Stability of surrounding rock mass and safety of supporting system in mining tunnel are always dominated by fault near excavation limit. The influence degree of the fault varies significantly under different fault distances to mining tunnel and initial stresses condition. In this paper, a series of numerical experiments are conducted to study the displacement, stress, and plastic region in surrounding rock mass. The stress characteristics in sprayed concrete layer are analyzed to study the influence of the fault distance on excavation limit and the initial stress in ground. The fault distance is defined geometrically by a mutual perpendicular line between the excavation limit and fault line nearest to the mining tunnel to quantify the influence. In order to simulate fault specialty reasonably, an interface model based on the theory of cohesive joint element is proposed in the numerical experiment model. Some other disciplinary and quantificational regularities about the influence of the fault distance and tectonic stress are summarized based on the numerical results. The results show that there is a critical distance to judge whether a fault has a prominent influence to mining tunnel. The critical distance depends on the quality of mining tunnel surrounding rock mass, initial stress, and the degree of excavation limit curvature. With the increase of lateral stress coefficient, the vertical displacement at crown decreases, but the horizontal displacement increases in the both right and left sidewalls.

Joint surfaces are widely distributed in natural rock mass, and their shear mechanical properties play an important role in determining the safety and stability of rock mass. Previous studies rarely discussed the contribution degree of different joint protrusions to resisting shear stress. In this study, seven irregular dentate joint profiles were proposed to represent the geometric morphology of natural joints. Joint samples were subjected to direct shear tests under constant normal stress using the particle flow code (PFC). First, the reliability of the research scheme was verified by routine test results. Secondly, based on the microcrack tracking module and the force chain analysis, the local failure modes of the joint sample after and during the shearing process are discussed in detail. The relationship between the shear stress and the number of microcracks was studied. Finally, based on the measuring circle function, the variation law of the mean stress at different joint protrusions during the shearing process was tracked. The maximum stress of each protrusion before the shear stress peak was introduced to quantitatively describe the contribution of local protrusions to shear stress. There was an important link between the size of the protrusion and the stress it can withstand. During the shearing process, the local shearing mechanism of the joint surface is controlled by the distribution of joint protrusions. The research results of this paper can provide a good idea for the follow-up joint surface research.

The mechanical properties of joints are important factors affecting the safety and stability of rock mass. The joint roughness coefficient (JRC) is a parameter for describing the roughness morphology of the joint surface, and its accurate quantification is very important to predict the shear strength. In the current statistical parameter methods for the estimation of joint roughness, the size of different protrusions on the joint surface was completely ignored, which did not correspond to the real failure mechanism of rock joint during the shear process. In this study, a new statistical parameter WPA was proposed for the estimation of JRC considering the shear direction and the contributions of different protrusions. First, the 10 standard roughness joint profiles were digitized based on image processing technology, and the obtained coordinate data were proved to be reliable by the calculation results of existing parameters. Secondly, the WPA value of 10 standard roughness joint profiles was calculated at a 0.5 mm sampling interval in two directions. The functional relationship between WPA and JRC indicated that they should be established in the same shear direction to maintain a high correlation. The JRC values of 10 standard roughness joint profiles in direction 2 were obtained based on the functional relationship established between WPA and JRC in direction 1, and the roughness of these 10 joint profiles was confirmed to be influenced by direction. Next, the effect of sampling interval on WPA was investigated. As the sampling interval increases, the WPA values gradually decreased and the correlation between them and JRC gradually declined. In practical application, a smaller sampling interval was recommended for more accurate prediction. Finally, the geometric coordinate data of 21 joint profiles given in the literature and 4 natural joint surfaces were obtained by graphics processing technology and 3D scanning technology, respectively. The JRC values of them were separately estimated by WPA in different directions. The results showed that the new statistical parameter WPA proposed in this paper can well describe the joint roughness considering the shear direction and the contribution of different protrusions.

The joint roughness coefficient (JRC) is an important factor affecting the shear properties of rock joints, and its accurate estimation is a challenging task in rock engineering. Existing JRC evaluation approaches such as the empirical comparison method and the statistical parameter method have some unresolved defects. In this study, a new method is proposed for JRC estimation to overcome the deficiencies of existing approaches based on back calculation of shear strength. First, the 10 standard roughness joints are established in numerical rock samples generated by the bonded particle method (BPM). Secondly, the microscopic parameters of the intact rock and joints are calibrated, and a series of direct shear tests of joint samples are carried out under different normal stresses. Finally, the empirical relationships between shear strength and JRC are proposed under high correlation conditions. The results show that the modified smooth joint model (MSJM) is proved to better simulate the mechanical properties of rough joints than the smooth joint model (SJM). When the shear strength of target joint is substituted in the corresponding relationship, the JRC of joint along the shear direction can be conveniently obtained. In addition, the JRC values of 10 standard roughness joint profiles under shear direction of from right to left (FRTL) are obtained. By estimating the JRC of 9 target joints in the literature, it can be seen that the new method proposed in this paper can well reflect the directionality of roughness and it is convenient to apply.

Microwave-assisted rock breaking is a new and promising technology for the tunneling and drilling industry. Minerals in rocks have an important influence on the effect of microwave-assisted rock breaking. In this paper, common minerals in rocks such as potash feldspar (hereafter referred to as K-feldspar), calcite and pyroxene were selected as samples, and a lot of microwave irradiation tests were carried out by using a hamilab-v1500 microwave oven. The mass, strength and microstructure of the rock samples were tested before and after microwave irradiation. The change law of the mineral mass, strength and microstructure with regard to temperature was analyzed, and the influence mechanism was discussed. The results show that the strength of K-feldspar increases from 20 °C to 400 °C but decreases significantly when it is higher than 400 °C; the strength of pyroxene increases from 20 °C to 600 °C but decreases when it is higher than 600 °C; the strength of calcite decreases with the increase in temperature. As for the weakening pattern, pyroxene shows drawstring, step and flow with the increase in temperature, but K-feldspar and calcite show that failure occurs along the cleavage plane of the crystal structure. The higher the temperature of microwave irradiation, the finer the pattern at the fractured zone is, and the more fragmented it becomes; the loss of mineral mass increases with the increase in the temperature of microwave irradiation.

Rock burst is a common hazard during tunnel excavation in high-stress and hard rock strata. Microwave-assisted breaking has a great potential application in hard rock tunnel excavation, reducing the possibility of rock burst, and how to reasonably make the application on the TBM cutterhead is one of the critical issues. The waveguide aperture and distance between the rock face and waveguide have serious effects on its performance. In this paper, based on the arrangement of the microwave waveguide of the TBM cutterhead and the actual excavation situation, considering the reflection of microwave energy by the metal cutterhead and the scattering state of electromagnetic waves at the rock surface irradiation, a 2D model of rock irradiated by microwaves is established. The effects of waveguide aperture and distance on microwave irradiation performance of rock are studied, considering three different waveguide types: convergent waveguide, rectangular waveguide, and horn waveguide. The results show that the maximum temperature is located on the rock irradiation surface, rather than inside the rock. The rock temperature decreases in a cosine pattern with irradiation distance, rather than in linearity, which is consistent with the characteristics of electromagnetic wave propagation. The interval of irradiation distance where the rock temperature local maximum value appears is 1/4 of the electromagnetic wavelength, corresponding to the crest and trough of the electromagnetic wave. The rock temperature at the wave trough distance is lower than that of the wave crest distance, but the high-temperature zone is wider. In the range of 50~200 mm waveguide apertures, the rock temperature and damage decrease with the increase in waveguide aperture when irradiated at the crest distance, while the valley distance is opposite. A convergent waveguide and short irradiation distance enhance the energy focusing performance, so the temperature rise characteristics and rock damage are more concentrated. A large-waveguide-aperture horn waveguide and long irradiation distance form a wide range of high-temperature zones and rock damages.