Key Technologies for Safe Mining Under Thin Bedrock with Water-Rich Unconsolidated Layers: A Case Study of Ground Pre-Grouting Application

Abstract

Significant risk of water and sand inrushes is commonly encountered during coal seam mining when thin bedrock is directly overlain by thick, water-bearing, unconsolidated layers. Achieving effective strata control and establishing reliable water-isolating mechanisms under these conditions represent critical scientific and technological challenges for safe mining operations. Furthermore, this is a vital research direction for advancing the extraction limit (or recovery height) in coal seams. Initially, drawing on key stratum theory, ground pressure behavior patterns, and mining operation characteristics, the weathered zone was identified as the critical grouting horizon. During the initial mining stage, the first two periodic weighting intervals (approximately 60 m) were identified as the key area. Subsequently, a strategy of high-pressure grouting was proposed to modify the weathered stratum. Numerical simulation methods were employed to optimize the grouting parameters, with the core specifications determined as follows: grouting pressure ≥30 MPa, water–cement ratio of 0.7:1, and grouting hole spacing ≤30 m. Ultimately, a grouting system was designed that used directional drilling from the surface to access the weathered zone, followed by branched horizontal boreholes for staged high-pressure grouting. The borehole trajectory was predominantly L-shaped. Field implementation demonstrated that the grouting intervention increased the first weighting span by an average of 17.3%. Critically, no water inflow was observed throughout the initial caving period, and significant roof falls or rib spalling were effectively mitigated. This confirmed a substantial improvement in key stratum stability, ensuring the safe and efficient advancement of the mining face. This study provides essential technical support and a practical model for safely and efficiently extracting coal seams under thin bedrock under similar complex hydrogeological conditions.

1. Introduction

In central and eastern China, coalfields are mainly characterized by concealed coal measures overlain by unconsolidated layers, with regionally distributed sandy aquifers exhibiting moderate to high water-yielding capacity [1]. Where the bedrock is too thin to effectively isolate mining-induced disturbances from the overlying unconsolidated layers, the conditions are conventionally defined as thin bedrock conditions. Under such conditions, the thin bedrock fails to form a stable load-bearing structure, allowing caved or fractured zones to penetrate the aquifuge and reach unconsolidated aquifers. This creates hydraulic connectivity pathways. Concurrently, the overlying thick unconsolidated layers (including weathering zones, aquifers, sand, and soil) are imposed directly onto the coal face roof. This triggers synchronous subsidence of both the bedrock and the unconsolidated layers, resulting in rapid or intense pressure surges on supports, irregular loading patterns, and increased hydraulic connectivity [2,3]. Traditional mitigation strategies rely on passive hazard prevention, such as leaving large-scale water-protective coal or rock pillars. While this approach can help isolate aquifers, it also causes substantial retention of coal resources, imposes artificial limits on mining height, and significantly reduces the coal recovery rate. Therefore, controlling roof stability and water-conducting fracture development under thin bedrock conditions is essential for disaster prevention and the safe extraction of coal faces. It also represents a major scientific and technological challenge for mine safety under these geological conditions [4,5].

Grouting, a primary technique for strata reinforcement and water sealing [6,7], effectively modifies unconsolidated aquifers overlying the coal face [8,9]. This enhances the structural integrity of the roof strata and the overlying rock formations, making it possible to increase mining height under thin bedrock conditions [10]. Chinese researchers have extensively analyzed and experimentally investigated grout modification horizons, recommending grouting pressures 1.5 to 3.0 times the hydrostatic pressure of the target stratum to ensure an adequate grout penetration radius and optimize efficacy [11,12]. Li et al. developed a diffusion control equation for variable-rate grouting slurry in fractures. Through numerical simulation and field validation, they showed the kinetic mechanism through which stepwise reductions in the grouting rate can decrease the final grouting power by 77.4%, with minimal effect on diffusion [13]. Zhu et al. demonstrated through simulated experiments that grout compaction is the predominant mechanism governing soil strength enhancement [14]. Zhu et al. investigated the seepage and diffusion mechanisms of power-law fluids, considering both porous media tortuosity and slurry water displacement effects. Using COMSOL Multiphysics simulations (6.2), they analyzed the seepage and diffusion characteristics of power-law grouting slurry in water-bearing sand layers. They showed that slurry water displacement grouting technology can effectively address challenges like limited diffusion range and poor grouting effectiveness in high-pressure, underwater, and water-bearing sand formations [15]. For borehole layout design, Zhou et al. developed single-hole and multi-hole grouting diffusion models based on fractal geometry and filtration effects. They examined diffusion mechanisms in porous media and assessed parametric impacts (e.g., hole arrangement patterns, spacing) on reinforcement efficacy [16]. Using visualized single-fissure grouting experiments and VOF gas–liquid coupling numerical simulations, Cao et al. revealed the radial diffusion law of grouting slurry in mining-induced cracks in overlying strata under constant-rate grouting. The results showed that increasing the number of grouting holes and reducing hole spacing are more effective than increasing the grouting rate [17]. Qin and Wang et al. conducted mechanical experiments to examine grouting reinforcement mechanisms and efficacy in fractured rock masses from microstructural and macroscopic perspectives [18,19].

Internationally, research on large-scale grouting for strata modification covers grouting technologies, implementation methods, grouting materials, and the microstructural properties of post-reinforcement rock masses. For instance, Sha et al. used sulphoaluminate cement-based grouting material (SCGM) to reinforce water-bearing fractured strata. Their experimental results confirmed that SCGM exhibits exceptional integrated reinforcement efficacy, impermeability, and interfacial bonding properties, demonstrating superior applicability in fractured or weakly cemented formations under high water-to-solid (W/S) ratios [20]. Mu W et al. investigated high-pressure grouting diffusion mechanisms. They designed an experimental setup that integrated an acoustic emission (AE) monitoring system to simulate and analyze grouting pressure variations across fracture characteristics, hydraulic fracturing effects after high-pressure implementation, and computational modeling of grout propagation, achieving grouting pressures exceeding 40 MPa [21].

While previous studies provide valuable technical guidance for grouting-based roof reinforcement in thin bedrock conditions, they lack rational pre-intervention delineation of target strata and optimal grouting domains based on scientific characterization. In the Huainan Coalfield of Anhui, China, total coal resources under thin bedrock exceed 4 bil-lion metric tons. Research on grouting-based roof modification for safe extraction in such settings is therefore critical for improving resource recovery rates. This study investigates thin bedrock mining in the Nan’er Panel of Gubei Mine, Huainan, Anhui Province. Drawing on key stratum theory, strata behavior laws, and mining layout characteristics, the weathered zone was identified as the critical grouting stratum and the key area. Grouting parameters were optimized through numerical simulation and validated in the field. This case provides a valuable reference for safe mining under similar conditions.

2. Engineering Profile

The 1512(3) coal face in the Nan’er Mining Area of Gubei Coal Mine, Huainan, Anhui Province, primarily extracts the No. 13-1 coal seam at depths of 473–478.4 m, with an average thickness of 5.4 m. As illustrated in Figure 1, the ascending roof strata above the No. 13-1 coal seam include an immediate roof predominantly composed of mudstone, silty mudstone, and carbonaceous shale. The thickness of this layer ranges from 0.7 to 6.2 m and progressively thickens eastward. The main roof, overlying the immediate roof, is predominantly composed of medium- to fine-grained sandstone. Its thickness ranges from 2.9 to 15.1 m, with an average of 6.5 m. Above this are unconsolidated layers and a weathered rock stratum, where the weathered zone measures approximately 25–28.8 m in thickness.

The designed Plan 1512(3) coal face in the 13-1 coal seam is shown in Figure 1c. The vertical distance between the roof south of the open-off cut and the weathered zone is approximately 24 m. The vertical distance between the roof north of the open-off cut and the weathered zone is approximately 15 m.

Drilling fluid consumption data from boreholes surrounding the working face and pumping test results of the roof sandstone in the 13-1 coal seam indicate that the inflow (q) of the sandstone is generally less than 0.1 L/(s·m). Some boreholes recorded values as low as 0.055 L/(s·m). Pumping tests indicate that the roof sandstone exhibits high water richness but strong heterogeneity. According to the confined aquifer formula of the Large-Well Method, the predicted maximum water inflow during mining is estimated to be 65 m³/h. Under static conditions, the fractured sandstone aquifer in the coal measures and the middle-lower aquifer of the overlying Cenozoic strata function as relatively independent water storage units. However, mining-induced disturbances may cause the lower aquiclude to develop new water-conducting channels, compromising mining safety and efficiency. According to the Guide to Coal Pillar Retention and Coal Extraction under Buildings, Water Bodies, Railways, and Main Wells (hereinafter referred to as the “Specifications”) [22], the water body mining-induced level is classified as Category II. This classification prohibits the caving zone from extending into the overlying aquifer [22]. Additionally, five faults were exposed in the gate roads and the open-off cut, with two faults affecting coal mining. The maximum fault throw reaches 7.2 m, which compromises the integrity of the coal seam and its roof strata.

Comprehensive analysis of the geological conditions at the crosscut of coal face 1512(3) indicates that this face exhibits typical characteristics of a thick weathered zone with thin bedrock. The strength and water-resisting properties of the weathered zone characterize the key stratum that governs roof stability during the initial mining period.

3. Analysis of the Key Stratum Influencing Water-Resisting Performance

3.1. Key Stratum

According to the Specifications [22], a roof sand-control coal (rock) pillar must be installed for Category II water bodies. The vertical height (H_s) of this sand-control pillar should be at least equal to the sum of the maximum height of the caving zone (H_k) and the protective layer’s thickness (H_b), expressed as

$$H_s\ge H_k+H_b$$

(1)

where H_s is the vertical height of the sand-proof coal pillar (m), H_b is the thickness of the protective layer (m), and H_k is the maximum height of the caving zone (m).

Based on the borehole columnar section (Figure 1a) at the crosscut of working face 1512(3), mining height M = 4.5. According to the Specifications [22], the calculation formulas for the caved zone and the water-conducting fracture zone are as follows:

$$H_k={\displaystyle \frac{100\sum M}{4.7\sum M+19}}\pm 2.2=11.21\pm 2.2$$

(2)

$$H_l={\displaystyle \frac{100\sum M}{1.6\sum M+3.6}}\pm 5.6=41.67\pm 5.6$$

(3)

The calculated maximum heights under the most unfavorable conditions are as follows: maximum height of caving zone H_k = 13.41 m; maximum height of water-conducting fracture zone H_l = 47.27 m.

According to the Specifications [22], the protective layer’s thickness should be taken as H_b = 12 m. Thus, the minimum required vertical height of the waterproof coal pillar is

$$H_s\ge H_k+H_b=13.41+12=25.41\mathrm{m}$$

As shown in Figure 2, H_l > H_s. After mining, this fracture zone will penetrate the bedrock and extend into the weathered zone and the overlying unconsolidated aquifer, creating a significant water inrush risk. To ensure safe mining in coal face 1512(3), the 25–28.8 m thick weathered rock stratum is designated as the target key stratum for engineering modification, where grouting techniques will be implemented to enhance its structural integrity and impermeability.

3.2. Key Stratum Analysis

Coal mining constitutes a complex, systematic engineering endeavor. The initial mining period is the most critical and risk-intensive phase of the working face extraction cycle. This phase requires not only the coordinated execution of fundamental tasks, such as comprehensive equipment commissioning and ventilation system realignment, but also the simultaneous mitigation of major hazards, including violent initial roof fracturing pressure and water–sand inrush risks. Completing these overlapping tasks within a constrained spatiotemporal framework hinges on roof stability and impermeability. Violent strata pressure can readily induce support crushing. Under thin bedrock conditions (15–25 m), initial roof caving and fracture zone development may establish hydraulic connections with the overlying thick weathered zone and the unconsolidated aquifer. This significantly elevates the risk of water inrush or even catastrophic water–sand mixture inrush at the working face.

Based on the fundamental conditions of coal face 1512(3), the key stratum analysis for safe extraction is as follows:

(1)

Empirical data from analogous thin bedrock conditions in China indicate that the first weighting interval typically ranges from 20 to 30 m [23,24]. In the 1512 (2) coal face’s first weighting section, where roof bedrock thickness is minimal (approaching the critical threshold of 15 m), pressure manifestations are most intense. This creates a high-risk scenario for developing water-conducting fractures that connect to the overlying unconsolidated aquifer. Grout modification of the key stratum in this high-risk core zone is critical for safe commissioning of the working face.

(2)

As the coal face advances, the bedrock’s thickness gradually increases along the extraction direction (>25 m). Increased bedrock thickness means improved self-stability and greater resistance to deformation and fracturing while also limiting the development height of the fracture zone. Beyond a critical advance distance, mining-induced disturbances have a less direct impact on the overlying weathered zone, and the risk of hydraulic fracture communication is greatly reduced.

In summary, the grouting reinforcement zone within the roof strata’s weathered zone for the initial 60 m of mining advance (equivalent to 2–3 weighting intervals) is determined based on a comprehensive risk assessment during the initial mining stage, fracture development patterns in the overlying strata, and engineering economics.

4. Grouting Methodology and Parameters

4.1. Groutability Analysis of Rock Strata

Core samples extracted from the weathered rock stratum, shown in Figure 3, reveal that the sequence primarily comprises highly weathered mudstone and moderately weathered sandstone. The highly weathered mudstone is pale yellow to yellowish gray, friable, and disintegrated. Upon retrieval to the surface, specimens typically fracture into blocky or angular fragments. The moderately weathered sandstone displays well-developed jointing, with joint surfaces coated by argillaceous infilling exhibiting distinct brownish-red coloration.

First, weathering progressively increases porosity within the rock mass, resulting in more intricate pore networks and additional adsorption sites for water molecules. Second, micro-fissures generated through weathering-induced oxidation create preferential pathways for fluid percolation, increasing the hydraulic conductivity of the weathered zone. Third, the elevated argillaceous content in weathered strata exhibits pronounced hydro-softening and slaking disintegration upon hydration. These mechanisms, coupled with the inherently low structural strength of the rock mass, result in hydration-induced swelling, which significantly enhances aquiclude properties. In summary, weathered rock strata exhibit dual porosity–fracture characteristics, making them significantly more groutable than intact bedrock. Before mining-induced disturbance, these strata function mainly as primary porosity-dominated media. Depending on the relationship between grouting pressure, rock mass tensile strength, and in situ stress, grout diffusion modes can be classified into three types, as detailed in

Table 1. Division of diffusion patterns in mudstone layers.

Grouting Pressure	Diffusion Mode	Diffusion Mechanism	Diffusion Characteristics
P < P0	Permeation Diffusion	Flow in pores or fractures depends on pressure gradient	Diffusion range governed by porosity, grout viscosity, and grouting pressure.
P0 < P < P0 + σt	Hybrid Permeation-Compaction Diffusion	Grout permeates pores, high-pressure grout compacts the rock mass and propagates micro-fractures	Diffusion range governed by: initial porosity, grouting pressure
P > P0 + σt	Fracturing Diffusion	Grouting pressure exceeds fracturing threshold, creating new fracture networks	Dominant fracture orientations. Linear/network-shaped, diffusion paths

In the table, P, grouting pressure; P₀, in situ stress; and σ_t, tensile strength.

. After disturbance, the formation of macroscopic apertures creates water-conducting channels. Therefore, considering grouting coverage, injection objectives, and cost-effectiveness, cement-based grouts are preferentially selected. High-pressure fracture grouting—specifically targeting weak structural planes through controlled hydraulic fracturing—is the optimal approach.

4.2. Single-Borehole Grouting Behavior

Classical theoretical formulations show that the grout penetration radius is proportional to grouting pressure and grouting duration and inversely proportional to grout viscosity and formation permeability [25,26]. Selecting grouting pressure, permeability, and the water–cement ratio as investigation parameters, we employed numerical simulations to assess their effects on grout penetration efficiency.

4.2.1. Modeling Approaches

A large-scale finite element model with dimensions of 200 × 100 m was established in COMSOL Multiphysics using the porous media phase transport module based on Darcy’s law to simulate single-hole grouting scenarios. The injection borehole radius was set to 150 mm. Local mesh refinement was applied within radial distances of 10 and 30 m from the borehole to improve computational accuracy, with the mesh configuration illustrated in Figure 4. The borehole boundary was set as a constant pressure condition to simulate grout injection under consistent pressure. Based on the stratum’s burial depth, the in situ stress was approximated at 8 MPa. The model’s outer boundaries were assigned constant pressure (8 MPa) outflow conditions, allowing grout and water to exfiltrate freely, as shown in the referenced figure. The groundwater density was set to 1000 kg/m³ with a dynamic viscosity of 0.001 Pa·s. Ordinary Portland Cement P.O 42.5 was selected as the grouting material.

The mechanical and seepage parameters of the overlying rock strata obtained through laboratory tests are presented in

Table 2. Mechanical parameters of rock masses.

	Young’s Modulus (GPa)	Poisson’s Ratio	Uniaxial Compressive Strength (MPa)	Tensile Strength (MPa)	Permeability (×10–12 m2)	Porosity
Overburden	27.81	0.31	42.75	2.85	5.75	18%
Unconsolidated Aquifer	23.11	0.25	11.81	0.52	28.57	25%
Weathered Rock Stratum	4.52	0.35	3.45	0.72	8.00	8%
Sandstone–Mudstone Intercalation	17.31	0.21	35.51	2.04	3.85	7%

.

For multiphase transport in porous media, the governing two-phase flow equations are expressed as

$${\displaystyle \frac{\mathsf{\partial }{ϵ}_{\mathrm{p}}{\rho }_{\mathrm{s}}S}{\mathsf{\partial }t}}+\nabla \cdot N=0$$

(4)

$$N={\rho }_{\mathrm{s}}u$$

(5)

$$u=-{\displaystyle \frac{{\kappa }_{\mathrm{r}}}{{\mu }_{\mathrm{s}}}}\kappa \nabla {\rho }_{\mathrm{s}}$$

(6)

$$p_{\mathrm{s}}=p+p_{\mathrm{c}}$$

(7)

where ε_p is the porosity of the porous material; ρ_s is the density of the current phase; S is the volume fraction of the current phase; μ_s is the dynamic viscosity of the current phase; u is the Darcy velocity of the current phase; κ_r is the effective relative permeability of the current phase fluid, which is a function of saturation; κ is the intrinsic permeability tensor of the porous medium; p_s is the phase pressure of the current fluid; p is the phase pressure computed via Darcy’s law; p_c is the capillary pressure at fluid–fluid interfaces; $\nabla \cdot$ is the divergence operator; and $\nabla$ is the gradient operator.

Building on prior research and theoretical analysis, grouting pressure, formation permeability, and the water–cement ratio were discretized into six levels (

Table 3. Parameter level table for numerical simulation.

	Effect Factor
Level	Factor 1: Grouting Pressure (P) (MPa)	Factor 2: Formation Permeability (κ) (m2)	Factor 3: Grout Water–Cement Ratio (W/C)
1	10	7.0 × 10−12	0.5
2	15	7.5 × 10−12	0.6
3	20	8.0 × 10−12	0.7
4	25	8.5 × 10−12	0.8
5	30	9.0 × 10−12	0.9
6	35	9.5 × 10−12	1.0

). A three-factor, six-level numerical simulation of grout propagation was performed using the controlled variable approach. The transition in the rheological model of cement grouts is primarily governed by w/c, reflecting variations in particle size distribution, yield stress, and shear-rate-dependent viscosity. In the w/c range of 0.5–0.7, grouts exhibit power-law fluid behavior; when w/c increases to 0.8–1.0, the emergence of yield stress triggers a transition to Bingham plastic behavior [27]. Accounting for time-dependent rheological properties, constitutive models and dynamic viscosities for different w/c grouts are summarized in

Table 4. Rheological constitutive models and dynamic viscosity of grout mixtures for various water–cement ratios.

Water–Cement Ratio	0.5	0.6	0.7	0.8	0.9	1.0
Rheological Constitutive Model	Power-Law Fluid $\tau =K{\gamma }^n$			Bingham Fluid $\tau ={\tau }_0+{\mu }_p\dot{\gamma }$
Dynamic Viscosity (Pa·s)	0.080	0.050	0.030	0.025	0.020	0.015

.

In the table, τ is shear stress, K is the power-law consistency coefficient, n is the flow behavior index (power-law exponent), τ₀ is yield stress, μ_p is plastic viscosity, and $\stackrel{.}{\gamma }$ is the shear rate.

During cement grout injection into unconsolidated aquifers, the grout and groundwater are modeled as undergoing transient two-phase flow through porous media. In the aquifer domain, phase saturations differ spatially, but satisfy the following constraint [28]:

$$S_w+S_n=1$$

(8)

where S_w is water saturation in the unconsolidated aquifers, and S_n is grout saturation. Under initial conditions (simulation time t = 0), the aquifer pores are fully water-saturated, where S_w = 1 and S_n = 0. At complete displacement, the theoretical limit state yields S_w = 0 and S_n = 1.

4.2.2. Results Analysis

(1)

Influence of Grouting Pressure (P)

Using baseline parameters of permeability κ = 8 × 10⁻¹² m², water–cement ratio of 0.7, and grouting duration t = 60 min, grout saturation distribution was used to characterize grout propagation within the seepage field. The resulting contour plots of grout penetration under different injection pressures are shown in Figure 5. Figure 6 illustrates the temporal evolution of grout penetration distance under varying grouting pressures, while Figure 7 quantifies the corresponding changes in grout penetration distance and cumulative grout volume.

① Figure 5 provides a visual comparison of grout diffusion patterns at different injection pressures. Higher grouting pressures promote greater grout propagation distances. Under constant grouting pressure, the penetration radius of the grout increases with time, but its penetration rate gradually slows. This is primarily because as the penetration radius increases, the grout must travel longer seepage paths, which increases flow resistance. According to Darcy’s law, flow velocity is proportional to pressure gradient. As the penetration radius increases, the pressure gradient at points far from the grout hole decreases, which weakens the driving force.

② The final grout propagation radius is positively correlated with the total grout volume, grouting pressure, and grouting time. The penetration radius at t = 30 min is approximately 75~80% of that at t = 60 min. Higher grouting pressure can more effectively overcome the formation’s low initial permeability and capillary resistance, increasing the seepage velocity of the grout within the formation’s pores or micro-fractures and promoting greater penetration distances.

③ Considering the time-dependent characteristics of grout rheology under a pressure of 30 MPa, the seepage penetration radius at t = 30 min is 12.3 m.

(2)

Permeability Impacts (κ)

Using fundamental grouting parameters (grouting pressure P = 30 MPa, water–cement ratio of 0.7, duration t = 30 min), Figure 8 presents the simulated contour plots of grout seepage penetration. Figure 9 illustrates the influence of permeability variations on the grout penetration distance. As shown, the penetration radius increases with rising permeability, but the rate of increase slows. Permeability alone has limited impact on grout penetration radius, necessitating synergistic enhancement through coordinated regulation of grouting pressure and optimization of grout rheology.

(3)

Water–Cement Ratio Impacts

With grouting pressure at 30 MPa, permeability at 8 × 10⁻¹² m², and a duration of t = 30 min, Figure 10 shows the simulated contour plots of grout seepage penetration. Figure 11 demonstrates the influence of varying water–cement ratios (W/C) on grout penetration distance. For the same grouting duration, the penetration distance increases nonlinearly as W/C rises, consistent with Hu and Xi [29,30].

For power-law fluids with a lower W/C (≤0.6), viscosity dominates flow resistance. The grout exhibits high dynamic viscosity, making initial injection difficult. The high solid content in low-W/C grout intensifies filtration effects, causing rapid particle accumulation at pore throats or surfaces. This forms low-permeability “filter cakes” and causes bridging blockage at constrictions, restricting effective penetration distance. When W/C reaches 0.7, shear-thinning enhancement becomes significant [31]. Under high-pressure grouting, localized shear rates surge sharply, reducing effective viscosity by 40–60% compared with static viscosity [32]. This mitigates pressure loss and enables a 41.4% increase in penetration distance when the water–cement ratio (W/C) rises from 0.6 to 0.7, significantly enhancing operational efficiency in low-permeability zones.

When W/C exceeds 0.7, grout transitions from a power-law to a Bingham fluid, significantly changing its rheological properties. Consequently, the penetration radius exhibits nonlinear growth. Increasing W/C further raises bleeding rates, yielding diminishing radius gains while compromising the integrity of the set grout [30,33]. Integrated analysis balancing penetration radius and grouting quality identifies ordinary Portland cement with W/C = 0.7 as the optimal material.

4.3. Stress Field Distribution Characteristics in Multi-Borehole Grouting Strategy

Building on the optimized single-borehole grouting parameters, this section investigates the stress field distribution of multi-borehole grouting to further assess the pressure superposition effects and the formation mechanism of continuous reinforcement zones at the engineering scale. Using a grouting pressure of 30 MPa, duration t = 30 min, permeability of 8 × 10⁻¹² m², and a water–cement ratio of 0.7:1, Figure 12 shows the calculated stress field characteristics for borehole spacings of 10, 20, 30, and 40 m.

The results show that continuous high-pressure compaction zones (>25 MPa) consistently form at borehole spacings of 10 to 30 m. However, spacings exceeding 40 m fail to ensure zone continuity. Thus, the optimum grouting borehole spacing is identified as p ≤ 30 m.

5. Field Implementation and Efficacy

5.1. Grouting Methodology

Grouting was implemented in the key weathered zone overlying Panel 1512(3) of the Guqiao Coal Mine. The target stratum is 25 m thick, 200 m long, and 60 m wide. To achieve high-pressure grouting, directional drilling from the surface to the target stratum was employed, with grouting performed through staged perforations. The drilling trajectory comprised four L-type directional boreholes and two lateral branches (Figure 13). The borehole trajectories of holes No. 1 and No. 2 comprised a vertical section (230 m) + a build-up section (330 m) + a horizontal section (120 m). For the No. 3 and No. 4 holes, there was a vertical section (160 m) + a build-up section (430 m) + a horizontal section (100 m). The horizontal laterals extended 100 m for the No. 1 and No. 2 holes. The total drilling footage was 680 m + 680 m + 690 m + 690 m + 100 m + 100 m = 2940 m. Within the depth range of 0–590 m, a φ251 mm four-wing alloy bit was employed. A Φ193.68 mm oil casing (wall thickness: 9.52 mm) was installed in the 0–160 m interval, while a Φ193.68 mm oil casing (wall thickness: 10.92 mm) was deployed in the 160–590 m interval, followed by cementing operations. The 590–690 m section was drilled with a φ152 mm roller cone bit for open-hole grouting.

A gyroscopic measurement while drilling (GMWD) system was used to drill the directional boreholes to achieve section-by-section precision control. Enhanced construction management kept the deviation of grouting points within ±1.5 m of the target horizon.

Progressive grouting was implemented in the horizontal section, with grouting performed every 30 m of advance. Grouting termination required a final pressure ≥ 30 MPa and a grout flow rate < 100 L/min for ≥ 30 min. The grout material comprised P.O 42.5 ordinary Portland cement, mixed as a single-fluid ordinary cement grout with a water–cement ratio of 0.7.

5.2. Grouting Effectiveness Assessment

(1)

The total grout volume injected into the weathered rock stratum during surface pre-grouting treatment reached 30,252 m³, using 23,203 tons of cement. Accounting for a consolidated volume of 14,501 m³ after hydration and assuming uniform distribution across the designated 200 × 60 m treatment area, this yielded an engineered consolidation layer averaging 1.2 m in thickness.

(2)

Core sampling of the coal face overburden was performed via 17 boreholes, with 8 positioned outside of the grout-treated weathered rock stratum and 9 within it. Each borehole penetrated 25–31 m of roof strata. The results showed that following grout reinforcement, mudstone cores from the weathered zone exhibited blocky fragments with distinct cementation textures. Prior to grouting, within ≈130 m of the weathered zone across nine boreholes, cumulative core recovery measured only 104 mm for fragments sized 16.5–24.5 mm. The core recovery rate reached only 0.35% of the total drilled length. Following grouting, cemented rock masses >30 mm accounted for 3–5% of the drilled length, indicating significantly enhanced integrity. High-pressure grouting induced compaction and effectively filled voids in micro-defects within the weathered rock stratum.

(3)

Mining performance of Panel 1512(3): The panel utilized inclined longwall retreat mining with full-height mechanized extraction (maximum height: 4.8 m). Hydraulic supports (four-pillar shield type Z13000/24/50) were selected based on rock-load theory and numerical modeling. Support resistance monitoring indicated a main roof first weighting interval of 27.7–36.5 m (avg. 31.5 m) across longwall face sections. Periodic weighting intervals measured 9.9–18.3 m (avg. 13.2 m). These intervals exceeded those in the non-grouted control panels under comparable geomechanical conditions in the same coal seam. During the first main roof weighting, hydraulic support resistance ranged from 9206 to 13,016 kN. Within seven days of mining commencement, the immediate roof completely collapsed, triggering the activation of safety valves on multiple hydraulic supports in the working face. Concurrently, frequent safety valve activation occurred in the longwall hydraulic supports. Surface pre-grouting interventions modified the roof structure and the surrounding rock mass, preventing water dripping during the initial caving period. Significant roof falls or rib spalling were absent. This strategy successfully controlled roof behavior during both the first weighting and subsequent periodic weightings, ensuring safe extraction of the working face.

6. Discussion

This study addresses the risks of water inrush and sand bursting during coal mining beneath aquiferous loose layers with thin bedrock. By identifying key strata and critical zones, secure extraction was achieved through reinforcement and water blockage via L-shaped directional drilling + high-pressure sectional grouting. Internationally, research has shown that advanced face pre-grouting using high-performance materials can reduce drilling costs. However, these methods often require relatively low grouting pressures, achieve limited slurry diffusion ranges, occupy valuable underground space, and incur high material transportation costs.

Due to budget constraints, this study did not incorporate advanced technologies, such as real-time grouting monitoring or feedback control based on machine learning. Instead, digital pressure gauges and flow meters were used, with termination criteria set at a final pressure ≥ 30 MPa, a grout flow rate < 100 L/min, and stability for ≥30 min. Future integration of advanced grouting monitoring technologies is expected to enhance the comprehensiveness of this technical approach.

7. Conclusions

To address the risks of water and sand inrush during coal mining beneath auriferous loose layers with thin bedrock, this study identified the critical strata and zones influencing safe extraction. A methodology was developed involving targeted grouting reinforcement and water sealing in confined zones to ensure mining safety.

Surface-based L-shaped directional lateral drilling technology enabled high-pressure grouting exceeding 20 MPa in critical strata and target zones. This technical approach represents a significant advancement in achieving grouting reinforcement and hydraulic sealing objectives.

Engineering practice validation: Comparative analysis with panels under analogous geological conditions in the Huainan Mining District demonstrates that grout-reinforced Panel 1512(3) exhibited a main roof first weighting interval of 27.7–36.5 m (mean: 31.5 m) and a periodic weighting interval of 9.9–18.3 m (avg 13.20 m). In contrast, non-grouted panels under similar conditions averaged only 26 m in the first weighting interval and 10–12 m in the periodic weighting interval. The grouting intervention increased the first weighting span by 17.3% on average. Critically, no water inflow was observed throughout the initial caving period, and significant roof falls or rib spalling were effectively mitigated, confirming substantial enhancement of key stratum stability.