Safety Mining Technology of Coal Seams in Weathered Zone Based on Ground J-Type Pregrouting Reinforcement

A thin basement coal seam mining model under different overlying strata conditions was developed using the discrete element software UDEC. ,is approach is used to discuss the safety mining of the thin basement coal seams. Fracture development in overlying and rock strata movement law in the stope was discussed. ,e relationship between support and surrounding rocks under different overlying strata conditions was analyzed. Lastly, a field industrial test was conducted based on the research results. A fewmajor conclusions could be drawn. Under load transmission in loose water-bearing strata, causing a large-scaled rock strata movement to advance into the working face is easy when only one bearing stratum exists in the overlying strata. Meanwhile, the support bears strong loads, which can easily be collapsed. When two bearing strata exist in the overlying one, the upper bearing stratum can form a voussoir beam structure. Loads on the support decreased substantially compared with those under single bearing stratum, whilst the probability of pressing frame was reduced accordingly. A weathered zone above the stope was reinforced by ground J-type drilling pregrouting, thereby improving the physical and mechanical properties and increasing the bearing capacity of the rock strata in the grouting range and safety mining of the working face in the lower coal seams. Research results provide important references for the safety mining of thin basement coal seams under similar conditions.


Introduction
Given the continuous increase of upper mining limit in mines in eastern and northern China, water and sand inrush accidents induced by mining of the working face in thin basement coal seams have become increasingly prominent in recent years [1].Chinese scholars have reported extensive studies on underwater mining and accumulated substantial practical experiences given the mining technology development.However, mining in special geological conditions, such as thick loose water-bearing stratum and thin basement, continuously face a series of problems, which attributes to hydrogeological differences in various regions, water abundance in loose stratum, and increased upper mining limit.Water and sand inrush accidents occur occasionally, thereby considerably threatening the safety production of coal mines [2][3][4].Safety mining in thick loose water-bearing strata also becomes a hindrance.
A series of Chinese studies have been conducted on the safety mining of a thin basement shallow seam in thick loose water-bearing strata.It is widely acknowledged that once mining-induced fractures extend upward into confined aquifers, these fractures can serve as passageways between aquifers and workings, triggering water and sand inrush.Xu et al. [5] proposed the design method of sand-prevention coal and rock pillar for the safety of the working force in loose water-bearing strata and thin basement under hydraulic loads.Guo et al. [6] suggested to maximize thin basement coal seams by reducing the mining height.Liu and Song [7][8][9] recommended mining based on water drainage.Guo et al. [10] and Xu et al. [11] studied the development law of fractures in overlying strata in a thin basement stope.Tan et al. and Liu et al. [12,13] given the height of the waterconducting fractured zone is highly related to mining conditions and the lithology of the overlying strata, and empirical formulae for height estimation were given based on numerous in situ observations.e design of waterproof pillars, water drainage mining, and mining with filling can assure safety mining of thin basement coal seams.However, given their limitations, the design of waterproof pillars is a waste of coal resources, and water drainage destroys the groundwater environment, while mining efficiency with filling has to be increased.Hence, a new thin basement mining technique should be explored to realize highefficiency safety mining.
A hydraulic pressing frame accident occurred on the completely mechanized coal mining face 1202(3) in a thin basement coal seam in Gubei Coal Mine.e working face 1512 (3) in this coal mine has similar geological conditions with those of the working face 1202 (3).
e manner of protecting the safety mining of the working face 1512(3) becomes one of the problems that engineering technicians have to resolve immediately.e coal bed pitch is 5 °-8 °.Oblique mining is adopted, which has +17.32 to +22.12 m corresponding elevation.e comprehensive column graph in drilling holes #12-13 Kz4 close to the open-off cut on the working face is shown in Figure 1.

Brief Introduction to the Project
e lithology of the weathered zone is mainly manifested as mudstone and sandy mudstone, which have low compressive strength values (<10 MPa).

Establishment of the Model.
e working face 1512(3) was used as an engineering background in this study.A horizontal mining model was developed for the working face owing to the small coal bed pitch and for calculation convenience.e discrete element software UDEC was applied in the simulation, while the Mohr-Coulomb model was used as well.Mining models with single and double bearing layers in the overlying strata were developed (Figure 2).e model size was length × height � 200 m × 80 m.
e rock mechanical parameters and the physical parameters of each rock stratum are shown in Table 1.e models were used to simulate fracture development and movement laws in the overlying strata after mining of the working face.Moreover, the models adopted fixed left, right, and lower boundaries but not free upper boundary.
e measured hydraulic pressure and buried depth in a loose water-bearing layer indicated that a 11.5 MPa vertical load was applied on the upper boundary to simulate load transmission in the loose strata.

Analysis of Numerical Simulation Results.
e fracture development in the overlying and rock strata movement laws when single and double bearing layers exist in the working face is shown in Figures 3 and 4, respectively.e fracture in the rock strata above the main roof further developed as a response to the first periodic fracture of the main roof (bearing layer) when the advancing distance in the working face was 40 m (see Figure 3).When the advancing distance was 50 m, the fractures above the main roof run through a hydraulic load transmission in the loose overlying strata.
is condition resulted in a large-scale subsidence of rock strata above the main roof.Under this circumstance, causing crushed hydraulic support in the working face is easy [14].Subsequently, the rock strata above the main roof broke down with the main roof.
Fractures began to develop in the overlying rocks above the main roof (low bearing strata) when the advancing distance was 45 m (see Figure 4).When the advancing distance was 50 m, the rock strata controlled by the main roof began to sink substantially because of the periodic fracture and rotary subsidence of the main roof.e rock strata below the high bearing strata developed transverse delamination fractures, while the high bearing strata did not subside with the rotary sinking of the lower main roof. is condition was considerably different from the movement characteristic of the overlying rocks under single bearing strata.Under this circumstance, the hydraulic support only has to assume fractured rock strata from the position above the support to the positive below the high bearing strata.Subsequently, the transverse crack range expanded with the periodic fracture of the main roof, while the high bearing strata began to bend with an increase in internal cracks.When the advancing distance was 90 m, the high bearing strata began to fracture and subside, thereby forming the "voussoir beam" structure.When the pressure transmits from the overlying strata to the distant working face, the stress on the hydraulic support on the working face declined, thereby decreasing the probability of pressing frame [15].
In summary, stress on the hydraulic support decreased substantially when at least two bearing strata exist in the overlying strata in thin basement coal seams.Similarly, the probability of pressing frame was decreased.Hence, causing crushed hydraulic support on the working face when only one bearing stratum exists in the overlying strata is easy.e preceding research conclusions and previous engineering experiences indicated that a thin basement coal seam mining test should be performed by creating a high bearing stratum in the weathered zone through artificial pregrouting.

Grouting Reinforcement and Effect Evaluation
4.1.Grouting Scheme.A distribution pattern of J-type horizontal grouting holes on winds was designed in the weathered zone on the roof of the working face 1512 (3) in the Naner Mining Area of Gubei Coal Mine.Ground pregrouting was performed on the weathered zone at the working face roof by combining oriented drilling and horizontal pregrouting technologies.After the preset depth was reached by drilling, horizontal drilling was initiated, while the vertical profile was in J-shape.e horizontal drilling holes were in wing-shaped distribution.is distribution of pregrouting holes was novel and applicable to 2 Advances in Civil Engineering pregrouting reinforcement in the weathered zone at the working face roof.

Grouting Parameters.
e grouting adsorption capacity of the surrounding rocks was considerably di erent owing to various lithologies, fracture development degrees, and loose ranges.Moreover, such capacity was in uenced by grouting pressure and time.In principle, grouting will be continued until the termination of the grouting adsorption to assure compact lling in fractures.Advances in Civil Engineering 3 (1) A pore diameter in the grouting section was designed φ152 mm.(2) Follow-up grouting: grouting was performed every 30 m and advances continuously thereafter.(6) e single liquid was adopted and the rock fracture rate was 8%.
(7) e grouting amount was calculated as follows: where λ is the loss coefficient of the grouting liquid (λ � 1.1), V is the grouting volume (V � 386, 460 m 3 ), Η is the fracture rate (η � 9%), Β is the filling rate of the grouting liquid (β � 0.8), and n is the setting percentage of the grouting liquid (0.9). erefore, Q � 1.1 × 386, 460 × 8% × 0.8/0.9� 30,230 m 3 .e theoretical and numerical calculations indicated that the grouting pressure was set to be 10 MPa initially.e grouting material with the P.O42.5 ordinary Portland cement and single-liquid ordinary Portland cement slurry was prepared.e water-cement ratio was 0.6 : 1-0.75 : 1, while the single liquid was the dominant material.

Grouting E ect.
e comprehensive column graphs of the #13-#13 Kz4 holes in Figure 1 show that the region in    Advances in Civil Engineering 20-50 m above the coal seam roof was the weathered zone.e characteristics of the grouting di usion indicated that this grouting was implemented in the 434.43-447.55 m deep weathered mudstone layer.
Rocks before and after the grouting were observed through SEM.Rock cores in the same layer in the grouting and nongrouting sections were selected in preparation of the scanning samples to observe the microscopic changes of rocks before and after grouting (Figure 6).In the weathered zone without grouting reinforcement, rock mass showed a rough surface and an evident defect.Rock mass was relatively smooth in the weathered zone with grouting reinforcement.High-pressure grouting relatively compacts and lls the microscopic defects in the weathered rock mass.
e mudstone core in the weathered zone was crushed, while the nonrock core conformed to the experimental standards.A shearing resistance experiment was conducted with a few nonstandard specimens with approximately 16.5-45 mm height.
e experimental results re ected that the mean values of the shear strength and cohesion were increased by approximately 40%.Grouting can substantially improve the strength of the mudstone in the weathered zone.Advances in Civil Engineering

Stress Analysis on Supports before and after Grouting
e relation curve between the support and the surrounding rocks was drawn based on the numerical calculation results and reference [16] to compare the stresses on the hydraulic supports before and after the grouting (Figure 7).
When two key layers exist in a thin basement below the loose water-bearing strata, the calculation formula of the working resistance (P 1 ) of support in Figure 7(a) is as follows [17]: where Q 1 is the weight of rock mass ① from the region above the support to the direct roof below key layer 1 (kN/m 3 ) and ξQ 2 is the force applied by the rotation of knife-shaped rock mass ② below the key layer 2 on rock mass ① (kN/m 3 ).e calculation formula of working resistance of support in Figure 7(b) is as follows: where Q 1 is the weight of rock mass ① from the region above the support to the direct roof below key layer 1 (kN/ m 3 ), ξQ 2 is the force applied by the rotation of the knifeshaped rock mass ② below the key layer 2 on rock mass ① (kN/m 3 ), and μQ is the force applied by the effect from the loose water-bearing strata on the roof of rock mass ②.
Figure 7 shows that the range of rock mass ② above the single bearing stratum in Figure 7(b) was substantially larger than that above the two bearing strata in Figure 7(a).Hence, ξQ 2 is substantially smaller than ξQ 2 .Moreover, P 2 involves the additional pressure from the loose water-bearing strata μQ compared with P 1 .Accordingly, the working resistance of support when two bearing strata exist in a thin basement is considerably lower than that under single bearing stratum.

Roof Stability and Strata Behavior Characteristic
Real-time monitoring of the working resistance of support in the working face was conducted to the protect safety mining of the working face.e three-dimensional contour map of the working resistance of the hydraulic support was drawn based on the monitoring results (Figure 8).When the advancing distance was 70 m (i.e., the third period pressure on the working face), the working resistance of the hydraulic support was substantially higher than those of the previous two periods (see Figure 8).Accordingly, this hydraulic stress is a large period pressure.At this moment, the working face experienced three small periodical weightings.When the advancing distance is 120 m, the working face developed the second strong periodical weighting (see Figure 8).At this point, the working face experienced seven periodical weightings, thereby conforming to previous numerical simulation results.
e step distance of periodical weighting during the initial period at different positions of the working face was 13-15 m.With regard to the advances to the working face, the step distance of periodical weightings at different positions of the working face began to decrease to 11-13 m after the 8th periodical weighting.e initial weighting step distance on the basic roof at different positions of the working face was 27.7-36.5m, with an average of 31.5 m. e periodical weighting step distance on the basic roof at different positions of the working face was 9.9-18.3m, with an average of 13.20 m.In particular, the weighting step distance in the middle range of the stope was small, but the step distances at the upper and lower ends were relatively large.
e initial and periodical weighting step distances were slightly higher than those on the working face without grouting reinforcement under similar conditions.

Conclusions
In this study, the working face 1512(3) in a thin basement coal seam in Gubei Coal Mine is used as the research object.
e roof stability of different strata structures on this working face is discussed through numerical analysis, laboratory test, and field investigation.
e following major conclusions are drawn: (1) Under the load transmission in loose water-bearing strata, causing a large-scaled roof subsidence in the overlying strata when a single bearing stratum exists

Advances in Civil Engineering
e upper and lower limit heights of coal mining in the working face 1512(3) are −391 m and −497 m, respectively.

( 3 )
e diffusion radius of grouting was 15 m.(4) Grouting range: 60 m long along the stride of the working face and 200 m wide along the inclined direction.(5) Grouting volume: 386, 460 m 3 .

e
grouting material used P.O42.5 ordinary Portland cement.e single-liquid ordinary Portland cement slurry was prepared, while the water-cement ratio ranged between 0.6 : 1 and 0.75 : 1. e single liquid occupied the dominant role.e cement content was approximately T 35,200 × 0.75 22,673 (tons).Four grouting holes were designed.e distances of the #1 to the #2 and 3# holes were 30 m and 40 m, respectively.e distribution pro le of the grouting holes is shown in Figure 5. e grouting depth in the weathered mudstone strata was designed to be 434.43-447.55 m. e grouting length from the airway 1512(3) to the region above the machine land 151(3) was designed to be 200 m.e grouting width from the open-o cut of the working face to the region 60 m above the stope direction was designed to be 60 m.

Figure 3 :
Figure 3: Movement law of the overburden strata when only one hard rock exists: advancing distances of the working face are (a) 30 m, (b) 40 m, and (c) 50 m.

Figure 4 :
Figure 4: Movement law of the overburden strata when two hard rocks exist: advancing distances of the working face are (a) 35 m, (b) 45 m, (c) 50 m, and (d) 90 m.

Figure 5 :
Figure 5: Ground J-type grouting hole distribution: (a) plan and (b) pro le views.

Figure 6 :
Figure 6: Comparison of the microstructures of rock masses in the weathered zone (2000 magnifications, 20 με): microstructure of the samples in the (a) unreinforced section and (b) after grouting reinforcement.

Figure 8 :
Figure 8: Face hydraulic support working resistance contour map.

Figure 7 :
Figure 7: Working face hydraulic support that surround the rock relations: (a) double and (b) single bearing layer structures.

Table 1 :
Mechanical properties of coal and rock.