In response to the complex geological conditions and developed faults in the Kongzhuang 7,436 working face, experimental studies on rheological and mechanical properties were conducted to meet the requirements for grouting material selection and parameter determination, providing laboratory-measured parameters for subsequent numerical simulations. The cementitious materials used in this experiment were three types of grouting materials previously applied for reinforcement at Kongzhuang Coal Mine: Jumina ultra-fine cement (JMN), inorganic two-component material (IBC), and organic two-component material (OBC). Portland cement PO42.5 (PC) was selected as the control group for comparative study.

Based on the grout mix ratio applied in the early-stage engineering of the 7,436 working face, the slurry was prepared for this experiment. PC and JMN dry materials were uniformly mixed with water at a water-cement ratio of 0.35 and fully stirred. For IBC, the dry materials of Component A and Component B were proportioned at a mass ratio of 1:1, uniformly mixed, then combined with water at a water-cement ratio of 0.35 and fully stirred. For OBC, the grouts of Component A and Component B were mixed at a volume ratio of 1:1, rapidly and thoroughly stirred to ensure uniform mixing, completing the slurry preparation. The fluid properties of the prepared slurries were characterized through setting time, bleeding rate, and fluidity tests.

Using a round-hole sieve at the 7,436 working face, coal particles and roof rock particles with a particle size of 10–20 mm were screened as preparation materials for the cemented specimens. The four different slurries were uniformly mixed with coal particles and rock particles in molds, respectively, to prepare 100 mm × 100 mm × 100 mm cemented specimens for uniaxial compression tests and 40 mm × 40 mm × 160 mm cemented specimens for flexural strength tests. (The average weight of coal in a single cube mold was 1.00 kg, the average weight of rock was 2.10 kg; the average weight of coal in a single prism mold was 0.25 kg, and the average weight of rock was 0.55 kg) After demolding, the specimens were cured in a YH-40B standard constant-temperature and humidity curing chamber at 20°C ± 2°C and 95% humidity for 7 days, 14 days, 21 days, and 28 days. This compared the bonding performance of different cementitious materials on the fractured coal-rock mass in the Kongzhuang 7,436 working face and the mechanical properties of the cemented bodies under the same curing environment but different curing times. The main experimental procedures are shown in Figure 2.

Following laboratory tests to select the most suitable grouting material for Kongzhuang Coal Mine’s 7,436 working face, research was conducted on the grout diffusion mechanism and optimization of grouting process parameters for advanced grouting in the working face’s fault fractured zone. This study established a two-dimensional numerical model using UDEC software based on the discrete element method (DEM). DEM treats the whole medium as a discontinuous system composed of independent sub-entities, characterizing fractured rock masses as a combination of blocks and joints. This approach best reflects the essence of grout fluid diffusion in the fractured rock network space within actual engineering scenarios, enabling intuitive and accurate simulation of macroscopic diffusion paths and distribution differences of grout within the complex fracture networks of fault fractured zones and intact surrounding rock. The flow of mine grouting materials represented by cement-based grouts in coal-rock fractured zones is typically modeled as a non-Newtonian Bingham fluid, characterized by yield shear strength and dynamic viscosity. The grout behaves as a rigid body under low stress and a viscous fluid under high stress. The fluid characteristics were simulated using UDEC software to realize the initiation and flow processes of grout within the fractures. The flow behavior follows a modified Bingham fluid model, with flow rate calculations described by Equations 1, 2: q = 0   J < J 0 − a 3 12 μ g J − J 0   J > J 0 (1)

Where J 0 = 2 τ 0 a   a n d   a = a 0 + μ n (2)

Where a is the joint aperture, μ g is the grout viscosity, J 0 is the yield stress, τ 0 is the yield strength, μ n is the normal displacement.

Based on the geological and mining conditions of the 7,436 working face, a two-dimensional numerical model of the Kongzhuang 7,436 working face was established using UDEC software. The model dimensions are 250 m × 55 m. To reduce model complexity and computational scale, a single key fault (FB19; dip angle 50°, H = 3.5 m) was simulated. Coal pillars of 50 m width were reserved on both lateral boundaries to eliminate boundary effects. To ensure the grout diffusion pattern realistically reflects the discontinuous characteristics of fractured rock masses, the coal seam and roof/floor were discretized using random polygonal meshes. The upper boundary of the model was set as a stress boundary with a vertical stress of 23.6 MPa and a lateral pressure coefficient of 1.5. The bottom boundary was fixed with zero velocity in both the X and Y directions, as shown in Figure 3. The Mohr-Coulomb criterion was employed to simulate the failure process. Model parameters were determined based on the grouting materials used and the characteristics of the complex structural surrounding rock mass, as listed in Table 1. These include a fracture permeability factor of 1 × 10⁸, zero-stress fracture aperture of 0.1 mm (1 mm in the fault zone), and residual fracture aperture of 0.02 mm (Wu et al., 2021). This setup significantly highlights the fault fractured zone as the core preferential seepage pathway for grout, enabling the study of its dominant influence on grout diffusion.

The basic assumptions for grout diffusion in the model are as follows:

(1) Joints are in a dry state, with impermeable strata above the joints and impermeable boundaries at the model edges.

The spatial positioning of grouting holes exerts a significant controlling effect on the reinforcement efficiency of fractured surrounding rock. Two critical dimensions include the strike-space relationship between the final hole position and the fault zone, as well as the vertical space relationship between the hole layout layer and the coal-rock strata. In this study, quantitative analysis was conducted by constructing different numerical models, as listed in Table 2 and shown in Figure 4. All simulation schemes adopted identical and constant grouting pressure, water-cement ratio, and contact parameters. Specifically, the horizontal height and length of grouting holes were unified in the model group with different final hole positions, while the length of grouting holes was unified in the model group with different hole layout layers. Based on laboratory measurements and the actual grouting engineering in the 7,436 working face of Kongzhuang Coal Mine, the grouting pressure was set at 12 MPa, the water-cement ratio of Jumina grout was 0.35:1, and the density was 2000 kg/m³.

To optimize the grouting parameters in the advanced grouting reinforcement scheme for the 7,436 working face of Kongzhuang Coal Mine, an orthogonal test was conducted using the numerical simulation results of different schemes to select the most ideal grout diffusion scheme. The orthogonal test was introduced to investigate grout diffusion under different water-cement ratios and grouting pressures, without considering the interactions between factor levels. Representative parameters were selected as the values for each level, and the final orthogonal test design table is presented in Table 3.

In this study, the total grout seepage length ( L ), relative grouting volume ( Q r ), and relative unit grouting efficiency ( η r ) were monitored to quantify the influence of grouting hole positions on grouting performance. To intuitively compare differences in grouting volume and unit grouting efficiency among schemes, relative grouting volume and relative unit grouting efficiency were used for result characterization and analysis. The total grout seepage length is defined as the sum of the lengths of each joint through which the grout flows. The calculation of relative grouting volume is shown in Equation 3, and the calculation of relative unit grouting efficiency is shown in Equation 4: Q r = Q Q min (3) η r = Q r L · η min (4)

Where Q is the actual grouting volume, Q min is the minimum grouting volume across all schemes, η min is the minimum unit grouting efficiency across all schemes.

Based on the mechanical properties of Jumina-cemented fractured coal-rock masses, modified reinforcement was implemented for the fault fractured zone. A comparative study was conducted by simulating two coal seam excavation scenarios—with and without reinforcement—to investigate the positive impacts of reinforcement on coal mining. When the working face advanced to the critical area 10 m from the fault fractured zone, three indicators were monitored within the 5 m range ahead of the coal wall: the number of shear cracks, the number of tensile cracks, and the maximum horizontal displacement of the coal wall. Comparative analyses were performed before and after grouting reinforcement, as shown in Figure 11. The research results show that in terms of the number of shear fractures, 39 shear fractures developed in this range without grouting reinforcement measures; after grouting reinforcement, the number of shear fractures was reduced to 23, a decrease of 41.0% compared with that before reinforcement. In terms of the number of tensile fractures, after grouting reinforcement, the number of tensile fractures was reduced from 70 to 33, a decrease of 52.9%. In terms of the maximum horizontal displacement of the coal wall, after grouting reinforcement, the maximum horizontal displacement was reduced from 63 cm to 35 cm, a decrease of 44.4% compared with that before reinforcement. Therefore, the grouting reinforcement technology for the fault fractured zone significantly reduced the complexity of the crack network ahead of the coal wall and the horizontal displacement of the coal wall by improving the integrity of the fault fractured zone, thus effectively enhancing the load-bearing performance of the coal wall and reducing geological hazards such as coal wall spalling and roof collapse.

Based on experimental and simulation studies, the optimized grouting reinforcement scheme includes: using Jumina cementitious material to enhance long-distance conveying capacity, fracture filling efficiency, and cementation strength; extending the final hole 2 m beyond the fault zone in the strike direction to reduce grout leakage rate and expand grout diffusion; positioning the final hole vertically within 1 m above the coal-rock interface to achieve bidirectional grout diffusion in coal and rock, cover the working face crossing the fault zone, and reduce floor seepage; adopting graded pressure design with a water-cement ratio not exceeding 0.6, increasing grouting pressure from 12 MPa to 15 MPa during the high-pressure diffusion stage to improve unit grouting efficiency; and extending the sealing segment of grouting holes to reduce grout loss and pressure dissipation along mining-induced fractures.

Considering the geological conditions and on-site construction environment, the advanced grouting drill field for the 7,436 working face is determined to be 37.4 m ahead of Point S13 in the material roadway. Grouting holes are arranged in a fan shape toward the open-off cut, with a total of 22 drill holes, including 13 reserved holes. Long and short holes form a group, with the long hole approximately 15 m longer than the short hole and the same azimuth. Borehole angles range from 9° to 16°, hole depths from 110 m to 170 m, final hole positions within 1 m above the coal-rock interface, and hole spacing no greater than 15 m. The grouting material is Jumina ultra-fine high-permeability single-component grouting reinforcement material, with a grouting pressure of 12–16 MPa. Curtain grouting is applied to the coal wall in the material roadway to prevent grout loss. Within the structural fractured zone, the hole spacing is reduced from 15 m to 10 m for key grouting, as shown in Figure 12.

Six inspection holes were arranged within 30 m ahead of the working face in the 7,436 material roadway to evaluate the grouting effect. The boreholes have a depth of 10 m, with final hole positions within 2 m above the roof. Solidified grout filling was found in most fractures of the fractured rock and coal seam regions, as shown in Figure 13A. After implementing the surrounding rock control scheme, the integrity of the coal wall was significantly improved. The control effects are demonstrated in Figures 13B,C, with fracture reinforcement effects meeting expectations.

Figure 14 shows the statistical results of coal wall spalling areas in the working face. It can be observed that after treatment, the occurrence range of coal wall spalling has decreased from the original 20#–120# support interval to 42#–78#, transitioning from long-distance continuous spalling to short-distance sporadic spalling. The quantity and scope of coal wall spalling have significantly reduced, enhancing the supporting effect on the roof and nearly eliminating roof leakage. The face advancing rate has increased from 1 cycle every 2 days to 2–3 cycles per day, forming a virtuous cycle.