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Water inrush represents a significant hazard in karst tunnels, and the water-resistant rock mass plays a pivotal role in water inrush incidents. It is of great significance to explore the coupled hydro-mechanical failure process of water-resistant rock mass under excavation. A series of triaxial compression tests with different pressure conditions and loading rates were conducted in this study to investigate progressive failure characteristics and permeability evolution of limestone specimens. The experimental findings demonstrate that loading rate and confining pressure significantly influence progressive failure characteristics and permeability evolution. Additionally, a decreasing-then-increasing trend is observed in the crack initiation threshold (σci), and the crack damage threshold (σcd) exhibits a decreasing tendency. The resulting σci/σp ratio increases while the σcd/σp ratio decreases, indicating a diminishing stable fracture development stage. In general, permeability experiences an increase within an order of magnitude during the progressive failure of rock specimens, and the permeability of the rock mass decreases with rising initial confining pressure. The rate of deformation induced over the duration of permeability measurement exhibits a positive correlation with the stress ratio. Lastly, we propose a potential hysteresis water inrush mechanism, drawing upon the aforementioned observations. It is deduced that the creation, expansion, and ultimate penetration of fractures within the rock mass, resulting from the combined effects of excavation stress disturbance and changed pore pressure, give rise to water inrush events in karst tunnels.HighlightsStudy of progressive failure characteristics and permeability evolution of limestone.Loading rate and confining pressure significantly influence progressive failure characteristics.Permeability experiences an increase within an order of magnitude.Deformation induced by permeability measurement exhibits a positive correlation with the stress ratio.Investigation of water inrush mechanism linking construction disturbance and pore pressure.

Along with the advance of the working face, coal experiences different loading stages. Laboratory tests and numerical simulations of fracture and damage evolution aim to better understand the structural stability of coal layers. Three-dimensional lab tests are performed and coal samples are reconstructed using X-ray computer tomography (CT) technique to get detailed information about damage and deformation state. Three-dimensional discrete element method (DEM)-based numerical models are generated. All models are calibrated against the results obtained from uniaxial compressive strength (UCS) tests and triaxial compression (TRX) tests performed in the laboratory. A new approach to simulate triaxial compression tests is established in this work with significant improved handling of the confinement to get realistic simulation results. Triaxial tests are simulated in 3D with the particle-based code PFC3D using a newly developed flexible wall (FW) approach. This new numerical simulation approach is validated by comparison with laboratory tests on coal samples. This approach involves an updating of the applied force on each wall element based on the flexible nature of a rubber sleeve. With the new FW approach, the influence of the composition (matrix and inclusions) of the samples on the peak strength is verified. Force chain development and crack distributions are also affected by the spatial distribution of inclusions inside the sample. Fractures propagate through the samples easily at low confining pressures. On the contrary, at high confining pressure, only a few main fractures are generated with orientation towards the side surfaces. The evolution of the internal fracture network is investigated. The development of microcracks is quantified by considering loading, confinement, and structural character of the rock samples. The majority of fractures are initiated at the boundary between matrix and inclusions, and propagate along their boundaries. The internal structure, especially the distribution of inclusions has significant influence on strength, deformation, and damage pattern.

The strain rate-dependent mechanical behavior of shale is characterized using triaxial compression tests under a constant confining pressure of 50 MPa and axial strain rates \\[\\dot{\\varepsilon }_{1}\\] ranging from 5 × 10−6 s−1 to 1 × 10−3 s−1. This study is conducted on the Longmaxi shale from Dayou in China, which is predominantly composed of brittle minerals including quartz (55%), albite (15%) and cristobalite (3%). The experimental results show that higher axial loading strain rates \\[\\dot{\\varepsilon }_{1}\\] lead to higher elastic modulus and higher peak shear strength, both following exponential relationships with \\[\\dot{\\varepsilon }_{1}\\]. When \\[\\dot{\\varepsilon }_{1} \\le 1 \\times 10^{ - 5} {\\text{s}}^{ - 1}\\], failure results in a single linear fracture, whereas a more complex multiple crisscrossing fracture network is formed when \\[\\dot{\\varepsilon }_{1} \\ge 1 \\times 10^{ - 4} {\\text{s}}^{ - 1}\\]. Failure in shale specimens can be described by a damage parameter \\[D\\], which is strongly affected by the axial strain \\[\\varepsilon_{{1{\\text{s}}}\\]. In addition, the strain rate \\[\\dot{\\varepsilon }_{1}\\] had different effects on \\[D\\], which also depends on axial strain \\[\\varepsilon_{{1{\\text{s}}}\\]. Energy accumulation and dissipation are also closely related to \\[\\dot{\\varepsilon }_{1}\\] with the total absorbed energy \\[U_{\\text{A}}\\], the recoverable elastic strain energy \\[U_{\\text{A}}^{\\text{e}}\\] and the dissipated energy \\[U_{\\text{A}}^{\\text{d}}\\] at the peak stress increasing with \\[\\dot{\\varepsilon }_{1}\\]. As for the total energy accumulation \\[U_{\\text{A}}\\], the recoverable elastic energy \\[U_{\\text{A}}^{\\text{e}}\\] decreases while the dissipated energy \\[U_{\\text{A}}^{\\text{d}}\\] increases with increasing strain rate.

Experimental results provide strong evidence that the deformation and strength of rocks are closely related to the damage suffered during loading. The classic constitutive models such as Mohr–Coulomb criterion and the Drucker–Prager criterion are usually unable to describe the non-linear deformation behavior of rocks, including strain hardening and softening. Their applications in numerical analysis and practical engineering are limited. For this purpose, an elastoplastic damage constitutive model that takes into account the competition mechanism between damage and strain hardening or softening during rock compression is proposed herein. This model is used to investigate the deformation behaviors and damage evolution of rocks. More importantly, this model has been implemented by finite element programming code and verified by a series of triaxial compression tests. The comparison results between theoretical analysis and experimental data indicate that this model can well describe the stress–strain curves and damage–strain curves of the investigated rocks (sandstone and salt rock), especially the characteristics of softening, hardening and residual strength. Based on parametric analysis, the influence of confining pressure, scale parameter and shape parameter on rock damage is revealed. It is found that the rock scale parameter in this model has a power function relationship with confining pressure. The ultimate plastic deformation that rocks can withstand is related to the scale parameter. The shape parameter controls the residual strength and deformation of rocks. Model results demonstrate that the strength and deformation vary with rock properties, and are strongly dependent on the stress-induced damage and strain characteristics.

The purpose of the present study is to investigate the initial irreversible critical damage state of red mudstone during the elastic stage of the stress–stain curve. First, a series of uniaxial and triaxial compression tests were carried out using a digital image correlation (DIC) based high spatial and temporal resolution 3D visualization test system of multi-field coupling damage of rocks. The time-dependent strain on the surface of rock samples during the test process was captured by DIC technology. The initial irreversible critical damage state of red mudstone was then investigated by analyzing damage process in macroscopic and mesoscopic scales in an integrated manner through developing theoretical models based on critical deceleration theory. It demonstrates that the developed models can recognize the initial irreversible critical damage point of the rocks by analyzing the DIC-measured deformation behavior of the samples in both macroscopic and mesoscopic level.HighlightsStudied the variance increase characteristics of mesoscopic strain at the crack site of red mudstone during the elastic stageProposed the initial irreversible critical damage state of red mudstone under the influence of strength and confining pressureStudy results enrich the theory of rock mechanics and can be applied to tunnel design or slope stability investigation, etc.

The acoustic emission b value is an important and widely used parameter for the early prediction of rock fractures. In this study, five groups of true triaxial compression tests were conducted on granite specimens to analyze changes in b value during the process of rock failure, and to investigate the b value characteristics of acoustic emission events. First, the acoustic emission events that simultaneously triggered at least four sensors were located using P-wave arrivals and sensor coordinates. Then, considering various intervals of acoustic emission event counts, stress magnitude, and stress proportion, b values were calculated using the values of the maximum amplitude, average amplitude, maximum absolute energy, and average absolute energy of the acoustic emission events. In addition, the goodness of the fitting curves was used to evaluate the fitting reliability of the b values. The results indicated higher accuracy of b value when calculated using the average amplitude setting for intervals of acoustic emission event counts of 200 or greater, stress magnitude of 20 MPa or greater, and stress proportion of 10% or greater. Moreover, the interval of event counts of 200 is suggested as a window parameter for b value calculations, and the b values are observed to exhibit a decreasing trend before fracture for more than 80% of the specimens. Furthermore, the b value tends to decrease with an increase in confining pressure. Thus, the b value can be used as an indicator for validating the stress concentration area, including magnitudes and accumulative probability density distribution of events, which is a beneficial complement to clarifying precursor information of rock mass instability.

We developed a novel measurement system to directly visually observe the sliding behavior of rocks inside a triaxial test apparatus. The proposed visual measurement system consists of a steel housing that contains a commercially available digital camera. A series of triaxial compression tests was conducted to visually observe the progressive sliding behavior of precut samples and to measure the evolution of the strain and displacement fields near an artificial joint, using the digital image correlation method. The recorded video frames and their associated digital image correlation results allow a better understanding of the effect of confining pressure on the sliding mode of precut joint asperities. The implementation of the proposed measurement system is simple, safe, and inexpensive, and has the potential to become widely used as a new measurement system because it can be installed into existing triaxial test equipment without modification.HighlightsA new measurement system is proposed for observing the sliding behavior of rock-like materials from the inside of the pressure vessel of a triaxial test apparatus.The measurement system is simple, safe, and inexpensive, and consists of a steel housing containing a digital camera.We successfully observed the effect of confining pressure on the sliding mode of the precut joint asperity.

In order to evaluate the damage deterioration degree of freeze–thaw rock under in-situ stress in cold area engineering, it is necessary to establish the triaxial compression damage constitutive model and study on meso-failure characteristics of freeze–thaw rock. The triaxial compression tests under confining pressure of 3 MPa, 5 MPa, and 10 MPa were carried out on saturated sandstone after 0, 20, 40, and 60 freeze–thaw cycles. The results show that with the increase of freezing–thawing times, the peak deviatoric stress and elastic modulus under the same confining pressure decrease gradually, and increase gradually with the increase of confining pressure. The triaxial compression damage constitutive model of freeze–thaw rock was established based on the dissipation energy ratio and the principle of minimum energy dissipation. Based on this model, the evolution law of energy ratio in different stages of compression process was studied, and the influence characteristics of confining pressure and freeze–thaw on rock failure were analyzed. PFC2D was used to establish the numerical method on triaxial compression of frozen-thawed rock and determine the calculation parameters. The energy evolution law and the number growth law of different cracks in the triaxial compression process of frozen-thawed rock were studied. It is found that the energy value at the peak and the ratio of tensile crack to shear crack decrease with the number of freezing–thawing cycles. Meanwhile, the characteristics of freezing–thawing cycles promoting the shear failure of rock were clarified.

The Donghekou landslide triggered by the 2008 Wenchuan earthquake was a typical large-scale rapid and long runout landslide. The saturated deposit material in runout path was liquefiable under the dynamic loading, which may play a key role in the hypermobility of the Donghekou landslide. The dynamic loading triaxial compression tests were performed to investigate dynamic responses of the soil samples from the runout path. The influence of variation of pore water pressure of deposit in runout path on the movement process of the landslide was investigated and analyzed by discontinuous deformation analysis (DDA). The results show that the impact loading could cause a sharp rise of the pore water pressure of the soil, but it cannot trigger liquefaction before the sample failure. Seismic loading is the main loading that causes the liquefaction of deposit material, while landquake loading has little effect on soil liquefaction. During the earthquake, liquefaction of the shallow soil (the depth was < 10 m) of deposit in the runout path was induced by the seismic loading. Thus, the shallow runout-path material and landslide debris can slide faster and farther under a significantly small friction.

Soil-rock mixtures in fault fracture zones are composed of rock blocks with high strength and fault mud with low strength. In this paper, in order to study the mechanical properties of the soil-rock mixture with non-cohesive matrix, a large-scale laboratory triaxial compression test with a specimen size of 500 mm×1000 mm is conducted, combined with numerical simulation analyses based on the two-dimensional particle flow software PFC2D. The macroscopic mechanical response and mesoscopic fracture mechanism of soil-rock mixtures with varying rock block proportions, block orientation angles and matrix strengths are studied. The results indicate the following: (1) When the proportion is less than 30%, the shear characteristics of the mixture are similar to those of its non-cohesive matrix. When the proportion is in the range of 30-70%, the internal friction angle and cohesion increase rapidly, and the softening characteristics of the mixture become more apparent. When the proportion exceeds 70%, the aforementioned effect slows. (2) The strength of the mixture is positively correlated with its matrix strength, and the influence of the matrix strength on the loading curve of the mixture is related to the block proportion. (3) When the block orientation angle is 0°, the cohesion and internal friction angle are slightly greater than those at an angle of 90°. Based on the above, for the soil-rock mixture with non-cohesive matrix, a strength prediction model based on the block proportion is given when the block orientation angle and matrix strength are consistent.