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Cement grout is widely used in civil engineering and mining engineering. The shear behaviour of the cement grout plays an important role in determining the stability of the systems. To better understand the shear behaviour of the cement grout, numerical direct shear tests were conducted. Cylindrical cement grout samples with two different strengths were created and simulated. The numerical results were compared and validated with experimental results. It was found that, in the direct shear process, although the applied normal stress was constant, the normal stress on the contacted shear failure plane was variable. Before the shear strength point, the normal stress increased slightly. Then, it decreased gradually. Moreover, there was a nonuniform distribution of the normal stress on the contacted shear failure plane. This nonuniform distribution was more apparent when the shear displacement reached the shear strength point. Additionally, there was a shear stress distribution on the contacted shear failure plane. However, at the beginning of the direct shear test, the relative difference of the shear stresses was quite small. In this stage, the shear stress distribution can be assumed uniform on the contacted shear failure plane. However, once the shear displacement increased to around the shear strength point, the relative difference of the shear stresses was obvious. In this stage, there was an apparent nonuniform shear stress distribution on the contacted shear failure plane.

As the mining environment gradually moves toward deep, the form and occurrence mode of gangue are more complex. The occurrence of non-breathable gangue seriously worsens gas migration conditions, which greatly reduces the gas extraction efficiency and increases the possibility of coal and gas outburst. Therefore, in order to explore the effect of multiple non-breathable gangue occurrence on gas migration in coal seams, the self-developed " the constant pressure coal and rock mass gas seepage test system " is used to systematically study the characteristics of coal seam gas migration under different layers of gangue and the differentiated occurrence of multiple gangue. The equivalent characterization of coal seam gas migration under the condition of multiple inclusions is proposed through theoretical analysis and numerical simulation. Research shows: the permeability factor of the whole coal sample, sample containing a single layer gangue and sample containing double layers gangue exhibits a “V"-shaped variation law with the increase of the gas adsorption equilibrium pressure. There is a critical gas pressure point when the permeability factor is the lowest, and with the increase in the number of gangue layers, the greater the pressure value of the critical gas pressure point. According to the characteristics of gas pressure distribution, the sample containing gangue can be roughly divided into four influence areas: gas expansion affected area, gas stable seepage area and upper and lower effective seepage area. The more the number of gangue layers and the larger the area of gangue layers, the upper and lower effective seepage area is smaller, the closer the positions of gangue are to the gas source, gas expansion affected area is smaller, both of them will cause more difficult in gas migration and the lower of the sample permeability factor. When different sizes of gangue are used at two positions of the gangue in the samples, with the increase of gas adsorption equilibrium pressure, the attenuation rate of permeability factor in the early stage of the sample is significantly higher than that of the sample with the same size of gangues in the front and at the back. The results of the paper have certain actual guiding significance for improving gas extraction efficiency and preventing mine gas disasters.

Rock masses with a distinct structure may present a transversely isotropic character, thus, the stress state in a transversely isotropic elastic half-plane surface is an important way to assess the behavior of the interaction between the distributed loading and the surroundings. Most previous theoretical analyses have considered a loading direction that is either vertical or horizontal, and the stress distribution that results from the effect of different loading directions remains unclear. In this paper, based on the transversely isotropic elastic half-plane surface theory, a stress solution that is applicable to distributed loading in any direction is proposed to further examine the loading effect. The consistency between the analytical solution and numerical simulations showed the effectiveness of the proposal that was introduced. Then, it was utilized to analyze the stress distribution rule by changing the Poisson’s ratio and Young’s modulus of the model. The effects of the formation dip angle on the stress state are also examined. The stress distribution, depending on the physical property parameters and relative angle, is predicted using an analytical solution, and the mechanisms associated with the transversely isotropic elastic half-plane surface subjected to the loading in any direction are clarified. Additionally, extensive analyses regarding this case study, with respect to the mechanical behavior associated with changes in the stress boundary, is available. Hence, the proposed analytical solution can more realistically account for the loading problem in many engineering practices.

Fully encapsulated rock bolts are widely used in keeping the stability of underground openings and slopes. However, bonding failure at the bolt/grout (B/G) interface still occurs. Thus, a theoretical solution was proposed to investigate the rock bolting performance. The coupling and decoupling behaviour of the B/G interface is simulated with a bonding-slipping formula. The feasibility of this bonding-slipping formula was confirmed with physical experiments on rock bolts. Then, the rock bolt loading procedures were analysed. The advantage of this theoretical model is that all input parameters can be directly acquired from the experimental pullout scenario. Three physical pullout experiments, including laboratory and field tests, were used to validate this theoretical modelling. After validation, the deduced theoretical model can calculate the rock bolting performance. Then, this model was adopted to perform the parameter analysis. The results reveal that the loading phases of the B/G interface are independent of the encapsulated length. With the increasing encapsulated length, the B/G interface encounters the elastic, elastic-debonding and debonding phases in order. Moreover, with the increasing grouting material modulus, the rock bolt shows more brittle performance.

Taking red sandstone specimen as the research object, this paper makes a rather systematic research on the formation and morphology of the main control fracture surface of the red sandstone under restrictive shear creep based on the basis of fractal theory by using three major self-designed research devices, that are, shear creep experimental device for soft coal or rock material, three-dimensional automatic measure and control experimental device for microstructure evolution based on micro-analysis, and three-dimensional mesoscopic monitoring software for dynamic evolution of micro-structure on the surface of coal-rock. The results show that restrictive shear creep of red sandstone experiences three stages, from the initial, steady to accelerated creep stage. The mechanism of creep action is the crystal slip under shear creep load and the slip due to dislocation. The formation of the main crack on the surface of the red sandstone under restrictive shear creep consists of four kinds of crack propagation modes: merging propagation, tensile propagation, intergranular propagation, bypassing crystal propagation. The changes of the profile along the shear direction are great in the distribution of the fracture surface, which shows a double crest trend in the second half, but the profile along the vertical shear direction declines relatively gently, slightly lower in the middle; in addition, the complexity of the roughness in the main fracture surface of the red sandstone specimen along the shear direction is greater than that in the vertical shear direction. A new method is proposed to calculate the fractal dimension of the fracture surface, and based on this method, it is found that the fractal characteristics of the main fracture surface of the red sandstone specimen show that the fractal dimension along the shear direction increases first and then decrease, and appears maximum value in the middle. And the changes of fractal dimension of the vertical shear profile are smaller than that in the shear direction, so does the fractal dimension.

The initiation and evolution of the cracks around the hole in coal specimens with a central hole under different uniform loads are studied by the rock failure process analysis software RFPA2D. The results show that homogeneity not only has an influence on the peak strength of coal specimens, but also on the crack propagation directly. The lower homogeneity, the more extensive the stress concentration areas, the rougher the surface crack, and the more irregular the crack propagation path. However, homogeneity has a certain impact on acoustic emission (AE) number and instantaneous effective coalescence rate of cracks, but the influence degree is significantly lower than that under nonuniform load gradient and original cracks. Nonuniform load can cause the obvious stress concentration on the higher load area of the coal specimens (right side), which mainly reflects the micro-crack initiation in the area, and the greater the nonuniform load gradient, the more significant is this trend. The peak stres...

The creep characteristics of the key unit rock in a slope potential slip surface play an important role in the evolution of slope slip. In this research, a systematic restrictive shear creep experimental study was conducted to investigate the restrictive shear creep characteristics of the key unit rock in a slope potential slip surface. A new plastic nonlinear model (PFY model) was developed to characterize the progressive-failure creep characteristics reflected in the process of the restrictive shear creep of rock. A variable-parameter restrictive shear creep model was established to reflect the progressive-failure characteristics and describe the whole process of restrictive shear creep of the rock and the two failure mechanisms of the rock, which can provide the reference base for the prediction of the time of shear creep failure of the rock. The constitutive equations characterizing the shear creep characteristics of the rock were established, and the methods to solve model parameters were determined. In addition, according to the obtained experimental data, the model parameters were solved, and the theoretical curves of the model were fitted with the experimental curves. The new model was validated, and the sensitivity of the model parameters to creep-failure time were analyzed. The results show that the overburden pressure and the maximum creep strain of the key unit rock in a slope play a leading role in the progressive failure and instability of the rock, and the shear creep rate with time fits the power function well. The theoretical curves of the new model match well with the experimental curves, and the differences in the creep-failure time between the model calculation values and the experimental values are small, which indicates the new model is appropriate in describing the creep characteristics of the rock. For the shear creep-failure time, the viscous coefficient η_2 is the most sensitive, the accelerating point strain γ_a comes next, and the progressive-failure coefficient β is the least sensitive.

The deformation and the damage characteristics of coal and rock under different restraint static load and different impact velocity were studied with the FEM software LS-DYNA. Based on this, the influence of impact load on the damage of coal rock under one-dimensional constraint condition was studied by using the self-developed constrained pendulum impact dynamic loading test device and ultrasonic testing device. The results show that the deformation of coal and rock increases with the increase of impact velocity and decreases firstly and then remains constant with the increase of constraint static load; when the constraint static load exceeds the compression strength, the displacement increases rapidly and the coal collapses at a strike; the damage quantity of coal and rock has a cumulative effect, and the damage quantity increases with the increase of impact number; when the constraint static load is identical, the increase of impulse contributes to microcrack propagation, and the damage quantity of coal and rock accelerates with the increase of single impact impulse; when the impulse is identical, the constraint static load restrains the microcrack propagation, and the damage quantity of coal and rock decelerates with the increase of constraint static load; the complete damage quantity range of coal and rock is 0.65∼0.75. In order to fully destroy the coal and rock, if the single horizontal impulse is greater, the number of shocks needed is less; if the constrained static load is greater, the more impact times are needed.

Taking the briquette sample as research object, the influence of the incremental impulse (momentum) on the damage of coal-rock under different uniaxial axial pressure was studied by using the self-developed pendulum impact dynamic loading test device, cooperating with the ultrasonic detection device. Meanwhile, the influence of constant impulse on the damage degree of coal-rock was compared. The results show that the damage degree of coal-rock increases with the increase of the impulse, and the damage fitting curve is upward concave, indicating that the coal sample tends to accelerating failure. Moreover, with the increase of axial pressure, the variation gradient of the damage degree of coal-rock tends to moderate and the cumulative damage degree decreases under the same impulse, and the impact resistance of coal-rock increases. When the impulse is constant, the damage degree of coal-rock increases with the number of impact, and the damage curve is upward convex, indicating that coal-rock has a tendency to slow down the damage. The cumulative damage degree of coal-rock decreases with the increase of axial pressure, and the number of impact needed to destroy coal-rock is increased. In addition, the damage model of coal-rock was proposed, and the criterion of coal-rock damage was obtained, which shows that the damage degree of coal-rock increases with the increase of impact load and decreases with the increase of static axial load.

Fully grouted rock bolts are widely used in civil engineering and mining engineering, playing a significant role in keeping the stability and safety of excavations. In this paper, the load transfer mechanism of fully grouted rock bolts was studied with an analytical model. A trilinear model was used to depict the bond-slip behaviour of the bolt/grout interface. The displacement of the confining medium was involved in this analytical model. Then, the shear stress propagation along the bolt/grout interface was analysed in the elastic, elastic-softening, elastic-softening-debonding, softening-debonding, and debonding stages. Experimental pull-out tests were used to validate this analytical model. There was a good correlation between experimental and analytical results. A parametric study was conducted to evaluate the influence of Young’s modulus of the confining medium, the shear strength of the bolt/grout interface, and the residual shear strength of the bolt/grout interface on the load transfer performance of rock bolts. The results show that increasing Young’s modulus of the confining medium was beneficial for improving the load transfer performance of rock bolts. However, once Young’s modulus of the confining medium was beyond a critical limit, it had marginal effect on the peak load of rock bolts. Furthermore, increasing the shear strength of the bolt/grout interface and the residual shear strength of the bolt/grout interface led to rising of the peak load of rock bolts. However, compared with the residual shear strength of the bolt/grout interface, increasing the shear strength of the bolt/grout interface had more apparent effect in improving the peak load of rock bolts.

In this paper, the digital image correlation was innovatively applied to study the deformation and damage process of raw coal and briquette under a complex stress environment. The results show that under symmetrical loading, briquette coal shows tensile failure and that the strain field goes through three stages. The raw coal shows shear failure; the stage characteristic of the strain field is not obvious. Under asymmetric loading, the strain field evolution of raw coal and briquette shows three characteristic stages, but the briquette is more likely to form a localization phenomenon. The displacement value of the crack in the shear direction is greater than that in the tension direction, so the raw coal and briquette mainly undergo shear failure. The localized starting stress is determined by the defined statistical index function, and the localized starting stress of the raw coal and the briquette coal has a quadratic function relationship with the asymmetric coefficient.

The deformation regional characteristics and fractal characteristics of crack propagation in raw coal and briquette coal under local loading were studied. The results show that the stress-strain curves of the two samples under local loading have typical stage characteristics, the deformation of the raw coal samples in the compaction stage is greater than that of the briquette coal samples, and the peak strength of the two samples increases linearly with the increase of the loading area. With the increase of loading time, the strain of the nonloading zone shows a slow growth stage and a strain stable stage. The farther away from the loading area, the smaller the strain and the strain rate are. Under local loading, the relation of the fractal dimension of the raw coal surface and strain can be described as a cubic function, while the relation can be expressed by a quadratic function for the briquette sample, and the relation of the fractal dimension and loading area of the raw coal and briquette both can be described as a quadratic function. Under uniaxial compression, the briquette coal mainly undergoes tensile failure, and the raw coal mainly undergoes x-type shear failure. Under local loading, the raw coal and briquette coal microcracks started propagation from the easy fracture zone, and the main control crack is located in the interface between the loading zone and the nonloading zone.

Taking raw coal and briquette coal samples with preset center holes as research objects, this paper makes a systematic analysis and research of crack evolution laws of the two different coal samples under the local load. The results show that the raw coal and briquette coal samples are different mainly in number, dimension, and complexity of the internal microstructures, so it is not right to replace raw coal with briquette coal when performing observational study of the crack evolution of microstructures; under the effect of local load, local property, randomness of crack initiation position, and crack initiation stress of raw coal samples are greater than those of briquette coal samples; law of instantaneous maximum effective cut-through rate of raw coal samples is more complex than that of briquette coals; under the effect of uniformly distributed load, end effect factor Fe, sample microstructure influencing factor Fs, and preset center hole factor Fh are the major factors influencing crack growth, among which the amplified end effect factor Fe and sample microstructure influencing factor Fs are dominant factors; under the effect of local load, local load influencing factor Fp, end effect factor Fe, sample microstructure influencing factor Fs, and preset center hole factor Fs are the major factors influencing crack growth, among which the local load influencing factor Fp, end effect factor Fe, and sample microstructure influencing factor Fs are dominant factors. Compared with briquette coal samples, raw coal samples are more sensitive to influencing factors, such as local load influencing factor Fp, end effect factor Fe, sample microstructure influencing factor Fs, and preset center hole factor Fh, and can aggravate the influence of these factors on the crack growth; the paper also puts forward a method for describing local load based on a coupling mechanical model of uniaxial compression and local shear.