The analytic solution for surrounding rock of roadway is of significance for stability analysis and roadway support. However, analytical solution for surrounding rock of roadway which took influences of water seepage, strain softening, dilatancy and intermediate principal stress all into account did not receive much reporting. To promote research in this aspect, a mechanical model simultaneously considering water seepage, strain softening, rock dilatancy, and intermediate principal stress was established based on porous elastoplastic mechanics, and then unified analytical solution for surrounding rock of roadway was obtained. Based on an example, influences of water seepage, strain softening, rock dilatancy, residual cohesion and intermediate principal stress on surrounding rock of roadway were thoroughly investigated using single factor analysis. The obtained results are as follows: radii of plastic zones and surface displacement of roadway would increase exponentially with water pressure increasing and their magnitudes are greater than corresponding values without water seepage considered; with softening modulus increasing, peak circumferential stress location would slightly shift to deeper surrounding rock, while broken zone radius and surface displacement of roadway would increase in a decay velocity; rock dilatancy has little effect on peak circumferential stress and plastic softening zone radius, while broken zone radius and surface displacement of roadway increase linearly with dilatancy coefficient α1 increasing indicating their magnitudes are overestimated if associated flow rule is adopted; with weighted coefficient increasing, stress components in plastic zones at the same distance from roadway center would increase, while radii of two plastic zones and surface displacement of roadway are reduced, i.e., self-bearing capacity of rock is enhanced considering intermediate principal stress effect compared to Mohr–Coulomb criterion; with residual cohesion increasing, peak circumferential stress remains unchanged, while stress components in plastic zones at the same distance from roadway center would increase and radii of two plastic zones decrease significantly. The above results implicated that water seepage effect should be carefully considered for roadway stability under groundwater environment; strain-softening and flow rule of rock should be reasonably analyzed and chosen to accurately predict surface displacement and broken zone radius of roadway; rock bolt length should be increased with softening modulus increasing, while it can be decreased with intermediate principal stress effect considered; grouting measure is an effective measure to improve roadway stability. In short, the research provides a theoretical basis and some practical engineering implication for roadway support.

Protective layer mining, as a dominating regional prevention measure, is generally adopted to prevent and control gas disasters in highly gassy or outburst mines of China. Interlayer distance is one of the most important factors that influences protection effect. However, how does interlayer distance affect the protection effect of steeply inclined upper protective layer mining is not understood fully. According to the engineering practice in Nantong mining district, a new method for similar material simulation experiment of steeply inclined upper protective layer mining is proposed, in which an orthogonal test of similar materials comprising of sand, cement (containing gypsum and fly ash), and water mixture is conducted to obtain relations between proportioning parameters and mechanical properties using a multiple regression method. And then the method is applied to study the protection effect of steeply inclined upper protective layer mining with varying interlayer distances. The results show the following. (1) The proportioning parameters of similar material have strong linear relations with its mechanical properties, and mechanical behaviors of such similar material denote that it can simulate most coal‐rock lithologies in coal mine. (2) Both pressure‐relief curves and swelling strain curves for protected layer present convex shapes; protection angles at lower excavation boundary are greater than those of upper excavation boundary; with the increase of interlayer distance, the pressure‐relief curve evolves from pattern “∩” to pattern “∧” and corresponding pressure‐relief region becomes narrower, the center of pressure‐relief region tends to transfer to the corresponding center of upper protective layer excavation region, the stress concentration coefficient decreases, the protection angles change little, and the length of the protection region reduces dramatically. (3) The protection region and protection angle calculated based on swelling strain of 3‰ are less than the empirical values based on the dip angle in Provisions, denoting that the method proposed in this study is safer than that in Provisions. The research results provide a useful guide for layouts of roadway and gas drainage boreholes to prevent gas disaster in Nantong coal mine district.

The mechanism of how hydraulic fracturing influences gas drainage in coal‐rock mass is still not clear due to its complex mechanism. In this work, statistical distributions are firstly introduced to describe heterogeneity of coal‐rock mass; a novel simultaneously coupled mathematical model, which can describe the fully coupled process including seepage‐damage coupling during hydraulic fracturing process and subsequent gas flow during gas drainage process, is established; its numerical implementation procedure is coded into a Matlab program to calculate the damage variables, and it partly uses COMSOL solver to obtain numerical solutions of governing equations with damage‐flow coupling; the mathematical model and its implementation are validated for initial damage pressure and mode of a single solid model without considering flow‐damage coupling, as well as fracture initiation pressure and influence of heterogeneity on damage evolution of hydraulic fracturing considering flow‐damage coupling; and finally, based on an engineering practice of hydraulic fracturing with two boreholes, the mechanism of how hydraulic fracturing influences gas drainage is investigated, numerical simulation results indicate that coal‐rock mass pore‐fissure structure has been improved, and there would exist a gas migration channel with characteristics of higher porosity and lower stresses, which demonstrates significant effects and mechanism of hydraulic fracturing on improving coal‐rock permeability and enhancing gas drainage. The research results provide a guide for operation of hydraulic fracturing and optimal layout of gas drainage boreholes.

Protective layer mining could not only significantly alter geostress but also increase the permeability of protected layer, which is beneficial for the coal gas extraction rate and ensures coal mining safety. However, due to unique geologic conditions, remote distance protective layer mining has its own characteristics. To investigate characteristic changes caused by remote distance protective layer mining, a suitable mathematical model that considered disturbance of unloading mining and solid-gas coupling effects during gas extraction was developed. The established mathematical model was implemented by combining FLAC^3D and COMSOL programs to study characteristic changes during remote distance protective layer mining of Chajiaotan coal mine. Numerical simulation results of unloading disturbance mining indicated that the protected layer would experience a process of stress loading, stress unloading, stress recovery, and stress stability as the working face of protective layer advanced; unloading disturbance has a greater influence on coal permeability than gas pressure; gas extraction measure should be further adopted to decrease gas pressure. Numerical comparisons of gas pressure distribution in the original protected layer and unloading protected layer revealed that gas extraction after unloading disturbance can reduce gas pressure more effectively, and appropriate borehole spacings in the fully pressure-relief and nonfully pressure-relief zones are 30m and 5m, respectively. The layout of field boreholes for gas extraction was designed according to numerical results. The results of site investigation showed that numerical simulation results of relative expansion deformation and gas extraction radii agree well with the results of site observation, demonstrating reliability of the mathematical model and its implementation. The proposed mathematical model is promising for assessing unloading disturbance and gas extraction in remote distance protective layer mining.