Serious floor heave in gob-side entry retaining (GER) with fully-mechanized gangue backfilling mining affects the transportation and ventilation safety of the mine. A theoretical mechanical model for the floor of gob-backfilled GER was established. The effects of the mechanical properties of floor strata, the granular compaction of backfilling area (BFA), the vertical support of roadside support body (RSB), and the stress concentration of the solid coal on the floor heave of the gob-backfilled GER were studied. The results show that the floor heave increases with the increase of the coal seam buried depth, and decreases with the increase of the floor rock elastic modulus. The development depth of the plastic zone decreases with the increase of the c and φ value of the floor rock, and increases with the increase of the stress concentration factor of the solid coal. The development depth of the plastic zone in the test mine reached 2.68 m. The field test and monitoring results indicate that the comprehensive control scheme of adjusting backfilling pressure, deep grouting reinforcement, shallow opening stress relief slots, and surface pouring can effectively control the floor heave. The roof-floor displacement is reduced by 73.8% compared to that with the original support scheme. The roadway section meets the design and application requirements when the deformation stabilizes, demonstrating the rationality of the mechanical model. The research results overcome the technical bottleneck of floor heave control of fully-mechanized backfilling GER, providing a reliable basis for the design of a floor heave control scheme.

To counter the technical difficulties faced by gob-side entry retaining (GER) under multiple complex mining geological conditions in China, this paper introduces a GER method with fully mechanized gangue backfilling mining. A similar materials simulation experiment was conducted to simulate the gob-backfilled GER process by using the similar model test system containing an independently developed horizontal pushing load device. The experimental results show that the compaction speed of the backfilling area (BFA) can be improved, and the main roof subsidence can be reduced by increasing the horizontal pushing load and reducing the attenuation rate of the stress in BFA. The designed roadside backfill body (RBB) containing a flexible cushion is adaptive to the given deformation of the main roof, thus reducing the stress concentration of the RBB. The field test results show that when a 2 MPa horizontal pushing load is exerted in the BFA, arranging a 200 mm high-water-material flexible cushion can cause the BFA to swiftly change to the compaction stage. After stabilized deformation, the roadway section satisfies the design and application requirements. The feasibility and rationality of the GER with the fully mechanized gangue backfilling mining are proved, providing a safe, efficient, and environmentally friendly mining method without using a coal pillar.

Development of a safe and economical roadside support body (RSB) material is the key to successful backfilling gob‐side entry retaining (GER). By means of laboratory tests, this paper studied the effects of the water‐cement ratio, aggregate content, and age on the contractibility and resistance increasing speed, compressive strength, and postpeak carrying capacity of the concrete with gangues as an aggregate. It also discussed the rationality and adaptability of gangue concrete as a RSB material for backfilling GER. The experimental results show that the compressive strength of gangue concrete increases with age, and that the strength of gangue concrete demonstrates a nonlinear decreasing trend with the increase of the cementing material’s water‐cement ratio. The water‐cement ratio in the range of 0.46–0.60 has the most significant regulation effect on the strength of gangue concrete. Mixing with a certain amount of coal gangue enhances the postpeak carrying capacity of concrete, preventing the sample from impact failure. The field experimental results report that as a RSB material, gangue concrete can meet the design and application requirements of GER with gangue backfilling mining. A RSB material featuring high safety, high waste utilization rate, fast construction speed, and low costs is provided.

In longwall mining of coal mines, the large deformation of small pillar retaining roadways creates difficulties for the safe and efficient retreating of the mining panel. Based on the engineering background of a small coal pillar retaining roadway in Wangzhuang coal mine, pressure relief technology for non-penetrating directional pre-splitting blasting with a deep hole ahead was proposed. The influence of the non-penetrating fracture length on the pre-splitting effect was studied by numerical simulation. The results showed that the vertical stress in the coal pillar center, the small pillar retaining roadway deformation, and the energy accumulation on the pillar decreased with an increase in the non-penetrating fracture length. The vertical stress at the working face end increased with an increase in the non-penetrating fracture length. The field application and monitoring results indicated that non-penetrating directional pre-splitting blasting could effectively control the deformation of small pillar retaining roadways. The roof-to-floor and rib-to-rib maximum convergences of the 6208 tail entry were reduced by 53.66% and 52.62%, respectively, compared to the results with no blasting. The roadway section met the demands of mining panel high-efficiency retreating, thereby demonstrating the rationality of the technical and numerical simulation results. The research results shed light on the improvement of small coal pillar retaining roadway maintenance theory and technology.

In underground mining and roadway support engineering of coal mine, the coal and rock layers bear loads together; therefore, the deformation and mechanical characteristics of the coal‐rock combined bodies are not the same as those of the pure coal or rock bodies. In this paper, conventional triaxial compression tests of coal‐rock combined bodies with different height ratios were conducted. And the stress and deformation characteristics of coal‐rock combined body were studied and the experimental results were analyzed with different strength criteria. The results show that the peak stress, elastic modulus, and strength reduction coefficient of coal‐rock combined body are negatively correlated with the ratio of coal to coal‐rock combination height and positively correlated with the confining pressure; the coal‐rock combination shows obvious ductility under 10 MPa confining pressure. Under the conventional triaxial condition, the shear failure was the main cause of the lateral deformation of the coal body in the coal‐rock combination, which was much larger than that of the rock body. The circle deformation value, volume strain value, and the deformation rate in the postpeak stage of coal‐rock combination are much higher than those in the prepeak stage. Mohr–Coulomb and general Hoek–Brown strength criterion fit the experimental results well.

Soft and hard composite rock strata are frequently encountered in transportation, geotechnical, and underground engineering. However, most of the current support is designed for homogeneous rock masses, which ignores the different anchoring effect in soft and hard composite rock strata. A numerical study is presented in this paper on the pull‐out behavior of fully grouted rock bolts in soft and hard composite rock strata. The nonlinear bond‐slip relationship of bolt‐grout interface that is anchored in soft rock and hard rock is obtained from laboratory test, respectively. Then, the nonlinear bond‐slip relationship is put into the numerical model. The numerical result shows a close match with the experiment tests and the proposed model. Lithological sequence, layer thickness ratio, and layer numbers are taken into consideration in numerical simulation models. Under the same layer number, the shallower‐soft and deeper‐hard composite rock strata (SHCRS) have a higher bearing capacity and deformation resistance than the shallower‐hard and deeper‐soft composite rock strata (HSCRS). As the soft‐to‐hard thickness ratio in SHCRS increases, the initial stiffness of the load‐displacement curve and peak load decreases continuously. The load‐displacement curve shows the same initial stiffness for different hard to soft thickness ratios in HSCRS. As the hard to soft thickness ratio increases, the load peak and the displacement at the peak load increase. Therefore, the closer the hard rock is to the loading end, the greater the initial stiffness of the load‐displacement curve is. The greater the hard rock thickness, the larger the peak load. Under the same anchor length, the peak load and the displacement at the peak load decrease with the increase of layer numbers, but the reduction magnitude decreases. This paper leads to a better understanding of the load transfer mechanism for the anchoring system in soft and hard composite strata and provides a reference for scientific support design and evaluation method.