displacements around a circular deep underwater tunnel with the consideration of the seepage forces and the damaged zone. The solutions are presented for tunnels excavated in pervious elastic–brittle–plastic rock masses with Mohr–Coulomb failure criterion. The damaged zone is assumed to have cylindrical shape with finite radius. The plastic zones may be formed in both damaged and undamaged rock masses, independently. In order to solve the proposed problem, three different paths for plasticity evolution including six different states that can possibly be encountered in the problem are considered. The results indicate that the seepage and the damaged zone have significant effects on the tunnel convergence and the distribution of stresses in the rock mass. [ABSTRACT FROM AUTHOR]

Copyright of Coal Science & Technology (0253-2336) is the property of Coal Science & Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Copyright of Coal Science & Technology (0253-2336) is the property of Coal Science & Technology and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Blasting has been widely used for economical and rapid rock excavation in civil and mining engineering. In order to study the influence of relative horizontal distance and relative vertical distance between two tunnels on the dynamical response of the two tunnels, 10 numerical simulation cases are done by LS-DYNA 3D models under surface explosion by controlling the clear distance and height difference of double-line tunnel, and the ALE multimaterial fluid structure coupling algorithm is applied to analyze the dynamic response characteristics of double-line tunnel under different conditions. The numerical results show that the dynamic response characteristics of the tunnel lining are affected by the change of the clear distance and height difference of the tunnel. With the increase of the height difference between adjacent tunnels, the peak value of vibration velocity at the top of the lining on the blast face increases, which is due to the upward elevation of the right tunnel, which is more conducive to the reflection and superposition of stress waves. When the height difference of tunnel is 4–6 m, the vibration velocity and displacement of monitoring point C on the back blasting side will change abruptly, and the variation range of vibration velocity is about 25%, while the variation range of displacement is about 60%. [ABSTRACT FROM AUTHOR]

In the convergence–confinement method, the longitudinal deformation profile (LDP) serves as a graphical representation of the actual tunnel convergence (both ahead of and behind the face); therefore, it is considered important for determining the distance of support installation from the face or the timing after excavation in this method. The LDP is a function of the rock mass quality, excavation size, and state of in situ stresses; thus, obtaining the LDP according to the rock mass conditions is essential for analyzing the complete behavior of convergence during tunnel excavation. The famous LDP shows that the best fit for the measured values of tunnel internal displacement reported simply expresses the ratio of the preceding displacement as approximately 0.3. This can lead to an error when predicting the ratio of the preceding displacement while neglecting the rock conditions; consequently, a complete tunnel behavior analysis cannot be realized. To avoid such error, the finite difference method software FLAC 3D is used to develop an expanded longitudinal deformation profile (ELDP) according to the rock mass conditions. The ELDP is represented by graphs featuring different shapes according to the rock mass rating (RMR), and the empirical formula of the LDP best fitted for the tunnel convergence measurement values is expanded. This expanded LDP formula is proposed in a generalized form, including the parameters α and β from the empirical equation. These parameters α and β are expressed as functions of the RMR and initial stress. Statistical analysis results of the 3D numerical analysis of 35 cases were analyzed in the ranges of α = 0.898–2.416 and β = 1.361–2.851; this result is based on the empirical formula of Hoek (1999) (α = 1.1, β = 1.7), which was expanded in the current study according to the rock quality and initial stress conditions. [ABSTRACT FROM AUTHOR]

In engineering blasting, the determination of the range of rock blasting fracture zone has important guiding significance for blasting construction. This paper proposes a method that can accurately and directly obtain the range of rock blasting fracture zone. Based on the theory of elastic wave propagation, test rods which are made of appropriate material are selected and prepared. A certain number of boreholes are drilled for subsequent insertion of the test rods along the direction perpendicular to the free surface of the excavation at a certain distance from the blast hole. Based on the field blasting test results, the deepest fracture position of the test rod is used as the boundary of the blasting fracture zone, and the range of the rock blasting fracture zone is obtained. A numerical analysis model is established according to the Mohr–Coulomb constitutive relationship and the Von Mises yield criterion. Then, the range of the fracture zone and the maximum horizontal radius of the fracture zone are analyzed and obtained. The numerical analysis results are compared with the field measured data. It is demonstrated that the range of the fracture zone obtained by the numerical simulation is in good agreement with the blasting test results of the pre-embedded test rods. The research results can provide references for the safety control of blasting and excavation of rock slopes. [ABSTRACT FROM AUTHOR]

Accurate orientation distributions are crucial to generating a reliable discrete fracture network (DFN) model for rock mass, while conventional one-dimensional (1D) and two-dimensional (2D) observation data have significant sampling bias. The study proposes a complete analytical method for estimating the orientation distributions of three-dimensional (3D) fractures in rock mass in conjunction with trace statistics in a sampling window, which is suitable for most continuous distributions by reducing the sampling bias. Traces are divided into three categories to derive the geometric probabilistic relationships between 2D trace statistics and 3D fracture orientation distributions. The moment estimation, number estimation, and normalization error functions are derived, and the distribution parameters are determined by minimizing the total error function. The proposed method is compared with the Terzaghi family methods and validated by multiple sets of stochastic fracture networks with different orientation distributions and sampling windows generated by the Monte Carlo method. The results indicate that the estimated continuous orientation distributions subjected to the error functions from a large single sampling window are well matched with the true distributions after removing the number estimation error functions of trace samples fewer than 20. Daxiagu tunnel is selected as a case study and the distributions estimated by the proposed method are more coincident with the field observation than those fitted by the orientations of the rock outcrops on the excavation face. [ABSTRACT FROM AUTHOR]

Strainburst is a kind of safety concern caused by the release of elastic strain energy from surrounding rocks under a tangential stress gradient. In this paper, laboratory rockburst model tests under four different stress gradients were conducted. Based on the acoustic emission (AE) data, the influence of tangential stress gradient on the energy evolution of strainburst was studied. The results indicate that (1) by controlling the tangential stress gradient loading at the top of the specimen, the change process of tangential and radial stress gradient from surrounding rocks caused by excavation disturbance, to a large extent, can be simulated. (2) The failure phenomena and failure stress of the specimen are both related to its tangential stress gradient distribution. As the tangential stress gradient increases, the failure stress is reduced, but dynamic failure phenomena become more evident. (3) With the increase of tangential stress gradient, the accumulation period of elastic strain energy lengthens, whereas the dissipation and release periods shorten during the loading process. As the tangential stress gradient rises, when the specimen rockburst occurs, the more dramatic decrease and faster decreasing rate of AE b-value reveal the increase in the specimen's proportion of shear failure. Also, the critical index (r) of probability density distribution shows a downward trend, which indicates that the AE energy in the low disturbance area decreases. In contrast, the AE energy span in the high disturbance area widens and the energy level becomes higher. [ABSTRACT FROM AUTHOR]

Soft rock masses represent a significant percentage of rock material on the earth's crust. The engineering properties of these rocks are related to their composition, geological history, and degree of weathering. Mudrocks (claystone, mudstone, argillite), marl, shale, sandstone, and conglomerates with weak cementing material are the most common example of these rocks which can be subjected to a wide variation of their engineering properties. Generally, they can be grouped in a broad sense, as geological materials with very low intact rock properties, namely, high deformability and low strength that span the range between soils and hard rocks. This paper is organized to suggest a classification for these rocks, dealing with the main geological and engineering aspects and features that characterize these materials. The two indices of soft rock number (SRN) and soft rock activity (SRA) have been introduced in this classification. The SRN is obtained based on the geomechanical properties of soft rocks, and it is within a range from 0 to 100 (0 for very weak rocks and 100 for strong ones). On the other hand, the SRA index, which indicates how rocks respond to disturbance, is calculated using an empirical graph. This index can have values from 0 to 1. If the index is closer to 0, the rock mass will be less sensitive to disturbance. Based on the two indices, the weakest group of soft rocks, which is called swelling soft rocks, has SRNs less than 20 and SRAs higher than 0.8. Excavation of these rocks can be accompanied by various types of failures. Moreover, in high-strength soft rocks, SRNs and SRAs have values higher than 80 and lower than 0.25, respectively. These rocks have good geomechanical properties and the lowest sensitivity to disturbance, so excavation them results in the fewest failures. [ABSTRACT FROM AUTHOR]

During the excavation of deep tunnels with high in situ stress, the stress unloading path caused by the rock mass excavation has important influences on the formation of the EDZ (excavation damage zone). In this study, the project background of the Jinping II diversion tunnels was firstly introduced. Then, the influences of stress unloading path induced by blasting excavation and tunnel boring machine (TBM) excavation on the EDZ were analyzed based on the acoustic data. Finally, the effects for different unloading paths of in situ stress induced by different excavation methods on the EDZ were studied by theoretical calculation and numerical simulation. The results show that compared with the quasi-static unloading, the transient unloading amplifies the effects of radial stress unloading and the tangential stress loading in surrounding rock masses. The EDZ depths of the blasting excavation tunnel are about 1.0 m greater than those of the TBM excavation tunnel. Moreover, the depths of the low-velocity band of the blasting excavation tunnel account for more than 50% of the total EDZ depths, while the depths of the low-velocity band of the TBM excavation tunnel account for about 30% of the total EDZ depths. The distribution of the low-velocity band along the excavation contour is closely related to the secondary stress field, indicating that the transient unloading of in situ stress is one of the important causes of the low-velocity band. After considering the post-peak mechanical properties of marble, the EDZ extents induced by different excavation methods can be estimated through numerical simulation. [ABSTRACT FROM AUTHOR]

It is very important to accurately determine the depth of excavation damaged zone for underground engineering excavation and surrounding rock stability evaluation, and it can be measured by acoustic test, but there is no quantitative method for analysis of the results, and it relies heavily on the experience of engineers, which leads to the low reliability of the results and also limits the application of the acoustic method. According to substantial field test data and the feedback of surrounding rock support parameters, the boundary method is proposed to determine the depth of excavation damaged zone in surrounding rock based on the relation between the ultrasonic velocity of measured point and the background wave velocity of rock mass. When the method is applied to the columnar jointed rock mass of Baihetan and the deep-buried hard rock of Jinping, the excavation damaged zone was well judged. The results in the Baihetan project show that the proposed method of determining excavation damage zone by the acoustic test can well demonstrate the anisotropy characteristics of the columnar jointed rock mass, and the damage evolution characteristics of jointed rock mass at the same position can also be obtained accurately. Moreover, the method also can accurately reveal the damage evolution process of the deep-buried hard rock under the condition of high ground stress, which proved the applicability of this method in jointed or nonjointed rock masses. [ABSTRACT FROM AUTHOR]

The influence of groundwater on tunnel engineering is very complicated. Due to the complexity of water flow water pressure transfer and uncertain defects in the stratum, all of which are key factors with regard to the design of tunnel engineering. Therefore, the variation of surrounding rock during excavation and the deformation and failure of soft surrounding rock under different seepage paths of underground water after excavation systematically. Experimental results showed that the stress change of surrounding rock caused by tunnel excavation can be divided into 3 stages: stress redistribution, stress adjustment, and stress rebalancing. In the process of water pressure loading, water flow rate is closely related to the experimental phenomenon. The between stable loading water pressure pore water pressure of the tunnel surrounding rock and the distance from the measuring point to the edge of the tunnel obey the exponential function of the decreasing growth gradient. With the increase of loading pressure, the pore water pressure and stress at the top of the tunnel increase, and the coupling of stress field and seepage field on both sides of surrounding rock more and more intense. The failure process of the tunnel can be divided into 6 stages according to the damage degree. The final failure pattern of the surrounding rock of the tunnel is mainly determined by the disturbed area of excavation. The arched failure area and the collapse-through failure area are composed of three regions. The surrounding rock is characterized by a dynamic pressure arch in the process of seepage failure, but it is more prone to collapse failure at low water pressure. The results of this study are the progressive failure mechanism of tunnel under different groundwater seepage paths and would be of great significance to the prevention of long-range disasters. [ABSTRACT FROM AUTHOR]

The use of explosives in cities is under strict control due to the risks associated with blasting. It is difficult to perform nonexplosive continuous excavation on hard rock connecting passage in crowded cities with dense high buildings and populations safely and rapidly. Therefore, a new method of rock fracturing through high-pressure gas expansion is proposed. The method was applied for the excavation of a connecting passage on the Hangzhou-Lin'an Intercity Railway in China. Analysis was conducted on the parameters of rock fracturing, rock fracturing effects of this method, and its impact on the vibration of surrounding buildings. Overall, when the compressive strength of rock reaches 100 MPa, and the surrounding rocks are dense, this method of rock fracturing can fully meet the requirements of rapid excavation in a connecting passage, and the vibration resulting from rock fracturing causes only a slight disturbance to the adjacent buildings. After optimizing the rock fracturing parameters, better, smoother rock fracturing effects are obtained, and a set of safe and efficient solutions to the excavation of hard rock in cities are created. It can provide a new approach for rock fracturing in similar projects. [ABSTRACT FROM AUTHOR]

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To study the influence of initial unloading damage on the mechanical properties of sandstone, the repeated loading test of unloading damaged sandstone was carried out considering 8 initial unloading quantities of 100%, 93.33%, 86.67%, 80%, 73.33%, 66.67%, 40%, and 0%. The results were compared with those of the triaxial compression test of intact samples. The results show that the peak strength of intact samples is higher than that of unloading damaged samples, and the difference is more obvious when the unloading quantity is more than 80%. During the unloading process, the strain increasing rate of rock samples is obvious, and the lateral dilatation is significant, and the deformation modulus and compressive strength of the rock sample deteriorate obviously. From the energy point of view, the greater the unloading damage, the smaller the stored elastic strain energy, which leads to the decrease of peak strength. At present, the unloading fracture inside the rock sample has developed, and the failure mode of the sample gradually changes from compression shear failure to tensile shear failure. In the process of engineering rock mass excavation, the unloading area and unloading damage amount of the rock mass is a dynamic adjustment process. To ensure the safety of the engineering rock mass, it is suggested to determine reasonable reinforcement time, reinforcement area, and reinforcement measures. [ABSTRACT FROM AUTHOR]

The construction of tunnels is often long and deep buried tunnels, and the geological conditions are more complex. Based on Jianshan tunnel in Gansu Province, the special geological conditions such as high ground stress and weak interbedded surrounding rock make the excavation of tunnel easy to produce large deformation. In this article, the software finite difference software FLAC3D was used to establish numerical models and select the best construction method by comparing the deformation of the tunnel under different construction methods. Aiming at the deformation characteristics of soft rock tunnels in highland interbedded layers, the control measures of tunnel deformation are discussed. Mainly consider the two aspects of the bolt support plan and the second lining construction time, comprehensively compare the deformation characteristics of the tunnel, and select the best working condition. The research results show that the combination of three-step temporary invert method and three-step ultra short bench method is recommended for the tunnel construction; when the bench length is 4 m, the deformation control effect of the tunnel is the best; by improving the length and angle of the anchor rod, the deformation of the tunnel can also be well controlled. [ABSTRACT FROM AUTHOR]

The chipping and grinding processes are the basis for understanding rock cutting during TBM excavation in hard rock conditions. The chips are produced when tensile fractures created by adjacent cutters propagate parallel to the tunnel face and coalesce whereas grinding occurs when fractures do not fully propagate through the rock and only fines are generated. Chipping is the normal and efficient boring process using machines equipped with disc cutters. Consequently, almost all of the models proposed for predicting the TBM penetration have been developed by the assumption that chipping is the dominant mode. It is obvious that these models are unable to predict correctly TBM performance when grinding is dominated in the rock cutting process. In this study, the relationships between engineering geological properties of the rock units (Cretaceous limestones) and operating and performance parameters of a refurbished EPB-TBM, employed for completion of SEL6 (the Southern Extension of Line 6, Tehran Metro) tunnel, were evaluated and analyzed. In this project due to limitations in the load capacity of the employed cutters and insufficient initial penetration in rock, the excavation was mostly carried out on grinding-dominated mode. So, the suitable data for establishing a special database and consequent analyses were provided. The established database composed of actual operating and performance parameters of the machine (i.e., cutterhead penetration and RPM and cutter load) and also engineering geological properties (i.e., UCS, alpha angle, GSI, and RQD) of limestone strata. Finally, by statistical analyses performed on collected and screened data (41 data sets), some simple, site-specific, empirical equations were developed to predict the TBM performance on grinding mode. In addition, according to the multivariate regression method, a significant relationship between the cutterhead penetration (PRev), the cutter load (Fn), intact rock strength (UCS), and geological strength index (GSI) was established, statistically. The obtained results demonstrate that this engineering geological-based model could provide a new applicable equation for accurately predicting TBM performance for similar geological conditions where grinding is dominant. [ABSTRACT FROM AUTHOR]

Designing the cutterhead of mechanized rock excavation systems and prediction of machine performance for these units often requires reliable estimation of cutting forces. The magnitude and variability of cutting forces depend on the rock and cutter type, as well as geometrical parameters in cutting such as spacing between the cuts, and depth of penetration. In most application, various components of cutting forces are determined from the force penetration curve, which can be generated by available formulas and more accurately by full scale cutting test on linear cutting machine (LCM) which mimics field conditions. The trend of varying cutting forces as a function of rock properties, cutter size and shape, and cutting geometry has been studied in the past and is well understood. However, all the tests in the past on various cutters, and especially disc cutters, have been performed at discrete penetration depths and used to develop the best-fit trend line of force – penetration curve. The current study was performed to understand the effects of cutting forces on a continuously increasing or decreasing (variable) penetration since this mode of operation is very common in partial-face machines. A modified testing method was developed and used to perform the LCM tests under variable penetration mode and generate a continuously increasing and decreasing force-penetration plot. The main goal was to compare the force-penetration trends with traditional constant or discrete penetration tests in the same rock. Five rock blocks were successfully tested to generate data for comparing the two methods. Overall, the tests show that the past practice of using the increments of constant penetration test results to develop force-penetration trend was valid. However, in the limited testing, the measured forces using the variable penetration method were slightly lower than the forces measured in respective constant penetration tests. [ABSTRACT FROM AUTHOR]

• The elastic modulus which is more consistent with the stress–strain relationship is defined. • There are stress thresholds for dividing different stages of ultimate strain and dynamic strength. • The linear variation law of strain rate and fractal dimension is obtained. • The law of energy evolution shows that the threshold is the best position. To investigate the impact of unloading on the dynamic mechanical properties of surrounding rock in roadway excavation, sandstone specimens were subjected to loading and unloading at six levels ranging from 0% to 80% of the uniaxial compressive strength (UCS). Subsequently, an improved SHPB was employed to perform dynamic compression tests, aiming to analyze the mechanical characteristics of the sandstone at various degrees of loading and unloading damage, investigate fractal attributes, and examine the dynamic failure process. The study uncovered that the stress–strain curve exhibits three distinct stages. Correspondingly, a more suitable set of dynamic elastic modulus denoted as E d 1 , E d 2 , and E d 3 , were defined. Using 45% of the UCS as the threshold for unloading stress (75 MPa), both the dynamic elastic modulus and dynamic strength displayed an initial upward trend, succeeded by a subsequent decline. In contrast, the defined damage peak strain ratio η ε p and damage ultimate strain ratio η ε m demonstrated the opposite trend, amounting to 45.64% and 51.03% at the threshold. In addition, increasing unloading damage stress (UDS) decreases the sandstone strain rate( ε ̇ max and ε ̇ ave) gradually. Simultaneously, the fractal dimension of the cracks exhibited a linear increase, ranging from 1.159 to 1.481. Similarly, the rock exhibited the lowest proportion of reflected energy and absorbed energy at the threshold. This holds noteworthy implications for examining the dynamic load response behavior of surrounding rock in roadway unloading, ensuring the dependability of blasting unloading techniques. [ABSTRACT FROM AUTHOR]

Deep roadways within fractured rock mass present substantial challenges for mining and construction due to the risk of large deformations. This study focuses on investigating the formation and evolution of cracking zones in the surrounding rock, specifically analyzing the 790-m level segment within the Jinchuan Mine No. 2 Mining area. Our comprehensive approach integrates field investigations, laboratory tests, in-situ monitoring, and numerical simulations. The findings highlight the vulnerability of surface surrounding rock in the σ 3 direction, leading to spalling and failure after excavation, while shallow surrounding rock acts as a natural barrier against deeper crack propagation. Over time, the elliptical cracking zone, aligned with the σ 1 direction, expands, which progressively degrades the internal rock mass, ultimately resulting in support failure due to instability and increasing deformation pressures. To mitigate these challenges, strategies were proposed to minimize the initial cracking zone and halt its growth. The insights and guidance provided herein are invaluable for enhancing project safety and efficiency in designing and constructing roadways in similar geological settings. • Shallow rock, a barrier against deep cracks, shields excavation; surface rock in σ 3 is prone to spalling and failure. • Elliptical cracking zone along σ 1 grow, leading to rock degradation, support failure, and heightened deformation over time. • Controlling large deformation hinges on minimizing the initial cracking zone and halting its ongoing evolution. [ABSTRACT FROM AUTHOR]

Predicting the cutter consumption rate of mechanical excavators using the Cerchar abrasivity index (CAI) results is possible. However, changing operational parameters in the field may cause differences between the estimated cutter consumption rate based on CAI test results and the real cutter consumption rate observed in the field. One of the essential parameters in the field that affects the cutter consumption rate is the increase in the water content of the excavated rocks. In theory, an increase in the water content of rocks is expected to reduce their abrasivity. In addition, the prediction of cutter consumption rate is based solely on CAI test results conducted on dry surfaces. However, especially during the excavation of low-strength clay-rich rocks, the excavated rock material can adhere to the cutters and block them, leading to a higher cutter consumption rate than initially predicted. This study aims to analyze the rock-pin interaction in CAI tests conducted on wet surfaces of clay-rich rocks and investigate the effect of water saturation on CAI at different scratch lengths. CAI tests were conducted on dry and wet surfaces of 20 clay-rich rocks with low strength at 10, 30, 50, and 70 mm scratch lengths, respectively. The relationships between the CAI test results obtained on dry and saturated rock surfaces were statistically analyzed for each scratch length. A positive linear correlation was found between CAI values obtained on dry (CAI Dry) and saturated surfaces (CAI Sat) at each scratch length. The percentage variations in CAI values at each scratch length were analyzed in detail. It was found that 90 % of samples have higher CAI Dry than CAI Sat at 10 mm scratch length. However, it was observed that as the scratch length increased, the relationship between the CAI values obtained in dry and saturated conditions changed, and the proportion of samples with CAI Dry higher than CAI Sat decreased to 65 %, 55 %, and 25 % at scratch lengths of 30 mm, 50 mm, and 70 mm, respectively. In CAI tests conducted on saturated surfaces, it was observed that the excavated rock material adhered to the test pins, and as the scratch length increased, the increase in CAI Sat values was considered to be due to this situation. This study revealed the importance of considering the adhesion potential of rocks to cutters at water-saturated conditions for the accurate prediction of cutter consumption in the excavation of clay-rich rocks. • A problem encountered during the excavations in the field was investigated through the approximately 800 CAI tests. • The effect of rock materials adhering to the test pin on CAI was investigated. • Standard CAI test results may be inadequate for predicting cutter consumption in water-saturated clay-rich rocks. • To estimate an accurate cutter consumption rate, the adhesion potential of clay-rich rocks has to be considered. [ABSTRACT FROM AUTHOR]

In this study, an enhanced numerical manifold method (NMM) simulator is developed to determine the combined impacts of reinforcement range and grouting timing on the effectiveness of grouting reinforcement for preventing the large deformation geo-hazard during the tunnel excavation through a fault rupture zone. To accomplish this objective, the limitations of the current two-dimensional simulation method, which lacks connection to the actual tunnel excavation process, are overcome by proposing a novel softening path function. Using this novel softening path function, each simulation step is given a specific physical meaning, establishing a connection to the actual tunnel excavation process. This improvement enables more accurate and analyzable simulation outcomes. To verify the accuracy of the proposed softening path function, a series of simulations of tunnel excavation are conducted using this softening path function. These results align closely with the results obtained by other reliable numerical methods and previous studies. After validation, several sets of numerical cases are conducted, considering 6 different geo-stress conditions, 10 different grouting reinforcement ranges, and 7 different grouting timings. Based on these simulation results, a critical value for reinforcement range, which equals to the diameter of tunnel face, is identified, and it is founded that delaying the grouting timing can increase the slurry migration range up to 94.6%. Furthermore, delaying the grouting timing can also reduce the convergence displacement of reinforced surrounding rocks after excavation up to 18.2% within a geo-stress range of 10 to 20 MPa. • A novel softening path function with clear physical meanings for tunnel excavation simulations is proposed. • A critical value of the grouting reinforcement range is identified. • Influences of grouting timing on the slurry migration range are revealed. • Influences of grouting timing on the convergence displacement of tunnels are revealed. [ABSTRACT FROM AUTHOR]

• Rock fracturing in smooth blasting under initial stress is experimentally tested. • The fractal characteristics of rock fractures in smooth blasting are investigated. • Rock fracturing in smooth blasting under initial stress is 3D numerically modelled. • The initial stress is beneficial to the performance of smooth blasting. In practical smooth blasting, rock fracturing is influenced by in-situ stress, which frequently leads to underbreak/overbreak and excavation damaged zone and consequently poses a serious threat to the safety of the excavated structure. This study systematically investigates the behaviour and mechanism of rock cracking under coupled initial stress and smooth blast utilizing experimental, theoretical, and numerical methods. Model experiments involving three-hole smooth blasting on 200 × 200 × 50 mm granite specimens are conducted under various static stresses. The resulting fracturing patterns are identified using the digital image correlation technique. The model testing examines rock cracking behaviours, fractal characteristics of rock fractures, and underbreak/overbreak. Experimental results indicate that the extents of rock fracturing and the fractal dimension in both the excavation and reserved zones show an overall decreasing trend as the static stress increases. Meanwhile, the area and depth of underbreak/overbreak also present a decreasing trend, suggesting that static stress is beneficial to the performance of smooth blasting. Then, the variations of crack patterns in model tests are theoretically analysed based on the transmission and superimposition of detonation stress waves combined with the static stress distribution near perimeter holes. Theoretical results emphasize the crucial role of the second hole and indicate no evidence of the third hole influencing the generation of connection fractures. The initial stress induces hoop tensile stress concentration on borehole-wall and generates the smallest hoop compression stress distributed vertically to the excavation line, which controls the initiation and extension of fractures in smooth blasting along the connection line of boreholes. Subsequently, the fracture evolution under coupled initial stress and smooth blast is numerically modelled using LS-DYNA. The numerical results vividly illustrate the necessity of a small burden to efficiently fragment the rock within the excavation zone when smooth blasting is applied to rock excavation under high in-situ stress. [ABSTRACT FROM AUTHOR]

• This paper presents a new method for evaluating rock excavation efficiency. • The new method uses the excavated surface to obtain the optimal cutting geometry. • Volumes of underbreaks and overbreaks vs s-p ratio yield a unique turning point. • The turning point represents the optimal cutting geometry of the tested geology. Efficient rock excavation requires an optimal s-p ratio, where adjacent cuts interact to completely plane the rock surface. Traditionally, the specific energy (SE) methodology has been used to determine this ratio, but its U-shaped curve is sometimes incomplete or absent, making it difficult to find the optimal s-p ratio. In this research, an alternative methodology is proposed. This methodology relies on the morphology of the excavated rock surface, specifically, the amounts of overbreaks. To evaluate the proposed methodology, full-scale rock cutting experiments were conducted in the laboratory. Numerical simulations were also conducted in 3D particle flow code (PFC3D) and the results were validated by the full-scale laboratory rock cutting experimental data. The study also involved conducting sensitivity analyses to evaluate the effectiveness of downscaling and upscaling the PFC3D-based model to obtain results for full-scale applications. Following the excavation, laser profilometry was employed to characterize the excavated rock surfaces, and the resulting profiles were used to calculate the volume of overbreaks. Plots of these volumes against the s-p ratio were generated to identify the optimal s-p ratio. The findings indicated the presence of a noticeable optimal s-p ratio in both the PFC3D-based simulations and full-scale laboratory experiments. This optimal s-p ratio was not evident when the specific energy methodology was employed. This proposed methodology exhibits considerable potential and can prove beneficial in determining cutting geometry that is optimal for site-specific and TBM-specific, as well as in the design and production of tunnel boring machines. [ABSTRACT FROM AUTHOR]

The energy-absorbing bolt is designed to counteract both burst-prone and squeezing rock conditions in rock excavation. In this paper, the mechanical contribution of energy-absorbing bolts installed systematically around tunnels excavated in rock masses was investigated based on the assumptions of the reinforced rock unit (RRU) model. The time-dependent behavior of rock with a large deformation problem was simulated using a Nishihara model, while that of an energy-absorbing bolt was simplified by a rheological model connecting a Hookean elastic element and a Saint-Venant element in series. The mechanical model of RRU anchored by an energy-absorbing bolt was established by connecting the rock mass model and the energy-absorbing bolt model in a parallel combination. The closed-form solution presented the time-dependent behavior of RRU under an instantaneously applied tension stress and an linearly increasing load, respectively. The results showed the reinforcement could reduce the time-dependent deformation of the reinforced region of tunnels significantly. A preliminary estimate of NPR-bolt reinforcement for large-deformation controls in the Muzhailing tunnel was carried out using the proposed model. The results showed that the NPR cables could have critical effects in controlling the large deformation of the tunnel. • Time-dependent behavior of the reinforcement rock unit (RRU) model was proposed. • Analytical investigations were carried out to understand the RRU model. • A case study was carried out to illustrate the application of proposed model. [ABSTRACT FROM AUTHOR]

Reasonable and accurate identification of the failure mechanism of rock masses is of great significance for achieving a comprehensive and thorough understanding of the fracture process in underground caverns under high geostress. Sophisticated microseismic (MS) monitoring was established in underground cavern groups to capture the excavation-induced microfracture signals of intact rock masses and rock masses with weak interlayer zones (WIZs). To clarify the mechanical mechanisms controlling the surrounding rock failure modes, the moment tensor inversion method is adopted to fully reveal the fracture types and focal mechanism solutions (strike, dip and rake) of the rock masses. The spatial distribution of the MS events shows that the excavation-induced microfractures of intact rock masses are concentrated in the working face of underground intersecting chambers. The fracture mechanisms of intact rock masses are dominated by tensile fracturing. Compared with those in intact rock masses, the microfractures induced by excavation in rock masses with WIZ form a strip-like cluster area along the WIZ, and the proportion of shear fractures is significantly higher. The moment tensor method quantitatively reveals the failure mechanism of excavation-induced microseismicity of rock masses with complex geological conditions. • A microseismic monitoring system was established in the Baihetan Hydropower Station. • The microfractures of intact rocks and rock masses with weak interlayer zones were studied using moment tensors. • The fracture mechanisms of tensile, mixed and shear fracture were revealed by moment tensor inversion. [ABSTRACT FROM AUTHOR]

• A novel thermo-mechanical coupling scheme is developed based on the combined finite-discrete element method (FDEM) to capture the mechanical response of the rock to the isotropic/anisotropic thermal conduction process. • The mechanisms of the creation and development of the excavation damaged zone (EDZ) around the wellbore subject to excavation unloading, drilling mud support, and thermal change in the surrounding rock are analyzed. • The influences of the thermal-mechanical properties of the rock mass, such as the isotropic/anisotropic rock strength and thermal conductivity, thermal expansivity, as well as drilling mud-rock thermal interaction process, on the EDZ development, are discussed. Insights into fracture development and excavation damaged zone (EDZ) formation around wellbore excavation sites are essential for understanding wellbore stability. In this study, a novel thermo-mechanical scheme is implemented in the 2D combined finite-discrete element method (FDEM) to investigate the fracturing process around a wellbore in a high-temperature subsurface environment. The developed scheme captures the isotropic/anisotropic thermal conduction characteristics within the rock formation. The coupled scheme and stress distributions around the wellbore subject to in-situ stress, drilling mud pressure, and temperature changes are validated by closed-form solutions. Fracture development and progressive EDZ formation by various mechanisms are analyzed. The results show that for an unsupported wellbore, fractures initiate in the region of the most severe stress concentration due to excavation unloading and constitute an EDZ that resembles the logarithmic-spiral rupture zone captured by experimental observations and Mohr-Coulomb models, resulting in potential water inrush due to the significantly increased fracture transmissivity of the EDZ and wellbore collapse. The shape and extent of the EDZ for wellbore excavation in the layered rock mass are dominated by the stress redistribution and the presence of low-strength bedding planes favorably oriented for bedding slippage. The excavation unloading-induced stresses can be effectively counteracted by properly applying drilling mud pressure, and the disturbance of the stress field occurs only within a small region, greatly reducing the extension of the EDZ. Thermal contraction of the surrounding rock due to its convective interaction with the cold drilling mud induces extra fractures around the wellbore. The extension of the EDZ is greater with increased thermal expansivity of the rock formation. Overall, the results indicate that the adopted thermo-mechanical FDEM simulation can provide unique geomechanical insight into wellbore stability behavior analysis, in which explicit consideration of the fracturing and fragmentation processes is of great significance. [ABSTRACT FROM AUTHOR]