Key layer presplitting settlement control method based on overlying strata fracture angle arrangement

Abstract

The invention discloses a key layer presplitting settlement control method based on overlying strata fracture angle arrangement. The method comprises the steps that geological mining parameters of a target mine are collected and determined; based on the physical and mechanical parameters of each rock stratum, determining the horizon height of the key stratum in combination with a thick and hard rock stratum judgment criterion; according to coal seam mining parameters and overlying strata lithology, the height of a water flowing fractured zone is judged, and high and low horizon classification is conducted on the water flowing fractured zone by combining the horizon of the key horizon; according to the development law of overlying strata damage along the fracture angle from bottom to top, the presplitting position in the horizontal direction of each layer key layer in the presplitting range is calculated and determined, and full-section directional presplitting cutting is carried out; based on thick and hard rock stratum mining-induced stress distribution characteristics and a damage deformation transmission mechanism, a working face moving boundary and a reduction distance of a building (structure) protection coal pillar after key stratum presplitting are calculated and determined. According to the method, presplitting implementation is guided based on the key layer fracture rule, and a new technical approach is provided for subsidence control of the mining area.

Description

Technical Field

[0001] This invention relates to the technical field of pre-fracture settlement control methods. Specifically, it is a pre-fracture settlement control method for key layers based on the arrangement of overburden fracture angles. Background Technology

[0002] Large-scale coal mining inevitably leads to surface subsidence, which can damage buildings, railways, highways, tunnels, and high-voltage power line towers, endangering the safety of the living environment and infrastructure in mining areas and severely restricting the development of green mines. my country has a large population, dense villages, a complex network of roads, and extensive infrastructure. To protect the ecological environment of mining areas and prevent mining subsidence from endangering surface buildings, mines typically leave protective coal pillars under these structures, resulting in significant coal-bearing problems under buildings, railways, and water bodies. Especially in the central and eastern mining areas such as Kailuan, Fengfeng, Xuzhou, Huainan, and Pingdingshan, the proportion of coal-bearing under these structures reaches 50% to 80%, with a large amount of coal resources being trapped, seriously affecting the mine's production continuity and sustainable development. Meanwhile, with the increasing depletion of coal resources in mines in the central and eastern regions, tapping the potential of coal resources under the "three underground" conditions is crucial to the survival and development of mining enterprises. It is urgent to develop economical and efficient sedimentation control technology to coordinate the needs of resource extraction and environmental protection.

[0003] Surface subsidence is a complex mechanical response process involving a chain transmission of "working face mining - overburden movement - surface deformation." Its subsidence range is influenced by various factors, including working face size, coal pillar stability, and overburden lithology (such as overburden structure and key strata). Currently, many technologies exist for controlling strata and surface subsidence, mainly including strip mining, backfilling mining, and coordinated mining, but all have certain technical limitations. Strip mining has low resource recovery rates and affects production efficiency; backfilling mining is costly and its effectiveness is difficult to guarantee; and coordinated mining has complex processes and is difficult to implement on-site.

[0004] Related studies have shown that thick, hard rock strata in the overburden (i.e., key strata) play a dominant role in stress transmission and strata movement during mining. Pre-splitting technology, by actively weakening the integrity of the key strata, can effectively block high-stress transmission paths and shorten the range of additional stress influence within the coal and rock mass. Furthermore, the selection of the pre-splitting location in the key strata is also crucial for controlling subsidence.

[0005] Chinese patent CN112879011A discloses a method for controlling the height of water-conducting fracture zones by pre-fracture weakening of hard overburden beneath an aquifer. Based on the structural characteristics of the overburden, this method determines the control height of the water-conducting fracture zone after pre-fracture weakening of the top stratum, and then operates on the selected pre-fracture weakening target layer using hydraulic fracturing, thereby effectively controlling the height range of the water-conducting fracture zone and achieving the goal of protecting the aquifer. However, this method lacks the ability to predict the fracture location of key layers, making it difficult to accurately implement directional pre-fracture operations.

[0006] Chinese Patent No. CN120159416A describes a system for classifying and pre-splitting hard roofs, using in-situ grouting modification and filling to prevent rockburst. This method controls rockburst by classifying and pre-splitting key layers and reinforcing the collapse zone with grouting. However, the pre-splitting process inevitably affects the distribution of mining-induced stress. Furthermore, the scheme does not fully consider how to scientifically directionally pre-splitting to optimize the distribution of overburden stress and deformation transmission, thus failing to effectively combine it with surface subsidence control.

[0007] The overburden fracture angle corresponds to the direction of the weak shear stress surface of the mined strata. If key layer pre-fracture is carried out along this direction, it can conform to the overburden mining fracture law, help improve the penetration of key layer pre-fracture, reduce the influence area of ​​additional stress, and achieve effective control over the surface subsidence range.

[0008] Therefore, if scientific and reasonable pre-splitting is carried out on the key layer above the working face boundary, it is expected to achieve optimized distribution of mining stress field, thereby effectively controlling the surface subsidence range, reducing the size of the protective coal pillar, and achieving the goal of improving coal resource recovery rate and mining efficiency. Summary of the Invention

[0009] Therefore, the technical problem to be solved by the present invention is to provide a key layer pre-fracture control method based on the arrangement of overburden fracture angle, which aims to implement directional artificial pre-fracture by predicting the fracture behavior of the key overburden layer, thereby achieving active and efficient control of the surface subsidence range.

[0010] To solve the above-mentioned technical problems, the present invention provides the following technical solution:

[0011] A method for controlling settlement by pre-fracture of key layers based on the arrangement of overburden fracture angles includes the following steps: Step 1, collecting and determining the geological and mining parameters of the target mine;

[0012] Step 2: Based on the physical and mechanical parameters of each rock layer and combined with the criteria for identifying thick and hard rock layers, determine the stratigraphic height of the key layer;

[0013] Step 3: Based on the coal seam mining parameters and overlying lithology, determine the height of the water-conducting fracture zone, and classify it into high and low strata based on the key strata positions;

[0014] Step 4: Based on the evolution law of overburden damage from bottom to top along the fracture angle, calculate and determine the horizontal pre-fracture position of each key layer within the pre-fracture range, and implement full-section directional pre-fracture cutting.

[0015] Step 5: Based on the stress distribution characteristics and damage deformation transmission mechanism of the thick and hard rock strata, calculate and determine the moving boundary of the working face and the reduction distance of the protective coal pillar of the building / structure after the pre-splitting of the key layer.

[0016] In the above-mentioned key layer pre-fracture and settlement control method based on the overburden fracture angle arrangement, the geological and mining parameters in step 1 include: coal seam mining parameters: working face width L, mining thickness M, and mining depth H0;

[0017] Overall lithology of overlying strata: Protodyakonov hardness coefficient f;

[0018] Rock strata physical and mechanical parameters: the thickness h of the i-th rock stratum i γ-weight i Elastic modulus E i internal friction angle φ i Tensile strength R Ti Shear strength R Ki .

[0019] The aforementioned method for controlling settlement by pre-fracture of key layers based on the arrangement of overburden fracture angles, in step 2, determines the number of key layers in the overburden and their location. The criterion is as follows: Assuming there are n layers above the coal seam, and the thickness of the i-th layer is h... i The bulk density is γ i The elastic modulus is E i The tensile strength is R Ti The criteria are determined layer by layer from bottom to top. The two criteria for each key layer are: stiffness condition and strength condition. The criteria for the stiffness condition are: (1); In equation (1), q i / m and q i / m+1 These represent the loads acting on the i-th rock layer, considering the m-th and (m+1)-th rock layers, respectively. , j is the j-th rock layer above the i-th rock layer;

[0020] The strength condition is determined as follows:

[0021] (2);

[0022] In equation (2), l i Let l be the fracture distance of the i-th rock layer. m+1 The fault distance of the (m+1)th rock layer is... If the i-th rock layer is a key layer, and both equations (1) and (2) are satisfied, the target rock layer m+1 is determined to be a key layer. Based on the results of the judgment of the stiffness and strength of each rock layer, the key layers in the overburden of the target mining area are determined to be layers t, which are denoted as s1, s2, ..., s from bottom to top. t Their layer heights are H s1 H s2 H st .

[0023] The aforementioned method for controlling settlement by pre-fracture of key layers based on the arrangement of overburden fracture angles includes the following steps in step 3:

[0024] Step 2-1: Use the Protodyakonov hardness coefficient f to determine the overall lithology of the overlying strata: when f > 8, it is a hard overlying strata; when 3 < f ≤ 8, it is a medium-hard overlying strata; when f ≤ 3, it is a weak overlying strata. Step 2-2: Under different lithological conditions, calculate the height of the water-conducting fracture zone in the mining-induced overlying strata, categorized by lithology as follows: When the overlying strata are hard: (3);

[0025] When the overburden is of medium-hardness: (4);

[0026] When the overlying strata are weak: (5); In equations (3)-(5), H f M represents the calculated height of the water-conducting fracture zone, in meters; M represents the coal seam thickness, in meters.

[0027] Steps 2-3: Classify the key layers according to their spatial relationship with the water-conducting fracture zone:

[0028] With the key layer height H si Calculated height H of the water-conducting fracture zone f Using the relative size as a criterion, key layers are divided into the following two categories:

[0029] Low-level critical layer: When H si <H f At that time, it was indicated that the water-conducting fracture zone had completely penetrated the key stratum s. i These critical layers are damaged, fractured, or cracked due to mining activities;

[0030] High-level critical layer: When H si ≥H f This indicates that the water-conducting fracture zone has not developed to the critical layer s. i The rock strata are in a bent and subsided state, and these key strata are less affected by mining.

[0031] In the aforementioned method for controlling settlement by pre-fracture of key strata based on the arrangement of overburden fracture angles, step 4 involves calculating the fracture angle based on the evolution of overburden damage along the fracture angle from bottom to top; the key strata and the soft rock above them are divided into a group, resulting in a total of t+1 groups, namely z1, z2, ..., z t+1 Then any rock stratum group z i The expression for calculating the breaking angle is:

[0032] (6);

[0033] Combined with the overburden load q on the key layer s(i-1) The calculation formula, equation (6), is further obtained as follows:

[0034] (7); In equations (6) and (7), θ zi rock strata group z i Breaking angle, °; R Kzi rock strata group z i The comprehensive shear strength, MPa; E s(i-1) For key layer s i-1 elastic modulus; φ zi The comprehensive internal friction angle of the rock strata is given in °; h s(i-1) For key layer s i-1 Thickness, m; E j h is the elastic modulus of the j-th rock layer above the key layer; j γ represents the thickness of the j-th rock layer above the key layer, in meters; j The unit weight of the j-th rock layer above the key layer is MN / m³. 3 λ is the critical layer fault distance standard, with a value of 1.5~2.5.

[0035] The aforementioned method for controlling settlement by pre-fracture of key strata based on the overburden fracture angle determines the horizontal pre-fracture locations of high and low key strata by calculating the evolution of overburden damage along the fracture angle from bottom to top, combined with the stratum height of each stratum group. The horizontal pre-fracture location of the low key strata is the horizontal distance from the pre-fracture location of the key strata to the coal face boundary, which is obtained using the following formula:

[0036] (8);

[0037] In equation (8), x dsi For the low-level key layer s i The horizontal distance from the pre-crack location to the working face boundary, in meters (m); H si For the low-level key layer s i The height of the layer, m, where H s0 =0; H s(i-1) For the low-level key layer s i-1The height of the stratum, m; θ z(i+1) rock strata group z i+1 breaking angle; R Kz(i+1) rock strata group z i+1 The comprehensive shear strength, MPa; E si For key layer s i The elastic modulus; h si For key layer s i Thickness, m; Horizontal pre-fracture location of high-level key strata: To effectively weaken the spatial transmission of overburden subsidence and optimize the control of surface subsidence range, a vertically upward pre-fracture path is adopted for high-level key strata to ensure the overall alignment and continuity of pre-fracture zones in the vertical direction; the horizontal pre-fracture locations of each high-level key stratum should be consistent and the same as the pre-fracture location of the first high-level key stratum above the water-conducting fracture zone; Assuming the total number of low-level key strata is c, the formula for calculating the horizontal pre-fracture location of any high-level key stratum is:

[0038] (9);

[0039] In equation (9), x gsi For high-level key layer s i The horizontal distance from the pre-fracture location to the working face boundary is in meters (m); c is the total number of low-level key layers within the water-conducting fracture zone; θ z(c+1) For rock group z c+1 The overlying rock breaking angle; H sc The uppermost low-lying key layer s within the rock strata group c The height of the floor; x dsc It refers to the horizontal distance between the pre-splitting location of the uppermost low-level key layer and the coal wall at the boundary of the working face.

[0040] The above-mentioned key layer pre-fracture and settlement control method based on the overburden fracture angle arrangement adopts directional long borehole hydraulic fracturing technology to carry out full-section pre-fracture cutting of key layers at high and low levels within the pre-fracture range according to the determined horizontal pre-fracture position.

[0041] After directional drilling is completed, the perforation tool string is pushed to the designed position, and high-pressure water jets are used to impact the borehole wall at a set angle to form initial holes in the rock strata. Subsequently, the tool string is dragged to achieve a dense directional arrangement of holes, thereby controlling the propagation direction and fracturing effect of subsequent hydraulic fracturing fractures.

[0042] In the aforementioned key layer pre-fracture and settlement control method based on overburden fracture angle arrangement, in step 5, the horizontal distance S between the surface subsidence boundary and the working face boundary is determined according to the stress distribution characteristics and damage deformation transmission mechanism of the thick and hard rock strata:

[0043] (10)

[0044] In equation (10): μ is the interlayer weak friction coefficient; φ is the comprehensive internal friction angle of the rock mass; γ is the average unit weight of the overlying strata, kg / m³ 3 H0 represents the mining depth, in meters; H si For key layer s i Stratum height, m; θ is the bedrock fracture angle, °; δ is the loose layer movement angle, °; β is the peak stress propagation angle, °; q di For high-level key layer s i Self-weight and superstructure load, MPa; h si For key layer s i Layer thickness, m; h n M is the thickness of the loose layer, in meters; L is the thickness of the mining layer, in meters; si For key layer s i The layer's own fracture distance; R Ki For key layer s i The overall shear strength of the layer.

[0045] The above-mentioned key layer pre-fracture settlement control method based on overburden fracture angle arrangement, the calculation method for the reduction distance ΔS of the working face moving boundary after pre-fracture is as follows:

[0046] After the top cutting and pre-splitting of the low-level key layer is carried out, the original hinge forces F1 and F2 acting on the boundary beam area disappear immediately. Continue to pre-splitting the high-level key layer in the bending and sinking zone to reduce the magnitude of the vertical additional stress F3.

[0047] The combined effect of pre-splitting of the low-level key layer on the misaligned beam zone is analyzed, and its influence is regarded as the vertical component F of the resultant force of the hinge forces F1 and F2. hin According to the critical layer theory:

[0048] (11)

[0049] In equation (11), q ki For the key block k of the i-th low-level key layer i Total vertical load; l ki The length of the key block; a ki ϕ' is the contact length between the two ends of the critical block. ki T is the rotation angle of the key block; ki and w ki These are the horizontal thrust and subsidence of the key block, respectively.

[0050] Assuming the highest critical layer at the pre-splitting point is the t1th critical layer, and there are c low-level critical layers, then the number of high-level critical layers requiring pre-splitting is (t1-c). After the high-level critical layers have completed pre-splitting, the critical layer load q in equation (10) is... di It will decrease, and the decrease (F'3-F3) is the weight of the overburden directly above the working face, which can be expressed as:

[0051] (12)

[0052] In formula (12), L is the working face width, in meters; H s(t1+1) For high-level key layer s t1+1 The height of the layer; H sc The topmost low-level key layer s c The height of the floor level;

[0053] After the pre-splitting and cutting of the low-level key layer c and the high-level key layer (t1-c) are completed, the working face moving boundary will shrink accordingly, and the shrinkage distance ΔS=S 前 -S 后 ,for:

[0054] (13)

[0055] In equation (13), S 前 and S 后 These are the horizontal distances, in meters, from the coal face to the surface movement boundary before and after pre-splitting of the key layer;

[0056] After pre-fracture of the key stratum, the width of the protective coal pillar is reduced by ΔD. m =ΔS.

[0057] The technical solution of the present invention achieves the following beneficial technical effects:

[0058] This invention proposes a novel method for controlling subsidence by pre-fracture of key strata based on the evolution law of overburden fracture and the characteristics of mining-induced stress distribution. The method mainly includes: First, classifying key strata into high-level and low-level key strata according to the relationship between the height of the water-conducting fracture zone and the stratum position; then, determining the horizontal pre-fracture positions of each key stratum based on the bottom-up evolution law of the overburden fracture angle, and implementing targeted pre-fracture for key strata at different strata positions; finally, calculating the reduction distance of the working face movement boundary and the reduction width of the protective coal pillar after pre-fracture based on the characteristics of mining-induced stress distribution and deformation damage transmission mechanism of thick, hard rock strata. This method guides pre-fracture implementation based on the fracture law of key strata, providing a new technical approach for subsidence control in mining areas.

[0059] 1) By identifying and grouping key layers and combining the evolution law of overburden fracture angle from bottom to top, reasonable pre-splitting positions of key layers in the horizontal direction are given for each layer. This allows for precise implementation of directional pre-splitting operations, making the pre-splitting layout more scientific and targeted, and effectively avoiding blind construction.

[0060] 2) This patent divides the key layers into high and low layers, clarifies the control mechanism of the pre-splitting measures of different key layers on the magnitude and range of mining stress in the boundary misalignment beam area, and gives the calculation formula for the working face movement boundary and the reduction distance of the protective coal pillar after the key layer pre-splitting, which can provide a basis for the optimization design and engineering practice of protective coal pillar.

[0061] 3) On the coal face of the protected object, pre-splitting is carried out from bottom to top along the fracture angle direction, targeting the weak shear stress surface (i.e., the hinge point of the edge rock block) in the low-lying critical strata. For the strata in the curved subsidence zone that are less affected by mining, i.e., the high-lying critical strata, a vertically upward pre-splitting path can be adopted to achieve better control of the subsidence range, reduce the size of the protective coal pillar, and thus improve the coal resource recovery rate and mining efficiency. In other words, different pre-splitting methods are used for low-lying and high-lying critical strata.

[0062] 4) This patent guides pre-fracture implementation based on the key layer fracture law, providing a new technical approach for subsidence control in mining areas. By optimizing the stress distribution and deformation transmission of overlying strata, it effectively combines with surface subsidence control.

[0063] This patented method involves pre-fracture of key strata along the fracture angle direction corresponding to the weak shear stress surface of the overburden, conforming to the fracture law of overburden mining and effectively improving the penetration of key strata pre-fracture and the control effect of surface subsidence. By performing directional pre-fracture along the fracture angle on the thick and hard key strata above the mining boundary, the area of ​​surface subsidence can be effectively reduced while alleviating rockburst. This provides a new technical basis for the optimized design of protective coal pillars for mining area buildings and structures and for releasing coal under pressure. Attached Figure Description

[0064] Figure 1 Schematic diagram of overlying strata distribution and key strata identification;

[0065] Figure 2 Schematic diagram of the classification of key layers in the overburden (high and low positions);

[0066] Figure 3 Schematic diagram of protective coal pillar retention and pre-fracture of key overburden strata;

[0067] Figure 4 Schematic diagram of horizontal drilling segmented hydraulic fracturing in a vertical cross section;

[0068] Figure 5 Schematic diagram showing the changes in stress and movement range of rock strata in the misaligned beam zone before and after pre-splitting;

[0069] Figure 6 Schematic diagram showing the reduction in the width of the protective coal pillar after pre-fracture of the key layer;

[0070] Figure 7 Flowchart of the key layer pre-fracture and settlement control method based on the overburden fracture angle arrangement. Detailed Implementation

[0071] An effective method for pre-fracture and settlement control of key layers based on overburden fracture angle arrangement is described below. (See attached document for details.) Figure 7 The flowchart. (Note: For distinction, in the patent, rock strata are represented by z) i The key layer uses s i Key blocks use k i 。

[0072] Step 1: Collect and determine the geological and mining parameters of the target mine, including: mining depth, mining thickness, mining width, rock strata lithology and physical and mechanical parameters, etc.

[0073] Collect and determine the basic geological and mining parameters of the mining area, mainly including:

[0074] ① Coal seam mining parameters: working face width L, mining thickness M, mining depth H0;

[0075] ② Overall lithology of overlying rocks: Protodyakonov hardness coefficient f;

[0076] ③ Physical and mechanical parameters of rock strata: thickness h of the i-th rock stratum i , with a bulk density of γ i The elastic modulus is E i internal friction angle φ i Tensile strength is R Ti Shear strength R Ki wait.

[0077] Step 2: Based on the physical and mechanical parameters of each rock layer and the criteria for identifying thick and hard rock layers, determine the stratigraphic height of the key layer.

[0078] The mining of underground coal seams disrupts the original stress balance of the surrounding rock, causing a redistribution of stress within a certain range. Stress concentration and release occur in the overburden, leading to overburden movement, deformation, and damage, which is then transmitted upwards layer by layer, causing large-scale surface subsidence.

[0079] Thick, hard rock strata in the overburden have greater stiffness and strength than other rock strata, playing a dominant role in stress transmission and strata movement during mining. Against this backdrop, pre-fracture of thick, hard rock strata at specific locations can reduce the stress transmission range of the coal and rock mass, regulate the surface subsidence boundary, and thus form a new subsidence control method that is easy to implement, effective, and cost-efficient.

[0080] First, determine the number of key layers in the overburden and their stratigraphic position. The criteria for this determination are as follows:

[0081] Assume there are n rock layers above the coal seam, and the thickness of the i-th rock layer is h. i The bulk density is γ i The elastic modulus is Ei The tensile strength is R Ti Judge from bottom to top, layer by layer, such as Figure 1 As shown. The two criteria for satisfying the key layer are:

[0082] (1) Stiffness condition:

[0083] (1)

[0084] In equation (1), q i / m and q i / m+1 These are the loads acting on rock layer i, considering the m-th and m+1-th rock layers respectively. , j is the j-th rock layer above the i-th rock layer.

[0085] (2) Strength condition:

[0086] (2)

[0087] In equation (2), l i Let l be the fracture distance of the i-th rock layer. m+1 The fault distance of the (m+1)th rock layer is... .

[0088] If the i-th rock layer is the key layer, when both equation (1) and equation (2) are satisfied, the target rock layer m+1 can be determined as the key layer.

[0089] Based on the combined results of stiffness and strength assessments of various rock strata, the key strata in the overburden of the target mining area are identified as stratum t, denoted from bottom to top as s1, s2, ..., s. t Their layer heights are H s1 H s2 H st .

[0090] Step 3: Based on the coal seam mining parameters and overlying lithology, determine the height of the water-conducting fracture zone, and classify it into high and low strata based on the key strata.

[0091] In coal seam mining, due to the different damage states of key strata in different strata and the differences in their distance from the working face or the surface, the pre-splitting locations and methods used in subsequent pre-splitting work are also different. Therefore, it is necessary to classify key strata.

[0092] During classification, the height of the water-conducting fracture zone in the overburden is first determined based on the coal seam mining parameters and overburden lithology. Then, the key layers in the overburden are classified into high and low layers based on the spatial relationship between the water-conducting fracture zone and the key strata.

[0093] (1) Comprehensive lithological judgment of the overlying strata.

[0094] The overall lithology of overburden can be characterized using the Protodyakonov hardness coefficient f, which is a dimensionless value. For ease of use in mining engineering, it is typically classified into three categories: ① when f > 8, it is hard overburden; ② when 3 < f ≤ 8, it is medium-hard overburden; ③ when f ≤ 3, it is weak overburden.

[0095] (2) Calculation of the height of the water-conducting fracture zone in the overlying rock under different lithological conditions.

[0096] After the working face is mined out, water-conducting fracture zones are formed inside the overburden due to mining disturbance. Their spatial development is roughly arched or saddle-shaped. To quantify the height of these fracture zones, the empirical formula for calculating the overburden failure height in the "Three-Underground" coal mining specifications can be used as a reference, calculated according to lithology as follows:

[0097] Hard overburden conditions: (3);

[0098] Medium-hard overburden conditions: (4);

[0099] Weak overburden conditions: (5);

[0100] In equations (3)-(5), H f M represents the calculated height of the water-conducting fracture zone, in meters; M represents the coal seam thickness, in meters.

[0101] (3) Classify the key layers according to the spatial relationship between the water-conducting fracture zone and the key layer.

[0102] With the key layer height H si Calculated height H of the water-conducting fracture zone f Using the relative size as a criterion, key layers are divided into the following two categories: Figure 2 As shown.

[0103] ① Low-level critical layer: When H si <H f At that time, it was indicated that the water-conducting fracture zone had completely penetrated the key stratum s. i These critical layers are damaged, fractured, or cracked due to mining activities;

[0104] ② High-level critical layer: When H si ≥H f This indicates that the water-conducting fracture zone has not developed to the critical layer s. i The rock strata are in a bent and subsided state, and these key strata are less affected by mining.

[0105] Furthermore, it is worth noting that during pre-splitting operations, drilling can be carried out from underground or on the surface, regardless of whether the critical stratum is located at a low or high position. To balance engineering economics, the most reasonable drilling path needs to be determined by considering the relative position of the critical stratum to the surface or coal seam, as well as the ease of construction.

[0106] Step 4: Based on the evolution law of overburden damage from bottom to top along the fracture angle, calculate and determine the horizontal pre-fracture position of each key layer within the pre-fracture range, and implement full-section directional pre-fracture cutting.

[0107] Existing research indicates that thick, hard key layers in the overburden play a controlling role in the damage to mining-induced rock masses. These rock layers are strong and not easily broken. After mining, they tend to form cantilever beams or articulated rock beam structures on both sides of the goaf, causing high static stress to accumulate and transfer to the deeper parts of the coal and rock walls. This not only exacerbates the damage and deformation of the coal and rock walls but also expands the range of overburden migration.

[0108] To address the aforementioned issues, pre-splitting technology can effectively optimize stress distribution and control the migration range of overburden. The determination of pre-splitting locations must be based on the fracturing patterns of the overburden during mining, taking into account both construction effectiveness and economic feasibility.

[0109] Specifically, on the coal face of the protected object, pre-fracture is carried out from bottom to top along the fracture angle direction, targeting the weak shear stress surface (i.e., the hinge point of the edge rock block) in the key stratum. For the strata in the curved subsidence zone that are less affected by mining, i.e., the high-level key strata, a vertically upward pre-fracture path can be adopted to achieve better control of the subsidence range, such as... Figure 3 As shown.

[0110] (1) Calculation of the breaking angle

[0111] Existing research indicates that the fracture angle of the overlying strata corresponds to the direction of the weak shear stress surface in the mined strata. The fracture angle of each stratum group is mainly affected by factors such as the shear strength, internal friction angle, elastic modulus, and overlying load of the strata. For ease of calculation and expression, the key strata and the soft rock above them are divided into one group, resulting in a total of t+1 stratum groups, namely z1, z2, ..., z t+1 Then any rock stratum group z i The expression for calculating the breaking angle is:

[0112] (6)

[0113] Combined with the overburden load q on the key layer above s(i-1) The calculation formula yields:

[0114] (7)

[0115] In equations (6) and (7), θ zi rock strata group zi Breaking angle, °; R Kzi rock strata group z i The comprehensive shear strength, MPa; E s(i-1) For key layer s i-1 elastic modulus; φ zi The comprehensive internal friction angle of the rock strata is given in °; h s(i-1) For key layer s i-1 Thickness, m; E j h is the elastic modulus of the j-th rock layer above the key layer; j γ represents the thickness of the j-th rock layer above the key layer, in meters; j The unit weight of the j-th rock layer above the key layer is MN / m³. 3 λ is the critical layer fault distance standard, which is generally 1.5~2.5.

[0116] (2) Determination of the horizontal pre-splitting location of the high and low key layers

[0117] ① The location of the horizontal pre-fracture of the low-lying key stratum (i.e., the horizontal distance between the pre-fracture location of the key stratum and the coal wall at the working face boundary) can be determined based on the evolution law of overburden damage along the fracture angle from bottom to top, combined with the stratum height of each rock group. The specific calculation formula is as follows:

[0118] (8)

[0119] In equation (8), x dsi For the low-level key layer s i The horizontal distance from the pre-crack location to the working face boundary, in meters (m); H si From the coal seam to the lower key strata s i The height of the layer, m, where H s0 =0. H s(i-1) For the low-level key layer s i-1 The height of the stratum, m; θ z(i+1) rock strata group z i+1 breaking angle; R Kz(i+1) rock strata group z i+1 The comprehensive shear strength, MPa; E si For key layer s i The elastic modulus; h si For key layer s i Thickness, in meters (m).

[0120] ② Given that the high-level key strata are located within a flexural subsidence zone and are relatively less disturbed by mining, their deformation mode is mainly characterized by overall flexural subsidence rather than shearing along the fracture angle. To effectively reduce the spatial transmission of overburden subsidence and optimize the control of surface subsidence range, a vertically upward pre-fracture path is adopted for the high-level key strata to ensure that the pre-fracture zones of each stratum are aligned and connected vertically. Therefore, the pre-fracture positions of each high-level key stratum in the horizontal direction should be consistent and the same as the pre-fracture position of the first high-level key stratum above the water-conducting fracture zone, i.e.:

[0121] (9)

[0122] In equation (9), x gsi For high-level key layer s i The horizontal distance from the pre-fracture location to the working face boundary is in meters (m); c is the total number of low-level key layers within the water-conducting fracture zone; θ z(c+1) For rock group z c+1 The overlying rock breaking angle; H sc The key layer s within the rock strata c The height of the floor; x dsc It refers to the horizontal distance between the pre-splitting location of the uppermost low-level key layer and the coal wall at the boundary of the working face.

[0123] (3) Based on the determined horizontal pre-fracture location, directional long borehole hydraulic fracturing technology is used to carry out full-section pre-fracture cutting of key layers in high and low strata within the pre-fracture range.

[0124] There are many existing rock strata pre-fracking technologies, including dense drilling, shaped charge blasting, and hydraulic fracturing. Taking directional long-hole segmented hydraulic fracturing, the most widely used in engineering, as an example, this technology guides fracture propagation directionally and works in conjunction with the mining stress field to effectively cut away thick, hard rock strata. The specific process is as follows: After directional drilling is completed, the perforation tool string is pushed to the designed position, and a high-pressure water jet (generally 15~40MPa) is used to impact the borehole wall at a set angle, creating initial cavities in the rock strata; subsequently, the tool string is dragged to achieve a dense, directional arrangement of the cavities, thereby controlling the propagation direction and fracturing effect of subsequent fracturing fractures. The perforation density in key strata should be comprehensively determined based on factors such as the rock strata fracture block size, mining fracture density, and the degree of natural fracture development, and should gradually increase from bottom to top. The spacing between perforations is typically in the range of 8~25m. Figure 4 As shown. For critical layers that are particularly thick and difficult to penetrate with a single borehole, the density of boreholes or the number of directional long boreholes can be appropriately increased to ensure overall fracturing effect across the entire cross-section.

[0125] Furthermore, critical layer pre-fracture technology is not limited to hydraulic fracturing in practical engineering. Depending on cost, construction conditions, and pre-fracture objectives, single or combined processes such as intensive drilling, shaped charge blasting, or hydraulic fracturing can be selected. For example, for low-lying thick and hard rock formations, effective top cutting can be achieved downhole using shaped charge blasting, intensive drilling, or hydraulic fracturing; while for high-lying thick and hard critical layers, due to the limitations of downhole construction depth, surface drilling is generally used for hydraulic fracturing or blasting pre-fracture.

[0126] Step 5: Based on the stress distribution characteristics and damage deformation transmission mechanism of the thick and hard rock strata, calculate and determine the moving boundary of the working face and the reduction distance of the protective coal pillar of the building (structure) after the pre-splitting of the key layer.

[0127] The stress state of the overburden caused by mining leads to stress concentration near the coal pillar boundary due to the huge load generated by the overburden and thick, hard rock layers, which induces damage to the coal and rock mass and is transmitted upward to the surface layer by layer with the overburden.

[0128] Starting from the peak stress concentration point within the coal pillar, the stress concentration is transmitted through the soft rock cushion layer to the thick, hard rock layer. Influenced by the inherent stiffness of the thick, hard rock layer and the additional load from the cantilever beam, the damage and deformation shift deeper into the original rock, causing the stress concentration location to migrate deeper, expanding the stress influence range, and continuing to be transmitted through the overlying cushion layer to the loose layer, ultimately causing the loose layer to slide at a certain angle and affect the surface. Therefore, based on the stress distribution characteristics and damage and deformation transmission mechanism of the thick, hard rock layer, the horizontal distance S between the surface subsidence boundary and the working face boundary is determined as:

[0129] (10)

[0130] Where: μ is the interlayer weak friction coefficient; φ is the comprehensive internal friction angle of the rock mass; γ is the average unit weight of the overlying strata, kg / m³ 3 H0 represents the mining depth, in meters; H si For key layer s i Stratum height, m; θ is the bedrock fracture angle, °; δ is the loose layer movement angle, °; β is the peak stress propagation angle, °; q di For high-level key layer s i Self-weight and superstructure load, MPa; h si For key layer s i Layer thickness, m; h n M is the thickness of the loose layer, in meters; L is the thickness of the mining layer, in meters; si For key layer s i The layer's own fracture distance; R Ki For key layer s i The overall shear strength of the layer.

[0131] Depend on Figure 5It is known that after the low-lying key strata are pre-splitting by cutting the top, the original hinge forces F1 and F2 (two forces perpendicular to each other) acting on the boundary overlapping beam area disappear. If the high-lying key strata within the bending subsidence zone are further pre-splitting, the magnitude of the vertical additional stress F3 can be reduced. Through the coordinated pre-splitting of the high and low-lying key strata, the overall overburden mining stress is weakened, thereby reducing the stress peak and influence range of the boundary misaligned overlapping beam area, shrinking the range of movement deformation caused by mining, and correspondingly reducing the width of the protective coal pillar, ultimately improving the coal resource recovery rate.

[0132] To facilitate the analysis of the combined effect of pre-splitting of the low-level critical layer on the misaligned beam zone, its influence can be regarded as the vertical component F of the resultant force of the hinged forces F1 and F2. hin According to the critical layer theory:

[0133] (11)

[0134] In equation (11), q ki For the key block k of the i-th low-level key layer i Total vertical load; l ki For key block k i Length; a ki For key block k i The contact length at both ends; ϕ' ki For key block k i The angle of rotation; T ki and w ki Key block k i The horizontal thrust and subsidence.

[0135] Assuming the highest critical layer at the pre-splitting point is the t1th critical layer, and there are c low-level critical layers, then the number of high-level critical layers requiring pre-splitting is (t1-c). After the high-level critical layers have completed pre-splitting, the critical layer load q in equation (10) is... di It will decrease, and the decrease (F'3-F3) is the weight of the overburden directly above the working face, which can be expressed as:

[0136] (12)

[0137] In formula (12), L is the working face width, in meters; H s(t1+1) For high-level key layer s t1+1 The height of the layer; H sc For high-level key layer s c The height of the floor level;

[0138] After the pre-splitting and cutting of the low-level key layer c and the high-level key layer (t1-c) are completed, the working face moving boundary will shrink accordingly, and the shrinkage distance ΔS=S 前 -S后 ,for:

[0139] (13)

[0140] In the formula, S 前 and S 后 These are the horizontal distances, in meters, from the coal face to the surface movement boundary before and after pre-splitting of the key layer.

[0141] Therefore, the reduction in the width of the protective coal pillar after pre-fracture of the key layer is ΔD. m =ΔS. For example... Figure 6 As shown.

[0142] Obviously, the above embodiments are merely illustrative examples for clear explanation and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations here. However, obvious variations or modifications derived therefrom are still within the scope of protection of the claims of this patent application.

Claims

1. A method for controlling settlement by pre-fracture of key layers based on the arrangement of overburden fracture angles, characterized in that, Includes the following steps: Step 1: Collect and determine the geological and mining parameters of the target mine; Step 2: Based on the physical and mechanical parameters of each rock layer and combined with the criteria for identifying thick and hard rock layers, determine the stratigraphic height of the key layer; Step 3: Based on the coal seam mining parameters and overlying lithology, determine the height of the water-conducting fracture zone, and classify it into high and low strata based on the key strata positions; Step 4: Based on the evolution law of overburden damage from bottom to top along the fracture angle, calculate and determine the horizontal pre-fracture position of each key layer within the pre-fracture range, and implement full-section directional pre-fracture cutting. Step 5: Based on the stress distribution characteristics and damage deformation transmission mechanism of the thick and hard rock strata, calculate and determine the moving boundary of the working face and the reduction distance of the protective coal pillar of the building / structure after the pre-splitting of the key layer.

2. The method for pre-fracture and settlement control of key layers based on the arrangement of overburden fracture angles according to claim 1, characterized in that, In step 1, the geological and mining parameters include: Coal seam mining parameters: working face width L, mining thickness M, mining depth H0; Overall lithology of overlying strata: Protodyakonov hardness coefficient f; Rock strata physical and mechanical parameters: the thickness h of the i-th rock stratum i γ-weight i Elastic modulus E i internal friction angle φ i Tensile strength R Ti Shear strength R Ki .

3. The method for pre-fracture and settlement control of key layers based on the arrangement of overburden fracture angles according to claim 2, characterized in that, In step 2, the number of key layers in the overburden and their stratigraphic position are determined, based on the following criteria: Assume there are n rock layers above the coal seam, and the thickness of the i-th rock layer is h. i The bulk density is γ i The elastic modulus is E i The tensile strength is R Ti The judgment is carried out layer by layer from bottom to top, and the two discrimination conditions of the key layer are satisfied respectively: stiffness condition and strength condition; The stiffness condition is determined as follows: （1）； In equation (1), q i / m and q i / m+1 These represent the loads acting on the i-th rock layer, considering the m-th and (m+1)-th rock layers, respectively. , j is the j-th rock layer above the i-th rock layer; The strength condition is determined as follows: （2）； In equation (2), l i Let l be the fracture distance of the i-th rock layer. m+1 The fault distance of the (m+1)th rock layer is... ; If the i-th rock layer is a key layer, when both equation (1) and equation (2) are satisfied, the target rock layer m+1 is determined to be a key layer; Based on the combined results of stiffness and strength assessments of various rock strata, the key strata in the overburden of the target mining area are identified as stratum t, denoted from bottom to top as s1, s2, ..., s. t Their layer heights are H s1 H s2 H st .

4. The method for pre-fracture and settlement control of key layers based on the arrangement of overburden fracture angles according to claim 3, characterized in that, Step 3 includes the following steps: Step 2-1: Use the Protodyakonov hardness coefficient f to determine the overall lithology of the overlying strata: when f > 8, it is a hard overlying strata; when 3 < f ≤ 8, it is a medium-hard overlying strata; when f ≤ 3, it is a weak overlying strata. Step 2-2: Calculate the height of the water-conducting fracture zone in the overlying rock under different lithological conditions. The calculations are categorized by lithology as follows: When the overlying strata are hard: (3); When the overburden is of medium-hardness: (4); When the overlying strata are weak: (5); In equations (3)-(5), H f M represents the calculated height of the water-conducting fracture zone, in meters; M represents the coal seam thickness, in meters. Steps 2-3: Classify the key layers according to their spatial relationship with the water-conducting fracture zone: With the key layer height H si Calculated height H of the water-conducting fracture zone f Using the relative size as a criterion, key layers are divided into the following two categories: Low-level critical layer: When H si <H f At that time, it was indicated that the water-conducting fracture zone had completely penetrated the key stratum s. i These critical layers are damaged, fractured, or cracked due to mining activities; High-level critical layer: When H si ≥H f This indicates that the water-conducting fracture zone has not developed to the critical layer s. i The rock strata are in a bent and subsided state, and these key strata are less affected by mining.

5. The method for pre-fracture and settlement control of key layers based on the arrangement of overburden fracture angles according to claim 4, characterized in that, In step 4, based on the evolution of overlying rock damage along the fracture angle from bottom to top, the fracture angle is calculated; the key layer and the soft rock above it are divided into one group, resulting in a total of t+1 groups of rock strata, namely z1, z2, ..., z t+1 Then any rock stratum group z i The expression for calculating the breaking angle is: （6）； Combined with the overburden load q on the key layer s(i-1) The calculation formula, equation (6), is further obtained as follows: （7）； In equations (6) and (7), θ zi rock strata group z i Breaking angle, °; R Kzi rock strata group z i The comprehensive shear strength, MPa; E s(i-1) For key layer s i-1 elastic modulus; φ zi The comprehensive internal friction angle of the rock strata is given in °; h s(i-1) For key layer s i-1 Thickness, m; E j h is the elastic modulus of the j-th rock layer above the key layer; j γ represents the thickness of the j-th rock layer above the key layer, in meters; j The unit weight of the j-th rock layer above the key layer is MN / m³. 3 λ is the critical layer fault distance standard, with a value of 1.5~2.

5.

6. The method for pre-fracture and settlement control of key layers based on the arrangement of overburden fracture angles according to claim 5, characterized in that, Based on the evolution of overburden damage from bottom to top along the fracture angle, and combined with the stratigraphic height of each stratum group, the horizontal pre-fracture locations of the key high and low strata are determined. The horizontal pre-splitting location of the low-level key stratum: that is, the horizontal distance between the pre-splitting location of the key stratum and the coal wall at the boundary of the working face, is calculated using the following formula: （8）； In equation (8), x dsi For the low-level key layer s i The horizontal distance from the pre-crack location to the working face boundary, in meters (m); H si For the low-level key layer s i The height of the layer, m, where H s0 =0; H s(i-1) For the low-level key layer s i-1 The height of the stratum, m; θ z(i+1) rock strata group z i+1 breaking angle; R Kz(i+1) rock strata group z i+1 The comprehensive shear strength, MPa; E si For key layer s i The elastic modulus; h si For key layer s i Thickness, m; Horizontal pre-fracture location of high-level key strata: To effectively weaken the spatial transmission of overburden subsidence and optimize the control of surface subsidence range, a vertically upward pre-fracture path is adopted for high-level key strata to ensure the overall alignment and continuity of pre-fracture zones in the vertical direction. The horizontal pre-fracture locations of each high-level key stratum should be consistent and the same as the pre-fracture location of the first high-level key stratum above the water-conducting fracture zone. Assuming the total number of low-level key strata is c, the formula for calculating the horizontal pre-fracture location of any high-level key stratum is: （9）； In equation (9), x gsi For high-level key layer s i The horizontal distance from the pre-fracture location to the working face boundary is in meters (m); c is the total number of low-level key layers within the water-conducting fracture zone; θ z(c+1) For rock group z c+1 The overlying rock breaking angle; H sc The uppermost low-lying key layer s within the rock strata group c The height of the floor; x dsc It refers to the horizontal distance between the pre-splitting location of the uppermost low-level key layer and the coal wall at the boundary of the working face.

7. The method for pre-fracture and settlement control of key layers based on the arrangement of overburden fracture angles according to claim 6, characterized in that, Based on the determined horizontal pre-fracture location, directional long borehole hydraulic fracturing technology is used to carry out full-section pre-fracture cutting of key layers at high and low levels within the pre-fracture range; After the directional drilling is completed, the perforation tool string is pushed to the designed position, and high-pressure water jets are used to impact the hole wall at a set angle to form the initial hole in the rock strata; Subsequently, by dragging the tool string, a dense directional arrangement of holes is achieved, thereby controlling the propagation direction and fracturing effect of subsequent hydraulic fracturing fractures.

8. The method for pre-fracture and settlement control of key layers based on the arrangement of overburden fracture angles according to claim 7, characterized in that, In step 5, based on the stress distribution characteristics and damage deformation transmission mechanism of the thick and hard rock strata, the horizontal distance S between the surface subsidence boundary and the working face boundary is determined as follows: （10） In equation (10): μ is the interlayer weak friction coefficient; φ is the comprehensive internal friction angle of the rock mass; γ is the average unit weight of the overlying strata, kg / m³ 3 H0 represents the mining depth, in meters; H si For key layer s i Stratum height, m; θ is the bedrock fracture angle, °; δ is the loose layer movement angle, °; β is the peak stress propagation angle, °; q di For high-level key layer s i Self-weight and superstructure load, MPa; h si For key layer s i Layer thickness, m; h n The thickness of the loose layer is in meters (m). M represents the thickness, in meters; l si For key layer s i The layer's own fracture distance; R Ki For key layer s i The overall shear strength of the layer.

9. A method for controlling settlement by pre-fracture of key layers based on the arrangement of overburden fracture angles, as described in claim 8, is characterized in that... The calculation method for the reduction distance ΔS of the working face moving boundary after pre-splitting is as follows: After the top cutting and pre-splitting of the low-level key layer is carried out, the original hinge forces F1 and F2 acting on the boundary beam area disappear immediately. Continue to pre-splitting the high-level key layer in the bending and sinking zone to reduce the magnitude of the vertical additional stress F3. The combined effect of pre-splitting of the low-level key layer on the misaligned beam zone is analyzed, and its influence is regarded as the vertical component F of the resultant force of the hinge forces F1 and F2. hin According to the critical layer theory: （11） In equation (11), q ki For the key block k of the i-th low-level key layer i Total vertical load; l ki The length of the key block; a ki ϕ' is the contact length between the two ends of the critical block. ki T is the rotation angle of the key block; ki and w ki These are the horizontal thrust and subsidence of the key block, respectively. Assuming the highest critical layer at the pre-splitting point is the t1th critical layer, and there are c low-level critical layers, then the number of high-level critical layers requiring pre-splitting is (t1-c). After the high-level critical layers have completed pre-splitting, the critical layer load q in equation (10) is... di It will decrease, and the decrease (F'3-F3) is the weight of the overburden directly above the working face, expressed as: （12） In formula (12), L is the working face width, in meters; H s(t1+1) For high-level key layer s t1+1 The height of the layer; H sc The topmost low-level key layer s c The height of the floor level; After the pre-splitting and cutting of the low-level key layer c and the high-level key layer (t1-c) are completed, the working face moving boundary will shrink accordingly, and the shrinkage distance ΔS=S 前 -S 后 ,for: （13） In equation (13), S 前 and S 后 These are the horizontal distances, in meters, from the coal face to the surface movement boundary before and after pre-splitting of the key layer; After pre-fracture of the key stratum, the width of the protective coal pillar is reduced by ΔD. m =ΔS.

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