A method for improving the upper limit of coal seam mining by grouting to transform thick water-bearing sand layer

Abstract

The present invention discloses a method for improving the upper limit of coal seam mining by grouting to transform thick water-bearing sand layer, comprising: step 1: determining the range that needs to be treated to improve the upper limit; 2) increasing the vertical height H of the upper limit section; 提 =H 裂 +H 保 +H 风 ‑H 垮 2) Scope of grouting reconstruction of thick water-bearing sand layers; Calculate the length and width of the grouting reconstruction based on the displacement angle between the bedrock and the loose layer; Step 2: Arrange the drill holes. The solution of this invention can be used to grout thick water-bearing sand layers into a water-proof layer, replacing waterproof coal rock pillars with anti-collapse coal rock pillars, raising the upper limit of mining and relieving the coal resources under the loose water-bearing body. Using different fly ash-cement slurry ratios for different sections ensures the grouting reconstruction effect while saving grouting material costs.

Description

Technical Field

[0001] The invention belongs to the technical field of coal mine roof water prevention and control and water-conserving coal mining, and relates to a method for grouting to transform a thick water-containing sand layer and improve the upper limit of coal seam mining. Background Art

[0002] Most coalfields in my country are concealed, with coal seams overlain by thick, loose sandy soil layers, some of which contain moderately to strongly water-rich aquifers. Shallow coal resources are typically mined using a safety pillar. With the rapid development of coal resources, shallow coal resources are becoming increasingly scarce and are nearing depletion. This pillar overburdens a significant amount of coal (over 500 million tons in the Huaibei coalfield alone). Raising the upper limit of coal mining beneath loose aquifers is crucial for improving coal resource recovery, extending mine service life, and optimizing the regional energy structure.

[0003] Technical and theoretical research on the use of safe coal (rock) pillars beneath loose strata to prevent water, sand, or collapse began in the 1960s. Preliminary theories were developed in the mid-1970s, and exploration of minimizing the pillars began. Research focused on establishing appropriate safe coal (rock) pillars, dewatering mining, and modifying mining methods. However, there has been limited practical and theoretical research on increasing the mining limit through grouting to transform thick, loose, water-bearing sand layers. The problems and shortcomings of the existing technology are as follows: (1) After the coal seam is mined, the safe coal (rock) pillar cannot be reused, making a large amount of coal pillar resources become idle resources and cannot be mined, wasting coal resources; (2) The conditions for drainage mining are limited. For the sand layer that receives the top supply of the strong water-rich karst aquifer (such as the Ordovician limestone aquifer), the drainage work is huge, and there may even be no drainage conditions. At the same time, secondary water inrush and sand burst disasters may occur after mining disturbance, and the uncertainty of mining safety is extremely high; (3) Drainage mining artificially drains water from the shallow loose aquifer into the well, destroying the groundwater circulation system, lowering the regional groundwater level, damaging the regional ecological environment and causing a series of secondary disasters such as ground subsidence and land salinization. (4) The change in mining method is mainly to limit the height of mining, which also causes a waste of some coal resources.

[0004] To this end, the present invention addresses the above problems and shortcomings through intensive research and design, integrating years of experience and achievements in related professions, and researching and designing a method for grouting to transform thick water-containing sand layers to increase the upper limit of coal seam mining, so as to overcome the defects of the existing technology. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for grouting to transform thick water-containing sand layers to increase the upper limit of coal seam mining, thereby solving the problems in the prior art such as large coal pressure in safety coal pillars and difficulty in recovering coal resources, poor applicability and reliability of drainage mining, and ecological environmental damage and secondary geological disasters caused.

[0006] The technical solution adopted in the present invention is:

[0007] A method for grouting to transform a thick water-bearing sand layer to increase the upper limit of coal seam mining comprises:

[0008] Step 1: Determine the scope of governance required to raise the ceiling;

[0009] 1) Increase the vertical height H of the upper limit section 提 =H 裂 +H 保 +H 风 -H 垮 ;

[0010] Where: H 提 To increase the vertical height of the upper limit, m; H 裂 is the maximum height of the water-conducting fracture zone, m; H 保 is the thickness of the protective layer, m; H 风 is the depth of the bedrock weathering aquifer zone, m; H 垮 is the maximum height of the collapse zone, m;

[0011] 2) Grouting transformation scope of thick water-bearing sand layer

[0012] Calculate the length and width of grouting reconstruction based on the movement angle of bedrock and loose layer;

[0013]

[0014]

[0015] Where: L 长 L is the length of the working face to be transformed, m; 宽 L is the width of the working face inclination transformation, m; A L is the horizontal projection length of the upper limit section of the working face, m; B is the horizontal projection length of the upper limit section of the working face, m; L1 is the horizontal projection length of the bedrock section, m; L2 is the horizontal projection length of the water-bearing sand layer section, m; H 基 H is the vertical height of the coal seam roof bedrock, m; 砂 is the thickness of the sand layer, m; α is the coal seam inclination, °; ​​δ is the bedrock movement angle, °; is the loose layer dynamic angle, °;

[0016] Step 2: Arrange the drilling holes.

[0017] Optionally, the arrangement of drilling holes specifically includes: the drilling holes are divided into a first sequence and a second sequence, the two sequences of drilling holes are staggered in alternate rows, the spacing D between drilling holes in the same row is 40 to 60 m, and the row spacing is 0.5D.

[0018] Optionally, the method further includes step three: drilling a hole and running a casing;

[0019] Drill a vertical hole at the hole position determined in step 2. The casing is set in a "long-short-leg" manner, that is, the casing depth increases along the coal seam dip direction. The casing depth of a certain borehole is H 套 At depth;

[0020] H 套 The calculation method is:

[0021]

[0022] Where: H 套 is the depth of the casing below the top of the thick water-bearing sand layer, m; M is the mining thickness of the coal seam, m; L d It is the horizontal distance from the borehole to the shallowest boundary of the treatment area, m.

[0023] Optionally, the casing section is cemented to seal the outer annulus.

[0024] Optionally, the method further includes step 4: sequential progressive segmented cyclic grouting;

[0025] Sequential progressive grouting: grouting is performed progressively in the first and second sequence boreholes, with odd-numbered rows representing the first sequence and even-numbered rows representing the second sequence. Pressure relief induction grouting is performed alternately in rows within the same sequence, meaning that while one row of boreholes is being grouted, the next row of boreholes acts as a pressure relief hole. During the grouting process, the grouting process is evaluated through the three stages of water return, sand return, and grout return through the drainage holes to determine whether the grout has spread to a point where it is connected. If grout return occurs, the drainage hole is sealed and grouting continues to expand the fracture channel.

[0026] Segmented cyclic grouting: The length of each grouting section is 4 to 6 meters. After each grouting reaches the end standard, it is extended to the next grouting section. If the end standard is not reached, the grouting is repeated; the final hole depth is 10 to 20 meters into the weathering zone.

[0027] Optionally, the grouting cement fly ash slurry ratio is as follows: the treatment section is divided into three sections from top to bottom according to the purpose of the transformation; the first section is from the bottom of the casing to twice the mining thickness of the bedrock top, and is filled and compacted with fly ash-cement slurry with a mass percentage of 20% to 50%, and the fly ash content is gradually reduced to 20% as the hole depth increases;

[0028] The second section is a sand layer twice as thick as the bedrock, which is transformed to serve as a protective layer. It requires a high stone rate and high strength, and uses fly ash-cement slurry with a mass percentage of 10% to 20%;

[0029] The third section is the bedrock weathering zone, where the weathered and broken rocks need to be grout-consolidated to give them higher integrity and water-resistance. Pure cement slurry or fly ash-cement slurry with a mass percentage of less than 10% is used.

[0030] Optional, grouting end standard: grouting drilling reaches the end pore pressure P i =P0+0.03h, stop grouting; or if the grouting time after the drainage hole returns to grouting reaches 2-4 hours and the terminal pore pressure has not been reached, stop grouting, sweep the hole and re-inject until it reaches P1;

[0031] Where: P i is the pore pressure at the end of grouting of the i-th segment of the hole, MPa; P0 is the splitting pressure of the first grouting of the hole, MPa; h is the lower extension of the segment, m.

[0032] Optionally, the method further includes step five: constructing a ground inspection hole;

[0033] After all grouting drilling is completed, ground inspection holes are constructed at the centers of four adjacent first-order drilling holes and second-order drilling holes. The number of inspection holes is 8% to 10% of the grouting drilling holes.

[0034] The beneficial effects of the present invention are:

[0035] (1) This scheme can be used to transform the thick loose water-bearing sand layer into an impermeable layer by grouting, and change the waterproof coal rock pillar to the anti-collapse coal rock pillar, thereby increasing the upper limit of mining and releasing the coal resources under the loose water body.

[0036] (2) Fly ash-cement slurry with different proportions is used according to different sections to ensure the grouting transformation effect while saving the cost of grouting materials.

[0037] (3) Sequential progressive grouting can control the diffusion of slurry toward the drainage holes. Progressive grouting can transform the sand layer and avoid disordered grouting that destroys the already formed slurry veins. Segmented cyclic grouting can ensure that the sand layer in each grouting section is fully transformed.

[0038] (4) 8% to 10% of ground inspection holes are set up, and the treatment effect can be fully verified through four inspection methods: core drilling observation method, moisture content test method, strength test method and permeability test method. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] The accompanying drawings are used to provide a further understanding of the present disclosure and constitute a part of the specification. Together with the following detailed description, they are used to explain the present disclosure but do not constitute a limitation of the present disclosure. In the accompanying drawings:

[0040] Figure 1 This is a schematic diagram of the vertical height calculation method for raising the upper limit;

[0041] Figure 2 This is a schematic diagram of the calculation method of the working face strike transformation length;

[0042] Figure 3 This is a schematic diagram of the calculation method for the inclined reconstruction width of the working face;

[0043] Figure 4 This is a schematic diagram of the scope of governance required to increase the upper limit;

[0044] Figure 5 It is a plan view of the drilling arrangement;

[0045] Figure 6 This is a schematic diagram of the casing's "long and short legs" type down to depth;

[0046] Figure 7 It is a three-dimensional schematic diagram of sequential progressive segmented cycle grouting;

[0047] Figure 8 It is a schematic diagram of slurry ratio at different depths;

[0048] Figure 9 This is a graph showing the increasing grouting pressure of the "four containments" with increasing burial depth;

[0049] The symbols in the figure represent:

[0050] 1-thick water-bearing sand layer, 2-weathering zone, 3-coal seam, 4-working face upper limit section, 5-collapse zone, 6-water-conducting fracture zone, 7-range that needs to be treated to raise the upper limit, 8-working face mining direction, 9-first-sequence drill hole, 10-second-sequence drill hole, 11-inspection hole, 12-sequence number, 13-casing, 14-grouting, 15-drainage, 16-grouting section, 17-injected slurry, 18-water-proof layer. DETAILED DESCRIPTION

[0051] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0052] The method of the present invention for improving the upper limit of coal seam mining by grouting to transform thick water-bearing sand layers comprises:

[0053] Step 1: Determine the scope of governance required to raise the ceiling;

[0054] 1) Increase the vertical height of the upper limit section

[0055] The goal of this plan is to change the original mining with waterproof coal rock pillar to mining with anti-collapse coal rock pillar; to increase the vertical height H of the upper limit section. 提 =H 裂 +H 保 +H 风 -H 垮 ;

[0056] Where: H 提 To increase the vertical height of the upper limit, m; H 裂 is the maximum height of the water-conducting fracture zone, m; H 保 is the thickness of the protective layer, m; H 风is the depth of the bedrock weathering aquifer zone, m; H 垮 is the maximum height of the collapse zone, m;

[0057] The upper limit elevation of the original coal seam mining plus the vertical height of the upper limit section is the coal seam mining elevation after the upper limit is raised;

[0058] 2) Grouting transformation scope of thick water-bearing sand layer

[0059] Considering the disturbance of mining to the roof bedrock and the four containments, the scope of the transformation should be expanded to outside the scope of mining at the working face; the length and width of the grouting transformation should be calculated based on the movement angle of the bedrock and the loose layer;

[0060]

[0061]

[0062] Where: L 长 L is the length of the working face to be transformed, m; 宽 L is the width of the working face inclination transformation, m; A L is the horizontal projection length of the upper limit section of the working face, m; B is the horizontal projection length of the upper limit section of the working face, m; L1 is the horizontal projection length of the bedrock section, m; L2 is the horizontal projection length of the water-bearing sand layer section, m; H 基 H is the vertical height of the coal seam roof bedrock, m; 砂 is the thickness of the sand layer, m; α is the coal seam inclination, °; ​​δ is the bedrock movement angle, °; is the loose layer dynamic angle, °;

[0063] Step 2: Arrange the drilling holes

[0064] The drilling holes are divided into the first and second sequences. The two sequences of drilling holes are staggered in alternate rows. The spacing D between drilling holes in the same row is 40 to 60 meters, and the row spacing is 0.5D.

[0065] Step 3: Drilling and running casing

[0066] Drill a vertical hole at the hole position determined in step 2. The casing is set in a "long-short-leg" manner, that is, the casing depth increases along the coal seam dip direction. The casing depth of a certain borehole is H 套 Depth. 套 The calculation method is:

[0067]

[0068] Where: H 套 is the depth of the casing below the top of the thick water-bearing sand layer, m; M is the mining thickness of the coal seam, m; L d It is the horizontal distance from the borehole to the shallowest boundary of the treatment area, m.

[0069] The casing section is cemented to seal the outer annulus.

[0070] Step 4: Sequential progressive segmented cyclic grouting

[0071] Sequential progressive grouting: Grouting is performed progressively for the first and second sequence boreholes, with odd-numbered rows being the first sequence boreholes and even-numbered rows being the second sequence boreholes. The boreholes of the same sequence are progressively and alternately grouted for pressure relief induction in rows, that is, when a row of boreholes is grouting, the next row of boreholes is used as drainage holes for pressure relief. During the grouting process, the slurry diffusion is judged through the three stages of water return, sand return, and grout return through the drainage holes until the connection is achieved. When grout return occurs, the drainage hole opening is closed, and grouting is continued to expand the splitting channel. (The so-called sequential progressive grouting means that the first row of first sequence boreholes is grouted, and the third row of first sequence boreholes are drained; the second row of second sequence boreholes is grouted, and the fourth row of second sequence boreholes are drained; after the grouting of the first and second rows of boreholes is completed, the third and fourth rows of boreholes are grouted, and the fifth, sixth, and sixth rows of boreholes are drained; and so on, until the grouting of all boreholes is completed.)

[0072] Segmented cyclic grouting: Each grouting section is 4 to 6 meters long. After each grouting reaches the end standard, it will be extended to the next grouting section. If the end standard is not reached, the grouting will be repeated. The final hole depth is 10 to 20 meters into the weathered zone.

[0073] Cement fly ash slurry ratio: The treatment section is divided into three sections from top to bottom based on the purpose of the renovation. The first section is from the bottom of the casing to twice the mined thickness (2M) of the bedrock top. Fly ash-cement slurry with a mass percentage of 20% to 50% is used for filling and compaction. The fly ash content gradually decreases to 20% with increasing hole depth. The second section is the sand layer twice the mined thickness (2M) above the bedrock, which will serve as a protective layer after renovation. A high stone concentration and high strength are required. A fly ash-cement slurry with a mass percentage of 10% to 20% is used. The third section is the weathered bedrock zone. Grouting is required to consolidate the weathered and broken rock to ensure high integrity and water-tightness. Pure cement slurry or fly ash-cement slurry with a mass percentage of less than 10% is used.

[0074] Grouting end standard: Grouting drilling reaches the end pore pressure P i =P0+0.03h, stop grouting; or if the grouting time continues for 2 to 4 hours after the drainage hole returns to grouting and the terminal pore pressure has not been reached, stop grouting, sweep the hole and re-inject until P1 is reached.

[0075] Where: P i is the pore pressure at the end of grouting of the i-th segment of the hole, MPa; P0 is the splitting pressure of the first grouting of the hole, MPa; h is the lower extension of the segment, m.

[0076] Step 5: Construction of ground inspection holes

[0077] After all grouting drilling is completed, ground inspection holes are constructed at the centers of four adjacent first-order drilling holes and second-order drilling holes. The number of inspection holes is 8% to 10% of the grouting drilling holes.

[0078] The grouting consolidation effect of the water-bearing sand layer was tested by drilling core observation method, moisture content test method, strength test method and permeability test method.

[0079] The present invention is described in detail below with reference to specific embodiments.

[0080] Example 1:

[0081] After decades of mining, a coal mine in Huaibei, Anhui Province, has gradually depleted its coal resources. However, a large number of coal pillars are located beneath the thick, loose, water-bearing sand layer "Sihan". The coal pillars are 2.2 meters thick, with a dip angle of 9° and good quality. According to the "Specifications for the Establishment and Pressure Mining of Coal Pillars in Buildings, Water Bodies, Railways, and Major Shafts and Tunnelings," there is no stable clayey aquiclude beneath the thick water-bearing sand layer "Sihan", which falls under Class I water bodies and requires the establishment of a waterproof coal (rock) pillar. According to measured data on the height of the mine's water-conducting fracture zone, the risk of mining is 2.3 to 4.7, with an average of 3.4, and the fracture mining ratio is 7.4 to 11.6, with an average of 9.5. Previously, the upper limit of mining for the waterproof coal (rock) pillar was -300 to -290 meters above sea level.

[0082] The floor elevation of the "four containments" in this coal mine averages -246 meters, with a thickness ranging from 25.3 to 47.88 meters, averaging 32.8 meters. The lithology is complex, consisting of gravel, sandy gravel, clayey gravel, coarse sand, medium sand, and clayey sand, interspersed with 0 to 4 thin layers of clay interbedded with gravel, clay, sandy clay, and calcareous clay. The thickness of the water-bearing sand layer ranges from 4.80 to 30.74 meters, averaging 14.42 meters. The "four containments" exhibit a coarse-bottomed, fine-topped sedimentary structure, with a prevalent gravel layer at the base. The porosity ranges from 25% to 31%. Pumping tests of the "four containments" indicate a unit water yield (q) of 0.00024 to 2.635 L / (s·m) and a permeability coefficient (K) of 0.0011 to 5.8 m / d. The water-rich soil is weak to moderate, primarily receiving recharge from lateral runoff. Horizontal permeability is strong, with vertical permeability being secondary. The "three barriers" above the "four containments" are 36 to 62.35 meters thick, with an average thickness of 47.84 meters. They are stable and well-developed, serving as a good aquiclude. The underlying bedrock wind-oxidized zone is 4.5 to 45.6 meters thick, with an average thickness of 22 meters. The strongly weathered zone is 0.41 to 37 meters thick, with an average thickness of 13 meters. These zones contain groundwater. The weathered rock has poor integrity, with RQD values ​​generally ranging from 0 to 50%. The compressive strength ranges from 0.31 to 17.03 MPa, making it a weak or extremely weak formation. The "four containments" are in contact with the coal seam and the weathered zone, serving as a direct source of water for shallow coal mining.

[0083] In order to extend the life of the mine and improve resource utilization, this solution was adopted to treat 1010 -1The thick water-bearing sand layer "four contains" and weathering zone on the roof of the working face have increased the upper limit of mining.

[0084] Step 1: Determine the scope of governance required to raise the ceiling

[0085] 1) Increase the vertical height of the upper limit section

[0086] like Figure 1 In this example, the elevation of the mining with waterproof coal rock pillar is -300m. If we want to change to mining with anti-collapse coal rock pillar, we need to increase the vertical height H of the upper section. 提 =H 裂 +H 保 +H 风 -H 垮 =(9.5+4-3.4)×2.2+13≈35m.

[0087] Where: H 提 To increase the vertical height of the upper limit, m; H 裂 is the maximum height of the water-conducting fracture zone, m; H 保 is the thickness of the protective layer, m; H 风 is the depth of the bedrock weathering aquifer zone, m; H 垮 is the maximum height of the collapse zone, m.

[0088] The mining elevation after raising the upper limit is -265m.

[0089] 2) Grouting transformation scope of thick water-bearing sand layer

[0090] like Figure 4 Considering the disturbance of mining to the roof bedrock and the four layers, the scope of transformation should be expanded to the outside of the mining range of the working face. The length of grouting transformation is calculated according to the movement angle of bedrock and loose layer (such as Figure 2 ) and width (such as Figure 3 ).

[0091]

[0092]

[0093] Where: L 长 L is the length of the working face to be transformed, m; 宽 L is the width of the working face inclination transformation, m; A L is the horizontal projection length of the upper limit section of the working face, m; B is the horizontal projection length of the upper limit section of the working face, m; L1 is the horizontal projection length of the bedrock section, m; L2 is the horizontal projection length of the water-bearing sand layer section, m; H 基 H is the vertical height of the coal seam roof bedrock, m; 砂 is the thickness of the sand layer, m; α is the coal seam inclination, °; ​​δ is the bedrock movement angle, °; is the loose layer dynamic angle, °.

[0094] Step 2: Arrange the drilling holes

[0095] like Figure 5 The drilling holes are divided into first sequence and second sequence. The two sequences of drilling holes are staggered in alternate rows. The spacing D between drilling holes in the same row is 40m, and the row spacing is 20m.

[0096] Step 3: Drilling and running casing

[0097] like Figure 6 , drill a vertical hole at the hole position determined in step 2, and set the casing in a "long-short-leg" manner, that is, the casing depth increases along the coal seam dip direction. The casing depth of a certain borehole is H 套 Depth. 套 The calculation method is:

[0098]

[0099] Where: H 套 is the depth of the casing below the top of the thick water-bearing sand layer, m; M is the mining thickness of the coal seam, m; L d It is the horizontal distance from the borehole to the shallowest boundary of the treatment area, m.

[0100] The casing section is cemented to seal the outer annulus.

[0101] Step 4: Sequential progressive segmented cyclic grouting

[0102] like Figure 7 , progressive grouting in sequence: the first and second sequence boreholes are progressively grouted, the odd-numbered rows are the first sequence boreholes, and the even-numbered rows are the second sequence boreholes. The boreholes in the same sequence are progressively and alternately grouted with pressure relief induction in units of rows, that is, when a row of boreholes is grouting, the next row of boreholes is used as drainage holes for pressure relief. During the grouting process, the slurry diffusion until the connection is determined through the three stages of water return, sand return, and grout return through the drainage holes. When grout return occurs, the drainage hole mouth is closed, and grouting is continued to expand the splitting channel. (The so-called progressive grouting means that the first row of first sequence boreholes is grouted, and the third row of first sequence boreholes are drained; the second row of second sequence boreholes is grouted, and the fourth row of second sequence boreholes are drained; after the grouting of the first and second rows of boreholes is completed, the third and fourth rows of boreholes are grouted, and the fifth, sixth, and sixth rows of boreholes are drained; and so on, until the grouting of all boreholes is completed.)

[0103] like Figure 7 , Segmented cyclic grouting: Each grouting section is 4m long. After each grouting reaches the end standard, it will be extended to the next grouting section. If it does not reach the end standard, the grouting will be repeated. The final hole depth is 10m into the weathering zone.

[0104] like Figure 8, the proportion of cement fly ash slurry: the treatment section is divided into three sections from top to bottom according to the purpose of the transformation. The first section is from the bottom of the casing to twice the mining thickness (2M) of the bedrock top, and 20-50% fly ash-cement slurry is used for filling and compaction. The fly ash content gradually decreases from 50% to 20% as the hole depth increases. The second section is the sand layer twice the mining thickness (2M) above the bedrock, which serves as a protective layer after transformation. It requires a high stone rate and high strength. 10-20% fly ash-cement slurry is used. The third section is the bedrock weathering zone. The weathered and broken rock needs to be grouting consolidated to give it high integrity and water-proof properties. Pure cement slurry or fly ash-cement slurry within 10% is used.

[0105] Grouting end standard: Grouting drilling reaches the end pore pressure P i =P0+0.03h, stop grouting; or if the grouting time continues for 2 to 4 hours after the drainage hole returns to grouting and the terminal pore pressure has not been reached, stop grouting, sweep the hole and re-inject until P1 is reached.

[0106] Where: P i is the pore pressure at the end of grouting of the i-th segment of the hole, MPa; P0 is the splitting pressure of the first grouting of the hole, MPa; h is the lower extension of the segment, m.

[0107] Step 5: Construction of ground inspection holes

[0108] After all grouting drilling is completed, ground inspection holes are constructed at the centers of four adjacent first-order drilling holes and second-order drilling holes. The number of inspection holes is 10% of the grouting drilling holes.

[0109] Drilling and coring observations revealed that the core recovery rates of the three "four-containment" core holes before grouting were 49%, 58%, and 65%, respectively. After grouting, the core recovery rates of the three inspection holes after grouting were 63%, 69%, and 69%, respectively. The integrity of the sand layer improved by approximately 17%, with cement slurry veins and permeated cement visible in the sand layer.

[0110] Table 1 shows a comparison of the moisture content and mechanical properties of the sand layer before and after grouting. After grouting, the true density increased slightly, the moisture content decreased by about one-third, and the strength increased by 2 to 3 times. This indicates that the porosity of the sand layer decreased after grouting, and the degree of cementation and strength increased significantly.

[0111] Table 1 Comparison of physical and mechanical properties of sand layer before and after grouting

[0112] project unit Before grouting After grouting True density <![CDATA[g / cm 3 ]]> 2.67～2.71 2.68～2.76 Moisture content ％ 6.55～18.04 2.53～12.27 Compressive strength MPa 0.56～3.05 0.17～10.3 tensile strength MPa 0.10～0.71 0.02～1.31 Shear strength MPa 0.23～1.17 0.12～4.85

[0113] The compressive strength of the weathered bedrock was tested before and after grouting, and the results showed that the compressive strength before grouting was 1.96-2.18 MPa, and after grouting was 3.03-11.10 MPa. The strength of the weathered bedrock increased by about 2-5 times, indicating that grouting has a good effect on the transformation of weathered bedrock.

[0114] After grouting, the pumping test of the inspection hole obtained a unit water yield q = 0.0000075 ~ 0.000013L / (s·m), a permeability coefficient K = 0.000 ~ 0.0069m / d, weak water-richness, and extremely slight to slightly permeable, indicating that the thick water-bearing sand layer "four contains" has been transformed into an impermeable layer.

[0115] Figure 9 The grouting pressure of grouting sections at different depths is statistically analyzed. The hole depth is used as the horizontal coordinate and the grouting pressure as the vertical coordinate. A scatter plot is drawn, and then a trend line and a linear formula are generated. The slope of the trend line in the figure is 0.0307, ​​indicating that the grouting pressure increases with the hole depth at a gradient of +0.0307 MPa / m, proving that the setting of the terminal pore pressure Pi changing with the hole depth is reasonable.

[0116] After the present invention was adopted for treatment, the fourth sand layer was successfully transformed into an effective aquiclude (Class III water body), and a coal rock pillar was set up to prevent collapse. The mining area of ​​the working face increased by about 40,000 m 2 The Jiefang coal pillar has a resource reserve of about 100,000 tons, and the coal quality is relatively good, which will increase the mine's revenue by about 200 million yuan.

[0117] The preferred embodiments of the present disclosure are described in detail above in conjunction with the accompanying drawings. However, the present disclosure is not limited to the specific details of the above embodiments. Within the technical concept of the present disclosure, various simple modifications can be made to the technical solutions of the present disclosure, and these simple modifications all fall within the scope of protection of the present disclosure.

[0118] It should also be noted that the various specific technical features described in the above specific embodiments can be combined in any appropriate manner without contradiction. In order to avoid unnecessary repetition, the present disclosure will not further describe various possible combinations.

[0119] In addition, the various embodiments of the present disclosure may be arbitrarily combined, and as long as they do not violate the concept of the present disclosure, they should also be regarded as the contents disclosed by the present disclosure.

Claims

1. A method for improving the upper limit of coal seam mining by grouting to transform thick water-bearing sand layers, characterized in that: include: Step 1: Determine the scope of treatment required to increase the upper limit of coal seam mining; 1) Increase the vertical height H of the upper limit section 提 =H 裂 +H 保 +H 风 -H 垮 ; Where: H 提 To increase the vertical height of the upper limit, m; H 裂 is the maximum height of the water-conducting fracture zone, m; H 保 is the thickness of the protective layer, m; H 风 is the depth of the bedrock weathering aquifer zone, m; H 垮 is the maximum height of the collapse zone, m; 2) Grouting transformation scope of thick water-bearing sand layer Calculate the length and width of grouting reconstruction based on the movement angle of bedrock and loose layer; L 长 =L A +L1+L2=H 提 / tanα+H 基 cotδ+H 砂 cotϕ; L 宽 =L B +2L1+2L2=L B +2H 基 cotδ+2H 砂 cotϕ; Where: L 长 L is the length of the working face to be transformed, m; 宽 L is the width of the working face inclination transformation, m; A L is the horizontal projection length of the upper limit section of the working face, m; B is the horizontal projection length of the upper limit section of the working face, m; L1 is the horizontal projection length of the bedrock section, m; L2 is the horizontal projection length of the water-bearing sand layer section, m; H 基 H is the vertical height of the coal seam roof bedrock, m; 砂 is the thickness of the sand layer, m; α is the coal seam inclination, °; ​​δ is the bedrock movement angle, °; is the loose layer dynamic angle, °; Step 2: Arranging drill holes in the thick water-bearing sand layer within the determined grouting transformation range, wherein the drill holes are arranged in a first sequence and a second sequence, and the drill holes in the two sequences are staggered in alternate rows, with a spacing D between the drill holes in the same row of 40 to 60 m and a row spacing of 0.5D; Step 3: Drilling and inserting casing; Drill vertical holes at the hole locations determined in step 2. The casing is set in a "long-short-leg" pattern, i.e., the casing depth increases along the coal seam dip direction. The casing depth of a certain borehole is H 100mm below the thick water-bearing sand layer. 套 At depth; H 套 The calculation method is: Where: H 套 is the depth of the casing below the top of the thick water-bearing sand layer, m; M is the mining thickness of the coal seam, m; L d L is the horizontal distance from the borehole to the shallowest boundary of the treatment area, m; 长 is the length of the working face to be transformed, m; H 砂 is the thickness of the sand layer, m; Step 4: Sequential progressive segmented cyclic grouting; Sequential progressive grouting: grouting is performed progressively in the first and second sequence boreholes, with odd-numbered rows representing the first sequence and even-numbered rows representing the second sequence. Pressure relief induction grouting is performed alternately in rows within the same sequence, meaning that while one row of boreholes is being grouted, the next row of boreholes acts as a pressure relief hole. During the grouting process, the grouting process is evaluated through the three stages of water return, sand return, and grout return through the drainage holes to determine whether the grout has spread to a point where it is connected. If grout return occurs, the drainage hole is sealed and grouting continues to expand the fracture channel. Segmented cycle grouting: Each grouting section is 4 to 6 meters long. After each grouting reaches the end standard, it will be extended to the next grouting section. If the end standard is not reached, the grouting will be repeated. The final hole depth is 10 to 20 meters into the weathering zone. The proportion of the cement fly ash slurry for grouting is as follows: the treatment section is divided into three sections from top to bottom according to the purpose of the transformation; The first section is from the bottom of the casing to twice the thickness of the bedrock top, and is filled and compacted with fly ash-cement slurry with a mass percentage of 20% to 50%. The fly ash content is gradually reduced to 20% as the hole depth increases; The second section is a sand layer twice as thick as the bedrock, which is transformed to serve as a protective layer. It requires a high stone rate and high strength, and uses fly ash-cement slurry with a mass percentage of 10% to 20%; The third section is the bedrock weathering zone, where the weathered and broken rocks need to be grout-consolidated to give them higher integrity and water-resistance. Pure cement slurry or fly ash-cement slurry with a mass percentage of less than 10% is used.

2. The method for improving the upper limit of coal seam mining by grouting to transform thick water-bearing sand layer according to claim 1 is characterized in that: The outer annulus of the casing section is sealed by cementing.

3. The method for improving the upper limit of coal seam mining by grouting to transform thick water-bearing sand layers according to claim 1 or 2, characterized in that: Grouting end standard: Grouting drilling reaches the end pore pressure P i =P0+0.03h, stop grouting; or if the grouting time after the drainage hole returns to grouting reaches 2-4 hours and the terminal pore pressure has not been reached, stop grouting, sweep the hole and re-inject until it reaches P1; Where: P i is the pore pressure at the end of grouting of the i-th segment of the hole, MPa; P0 is the splitting pressure of the first grouting of the hole, MPa; h is the lower extension of the segment, m.

4. The method for improving the upper limit of coal seam mining by grouting to transform thick water-bearing sand layers according to claim 1 or 2, characterized in that: It also includes step five: constructing a ground inspection hole; After all grouting drilling is completed, ground inspection holes are constructed at the centers of four adjacent first-order drilling holes and second-order drilling holes. The number of inspection holes is 8% to 10% of the grouting drilling holes.

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