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Frost crack evolution induced by cyclic freeze–thaw is responsible for rock deterioration in cold regions and poses major threats to public safety, engineering structures, and alpine slope stability. This paper presents experimental and numerical works aimed at investigating the frost crack evolution in fissured rock masses, as well as the interaction between frost cracks. A series of laboratory freezing experiments are conducted on rock-like specimens with various pre-existing fissures. Experimental results show that frost cracks initiate at the pre-existing fissure tips and propagate under the freeze–thaw treatment. Moreover, the frost crack evolution is significantly influenced by external stress conditions and frost crack interactions, forming several typical propagation patterns (e.g., deflection, coplanar and butterfly shape, etc.). Then, numerical simulations with a low-temperature thermal–mechanical coupled model, where the water/ice phase transition and hence volume expansion are explicitly simulated, are conducted to reproduce the experimental observation. The numerical results are consistent with the experimental observations and help to reveal the underlying mechanisms of the frost crack growth and frost crack interaction. This experimental and numerical investigation helps to improve the understanding of frost cracking mechanisms that can inform engineering design in cold regions with fissured rock masses. Highlights: Frost crack propagation and interaction under freeze-thaw cycles are investigated. Frost heaving pressure induced by ice expansion is large enough to drive crack propagation. Confining stresses alter the stress field around the crack tips, inducing crack orientation. Mutual interaction between adjacent frost cracks significantly affects the cracking pattern. Novel low-temperature thermal-mechanical coupling model is developed for frost cracking problems. [ABSTRACT FROM AUTHOR]

The morphological characteristics of rock joints have a significant impact on the peak shear strength of a rock mass. In this study, three-dimensional scanning tests have been conducted on 39 joint specimens. Based on the scanning data, the morphological parameter C′ has been developed to describe the joint roughness. With an explicit physical meaning, this index is negatively correlated with the joint roughness and can reflect the anisotropy of surface morphology. Using a newly developed derivation method, the morphological parameter C′ of each joint from the earlier studies has been determined. The scanning test results in this study have been used to confirm the feasibility of this new derivation method. By applying the least square fitting method to the Grasselli’s (Shear strength of rock joints based on quantified surface description. Dissertation, Swiss Federal Institute of Technology, 2001) test results, a new peak shear strength model has been developed. Finally, a comparison of the peak shear strength of rock joint predicted by the new model and other classic models shows that the new model, characterized by simplicity, relatively higher precision, and a definite physical meaning, is feasible to predict the peak shear strength of rock joints. [ABSTRACT FROM AUTHOR]

The combined finite-discrete element method (FDEM) is an advanced finite and discrete element coupling method, which can commendably simulate the large deformation and fracture process of materials. The explicit solution method in FDEM is naturally suitable for solving dynamic problems, and the key to dynamic analysis is to establish artificial boundary conditions that can be prepared to simulate infinite domain motion. At present, there is only the viscous boundary condition in the original FDEM, but this is only applicable for solving the internal source wave problems (e.g., blasting, mechanical vibration), and there are no artificial boundary conditions for solving the external source wave problems (e.g., earthquake). Therefore, three artificial boundary conditions have been developed in the present study to enhance the ability of FDEM to conduct dynamic response analysis. Firstly, the basic principle of FDEM and the existing viscous boundary condition are briefly introduced. Then, three newly added boundary conditions are introduced in sequence: (1) viscous-spring boundary condition, which can absorb stress wave energy at the boundary and restore the residual displacement to meet the actual engineering; (2) free-field boundary condition, which is coupled with free-field motion to absorb scattered waves at the lateral boundaries of the model, and the algorithms of mesh matching retrieval and coupling calculation are also introduced; (3) static-dynamic unified boundary condition, which can accurately transform the fixed boundary under the quasi-static analysis into the non-reflective boundary condition under the dynamic analysis. Finally, several classic models are built to verify the feasibility of the newly added boundary conditions. The comparison with the shaking table test also shows that the improved FDEM can be used for seismic response analysis under actual earthquakes. [ABSTRACT FROM AUTHOR]

Experiments for the rectangular rock-like samples (made of high-strength gypsum and water-cement ratio is 1) with two parallel pre-existing flaws subjected to uniaxial compression were carried out to further investigate the influence of varying flaw geometries on mechanical properties and crack coalescence behaviors. According to the tests results, eight crack types were characterized on a basis of the mechanisms of crack nucleation, formation and propagation, and seven coalescence modes occurred through the ligament, including S-mode, M1-mode, M2-mode, M3-mode, T1-mode, T2-mode and T3-mode. The AE and photographic monitoring techniques were adopted to further clarify the procedure of the crack coalescence and failure during uniaxial compression tests and in consequence the whole process of crack emergence, growth, coalescence and failure was recorded in real time. The results of AE technique revealed that the characteristics of acoustic emission energy associate with crack coalescence modes, and AE location method can emphasize the moments of crack occurrences and follow the crack growth until final failure. This study put forward better understanding of the fracture and failure mechanism of underground rock engineering, like rock burst. [ABSTRACT FROM AUTHOR]

In this study, a cohesive fracture model (CFM), which assumes rock material as assemblage of deformable grains joining together at their cohesive boundaries, is developed to simulate rock fracture and failure behavior in terms of mineral and cement properties. To better represent the structural bonds between the particles and the heterogeneity of rock material, a user-defined material model (UMAT) for the cohesive element with stochastic strengths is incorporated into the developed CFM. The capability of the developed model for rock fracture problems under both static and dynamic loads is first explored. Based on the developed model, the heterogeneity effect of rock on its dynamic failure process and dynamic strength is then studied. The results demonstrate that the developed CFM can be used to investigate the rock fracture problems under both static and dynamic loads. The heterogeneity of rock material not only affects the rock dynamic failure process but also leads to an increase of rock dynamic strength. [ABSTRACT FROM AUTHOR]

In this paper, to study the influence of grouting on meso-failure mechanism, acoustic emission (AE) localization technique was used to monitor the micro-cracking during the uniaxial compression tests on single flawed sandstone specimens with or without grouting. Due to the inhomogeneous geometrical structure of the flawed specimens, the mesoscopic damage evolution is also inhomogeneous during the loading process. As a result, the wave velocity of the specimens varied temporally and spatially, which greatly increased the difficulty in accurate AE localization. To solve this problem, the wave velocities versus the axial load for the studied sandstone were first measured, and the stress concentration effect near the flaw was also theoretically analyzed. Accordingly, the localization arithmetic was also improved to match the complex wave velocity field. Results show that compared to the specimens without grouting, more microcracks (particularly the shear microcracks) are induced in the grout-infilled specimens prior to the final failure of the specimens. Moreover, the effect of grouting on microcrack initiation also varies with the oblique angle of the flaw. For θ (flaw inclination) ≤30°, grouting significantly increases the shear microcrack at macrocrack propagation stage which states that the axial tensile cracking was limited by the supporting effect of the grout material and made the failure mode trend to shear mode. For θ = 45° and 60°, grouting improved the ratio of shear microcrack during the whole failure process, indicating that the direction of maximum shear stress is close to the flaw direction. Hence, failure develops in stable shear mode. For θ = 75° and 90°, the primary microcrack generated during the first macrocrack initiation changed to shear mode, but the grouting has little influence on the subsequent crack propagation. So that, the reinforcement effect is poor in this set of specimens. • The AE localization technique was improved by adopting a temporally and spatially varied wave velocity model. • Grouting decreases the stress concentration near the prefabricated flaw, which changes the mechanism of crack evolution. • The development of a macro crack in the flawed specimen follows a tensile-shear-alternating meso damage evolution mode. [ABSTRACT FROM AUTHOR]