Method for testing water resistance of remaining bedrock pillar after mining in coal seam outcrop area

Abstract

The invention belongs to the technical field of coal mining, and particularly relates to a coal seam outcrop area after-mining remaining bedrock column water resistance testing method, which comprises the following steps of: determining the height of outcrop area thin bedrock, the maximum height of a water flowing fissure zone and the bottom water pressure according to working face geology, hydrogeology and mining conditions of an outcrop area; the height of the remaining bedrock column is calculated, and the lithologic composition of the remaining bedrock column after mining is analyzed; adopting similar materials, and performing bed rock pillar height preparation according to a similar three-theorem; designing a combined model with different fracture widths and different remaining bedrock column heights for testing; and a test result is inversed into reality and is compared with the bottom water content pressure, and finally whether the coal seam outcrop area is safely mined or not is accurately obtained. According to the method, an important basis is provided for the prevention and treatment of the roof water damage in the outcrop area, the problem that the water resistance of the remaining bedrock columns is not affected by the development of the water-conducting fissure zone after the outcrop area is mined is solved, and an important reference value is provided for the safety production of the mine outcrop area in the future.

Description

Technical Field

[0001] The invention belongs to the technical field of coal mining, and in particular relates to a method for testing the water resistance of bedrock columns remaining in a coal seam outcrop area after mining. Background Art

[0002] With the increase of mining years, the coal resources of normal blocks in many mines of North China coalfield covered by thick loose layers have been mined out, and they have gradually turned to mining in shallow outcrop areas. The shallow outcrop area is covered by thick loose layers, and there are often aquifers (bottom aquifers) with uneven water-richness at the bottom. The existence of bottom aquifers directly affects the shallow mining of the outcrop area. At present, shallow mining is mainly carried out by leaving a safe coal (rock) pillar on the roof according to the allowable mining level corresponding to the mining level of the water body under the water body given in the "Code for the Retention and Compressed Coal Mining of Coal Pillars for Buildings, Water Bodies, Railways and Main Wells and Lanes" revised in 2017. There are three types of safe coal (rock) pillars: waterproof roof, sand prevention and anti-collapse safety coal (rock) pillars. However, this method does not take into account the rock composition and water blocking capacity of the bedrock pillars left after mining, and is generally conservative. The invention patent entitled "Evaluation of mining upper limit and mining hazard evaluation method for near-loose aquifers in coal mines" authorized by the first inventor Sui Wanghua in 2015 is a method for constructing a 6-factor evaluation index system for improving the mining upper limit of coal seams near loose aquifers in coal mines based on a systematic analysis of the geological and hydrogeological characteristics of coal mines using a three-dimensional GIS method; however, this method focuses on the evaluation of mining hazard and lacks research on the water resistance of overburden after mining. The invention patent entitled "A method for improving the upper limit of coal mining in thin bedrock areas" authorized by the first inventor Qian Ziwei in 2023 is a method for transforming the level II water mining level to the level III water mining level in thin bedrock areas through grouting; however, this method focuses on the evaluation of thin bedrock areas from sand prevention to collapse prevention mining using grouting and lacks research on the combination characteristics and water resistance of the overburden strata remaining after mining.

[0003] The above specifications and methods all consider raising the upper limit of mining based on the allowable mining degree corresponding to the water body mining grade, without considering the water-resistance problem of the remaining bedrock column that is not affected by the development of water-conducting fracture zones after mining in the outcrop area. Summary of the invention

[0004] In order to solve the above technical problems, the present invention provides a method for testing the water-resistance of bedrock pillars remaining after mining in coal seam outcrop areas. The method is closely integrated with the on-site test experiment, based on the three similarity theorems as the theoretical basis, and taking into account the development height of the on-site bottom content and water-conducting fracture zone, the water-resistance of the bedrock pillars remaining after mining is accurately analyzed by analogy, providing an important basis for the prevention and control of roof water hazards in the outcrop area.

[0005] To achieve the above object, the present invention adopts the following technical solution:

[0006] A method for testing the water resistance of bedrock pillars remaining after mining in coal seam outcrop areas is as follows:

[0007] ① According to the geology, hydrogeology and mining conditions of the outcrop area working face, determine the height of thin bedrock in the outcrop area (H), the maximum height of the water-conducting fracture zone (H li ) and bottom water pressure (P);

[0008] ② Calculate the height (L) of the remaining bedrock column and analyze the lithology composition of the remaining bedrock column after mining;

[0009] ③ Use similar materials and prepare the height of the bedrock column according to the geometric similarity ratio, motion similarity ratio, density similarity ratio and dynamic similarity ratio;

[0010] ④ According to the thickness of the coal seams mined at different locations in the coal seam outcrop area, bedrock height, lithology, height of the water-conducting fracture zone and fracture development characteristics, a combination model with different fracture widths and different remaining bedrock column heights was designed for testing;

[0011] ⑤ Invert the test results to reality and compare them with the bottom water pressure (P), and finally accurately determine whether the coal seam outcrop area is safe to mine.

[0012] Furthermore, the maximum height of the water-conducting fracture zone is 10 times the mining thickness, H li =10M, M is the mining thickness.

[0013] Furthermore, the remaining bedrock column height L = HH li .

[0014] Furthermore, the geometric similarity ratio (C l ) is 1:100 to determine the model height; the motion similarity ratio (C t ) is 1:10 to determine the mining speed; the density similarity ratio (C ρ ) is 1:1.5 to determine the formation density; the dynamic similarity ratio (C σ ) is 1:150 to determine the formation strength.

[0015] Furthermore, the similar material uses river sand as aggregate, clay and gypsum as binder, and mica powder is used to layer the rock layer.

[0016] Furthermore, similar materials of various rocks in the bedrock were mixed and made into cylinders with a diameter of 50 mm and a diameter-to-height ratio of 1:2, and uniaxial compressive strength tests were performed.

[0017] Furthermore, the combined model test calculates the compressive strength according to the rock composition in the height of the remaining bedrock column, and then obtains the proportion strength of similar materials according to the dynamic similarity ratio, prepares similar materials, designs prefabricated fractures, and then uses an external water pressure system to simulate the water resistance of similar materials in the bedrock column remaining after mining.

[0018] The present invention is closely integrated with the on-site testing experiments, based on the three similarity theorems as the theoretical basis, and taking into account the on-site bottom content and the development height of the water-conducting fracture zone, accurately conducts an analogy analysis of the water resistance of the remaining bedrock pillars after mining, providing an important basis for the prevention and control of roof water hazards in the outcrop area, solving the problem of water resistance of the remaining bedrock pillars that are not affected by the development of the water-conducting fracture zone after mining in the outcrop area, and providing important reference value for future safe production in the outcrop area of ​​mines. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 : Schematic cross-section diagram of the height zoning of the overburden rock after mining in the outcrop area under the thick topsoil layer;

[0020] Figure 2 : Structural diagram of the water-resistance test model of the bedrock column remaining after mining in the coal seam outcrop area. DETAILED DESCRIPTION

[0021] The present invention is described in detail below with reference to the accompanying drawings and embodiments.

[0022] like Figure 1 As shown in the figure, according to the geological, hydrogeological and mining conditions of the pre-mining working face in a certain outcrop study area, combined with the bottom content distribution characteristics of the overlying thick topsoil layer, the pre-mining coal seam belongs to a very thick coal seam with an average thickness of 9.0m. In thin bedrock areas, layered comprehensive mining is generally considered first, and the preliminary design of the first layer mining thickness (mining thickness) is 3.0m; the coal seam is covered by a thick topsoil layer composed of the Quaternary and Neogene systems, and the bottom water content is weak to medium, and the water content is uneven. Whether the height of the remaining bedrock column above the height of the water-conducting fracture zone generated by the first layer mining of the coal seam can prevent the bottom water from entering the working face is the key to ensuring the safe mining of the pre-mining working face in the outcrop area. To this end, a test method for the water resistance of the bedrock column remaining after mining in the outcrop area of ​​the coal seam is proposed, considering the actual bedrock height on site and the maximum height of the water-conducting fracture zone, and focusing on analyzing the lithology of the bedrock column remaining after mining. By proportioning similar materials from the perspectives of geometric similarity ratio, motion similarity ratio and dynamic similarity ratio, a model is established to conduct a water-resistance test on the bedrock column remaining after mining in the coal seam outcrop area. The test results are then inverted to reality and compared with the bottom water pressure. Ultimately, it can be accurately determined whether the coal seam outcrop area is safe to mine.

[0023] A method for testing the water resistance of bedrock pillars remaining after mining in coal seam outcrop areas is as follows:

[0024] ① According to the geology, hydrogeology and mining conditions of the outcrop area working face, determine the height of thin bedrock in the outcrop area (H), the maximum height of the water-conducting fracture zone (H li ) and bottom water pressure (P).

[0025] The height of thin bedrock in the outcrop area (H) was 40m, 50m and 60m; the maximum height of the first layer of water-conducting fracture zone in the first mining face (H li ) is 10 times the mining thickness (M), that is, Hli =10M,H li =30m; the bottom water pressure (P) is considered to be 5MPa, 6MPa and 7MPa.

[0026] ② Calculate the height (L) of the remaining bedrock column and analyze the lithology composition of the bedrock column remaining after mining.

[0027] Considering the height of thin bedrock (H) overlying the coal seam of 40m, 50m and 60m, and taking the stratigraphic lithology combination as the background, the height of the water-conducting fracture zone (H li ) is 30m, and the heights (L) of the bedrock columns remaining after mining are 10m, 20m and 30m respectively; this provides a practical reference for the proportions of bedrock columns of similar materials and bedrock columns remaining after mining.

[0028] Based on ①, L=HH li , the height of the remaining bedrock column after mining (L) is 10m, 20m and 30m respectively; according to statistics of a large number of supplementary drilling holes in the outcrop area, the bedrock column in the outcrop area is composed of three types of lithology: mudstone (mudstone, sandy mudstone, ferruginous mudstone and aluminous mudstone), sandstone (siltstone, fine sandstone and medium sandstone) and coal seam. Affected by weathering, the overall strength of the remaining bedrock is low. The actual compressive strength of each rock is shown in Table 1.

[0029] ③ Use similar materials and prepare the height of the bedrock column according to the geometric similarity ratio, motion similarity ratio, density similarity ratio and dynamic similarity ratio.

[0030] Geometric similarity ratio (C l ) According to the ratio of 1:100, the model heights are determined to be 40cm, 50cm and 60cm respectively; the motion similarity ratio (C t ) is 1:10 to determine the mining speed; density similarity ratio (C ρ ) is 1:1.5 to determine the formation density; the dynamic similarity ratio (C σ ) is 1:150 to determine the stratum strength. Similar materials use river sand as aggregate, clay and gypsum as cementing materials, and mica powder is used to layer the rock strata. The proportions of similar materials for different lithology are shown in Table 1.

[0031] After similar materials of various rocks in the bedrock are proportioned, 5 cylinders with a diameter of 50 mm and a diameter-to-height ratio of 1:2 are first made and uniaxial compressive strength tests are carried out. The average value is basically the same as the compressive strength value of similar materials of various rocks corresponding to Table 1. Otherwise, it is necessary to re-proportion and re-test until the requirements are met before the bedrock column height is prepared.

[0032] Table 1 Reference table of proportions of similar materials with different bedrock lithology (mass ratio)

[0033]

[0034] ④ According to the differences in coal seam thickness, bedrock height, lithology, height of water-conducting fracture zone and fracture development characteristics at different locations in the coal seam outcrop area, a combination model with different fracture widths (0.5mm; 1.0mm; 1.5mm) and different heights of remaining bedrock columns was designed for testing. There are 9 combinations, see Table 2. According to the lithology composition in the height of the remaining bedrock column, the compressive strength is calculated, and then the strength of the similar material ratio is obtained according to the dynamic similarity ratio. After the strength of different ratios is determined, river sand, clay and gypsum are added with water to prepare similar materials according to the mass ratio. The prefabricated fracture 1 is designed, and the water-resistant similar material simulation test of the remaining bedrock column after mining is carried out through the permeable stone 2 and the flange 3 external water pressure pressurization system 4, such as Figure 2 shown.

[0035] Table 2 Test combinations of materials with similar height in bedrock columns

[0036]

[0037] ⑤ Invert the indoor test results to the actual situation and compare them with the bottom water pressure (P), and finally accurately determine whether the coal seam outcrop area is safe to mine, which has very important reference value for future safe production in the outcrop area of ​​the mine.

[0038] This method is controlled by key parameters such as the height of the overburden rock on the roof of the mined coal seam in the outcrop area, the lithology composition and the height of the development of the water-conducting fracture zone in the mined coal seam. It requires close integration between the on-site and indoor work, and the ratio of similar materials in the indoor remaining bedrock column is very critical.

Claims

1. A method for testing the water resistance of bedrock columns remaining after mining in coal seam outcrop areas, characterized in that: ① According to the geology, hydrogeology and mining conditions of the outcrop area working face, determine the height of thin bedrock in the outcrop area (H), the maximum height of the water-conducting fracture zone (H li ) and bottom water pressure (P); ② Calculate the height (L) of the remaining bedrock column and analyze the lithology composition of the remaining bedrock column after mining; ③ Use similar materials and prepare the height of the bedrock column according to the geometric similarity ratio, motion similarity ratio, density similarity ratio and dynamic similarity ratio; ④ According to the thickness of the coal seams mined at different locations in the coal seam outcrop area, bedrock height, lithology, height of the water-conducting fracture zone and fracture development characteristics, a combination model with different fracture widths and different remaining bedrock column heights was designed for testing; ⑤ Invert the test results to reality and compare them with the bottom water pressure (P), and finally accurately determine whether the coal seam outcrop area is safe to mine.

2. The method for testing the water resistance of bedrock pillars remaining after mining in coal seam outcrop areas according to claim 1 is characterized in that: The maximum height of the water-conducting fracture zone is 10 times the mining thickness, H li =10M, M is the mining thickness.

3. The method for testing the water resistance of bedrock pillars remaining after mining in coal seam outcrop areas according to claim 1 is characterized by: The remaining bedrock column height L = HH li .

4. The method for testing water resistance of bedrock pillars remaining after mining in coal seam outcrop areas according to claim 1 is characterized in that: The geometric similarity ratio (C l ) is 1:100 to determine the model height; the motion similarity ratio (C t ) is 1:10 to determine the mining speed; the density similarity ratio (C ρ ) is 1:1.5 to determine the formation density; the dynamic similarity ratio (C σ ) is 1:150 to determine the formation strength.

5. The method for testing water resistance of bedrock pillars remaining after mining in coal seam outcrop areas according to claim 1 is characterized in that: The similar material uses river sand as aggregate, clay and gypsum as cementing materials, and mica powder is used to layer the rock layer.

6. The method for testing water resistance of bedrock pillars remaining after mining in coal seam outcrop areas according to claim 1 is characterized in that: After similar materials of various rocks in the bedrock are mixed, they are first made into cylinders with a diameter of 50 mm and a diameter-to-height ratio of 1:2, and then the uniaxial compressive strength test is carried out.

7. The method for testing water resistance of bedrock pillars remaining after mining in coal seam outcrop areas according to claim 1 is characterized by: The combined model test experiment calculates the compressive strength according to the rock composition in the height of the remaining bedrock column, then obtains the proportion strength of similar materials according to the dynamic similarity ratio, prepares similar materials, designs prefabricated fractures, and then uses an external water pressure pressurization system to simulate the water resistance of similar materials in the bedrock column remaining after mining.

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