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It is generally acknowledged that the stability evaluation of surrounding rock denotes nonlinear complex system engineering. In order to accurately and quantitatively assess the safety states of surrounding rock and provide a scientific basis for the prevention and control of surrounding rock stability, the analysis method of the synergetic theory of information entropy using the failure approach index has been proposed. By means of deriving the general relationship between the total two-dimensional plastic shear strain and the total three-dimensional plastic shear strain and obtaining the numerical limit analysis step of the plastic shear strain, the threshold value of the ultimate plastic shear strain can be determined, which has provided the key criterion for the calculation of the information entropy based on the failure approach index. In addition, combining with the synergetic theory of the principle of maximum information entropy, the evolution equation of the excavation step and information entropy based on the failure approach index of the surrounding rock system in underground mining space are established, and the equations of the general solution and particular solution as well as the expression of the destabilizing excavation step are given. To account for this, the method is applied to analyze the failure states of the floor surrounding rock after the mining of the 71 coal seam in Xutuan Coal Mine and involve the disturbance effect and stability control method of the underlying 72 coal seam roof from the macroscopic and microscopic aspects. Consequently, the validity of the analysis method of synergetic theory of information entropy based on the failure approach index has been verified, which presents an updated approach for the stability evaluation of surrounding rock systems that is of satisfactory capability and value in engineering applications.

It is extremely difficult to accurately predict the rock damage evolution during the underground space development or the deep excavation activity. In this paper, based on the statistical damage mechanics, a plastic strain-induced damage model of porous rock was established to describe the damage evolution and the constitutive behavior of porous rock under different stress paths. In the proposed model, the modified porosity was introduced which considered the effect of the generalized plastic shear strain. Besides, the proposed damage evolution function was also controlled by the generalized plastic shear strain. To validate the proposed damage model, the sandstone is selected as the experimental specimen due to it is a typical porous rock, and a series of conventional tri-axial compressive experiments (CTC) and confining pressure unloading experiments under constant deviatoric stress (UCP-CDS) were carried out. Furthermore, the confining pressure unloading experimental data under increscent deviatoric stress (UCP-IDS) was referenced to further validate the applicability of the proposed model. The results showed that the deviatoric strain-damage curves were an “S” shape, moreover, the relationship between the damage variable with the unloading ratio was exponential function. The proposed damage model could better reflect the void volume change and the radial dilation during the unloading process. Moreover, the model could successfully capture the damage evolution law and the mechanical behavior of sandstone by matching a set of tri-axial compressive experiments under different stress paths. Finally, it is found that the strength, strain-hardening and strain-softening characteristics were controlled by the Weibull distributed parameters m0 and F0.HighlightsThe modified porosity was introduced which considered the effect of the generalized plastic shear strain.The relationship between the damage variable with the unloading ratio was exponential function.The proposed model could reflect the void volume and the radial dilation.The proposed model could reflect the stress-strain behavior under different stress paths.

In a high‐pressure injection fault activation experiment conducted at the Mont Terri underground research laboratory in Switzerland, the transmissivity of the Opalinus Clay fault significantly increased due to opening and shearing. The fluid injection, spanning a few hours, generated a 10 m radius fault activation patch. Subsequent pressure pulse tests conducted bi‐weekly for a year revealed the gradual return of fault transmissivity to its initial state. The study utilized fluid pressure decay analysis, optical fiber monitoring, continuous active source seismic measurements and borehole displacement sensors for measuring fault displacements. The fault zone exhibited a dilation of approximately 1.4 mm, associated with both normal and tangential movements during activation, resulting in a sudden transmissivity increase from 1 × 10−12 to 3.2 × 10−7 m2/s. Early post‐activation, transient compaction and the subsequent slow compaction were observed, transitioning to an extension regime. The pressure pulse tests demonstrated a rapid transmissivity drop by more than two orders of magnitude within the first 10 days, followed by a gradual and less pronounced decrease. Plastic shear and compaction dominated the transmissivity evolution until 70 days after injection ended, followed by a period where additional factors, such as clay mineral swelling, influenced the behavior. Extrapolation suggested a sealing process taking at least 50 years after the initial activation. Plain Language Summary A field‐scale fault activation experiment offers valuable insights into the elasto‐plastic processes governing the sealing of shale faults. The experiment reveals a rapid increase in the fault's transmissivity by approximately five orders of magnitude during activation. Subsequent observations show a gradual transmissivity decrease by about three orders of magnitude post‐activation, with slow long‐term plastic shear and compaction of the fault competing against secondary processes, notably clay mineral swelling. All conceptual models employed to interpret these field data converge on the estimation that the fault's return to its initial low transmissivity state would require a minimum of 50 years. Key Points High‐pressure injection fault activation experiment at the Mont Terri underground research laboratory Continuous transmissivity measurements record self‐sealing inside a clay‐rich fault zone Transmissivity undergoes a phase of domination by slow plastic compaction and shearing during the initial post‐activation period, with mineral swelling exerting its influence over the long term

Dilation behavior is of great importance for reasonable modeling of the stability of the host rock of the repository for high-level radioactive waste disposal. It is a suitable method for carrying out direct shear experiments to analyze the dilation behavior of rock with well understood physical meanings. Based on a series of direct shear experiments on granite samples from the Alxa candidate area under different normal stresses, the shear stress‒shear strain and shear stress‒normal strain relations have been studied in detail. Five typical stages have been divided associated with the fracturing process and deformation behaviors of the granite samples during the experimental process, and the method to determine the typical stress thresholds has been proposed. It has also been found that the increasing normal stress may reduce the maximum dilation angle, and when the normal stress is relatively lower, the negative dilation angle may occur during the post-peak stage. According to the data collected from the direct shear tests, an empirical model of the mobilized dilation angle dependent on normal stress and plastic shear strain is proposed. This mobilized dilation angle has clear physical meanings and can be used in plastic constitutive models of the host rock of the repository, and this analysis can also be put forward to other types of geomechanical problems, including the deformation behaviors related to landslide, earthquake, and so on.

A series of nonlinear characteristics, such as strain softening, elastic–plastic coupling, and plastic deformation failure, have focused attention on postpeak deep rock masses. However, one-dimensional models, such as the Mohr–Coulomb (M–C) and strain-softening (SS) models, do not consider the influence of confining pressure, which can increase inaccuracies when used for simulating rock deformation in deep underground engineering. In this study, the composite effect of plastic damage accumulation on the mechanical properties of surrounding rock under different confining pressures was investigated through triaxial cyclic loading–unloading tests. A two-dimensional mechanical model for the cohesion, friction angle, elastic modulus, and dilatancy angle was developed by considering the combined effect of the confining pressure and plastic shear strain. The results showed that the cohesion and friction angle could be determined using an exponential function and a line function, respectively, with plastic deformation and confining pressure. The correlation coefficient of the two-dimensional function reached 95%. The elastic modulus increased with the confining pressure as a negative exponential function but decreased with the plastic deformation as a linear function. The dilation angle linearly decreased with the confining pressure and first increased and then decreased with the plastic deformation, which could be determined by the difference between two exponential functions. Through the use of FLAC3D software, the strain-softening model developed in this study was used to simulate a triaxial rock compression test, and the result demonstrated that the modified model was accurate and reasonable.

In this study, three typical limestones, including Tavel limestone, Indiana limestone, and Lixhe chalk, were selected from a large number of porous limestones. These limestones with different porosities have been largely studied in previous experimental investigations because of the complexity of mechanical behavior. According to previous experimental studies, porous limestones present two basic plastic mechanisms: plastic shear as a response at low confining pressures and plastic pore collapse at high confining pressures. In related to the plastic mechanisms, two types of plastic volumetric deformation are revealed: plastic compaction induced by pore collapse, and plastic dilatancy by plastic shearing. In this paper, a micromechanics-based plastic model is extended to describe the elastoplastic behavior of porous limestones. The plastic criterion of porous rock is explicitly dependent on the porosity in addition to being directly based on the relevant mechanical properties of solid matrix at the microscopic scale. An additional plastic hardening law for the solid matrix is proposed, in which two plastic deformation mechanisms are considered in hardening law of the solid matrix, including hardening effect caused by the local equivalent plastic deformation and weakening effect caused by the increase in porosity. Three typical porous limestones with different porosity are selected to validate the proposed model on both hydrostatic and triaxial compression tests. By comparing numerical predictions and experimental data, it is shown that the presented model can correctly describe the mechanical behavior of porous rocks.HighlightsLimestones with different porosities have been largely studied in previous experimental investigations because of the complexity of mechanical behavior.In related to the plastic mechanisms, two types of plastic volumetric deformation are revealed: plastic compaction induced by pore collapse, and plastic dilatancy by plastic shearing.A micromechanics-based plastic model is extended to describe the elastoplastic behavior of porous limestones.The plastic criterion of porous rock is explicitly dependent on the porosity in addition to being directly based on the relevant mechanical properties of solid matrix at the microscopic scale.

Based on the geometric analysis of the relationship between the stress state at a point and the yield surface defined in the principal stress space, a coefficient ω is set up as an estimation index to describe the stress-induced yield risk. After yield, the equivalent plastic shear strains is usually used to characterize the failure degree (FD) of the material and adopted here as an index of the damage degree for the surrounding rock masses. Then, a unified variable combining ω and FD, named failure approaching index (FAI), is constructed to estimate the stability of rock mass which may be at different deformation stages. The formulas of FAI are derived for some popular yield criteria in geomechanics. Details for such development are addressed in the paper. Its rationality is verified by numerical simulation and comparative analysis of the conventional triaxial compression tests and typical tunnel projects. In addition, the method for applying FAI to the stability estimation of surrounding rock mass is proposed. As examples, the stability of the underground powerhouse, access tunnels and headrace tunnels at the Jinping II hydropower station are estimated by making use of the method we presented. The results indicate that not only is the index rational in mechanics, but the theory also has good expansibility, and the estimation methods are simple and practical as well. It is easier for field engineers to analyze and understand the numerical results.

— The formation of misoriented substructures in plastically deformable metal materials is theoretically studied. Expressions are obtained for the intensity of the accumulation of low-angle and high-angle misorientation boundaries. Within the framework of a mathematical model of shear plastic deformation and hardening, numerical calculations of the dependences of the average characteristics of an imperfect medium on the degree of deformation under conditions of uniaxial compression with a constant strain rate at room temperature are performed. It is shown that the intensity of the generation of low-angle tilt walls depends significantly on the scenario of changes in the density of jogs on the screw segments of dislocation loops emitted by dislocation sources. The main mechanism for the formation of low-angle walls is the rearrangement of clusters of edge segments of dislocation loops into tilt dislocation walls under the influence of flows of interstitial atoms generated by moving screw segments. It is assumed that low-angle walls merge into one until the total misorientation angle of the merged walls reaches a critical value of about 10°, after which the distance between dislocations in the wall decreases to the corresponding critical value and the further penetration of individual dislocations into the wall becomes impossible. The expression for the intensity of the formation of high-angle boundaries is obtained as a consequence of continuation of the work of dislocation sources and the formation of clusters of low-angle walls, the total energy of which is higher than the energy of the equilibrium high-angle boundary at the same misorientation.

Using a gradient-type model, the authors solve a boundary-value problem on stress redistribution in rock mass during mining. The elastoplastic model takes into account local discontinuity. The condition of smoothness of the displacement field is essentially weakened—instead of one smooth field of displacements, two-dimensional kinematics is described using two independent smooth fields. As a consequence, the model receives a structural parameter including the dimension of length and characterizing local bends of unit volumes. The article gives examples of plastic strain calculations in adjacent rock mass with identification of the increased stress concentration zones. It is shown that inclusion of local bends leads, on the hand, to the reduction of plastic shears in adjacent rock mass and, on the other hand, to deeper expansion of high stress concentration zones in rock mass.

Metallic shear panel dampers (SPDs) have been widely adopted in seismic engineering. In this study, axial and torsional specimens of four types of metallic damping materials, including three conventional metallic steels as well as low yield strength steel 160 (LYS160), were tested in order to investigate the material response under repeated large plastic strain and low cycle fatigue between 10 and 30 cycles. The present study demonstrated that both the deformation capacity and fatigue performance of LYS160 were underestimated by the conversion from the traditional uniaxial tensile test. The main difference in the failure mechanism between LYS160 and the three conventional materials was determined from the scanning electron microscopy data. The dominant failure mode in LYS160 is stable interlaminate slip and not bucking. Our results provide physical insights into the origin of the large deformation capacity, which is an important foundation for the lightweight design of SPDs.