Abstract
Gob-side entry driving with small coal pillars in extra-thick seams improves resource recovery but is constrained by fractured, low-capacity surrounding rock and large asymmetric deformation. This study develops and validates a grouting-reinforcement technology that couples hollow high-pressure grouting cable bolts with a nano-modified, two-component microfine grout, and elucidates the evolution and control mechanisms governing stability. A process-based mechanical framework is first established, showing two stages of surrounding-rock evolution: excavation-induced redistribution/damage followed by retreat-mining-induced reactivation/restructuring. Mechanistic analysis indicates that grouting increases normal and tangential stiffness and shear strength along fracture surfaces, forms a consolidation skeleton, reduces the loosening zone, seals water–air pathways, and strengthens bolt anchorage to create a load-sharing rock-grout-bolt composite. Grout diffusion was examined using UDEC hydro-mechanical simulations with Bingham-type rheology across water-cement ratios, and by laboratory tests on a nano-modified grout (D90 < 10 µm) exhibiting rapid setting, early strength, and high adhesion. The technology was implemented in Roadway #5121, Tashan Coal Mine, using Φ22.4 mm hollow cables; field monitoring (surface convergence, bolt working resistance, anchorage tests, and borehole imaging) provided verification. Relative to the non-grouted control, floor heave decreased by 28%, solid-coal rib convergence by 43%, and coal-pillar rib convergence by 44%, while roof subsidence showed no notable change; bolt working resistance stabilized and the bolt failure rate declined markedly. The results demonstrate that stage-targeted grouting-prioritizing ribs and floor and integrating with cable-bolt systems-rebuilds the bearing ring and substantially enhances roadway reliability. The quantified reductions provide practical design benchmarks and calibrated inputs for future numerical and probabilistic evaluations in similar geological and mining settings.
1 Introduction
Gob-side entry driving with small coal pillars is a key roadway layout in underground coal mining. In this configuration, the roadway is excavated along the goaf (gob) after the overlying strata of the previously mined panel have stabilized, leaving only a narrow coal pillar to separate adjacent goafs (Bai, 2006; Liu et al., 2014; Kang et al., 2010a; Zhang et al., 2014). Relative to conventional wide-pillar layouts, this approach markedly reduces coal left in pillars, improves resource recovery, and mitigates hazards such as gangue collapse and air leakage between panels. Owing to these advantages, gob-side entry driving/retaining with small coal pillars has been widely implemented in thick and extra-thick seams in China and abroad (Kang et al., 2010b; Hai, 2017), and engineering studies on entry retaining, roof cutting, and coordinated support have formed an increasingly complete technical system.
Despite these advances, maintaining such entries remains challenging. The residual pillar is narrow and the surrounding rock is commonly fractured; the resulting bearing structure is weak, highly sensitive to mining disturbance, and prone to large, asymmetric deformation. Convergence of tens to hundreds of millimeters is frequent, and roadway closure may occur in severe cases, directly threatening stability and production safety. To address these problems, integrated control technologies have been proposed, including staged grouting reinforcement; combined “anchor–mesh–shotcrete–cable–grouting” systems; coordinated “support–pressure-relief” systems; and pre-reinforcement of coal pillars verified by numerical analysis and field tests (Zhao et al., 2010; Zhang et al., 2016; Li et al., 2024; Yang, 2017; Guan and Xiong, 2013). While these studies confirm the effectiveness of active, combined support, they also show that control performance is strongly governed by mining-induced stress transfer, the position of main-roof fractures, and loading asymmetry between the solid-coal and goaf sides. Recent investigations further indicate that when the main-roof fracture line deviates from the gob edge, the number and length of tensile cracks in the pillar and ribs increase sharply, accelerating deformation and complicating support design (Xu, 2017; Qi and Geng, 2021; He and Cui, 2019; Sui et al., 2018).
Within these integrated systems, grouting has become indispensable from both material and structural perspectives. Early engineering practice predominantly used ordinary cement-based slurries because they are inexpensive, simple to prepare, and effective for filling medium-to-large fractures; applications centered on roadway consolidation, water sealing, and reinforcement around small pillars (Kang et al., 2023). To improve penetration and adapt to water-bearing or severely broken coal–rock masses, subsequent work introduced water-glass–cement double-liquid grouts and polymer/chemical grouts, which provided faster setting, improved water- and air-sealing performance, and greater adaptability to complex mine environments (Huang et al., 2022). Collectively, these efforts demonstrated that grouting can (i) fill mining-induced fracture networks, (ii) strengthen the coal–rock mass and residual pillar, (iii) enhance bolt/anchor-cable bonding, and (iv) cooperate with surface support to form a more integral load-bearing structure.
However, when applied to gob-side entries with small pillars in extra-thick seams, the above materials exhibit clear limitations. Ordinary cementitious systems possess relatively coarse particles and poor injectability, hindering penetration into the fine, multiscale fractures generated by repeated roof movement; their slow strength development cannot restrain the high initial deformation rate. Double-liquid or chemical systems improve fluidity and setting rate, but the rheology–setting window becomes narrow when long diffusion distances and early strength are simultaneously required; some systems also have higher cost or environmental sensitivity. Moreover, the mining-induced fracture field in gob-side entries is strongly directional and often connects to the goaf, so grouting must cooperate with grouted anchor cables to seal leakage paths and rebuild a continuous bearing ring—yet this coupling has not been fully clarified in prior reports. Consequently, there is a need for finer, faster, and more durable cement-based or cement–chemical composite grouts, together with systematic field-scale verification under real mining disturbance (Kang et al., 2023; Huang et al., 2022).
Recent advances in microfine, ultrafine, and nano-modified grouting materials offer a promising pathway. Ultrafine or microfine cement–fly-ash–silica-fume systems incorporating nano-SiO2 can enhance fluidity, shorten setting time, and accelerate early strength by supplying abundant hydration-nucleation sites, thereby enabling deep penetration into small fractures and rapid load-bearing after injection (Zong et al., 2025; Zhang et al., 2019). High-fluidity, ultra-early-strength grouts designed for fractured sandstone and coal–rock masses show that suitably proportioned ultrafine binders with nano-modifiers and dispersants can simultaneously satisfy the requirements of long-distance diffusion, short setting time, and early strength in underground reinforcement (Oppong and Kolawole, 2025). Emerging nano-/magnetically modified and expansion grouts further improve interfacial bonding and long-term strength recovery of fractured rock masses (Yao et al., 2024). These developments reflect a clear trajectory: from ordinary cement grouting, to composite/double-liquid systems, to microfine and nano-modified, performance-oriented grouts tailored to complex, mining-induced fracture fields.
Against this background, the present study targets gob-side roadways with small pillars in extra-thick seams, using roadway #5121 of the Tashan Coal Mine as a case. We first analyze the structural characteristics and deformation–failure mechanisms of the surrounding rock under narrow-pillar influence, with emphasis on main-roof fracture position and loading asymmetry. Building on this analysis, we develop a grouting-reinforcement technology that integrates grouted anchor cables with a nano-modified, two-component microfine grout to improve injectability, accelerate early strength, and enhance grout–rock interfacial bonding. Laboratory tests and numerical simulations are used to elucidate grout diffusion and strengthening mechanisms, and field monitoring compares reinforced and non-reinforced sections. The results show that the proposed technology reconstructs the bearing structure, significantly reduces roadway convergence, and improves overall stability. In contrast to existing practices that rely mainly on conventional cement or simple double-liquid systems (Zhao et al., 2010; Zhang et al., 2016; Li et al., 2024; Yang, 2017; Guan and Xiong, 2013), this work demonstrates the innovative application of nano-modified two-component grouting to gob-side entries with small pillars in extra-thick seams, providing practical guidance for safer roadway maintenance, improved resource recovery, and higher mining efficiency.
2 Mechanical analysis
2.1 Mechanical model in gob-side entry driving
In this study, during the continuous retreat mining of the upper section working face, the upper section gradually transforms into a goaf (mined-out area), with the immediate roof above it collapsing and separating from the main roof (in Figure 1). The fixed support edge of the main roof in the upper section working face is located on the solid coal side. Fracturing of the coal mass primarily occurs in the lateral coal mass, with the fracture line above the coal seam serving as the central axis. This results in rotation or bending subsidence, forming a hinged structure comprising rock blocks A, B, and C. The formation of rock block B arises from the collapse of the main roof at the end of the working face (Yang, 2019).
FIGURE 1
2.2 Structural model diagram of key block B in gob-side entry driving
As shown in Figure 1, the semi-arch articulated structure comprises rock block A, arc-shaped triangular block B, and rock block C (Yang et al., 2021). Block A represents the main roof rock beam above the working face of this section; block B corresponds to the arc-shaped triangular block on the goaf side of the upper section working face; and block C is the fractured block in the upper section goaf. The stability of this semi-arch articulated structure is directly influenced by the fracturing of the main roof rock beam and the collapse of the immediate roof. Under the combined effects of advanced and lateral support pressure, within the extraction influence range of the lower section working face, rock block B rotates and subsides, altering its position. This movement leads to compression and subsidence of the gangue beneath rock block C, while also causing compression and subsidence of the coal mass and roof associated with rock block A. Rock block B plays a pivotal role in maintaining the stability of large-scale structures within the overlying rock mass during gob-side entry driving.
The formation and stability of large structures in gob-side entry driving are influenced by the thickness and mechanical properties of the immediate roof and main roof, as well as the extent of coal seam extraction. The key block B, specifically the main roof beam in the overlying rock structure, is characterized by four fundamental parameters: (1) the fracture position S0 of the key block in the coal mass; (2) fracture length L1 (Ci) of the main roof along the working face advancement direction; (3) the lateral fracture span L2 of the main roof beam; and (4) thickness h of the old roof beam (i.e., thickness of the main roof beam).
The fracture position S0 of the key block in the coal mass is calculated as Equation 1:
Where Sm is the advance distance of the working face (m); ME is the maximum thickness of the fractured main roof beam (m); and CE is the periodic weighting step distance of the fractured main roof beam (m).
The lateral fracture span L1 of the main roof beam represents the periodic weighting step distance of the main roof.
The lateral fracture span of the main roof beam L2 is calculated as Equation 2:
Where L0 is the length of the working face (m).
3 Core mechanisms of grouting technology
3.1 Grouting reinforcement mechanisms
Grouting reinforcement technology for roadway excavation along the goaf is an important method to improve coal pillar stability and bearing capacity. Its core mechanisms are reflected in numerous aspects (Zhang P. et al., 2018; Zhang H. et al., 2018; Guo et al., 2017), such as enhancing the self-supporting capacity of surrounding rock, creating a network skeleton effect through grouting consolidation, reducing the loosening zone, preventing weathering and water seepage, and improving the stress state of anchor cables.
3.1.1 Enhancement of self-supporting capacity
The deformation characteristics of the fracture surface can be described by its normal stiffness and shear (tangential) stiffness:
Where σ and τ are the normal stress and shear stress on the fracture surface, respectively, and μ is the tangential displacement of the fracture surface.
Based on the shear tests, the relationship between the shear stress τ and the tangential displacement μ can be fitted as follows:
Where KS0 and τm are the initial tangential stiffness and the maximum shear stress of the fracture surface, respectively. According to the Mohr–Coulomb theory, the relationship between τm and σ is:where c and f are the cohesion and the internal friction coefficient of the fracture surface, respectively.
From Equations 3–7, it can be derived as Equation 8:
The initial stiffness KS0 can be expressed by the following empirical formula (Equation 9):
Where F, s, and n are constants. According to empirical data, the normal stiffness Kn of the fracture surface can be expressed as Equation 10:where Kno is the initial normal stiffness of the fracture surface, and Vm is a constant. The initial tangential stiffness Kn0 (before and after grouting) and the tangential stiffness KS are shown in Equations 11–14.
After grouting, the initial tangential stiffness of the fracture surface increases by (3.778σ0.14-1) times.
The shear strength parameters of the fracture surface before and after grouting are: Before grouting: f = 0.6, c = 1.2 MPa; After grouting: f = 0.7, c = 1.8 MPa.
The shear strength of the fracture surface can be expressed as Equations 15, 16:
It can be seen that, after grouting, the shear strength of the fracture surface increases by (0.6 + 0.1σ). The engineering case further shows that grouting markedly enhances both the stiffness and shear resistance of the fracture surface. This enhancement becomes more pronounced as the normal stress on the fracture surface increases, with the improvement in stiffness being especially significant. As a result, the upgraded stiffness and strength properties of the fracture surface after grouting strengthen the rock mass and consequently increase the bearing capacity of the small coal pillar.
3.1.2 Network skeleton effect of grouting consolidation body
Grouting forms a network skeleton structure within the coal pillar. Although the compressive strength of the consolidation body may not match that of the coal mass, its excellent bonding properties enable it to deform without failure under increasing loads. This mechanism transforms the failure conditions of the coal pillar from weak surface strength to conditions approximating coal mass strength, thereby significantly improving residual strength and stability.
3.1.3 Reduction of loosening zone
Grouting reinforcement effectively reduces the loosening zone of the surrounding rock. Analysis shows that the size of the loosening zone is closely related to ground stress and rock strength. By increasing rock strength, grouting reduces the loosening zone, decreases coal pillar deformation risk, and further enhances stability.
3.1.4 Prevention of weathering and water sealing
Grouting seals fractures, preventing moisture and air penetration, which can deteriorate the structure. This reduces the risk of weathering and softening in the surrounding rock. Additionally, grouting materials block water flow channels, preserving the physical and mechanical properties of the rock mass and further consolidating coal pillar stability.
3.1.5 Enhancement of anchor cable stress state
Grout penetration strengthens the bond between anchor cables and coal mass, enhancing anchoring force and support effectiveness. In loose or fractured coal masses, grouting significantly improves the anchoring performance of cables, increasing the overall support system’s strength.
3.2 Grout diffusion in surrounding rock
Grouting methods are selected according to geological conditions and the penetration capacity of the slurry, and can generally be divided into filling grouting, permeation grouting, and splitting (fracturing) grouting. For gob-side roadways supported by small coal pillars, where a large number of mining-induced fractures are developed, filling and permeation grouting are the main techniques adopted. According to flow–sedimentation theory, when grout enters a fissure, its velocity and pressure decrease rapidly with increasing distance from the injection point. Once the flow velocity falls below a critical value, solid particles begin to settle under gravity, which reduces the effective flow cross-section. This further alters the local pressure gradient and velocity distribution, while the remaining liquid phase continues to migrate along the fracture until the voids are gradually filled.
Figure 2 illustrates the simulated diffusion and spatial distribution of grout under the same grouting pressure (3 MPa) but with two different water–cement ratios (0.5:1 and 1:1). The simulations show that grout migration is strongly controlled by the pre-existing fracture network of the rock mass. In both cases, the slurry preferentially propagates along interconnected joints and cavities, whereas penetration into intact rock blocks is negligible. When the water–cement ratio is increased to 1:1, the diffusion radius and total injected volume both increase significantly compared with the 0.5:1 case, indicating that higher water content improves fluidity and thus promotes percolation into finer or deeper fractures.
FIGURE 2
The numerical analyses were carried out using UDEC (Universal Distinct Element Code), which is well suited to modeling fractured rock masses as discrete block systems. In the model, the strata were represented by deformable blocks separated by joints, and hydro–mechanical coupling was introduced to simulate grout flow within these discontinuities. Grouting was modeled by prescribing a constant injection pressure of 3 MPa at the borehole and tracking the transient evolution of fluid pressure and flow velocity throughout the joint network. Joint aperture, spacing, and connectivity were calibrated using field fracture surveys and laboratory permeability tests to ensure that the model reproduced the actual fracture characteristics of the coal–rock mass. The grout was treated as a Bingham-type fluid, with viscosity and yield stress parameters determined from rheological tests at different water–cement ratios. Diffusion was therefore simulated as a coupled process in which grout penetration simultaneously improved joint stiffness due to consolidation.
The simulation results indicate that, when rock splitting or hydraulic fracturing is not considered, grout mainly diffuses radially around the injection point and propagates outward along the most continuous fracture paths. Flow velocity is highest near the borehole and in shallow, highly fractured zones, but it decays rapidly with increasing distance because of pressure attenuation and the gradual narrowing of fractures. This behavior is consistent with theoretical predictions for flow–sedimentation processes in fractured–porous media. The comparison of the two water–cement ratios further demonstrate the dominant role of grout rheology: the 1:1 mixture, having lower viscosity, achieves a larger diffusion range and better coverage, but its final strength is relatively lower; by contrast, the 0.5:1 mixture produces a denser filling zone near the borehole while exhibiting a smaller penetration radius. These observations provide a useful basis for optimizing grouting parameters in field applications, especially in extra-thick coal seams where fracture development is complex and strongly anisotropic.
Figure 3 shows the attenuation curves of grouting pressure in both the radial and tangential directions of the roadway for the 1:1 mixture. In the radial direction, the grouting pressure drops sharply near the rock surface and then more gradually at greater depths, and the attenuation can be described by a quadratic relationship with diffusion distance. In the tangential direction, the grouting pressure exhibits a nearly symmetrical attenuation pattern about the grouting hole, reflecting the lateral diffusion characteristics of the slurry along the roadway.
FIGURE 3
3.3 Nano-modified two-component grouting materials
Grouting materials play a pivotal role in achieving an effective grouting outcome. A nano-modified two-component grouting material has been developed. This material is primarily composed of ultra-fine inorganic mineral powder with a D90 particle size under 10 μm, which is one-sixth that of #42.5 cement. It exhibits strong wettability, high injectability, rapid setting, early strength development, and strong adhesion, making it suitable for both low- and high-pressure grouting processes. The material achieves a 100% consolidation rate and slight expansibility, effectively preventing post-solidification shrinkage. The weight ratio of the two components (material A and material B) is 1:1. For well-developed fractures, the recommended water-cement ratio ranges from 0.5:1 to 0.8:1, while for less-developed fractures, it ranges from 0.8:1 to 1:1.
Figure 4 illustrates the bleeding rates of material A and B grouts under varying water-cement ratios. As shown, both grouts demonstrate an initial increase in bleeding rates with time, stabilizing later. At a water-cement ratio of 0.6:1, neither grout exhibits bleeding, and the bleeding rate remains unaffected by time. For the material A grout, the bleeding rate reaches a maximum of 1.52% at a water-cement ratio of 1.5 after 120 min of settling. When the water-cement ratio is ≤ 1.0, the bleeding rates are below 1%. Similarly, the material B grout exhibits no bleeding at water-cement ratios of 0.6:1 and 0.8:1, while at 1.5:1, the bleeding rate is 1.6% after 120 min. At ratios <1.5, bleeding rates remain below 2%.
FIGURE 4
Figure 5 highlights the compressive strength characteristics of the grouting material at different water-cement ratios. A lower water-cement ratio correlates with higher compressive strength of the solidified body, while a longer curing period results in further strength gains. For example, at a ratio of 0.6:1, the 2-h compressive strength reaches 20.1 MPa, whereas at 1.5, it drops to 2.5 MPa. From a strength perspective, field grouting is recommended with water-cement ratios between 0.6:1 and 1.3:1 to meet engineering requirements.
FIGURE 5
Table 1 demonstrates a comparison of mechanical properties between two grouting materials. Material I (nano-modified two-component grouting material) is characterized by a micro-fine particle size (D90 = 7.8 μm), whereas Material II (low-viscosity two-component polyimine resin) is liquid without a defined particle size. The micro-scale of Material I is consequential: it promotes diffusion into microcrack networks and coal cleats that are inaccessible to coarser slurries or neat resins, improving grout curtain continuity and sealing effectiveness in low-permeability media.
TABLE 1
| Materials | Particle size (D90)/μm | Setting time (min) | Compressive strength (MPa) | Adhesive strength (MPa) | ||||
|---|---|---|---|---|---|---|---|---|
| Initial set | Final set | 2 h | 24 h | 7 d | Coal | Rock | ||
| I* | 7.8 | 2–5 | 40 | 9.5 | 15.8 | 20.3 | 1.5 | 2.13 |
| II* | Liquid | 2 | 30 | 21 | 36 | 42 | 3.2 | 3.5 |
Comparison of mechanical properties between new and conventional grouting materials.
I*stands for the nano-modified two-component grouting materials; II*stands for the low-viscosity two-component polyimine resin material.
In terms of kinetics, Material I exhibits a fast yet controllable setting profile (initial set 2–5 min; final set 40 min). The broader initial-set window provides practical adjustability for field operations—reducing the risk of premature line blockage while still limiting washout—and the longer final-set time allows deeper migration before immobilization. By contrast, Material II reaches initial and final set at ∼2 min and 30 min, respectively, offering less operational latitude during injection.
Mechanical testing shows that Material I develops adequate early strength (2 h compressive strength 9.5 MPa) and reaches 20.3 MPa at 7 d, with bond strengths of 1.5 MPa (coal) and 2.13 MPa (rock). Although these values are lower than those of Material II (21/36/42 MPa at 2 h/24 h/7 d and 3.2/3.5 MPa adhesion), the moderate stiffness of Material I can be advantageous in deformable coal–rock masses: it lowers the likelihood of stress concentration and secondary cracking at the grout–matrix interface, supporting energy dissipation rather than brittle localization.
Overall, the data indicate that Material I is optimized for scenarios where penetrability, controllable setting, and compatibility with weak or highly fractured matrices are prioritized over peak strength—e.g., sealing fine fissures, reducing permeability, and stabilizing coal measures without imposing excessive rigidity.
4 Case study
4.1 Project overview
Working face #8121 is located in the western sector of the first mining district’s west wing. Its northern end connects to three main roadways in the 1070 west wing, and its southern end terminates at the F1532 fault boundary. The western side adjoins the goaf of working face #8121, whereas the eastern side is solid coal. Roadway #5121 serves as the return-air and transport roadway for working face #8121. It was excavated along the floor of coal seams No. 3-5, with a design length of 1660.647 m and a coal-pillar width of 10 m. The layout of working face #8121 is shown in Figure 6.
FIGURE 6
4.2 Design parameters of the grouting anchor cable support
The reinforcement employed Φ22.4 mm hollow, high-pressure grouting cables fabricated from 1 × 9 steel strands. Each cable is 4,300 mm long; the drill bit diameter is 36 mm; and the ultimate tensile capacity is ≥ 450 kN. Two resin cartridges were used (MSK2860 and MSZ2860).
A high-strength bearing plate (300 × 300 × 16 mm) with a spherical washer was installed at each collar. Cables were arranged in a rib pattern of “2-0-2”, with 1,800 mm row spacing and 950 mm column spacing. All cables were installed vertically into the ribs and tensioned to 250 kN. The cross-sectional arrangement is shown in Figure 7.
FIGURE 7
4.3 Implementation of high-pressure grouting cables
After the baseline support (anchor cable-mesh-bolt plus shotcrete) was completed, high-pressure grouted cables were installed between steel bands #333 and #555 to reinforce the surrounding rock. A nano-modified, two-component microfine grout (D90 < 10 μm) was adopted. Key properties were: initial set 3–20 min, final set 5–30 min, 28-day compressive strength ≥30 MPa, and coal–grout bond strength ≥3.5 MPa. Component A and B powders were mixed at a 1:1 volumetric ratio with a water-to-cement ratio of 0.6:1–0.8:1, adjusted to site conditions.
Mixing and delivery used two-component synchronous electric pumps with high-speed mixers. Injection was performed through the hollow cables at a terminal pressure typically limited to 5–10 MPa and, where necessary to ensure grout take, briefly raised to 10–20 MPa. The locking device, mixing head, and plate matched those of the existing Φ21.8 mm cables. After installation, the plate, spherical washer, and lock were mounted and the cables tensioned to seal the collar and enable subsequent high-pressure injection via the grouting connector (Figure 8).
FIGURE 8
The grouting construction procedure comprised equipment setup, slurry preparation and ratio calibration, injection, shutdown and cleaning, and operational control. First, the pump and mixing tanks were installed on a level pad, and the suction and discharge lines were laid as straight as possible, avoiding twists, crossings and sharp bends. System priming and functional checks were conducted by circulating clean water for more than 1 min with the grouting valve open, followed by briefly running the pump dry to expel residual water. Slurry preparation involved adding the prescribed amounts of water to the A- and B-tanks, starting the mixers, and then introducing equal masses of A- and B-component powders under continuous agitation. After approximately 10 min of mixing, the pump suction was immersed in the blended slurries; with the relief valve closed and the grouting valve open, equal discharge at the A and B outlets (A:B = 1:1 by mass) was verified, after which the hoses were returned to the tanks for recirculation. For injection, the discharge hose was quickly connected to the grouting pipe, the grouting valve was opened with the relief valve closed, and pumping commenced. Upon completion of each hole, the relief valve was opened to depressurize the system and the grouting valve was closed before moving to the next hole. For batch changes or stoppages, the system was first depressurized (relief valve open, grouting valve closed), the suction line was transferred to clean water, and the pump was operated with the discharge valve open until the outflow ran clear; subsequently, the discharge valve was closed, the relief valve reopened, and circulation continued until clear water was observed at the relief outlet, while the mixing tanks were cleaned concurrently. During operation, leakage points were sealed immediately, using manual sealing for minor leaks and adjusting grout setting time in the case of large or continuous leakage to balance flowability and sealing effectiveness. A functional pressure gauge was mandatory, and any faulty gauge was replaced without delay. The injection rate was regulated within 10–25 L min-1 according to the pressure response, and a bottom-to-top grouting sequence with a pre-planned hole order was strictly enforced, with shift-by-shift verification to prevent omissions.
During field implementation, 176 of the 240 designed hollow grouting cables were successfully injected; the remaining cables could not be grouted owing to localized cable-head failures and excessively short exposed lengths after shotcreting. The total grout consumption was 7.24 t, corresponding to an effective uptake of approximately 25–50 kg per cable. The relatively uniform grouting volumes along the reinforcement zone, together with the absence of abnormally high local grout takes, indicate that no large open fractures or cavities were encountered within the treated coal mass. Throughout construction, the grouting pressure was generally maintained below 10 MPa and stabilized at approximately 5 MPa under normal operating conditions. The on-site grouting process for the hollow cables is illustrated in Figure 9.
FIGURE 9
4.4 Effectiveness evaluation
4.4.1 Borehole observations
Borehole imaging revealed material-B grout traces at 0.1–0.8 m, 1.2 m, and 1.7–1.9 m on the mining side, and at 0.1–0.3 m, 1.6 m, and 3.1–3.5 m on the coal-pillar side (Figure 10). The coal mass remained largely intact with few fractures, consistent with the moderate grout take. Nevertheless, the observed grout penetration along existing fissures indicates effective integration with the coal matrix, which is expected to improve integrity and strength. Follow-up monitoring was undertaken to quantify these effects.
FIGURE 10
4.4.2 Roadway surface displacement analysis
The surface convergence in grouted and non-grouted roadway sections is shown in Figure 11, from which the following conclusions are reached: a) In both the grouted and non-grouted sections, the surface convergence initially increases then stabilizes with the distance from the excavation face to the measuring station. When the measuring stations are closer to the excavation face, the rate of surface convergence rapidly increases. However, as the distance from the face increases, the rate of convergence gradually drops and eventually stabilizes. b) At a measuring station 200 m from the excavation face, the non-grouted section shows roof subsidence of 35 mm, floor heave of 145 mm, solid coal side convergence of 80 mm, and coal pillar side convergence of 101 mm. c) At a measuring station 200 m from the excavation face, the grouted section shows roof subsidence of 42 mm, floor heave of 104 mm, solid coal side convergence of 46 mm, and coal pillar side convergence of 57 mm d) When comparing the grouted section to the non-grouted section, the roof subsidence remains relatively unchanged, while the floor heave decreased by 28%, solid coal side convergence reduced by 43%, and coal pillar side convergence decreased by 44%.
FIGURE 11
4.4.3 Bolt load monitoring
Figure 12 shows the working resistance of bolts in both grouted and non-grouted sections, from which the following conclusions are reached: a) As the working face advances, after grouting is implemented at 60 m from the face in the grouted section, the bolt working resistance becomes quite stable. In the non-grouted section, the bolt working resistance gradually increases within 0–120 m from the face, then stabilizes in the range of 120–200 m from the face. b) Comparing the bolt loads between grouted and non-grouted sections reveals that the surrounding rock structure becomes more intact after grouting. This helps reduce roadway deformation pressure, thereby resulting in some reduction in bolt loads.
FIGURE 12
4.4.4 Bolt anchorage tests
Table 2 shows the pull-out test results for both the grouted and non-grouted sections, from which following conclusions are reached: a) All bolts in the grouted section achieve an anchoring force of 189 kN, while two bolts in the non-grouted section fail to meet the standard. b) Grouting helps restore the integrity of surrounding rock structure. The penetration and injection of grout enhances the bonding between bolts and coal body, effectively improving the anchoring performance of the bolts, thereby increasing the overall support system strength.
TABLE 2
| Cross-section location | Bolt position | Pull-out force (kN) | Remarks |
|---|---|---|---|
| Steel belt #350 | Coal pillar side | 189 | Grouted section |
| Steel belt #423 | Coal pillar side | 189 | |
| Steel belt #483 | Coal pillar side | 189 | |
| Steel belt #501 | Solid coal side | 189 | |
| Steel belt #524 | Solid coal side | 189 | |
| Steel belt #239 | Coal pillar side | 189 | Non-grouted section |
| Steel belt #309 | Coal pillar side | 189 | |
| Steel belt #604 | Coal pillar side | 150 | |
| Steel belt #621 | Solid coal side | 189 | |
| Steel belt #647 | Solid coal side | 120 |
Bolt anchoring force test results in an experimental roadway.
5 Conclusion
This study addresses the mechanical model in gob-side entry driving and implemented a grouting anchor cable support in #5121 roadway. The key conclusions are as follows.
The structural changes in the surrounding rock of small coal pillar roadways along the goaf progressed through two distinct stages. The first stage occurred during the excavation of the roadway, where the excavation process itself induced changes in the rock structure. The second stage took place during the retreat mining of the working face, where mining activities further altered and restructured the surrounding rock.
A comparative analysis of the grouted and non-grouted sections in small coal pillar roadway #5121 revealed that grouting had a significant impact on controlling deformation. While roof subsidence showed no notable change, the floor heave decreased by 28%, solid coal side convergence was reduced by 43%, and coal pillar side convergence decreased by 44%.
The implementation of grouting in the small coal pillar roadway notably stabilized bolt working resistance and significantly reduced the bolt failure rate. Grouting not only restored the integrity of the surrounding rock structure but also enhanced the overall strength of the support system, thereby improving roadway stability.
Statements
Data availability statement
The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.
Author contributions
XX: Conceptualization, Data curation, Formal Analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Software, Supervision, Validation, Visualization, Writing – original draft, Writing – review and editing. ZL: Formal Analysis, Methodology, Resources, Validation, Writing – original draft, Writing – review and editing.
Funding
The author(s) declared that financial support was not received for this work and/or its publication.
Conflict of interest
The author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
Generative AI statement
The author(s) declared that generative AI was not used in the creation of this manuscript.
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Summary
Keywords
extra-thick coal seam, gob-side entry driving, grouting, small coal pillar, surrounding rock control
Citation
Xu X and Li Z (2026) Research and application of grouting reinforcement technology for small coal pillar roadways along gob in extra-thick coal seams. Front. Earth Sci. 14:1760730. doi: 10.3389/feart.2026.1760730
Received
04 December 2025
Revised
08 January 2026
Accepted
06 February 2026
Published
02 March 2026
Volume
14 - 2026
Edited by
Faming Huang, Nanchang University, China
Reviewed by
Zizheng Zhang, Hunan University of Science and Technology, China
Changtai Zhou, City University of Hong Kong, Hong Kong SAR, China
Updates
Copyright
© 2026 Xu and Li.
This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
*Correspondence: Xiang Xu, xuxiang@tdkcsj.com
Disclaimer
All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.