Gob-side entry retaining method based on modular prefabricated hollow grouting wall

Abstract

The invention discloses a gob-side entry retaining method based on a modular prefabricated hollow grouting wall, and belongs to the technical field of coal-pillar-free mining in the coal industry. The gob-side entry retaining method comprises the following steps that S1, roadway measurement and wall parameter design are conducted; s2, a hollow wall is prefabricated, and underground transportation is completed; and S3, underground splicing installation and high-pressure grouting are carried out. The problems that a traditional flexible formwork wall is poor in durability, complex in construction and insufficient in bearing capacity are effectively solved through the prefabricated hollow wall body and the rapid construction technology. The C30 early strength concrete prefabricated wall body is high in strength and long in service life, and the durability is improved by more than two times compared with a flexible formwork wall; the modular design realizes ground prefabrication and underground rapid splicing, the construction period is shortened, the single-shift installation efficiency is improved, and the working face connection is obviously accelerated. The hollow structure and high-pressure grouting form a prefabricated shell and core concrete composite stress system, the mining-induced stress resistance is improved, the surrounding rock deformation is reduced, and strong stress concentration can be independently coped with.

Description

Technical Field

[0001] The present invention belongs to the technical field of coal pillar-free mining in the coal industry, and particularly relates to a goaf-side lane retaining method based on modular prefabricated hollow grouting walls. Background Art

[0002] Gob-side entry is a common tunnel layout method used in coal mining. During the recovery process of the coal face, a tunnel is reserved on one side of the coal seam gob for use by subsequent coal mining faces. This technology has gradually matured with the improvement of efficient fully mechanized mining and the degree of mine intensification. It is particularly suitable for mines with coal seams of a certain thickness, relatively stable roofs, and controllable surrounding rock. The advantages of gob-side entry are significant. First, it reduces the amount of excavation work, improves the coal recovery rate, and saves a lot of manpower and material resources. Second, it shortens the preparation cycle of the coal face and improves the efficiency of mining and excavation connection. Third, it facilitates the rational layout of the mine ventilation system, reducing air leakage and ventilation resistance. Fourth, by reducing the duplication of tunnel layouts, it reduces the impact of ground pressure and safety hazards, and improves the overall safety of the mine. Its core concept is to make tunnels reusable without further excavation through the rational arrangement of support structures and the control of surrounding rock stress, greatly improving mining efficiency and economic benefits. Overall, goaf-side entry retention technology is an important means to achieve efficient, safe and low-cost production in coal mines, and will play a greater role in intelligent and green coal mining in the future.

[0003] In gob-side entry retention, flexible formwork walls are often used to isolate the goaf from the entry space, effectively mitigating the impact of mining stress on the entry and reducing surrounding rock damage. They gradually deform under the influence of mining, delaying stress release and reducing stress on the entry support system. While flexible formwork walls offer excellent deformation capacity and cushioning effects, they also have several drawbacks. First, the durability of flexible formwork materials is poor, susceptible to aging and corrosion, resulting in a short service life. Second, their construction process is relatively complex, slowing down working face continuity and increasing costs. Third, the flexible formwork walls themselves have limited bearing capacity, making them unable to independently cope with the intense stress concentrations caused by intense mining, requiring the coordinated action of other support methods. Finally, their structural deformation is often irreversible, making repair difficult and impacting subsequent performance. To address these shortcomings of flexible formwork walls, a prefabricated hollow gob-side wall and a rapid construction process for gob-side entry retention have been developed. This wall boasts high strength, a long service life, and effectively mitigates the impact of mining stress on the entry and reduces surrounding rock damage. The simple and rapid construction process expedites working face continuity and improves mining efficiency. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for retaining a tunnel along the goaf based on modular prefabricated hollow grouting walls, which solves the technical problem that the traditional flexible formwork wall construction process is relatively complicated and leads to slow connection of the working surface.

[0005] The technical solution adopted by the present invention is a method for retaining a tunnel along the goaf based on modular prefabricated hollow grouting walls, comprising the following steps: S1: Conduct tunnel measurement and wall parameter design; S2: Prefabricate hollow walls and complete underground transportation; S3: Implement underground splicing installation and high-pressure grouting.

[0006] The present invention is also characterized in that: S1 specifically includes: measuring the height difference between the top and bottom plates of the supporting section through a total station or a tape measure, and taking the average value at different cross sections to determine the tunnel height; establishing a mechanical model based on the separated block method to calculate the load of the tunnel side support body and the width of the tunnel side wall.

[0007] The calculated load of the tunnel support body is:

[0008] Where, q is the load of the roadside support body; b B The distance from the inner side of the roadway support to the coal seam; x is the width of the tunnel side support; b C is the outer cantilever distance of the roadside support; L is the length of the roof separating the rock blocks, L=b B +x+b C ; c s is the direct top bulk density; h is the mining height; θ is the shear angle; α is the coal seam inclination; Calculate the width of the wall along the side of the lane as follows:

[0009] Where Q1 is the initial bearing capacity required per unit length of the filling body; oh is the width of the wall along the void; k 3 is the design wall safety factor; k 2 is the reliability coefficient of dynamic pressure influence; k 1 is the reduction coefficient of wall strength; s is the final setting strength of the wall along the void.

[0010] The wall length is 1.8m~2.2m, the hollow channel diameter is 80mm~100mm, the number of hollow channels is 2~4, the channel axis is parallel to the wall axis and the spacing is ≤500mm.

[0011] The prefabricated hollow wall in S2 is specifically: S2.1: Precast hollow walls shall be constructed using C30 early-strength concrete with an initial setting time of 2-4 hours, a final setting time of ≤8 hours, and a compressive strength of ≥30 MPa; S2.2: Embedded grouting splicing structures are set on both sides of the prefabricated hollow wall. The left side is a boss-type splicing surface with an embedded female grouting interface; the right side is a groove-type splicing surface with an embedded male grouting interface. The interface is connected to the hollow channel inside the wall to form a grouting network.

[0012] The concrete mix ratio is: cement: sand: crushed stone = 1:1.5:2.5, with 3% early strength agent and 0.3% polypropylene fiber added, and the water-cement ratio is 0.26~0.30. Through orthogonal test optimization, the flexural strength is ensured to be ≥4.5MPa and the ultimate compressive strength is ≥20MPa.

[0013] S3 is specifically: S3.1: The trench depth is 80mm~120mm, and a laser leveler is used to control the flatness error to ≤3mm; S3.2: When prefabricated hollow walls are spliced, the grouting interface is automatically aligned to achieve precise docking of the male and female heads. The grouting pipes are inserted simultaneously, and a dual-liquid grouting pump is used to pump cement-stone powder composite slurry until slurry emerges from the overflow holes of the adjacent walls.

[0014] The grouting slurry has an initial setting time of ≤45 minutes and a 28-day compressive strength of ≥30 MPa, and self-compacting filling is achieved by adding 1‰ anti-segregation agent to the slurry.

[0015] Also includes S4: S4.1: Embed vibrating wire stress gauges at the interface between the prefabricated hollow wall and the roof to monitor mining stress changes in real time. When the stress gradient is greater than 0.5 MPa / m, implement secondary compensatory grouting through the reserved grouting holes. S4.2: Use distributed optical fiber to monitor the internal strain of the wall. When the strain difference is greater than 150 with When the alarm is triggered, the sound and light warning will be automatically triggered.

[0016] Compared with the prior art, the present invention has the following beneficial effects: This invention effectively solves the problems of poor durability, complex construction, and insufficient bearing capacity of traditional flexible formwork walls through prefabricated hollow walls and rapid construction technology. The prefabricated walls are made of C30 early-strength concrete, which has high strength and long life, and is more than twice as durable as flexible formwork walls. The modular design enables ground prefabrication and rapid underground splicing, shortening the construction period, improving single-shift installation efficiency, and significantly accelerating the connection of working surfaces. The hollow structure and high-pressure grouting form a "prefabricated shell + core concrete" composite force system, which improves the ability to resist mining stress, reduces surrounding rock deformation, and can independently cope with strong stress concentration, reducing dependence on other supports. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 Schematic diagram of the physical model of the gob-side entry retaining filling in the gob-side entry retaining method of the present invention; Figure 2 Schematic diagram of the force calculation model of the filling body beside the gob-side entry retaining method of the present invention; Figure 3 Schematic diagram of prefabricated segmented gob-side walls in the gob-side entry retaining method of the present invention; Figure 4 It is a schematic diagram of splicing prefabricated segmented gob-side walls in the gob-side tunnel retaining method of the present invention. DETAILED DESCRIPTION

[0018] The following will be combined with the drawings in the embodiments of this application to clearly and completely describe the technical solutions in the embodiments of this application. Obviously, the embodiments described are only part of the embodiments of this application, not all of the embodiments. Based on the embodiments in this application, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of this application.

[0019] Example 1 The method for retaining a tunnel along the goaf based on modular prefabricated hollow grouting walls disclosed in the present invention comprises the following steps: S1: Conduct tunnel measurement and wall parameter design; S2: Prefabricate hollow walls and complete underground transportation; S3: Implement underground splicing installation and high-pressure grouting.

[0020] Example 2 Based on Example 1, S1 is specifically as follows: measuring the height difference between the top and bottom plates of the supporting section tunnel by a total station or a tape measure, and taking the average value at different cross sections to determine the tunnel height; establishing a mechanical model based on the separated block method, and calculating the load of the tunnel side support body and the width of the tunnel side wall.

[0021] Specifically, benchmarks are set up on both sides of the support tunnel. A total station can be used to measure the elevation difference between the roof and floor. This approach is suitable for high-precision measurements of longer tunnel sections. Alternatively, a tape measure can be used to manually measure the vertical distance from the tunnel top to the floor, averaging the values ​​across different cross sections. However, caution must be exercised regarding operational errors.

[0022] Furthermore, the wall length is 1.8m~2.2m, the hollow channel diameter is 80mm~100mm, the number is 2~4, the channel axis is parallel to the wall axis and the spacing is ≤500mm.

[0023] Specifically, when leaving a tunnel along the goaf in an inclined coal seam, after the coal mining face advances a certain distance, a tunnel side filling body (wall along the goaf) is constructed on the inclined goaf floor to support the roof. When the tunnel side filling body is just constructed, it has not yet supported the overlying rock layer, and the roof pressure can be ignored. At this time, it is most likely to slide or overturn; as the roof gradually rotates and sinks, the tunnel side filling body begins to cut the roof. At this time, the support resistance of the tunnel side filling body is its maximum support resistance. Figure 1 Physical model shown.

[0024] The mechanical model of gob-side entry retention generally includes five key points: ① The gob-side entry retention mechanical model is considered a planar problem; ② The roof of the gob-side entry retention is considered a load-bearing beam structure, and the thickness of the beam is usually based on the effective reinforcement thickness of the anchor cable; ③ The load borne by the upper part of the load-bearing beam is uniformly distributed, and the magnitude of the load is related to the buried depth of the roadway; ④ The change in the length of the cantilever beam on the gob side of the roadway support is the main factor causing the dynamic load of the roadway support, and the maximum length of the load-bearing beam is equal to the step distance of one cycle (the distance from the gob cantilever roof to the coal wall = the length of the load-bearing beam - the length of the coal wall - the width of the roadway retention wall); ⑤ The two ends of the load-bearing beam have the same shear angle. The most commonly used method for calculating the load of the side support body of the gob-side tunnel is the "separated block method". This method assumes that the weight of the separated rock blocks within a certain range above the gob-side tunnel and the support body constitutes the support body load. There are four assumptions when using the "separated rock block method" to calculate the roof pressure of the gob-side tunnel: ① The rock blocks on the side support body reach a static equilibrium state under the action of their own gravity and the supporting force provided by the side support body, and are no longer affected by other forces; ② The rock blocks have the same shear angle on both sides; ③ The rock blocks can be regarded as a rigid body in balance under the action of three forces, and the lines of action are parallel to each other; ④ The vector sum of the forces acting on the rock blocks is zero. The force calculation model of the side filling body is as follows: Figure 2 shown.

[0025] The calculated load of the tunnel support body is:

[0026] Where, q is the load of the roadside support body; b B The distance from the inner side of the roadway support to the coal seam; x is the width of the tunnel side support; b C is the outer cantilever distance of the roadside support; L is the length of the roof separating the rock blocks, L=b B +x+b C ; c s is the direct top bulk density; h is the mining height; θ is the shear angle; α is the coal seam inclination; The stability of the sidewalls along the gob determines the success of gob-side entry retention. A reasonable size of the sidewall fill fully leverages the inherent stability and bearing capacity of the roof strata. To ensure both the current and sustained bearing capacity of the fill, a reasonable width is required.

[0027] Calculate the width of the wall along the side of the lane as follows:

[0028] Where Q1 is the initial bearing capacity required per unit length of the filling body; oh is the width of the wall along the void; k 3 is the design wall safety factor; k 2 is the reliability coefficient of dynamic pressure influence; k 1 is the reduction coefficient of wall strength; s is the final setting strength of the wall along the void.

[0029] After the calculation is completed, it is necessary to check the strength of the wall along the tunnel: Gangue from the goaf of an inclined coal seam scatters and accumulates on the sides of the goaf wall, exerting a certain lateral pressure on it. This requires a stability check of the goaf wall. The maximum gangue lateral pressure should be used for this check, assuming the goaf side of the goaf wall is completely filled with gangue. Due to the inclination of the coal seam, when the angle is greater than the natural repose angle of the scattered top coal and gangue, the gangue from the upper and middle parts of the goaf slides down along the floor. To ensure the stability of the goaf wall, it is necessary to check its anti-slip and anti-overturning properties under the ultimate limit state.

[0030] a. Wall stability verification ① Calculation of gangue side pressure The calculation formula of the lateral pressure of gangue is:

[0031] Where: E a is the pressure of gangue on the support body, KN / m 2 ; 𝛾 is the bulk density of gangue, KN / m 3 ; H is the height of scattered gangue, m; Φ is the internal friction angle of scattered gangue, (°); The calculation formula for the bending moment borne by the wall is:

[0032] Where: M is the bending moment borne by the wall, kN·m; q is the wall load along the void, kN / m; l is the distance from the edge of the wall, m; ② Anti-slip stability verification When roof pressure is not considered, the goaf wall is subject to friction with the roadway floor and lateral pressure from the goaf's waste rock. Because the roadway roof is tilted, its normal direction points toward the goaf. Under the compressive force of the lateral pressure from the goaf's waste rock, the roadway roof exerts a force perpendicular to the roof on the goaf wall. If the goaf wall is intact, the horizontal component of the roof force, the friction from the floor, and the lateral pressure are balanced. Therefore, when roof load is not considered, the goaf wall will not experience sliding instability.

[0033] When considering the top plate pressure, the anti-sliding stability of the wall is calculated as follows:

[0034] Where, K a is the anti-slip safety factor; G is the deadweight of the wall per meter, kg / m; E is the lateral pressure of the gangue on the wall along the void, kN / m 2 ; Q x is the horizontal pressure of the roof of the gob-side entry, kN / m 2 ; Q y is the vertical pressure of the roof of the gob-side entry, kN / m 2 ; m is the friction coefficient between the base plate and the wall, which is set to 1.

[0035] Because the horizontal roof pressure Qx > E, the resulting horizontal force is directed toward the goaf. Therefore, the wall will not slide when subjected to both roof pressure and lateral pressure. Therefore, the wall can be considered to meet the anti-slip stability requirements.

[0036] ③ Overturning stability verification When the roof pressure is not considered, the force analysis shows that under the effect of the lateral pressure of the gangue, the overturning stability verification formula is:

[0037] Where: K t is the safety factor against overturning; G is the deadweight of the wall per meter, Kg / m; a is the distance between the center of gravity of the wall and the instability point of the support body, m; b is the distance between the gangue side pressure and the wall instability point, m; 𝛼 is the coal seam inclination, (°); z is the distance between the pressure acting point on the gangue side and the instability point of the wall, m.

[0038] When the wall is subjected to pressure from the top plate, the overturning stability calculation formula is:

[0039] Where: Q y is the vertical pressure of the roof of the gob-side entry, kN / m 2 .

[0040] b. Verification of wall bearing capacity The hollow wall is a prestressed composite structure. Therefore, its bearing capacity is composed of two components: the restraining reinforcement (precast hollow wall) and the core concrete (later grouting). Under axial pressure, the restrained core concrete undergoes lateral expansion deformation, causing the outer wall to undergo tensile deformation, thus generating lateral restraining forces on the core concrete, which is subjected to a triaxial compressive stress state.

[0041] The calculation formula for the bearing capacity of internally grouted mixed concrete walls is:

[0042] The calculation formula for the restraint stress of the external wall is:

[0043] Where, N 2 is the bearing capacity of the roadside support, MPa; s r ——Effective restraint force generated by external wall, MPa; A cor is the area of ​​concrete wrapped inside, mm 2 ; f c is the design value of concrete compressive strength, MPa; d is the internal hollow width, mm; s b is the design value of concrete tensile strength, N / mm 2 ; a 1, a 2 is the length and width of the hollow wall, mm.

[0044] To meet the need for precast hollow walls along the gob-side of tunnels to replace flexible formwork walls, the foundation materials should be high-strength and stable ordinary Portland cement (PO42.5 or higher), medium-coarse sand, crushed stone with a particle size of 5-20 mm, and clean water. These base materials ensure that the concrete has good strength, density, and durability, and are the foundation for achieving stable wall support.

[0045] To improve construction efficiency, mechanical properties, and durability, auxiliary materials such as high-efficiency water reducers, early strength agents, expansive agents, and anti-cracking fibers can be added to concrete. Early strength agents significantly shorten setting time, meeting the need for rapid connection; anti-cracking fibers and expansive agents help prevent wall cracking, enhancing overall stability and service life; and water reducers improve workability and ultimate strength, ensuring smooth on-site construction.

[0046] Based on the above requirements, it is recommended to use fast-hardening concrete with a strength grade of no less than C25, a compressive strength of at least 30 MPa, an initial setting time of 2 to 4 hours, and a final setting time of 8 hours or less. Incorporating appropriate water reducers, early strength agents, and anti-cracking materials can achieve an organic balance between structural strength and construction efficiency, meeting the practical needs of rapid and efficient support for gob-side entryways in underground coal mines.

[0047] Example 3 Based on Example 1, the prefabricated hollow wall in S2 is specifically: S2.1: Precast hollow walls shall be constructed using C30 early-strength concrete with an initial setting time of 2-4 hours, a final setting time of ≤8 hours, and a compressive strength of ≥30 MPa; S2.2: Embedded grouting splicing structures are set on both sides of the prefabricated hollow wall. The left side is a boss-type splicing surface with an embedded female grouting interface; the right side is a groove-type splicing surface with an embedded male grouting interface. The interface is connected to the hollow channel inside the wall to form a grouting network.

[0048] Specifically, the measured tunnel height data is used to determine the wall height, with a margin of -10-20mm from the minimum height to prevent the wall from being too high and making it difficult to support. Considering the thickness of the base layer, the wall should deduct the thickness of the foundation layer (such as the cushion layer and secondary grouting layer), which is generally 30-50mm. Construction allows for deviation adjustments. During prefabrication of the wall, a height adjustment margin of ±10-20mm can be reserved. Fine-tuning can be performed on-site using cushion layers or tightening devices to ensure a tight fit between the top and bottom of the wall.

[0049] The wall height h is the measured support tunnel height. Considering the comprehensive results of structural stability, construction convenience and transport and installation feasibility, the wall length is designed to be 2m. The wall width b can be calculated based on the required bearing capacity of the wall. Figure 3 shown.

[0050] The hollow wall of the present invention has a platform with a distance of 100mm from the boundary on the left side, two female water pipe interfaces with an inner diameter of 62mm in the middle part, and a 100mm deep groove on the right side, with two male water pipe interfaces with a diameter of 60mm in the middle of the groove.

[0051] The hollow wall of the present invention is cast on the ground through a formwork. When a certain strength is reached, the formwork is carefully removed to avoid chipping of corners and edges of the wall. The appearance quality, dimensional deviation and patency of the hollow holes of the wall are checked. After the pouring is completed, it is immediately covered with wet sacks or films and kept moist for initial setting (about 4 to 6 hours). The surface can be trimmed and covered. Normal curing is not less than 3 to 7 days. When using an early strength agent, the mold can be removed in advance according to the actual strength. The curing environment should prevent exposure to the sun, rain and frost.

[0052] Example 4 Based on Example 3, the concrete mix ratio is: cement: sand: crushed stone = 1:1.5:2.5, 3% early strength agent and 0.3% polypropylene fiber are added, and the water-binder ratio is 0.26-0.30. The orthogonal test optimization is used to ensure that the flexural strength is ≥4.5 MPa and the ultimate compressive strength is ≥20 MPa.

[0053] Example 5 On the basis of Example 1, S3 is specifically: S3.1: The trench depth is 80mm~120mm, and a laser leveler is used to control the flatness error to ≤3mm; S3.2: When prefabricated hollow walls are spliced, the grouting interface is automatically aligned to achieve precise docking of the male and female heads. The grouting pipes are inserted simultaneously, and a dual-liquid grouting pump is used to pump cement-stone powder composite slurry until slurry emerges from the overflow holes of the adjacent walls.

[0054] Specifically, after the walls are prefabricated and cured on the ground, each wall is numbered and marked, loaded into a cage and lifted and transported underground, unloaded underground and placed on a platform truck or transport vehicle, and finally transported through the tunnel to the tunnel support point.

[0055] A trench is dug before splicing and installing precast concrete hollow walls underground. Its main function is to provide a stable foundation for the wall to prevent it from slipping, tilting or settling during mining or construction. At the same time, the trench helps to eliminate unevenness and protrusions at the bottom of the tunnel and improve the fit between the wall and the ground. In addition, the trench can also seal the bottom gap to prevent slurry from seeping during grouting, improve the grouting fullness and surrounding rock reinforcement effect, and serve as an installation guide benchmark to ensure that the wall splicing position is accurate and straight, thereby improving the overall stress performance and construction quality.

[0056] After the trench is dug, the wall is spliced. The prefabricated segmented hollow wall on the ground has a platform on the left side that is 100mm away from the boundary. There are two female water pipe interfaces with an inner diameter of 62mm in the middle part. There is a 100mm deep groove on the right side, and there are two 60mm diameter male water pipe interfaces in the middle of the groove. The platforms and grooves on the left and right sides of the hollow wall match in size, and the positions of the male and female interfaces match. The wall can be spliced ​​by connecting and interlocking the platform grooves, male and female water pipe interfaces on the left and right sides, as shown below. Figure 4 shown.

[0057] Furthermore, the initial setting time of the grouting slurry is ≤45 minutes, the 28-day compressive strength is ≥30 MPa, and self-compacting filling is achieved by adding 1‰ anti-segregation agent to the slurry.

[0058] Specifically, grouting concrete is made by mixing cement, stone powder, and admixtures in a certain proportion on the ground, transporting them to the underground working surface, and adding water to mix them. Considering its characteristics of early strength at a young age, self-compacting and vibration-free, and pumpability, the following requirements are made for the raw materials: ① Cement uses 42.5 ordinary Portland cement; ② The maximum particle size of stone powder should not be greater than 20mm. The particle size should be 5-20mm continuously graded crushed stone. The content of needle-like particles in the stone should not be greater than 10%, the mud content (by mass) should not be greater than 1%, and the mud block content should not be greater than 0.5%. When crushed stone is used, the content of non-clay stone powder should not be greater than 1.5% to prevent adverse effects on the setting time and workability of the grouting concrete. ③ Use KTRHWJ type admixture to improve the early strength of grouting concrete, reduce the water consumption per unit of grouting concrete, and improve the mixing viscosity and pumping performance of grouting concrete, thereby effectively improving the anti-segregation and pumpability of grouting concrete.

[0059] The grouting concrete grade of the working face along the goaf tunnel is C25, and the mix ratio of cement to stone powder is 5:5 (the concrete mix ratio is optimized according to the mine pressure observation during construction). The mix ratio is shown in Table 1. It is prepared on site before construction to obtain the construction mix ratio.

[0060] Table 1

[0061] The compacted density of cement is 2.4t / m³, and the compacted density of stone powder is 2.4t / m³. The volume of concrete per cubic meter is calculated as shown in Table 2.

[0062] Table 2

[0063] The pouring process of the side of the goaf-retained tunnel is as follows: prepare the dry mix on the ground → transport it to the rear of the grouting concrete preparation and conveying unit in the working face tunnel → transport the dry mix to the mixer through the loader → add water and mix evenly → transport the concrete into the hollow goaf-retained wall through the concrete pump and pipeline.

[0064] During the construction of grouting concrete walls along gob-side tunnels, a KTRHZSJ-50 grouting concrete preparation and conveying unit was used: it includes a surface concrete dry material preparation system and an underground grouting concrete preparation and conveying system. The surface concrete dry material preparation system includes a surface concrete batching machine and a mixer with a production capacity of 50 m³ / h. The underground grouting concrete preparation and conveying system includes a coal mine concrete pump, coal mine concrete, a coal mine scraper loader, a mixer, and supporting piping. Due to the difficulty in transporting concrete mixtures and limited storage time, grouting concrete uses a secondary mixing process. The primary mixing process is performed at a surface mixing station, using a conventional mixer to mix cement, sand, gravel admixtures, and other dry materials. The dry materials are transported to the underground grouting concrete preparation and conveying system by mine cars and unloaded directly onto the loader. The main dry concrete mixing equipment includes concrete mixers and batching machines.

[0065] Example 6 On the basis of Examples 1-5, S4 is further included: S4.1: Embed vibrating wire stress gauges at the interface between the prefabricated hollow wall and the roof to monitor mining stress changes in real time. When the stress gradient is greater than 0.5 MPa / m, implement secondary compensatory grouting through the reserved grouting holes. S4.2: Use distributed optical fiber to monitor the internal strain of the wall. When the strain difference is greater than 150 with When the alarm is triggered, the sound and light warning will be automatically triggered.

[0066] Based on the above steps, during actual work, in the underground environment, the wall surface should be kept moist by regular water spraying, covering with wet sacks or plastic film, etc.; the curing time should be no less than 7 days to ensure that the cement slurry is fully hydrated and enhance the adhesion between the grouting and the wall; especially the areas around the grouting holes and the joints should be kept moist to prevent early cracking.

[0067] During wall maintenance, equipment collisions and pedestrian trampling should be avoided, and warning areas should be set up around the walls. In affected areas adjacent to goafs, temporary supports or flexible buffer layers should be added to prevent sudden stress shocks from causing wall cracks. Monitoring markers or strain gauges can be set up to dynamically observe whether the wall is displaced or cracked, facilitating early identification of abnormalities.

[0068] Grouting holes and wall joints should be sealed promptly with fast-hardening cement or special leak-proof materials; possible micro cracks can be repaired with structural adhesive or epoxy mortar to improve the anti-seepage and bonding properties; cracks in joints caused by shrinkage of grouting materials should be dealt with as soon as possible after discovery to prevent crack expansion and affect the stability of the overall structure.

[0069] Regularly check the wall surface for bulging, cracks, peeling, water seepage and other phenomena; once an abnormality is found, analyze the changes in surrounding rock stress and take timely reinforcement measures such as grouting, anchor reinforcement or secondary support; in areas close to the goaf, consider setting secondary grouting holes or observation holes behind or on the top of the wall to facilitate subsequent reinforcement.

[0070] The wall is not only subjected to stress independently, but also forms a joint support system with the anchor net, grouting body and top plate. The later maintenance should be coordinated with the stability inspection of the entire support system, such as whether the anchor tension is attenuated, whether the top plate sinks, etc. If a gap is generated between the wall and the top plate, grouting should be carried out to prevent the top plate pressure from being concentrated and transmitted to the wall.

[0071] Finally, it should be noted that, in this document, relational terms such as first and second, etc., are used only to distinguish one entity or operation from another entity or operation, and do not necessarily require or imply any actual relationship or order between these entities or operations. Moreover, the terms "comprises," "comprising," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or device comprising a series of elements includes not only those elements, but also other elements not explicitly listed, or elements inherent to such process, method, article, or device. In the absence of further limitations, an element defined by the phrase "comprising a ..." does not exclude the presence of additional identical elements in the process, method, article, or device comprising the element.

[0072] The various embodiments in this specification are described in a progressive manner, and each embodiment focuses on the differences from other embodiments. The same or similar parts between the various embodiments can be referenced to each other.

[0073] The above description of the disclosed embodiments is intended to enable one skilled in the art to implement or use the present application. Various modifications to these embodiments will be readily apparent to one skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the present application. Therefore, the present application is not limited to the embodiments shown herein, but is intended to conform to the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for retaining tunnels along the goaf based on modular prefabricated hollow grouting walls, characterized in that: The following steps are included: S1: Conduct tunnel measurement and wall parameter design; S2: Prefabricate hollow walls and complete underground transportation; S3: Implement underground splicing installation and high-pressure grouting.

2. The method for retaining a tunnel along the goaf based on modular prefabricated hollow grouting walls according to claim 1 is characterized in that: S1 specifically includes: measuring the height difference between the top and bottom plates of the supporting section through a total station or a tape measure, and taking the average value at different cross sections to determine the tunnel height; establishing a mechanical model based on the separated block method to calculate the load of the tunnel side support body and the width of the tunnel side wall.

3. The method for retaining a tunnel along the goaf based on modular prefabricated hollow grouting walls according to claim 2, characterized in that: The calculated load of the tunnel support body is: Where, q is the load of the roadside support; b B The distance from the inner side of the roadway support to the coal seam; x is the width of the roadside support; b C It is the outer cantilever distance of the tunnel side support; L is the length of the roof separating the rock blocks, L=b B +x+b C ; γ s is the direct top bulk density; h is the mining height; θ is the shear angle; α is the coal seam inclination; Calculate the width of the wall along the side of the lane as follows: Where Q1 is the initial bearing capacity required per unit length of the filling body; ω is the width of the wall along the void; k 3 is the design wall safety factor; k 2 is the reliability coefficient of dynamic pressure influence; k 1 is the reduction coefficient of wall strength; s is the final setting strength of the wall along the void.

4. The method for retaining a tunnel along the goaf based on modular prefabricated hollow grouting walls according to claim 3 is characterized in that: The wall length is 1.8m~2.2m, the hollow channel diameter is 80mm~100mm, the number is 2~4, the channel axis is parallel to the wall axis and the spacing is ≤500mm.

5. The method for retaining a tunnel along the goaf based on modular prefabricated hollow grouting walls according to claim 1, characterized in that: The prefabricated hollow wall in S2 is specifically: S2.1: Precast hollow walls shall be constructed using C30 early-strength concrete with an initial setting time of 2-4 hours, a final setting time of ≤8 hours, and a compressive strength of ≥30 MPa; S2.2: Embedded grouting splicing structures are set on both sides of the prefabricated hollow wall. The left side is a boss-type splicing surface with an embedded female grouting interface; the right side is a groove-type splicing surface with an embedded male grouting interface. The interface is connected to the hollow channel inside the wall to form a grouting network.

6. The method for retaining a tunnel along the goaf based on modular prefabricated hollow grouting walls according to claim 5, characterized in that: The concrete mix ratio is: cement: sand: crushed stone = 1:1.5:2.5, with 3% early strength agent and 0.3% polypropylene fiber added, and the water-cement ratio is 0.26~0.

30. Through orthogonal test optimization, the flexural strength is ensured to be ≥4.5MPa and the ultimate compressive strength is ≥20MPa.

7. The method for retaining a tunnel along the goaf based on modular prefabricated hollow grouting walls according to claim 1, characterized in that: S3 is specifically: S3.1: The trench depth is 80mm~120mm, and a laser leveler is used to control the flatness error to ≤3mm; S3.2: When prefabricated hollow walls are spliced, the grouting interface is automatically aligned to achieve precise docking of the male and female heads. The grouting pipes are inserted simultaneously, and a dual-liquid grouting pump is used to pump cement-stone powder composite slurry until slurry emerges from the overflow holes of the adjacent walls.

8. The method for retaining a tunnel along the goaf based on modular prefabricated hollow grouting walls according to claim 7, characterized in that: The grouting slurry has an initial setting time of ≤45 minutes and a 28-day compressive strength of ≥30 MPa, and self-compacting filling is achieved by adding 1‰ anti-segregation agent to the slurry.

9. The method for retaining a tunnel along the goaf based on modular prefabricated hollow grouting walls according to any one of claims 1 to 8, characterized in that: Also includes S4: S4.1: Embed vibrating wire stress gauges at the interface between the prefabricated hollow wall and the roof to monitor mining stress changes in real time. When the stress gradient is greater than 0.5 MPa / m, implement secondary compensatory grouting through the reserved grouting holes. S4.2: Use distributed optical fiber to monitor the internal strain of the wall. When the strain difference is greater than 150 με When the alarm is triggered, the sound and light warning will be automatically triggered.

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