Short-wall block filling mining (SBBM) technology has become an effective way to recover coal resources beneath the aquifer, which are unsuitable, or cannot be used by long-wall mining, such as corner coal pillars, industrial square pillars, and irregular coal blocks as well as the coal beneath buildings, railways, and water bodies. The SBBM method can not only enhance the recovery ratio but also provide a solution for the environment problems associated with gangues on the surface. However, whether the height of water flowing fractures will reach to the aquifer to cause water loss during SBBM has always been a key problem. Therefore, based on the theory of elastic foundation beam and SBBM characteristics, a mechanical model for calculating the height of a water flowing fracture zone in the overlying strata of SBBM was established, and this model calculated that the height of the water flowing fracture zone was 27.0 m in the experimental working face, and the height of the water flowing fracture zone was measured as 26.8 m according to washing fluid loss in the hole, core damage analysis, and drilling TV imaging detection. The comparison results demonstrated that the calculated value almost fit well with the field-measured data, validating the accuracy of the proposed mechanical model, while the predicted value (48.7 m) in the
During the coal mining process, overlying strata experience significant mining-induced movements and deformation, inevitably causing the collapse and fracturing of strata and forming a lot of fractures [
Coal-mining-induced severe environmental problems. (a) Land desertification; (b) dry-up of streams; (c) sparse ground vegetation; (d) development of fractures towards the ground; (e) stacking of gangues.
Currently, a great deal of research has been carried out on the protection of underground and ground water resource. Sun and Miao [
Based on the SBBM technological characteristics, a new mechanical analysis method was established for calculating the height of the water flowing fracture zone in the overlying strata during the SBBM process, and the related mechanical properties of the development rules of the water flowing fracture zone were analyzed. Furthermore, the applicability of the
The Shaanbei coal mine is located in Yulin City, Shaanxi Province, China. The area of the mine field is 1.52 km2, and the recoverable coal reserve is 4.98 million tons. The surface of the mine field is covered by driftsands. The region of the mine belongs to arid and semi-arid areas with deficient water resource, sparse vegetation, and serious water loss, featured by typical semi-acrid and semi-desert continental climate. Moreover, the burial depth of coal seams is generally within 112.8–184.5 m, which belongs to the shallow buried coal seam. The geological structure is simple and the coal quality is good, and long-wall mining is the main practiced method in the mine. The SBBM experimental region was to the north of the no. 10304 long-wall working face, with a mining length of about 150 m and a total experimental area of 19,500 m2. It is estimated that about 13,000 tons of coal in total can be recovered. The coal seam exhibits simple storage structures with slightly varying thicknesses and can be regarded as a nearly flat coal seam. The average thickness and burial depth of the coal seam are 5 m and 145 m, respectively. A layer of the aquifer can be observed on the mining coal seam, with an average distance of 105 m away from the coal seam. This aquifer serves as the main potentially protected water resource for the mine. The top of the coal seam mainly consists of mudstone, fine sandstone, siltstone, sandy mudstone, fine sandstone, medium sandstone, siltstone, red clay, and driftsand from the bottom up. The aquifer is above the red clay layer. Figure
Columnar illustration and physical/mechanical properties of the coal seam.
In combination with the occurrence of corner coal pillars, the experimented region can be divided into two blocks for mining. The advancing length of each block was 72.5 m, with a mining height of 5 m; in addition, the protective coal pillar between the blocks was about 5 m in length, and the backfilling ratio of gangues in the gob was about 80%. Figure
Geographic position of the mine and the detailed arrangement in the SBMM working face. (a) Geographical position of the mine; (b) detailed arrangement in the SBMM working face.
During the field of the SBBM process, the height of the water flowing fracture zone was comprehensively determined by observation of washing fluid loss in the hole, core damage degree analysis, and drilling TV imaging detection. After mining and backfilling in the working face, an observation hole, denoted as hole F, was arranged on top of the working face to measure the height of the water flowing fracture zone. The detailed arrangement of the hole is shown in Figures
Illustration of the positions of the aquifer and the observation hole.
Figure
Relationship between the loss of washing fluid in the hole and the drilling depth.
Drilling TV imaging detection results of the distribution of fractures in the strata. (a) No fracture; (b) first vertical fracture; (c) fractured zone.
The SBBM technique is mainly used for recycling corner coal pillars, industrial square pillars, and irregular coal blocks as well as the coal beneath buildings, railways, and water bodies, all of which are unsuitable or cannot be exploited by long-wall mining. The SBBM technique also exhibits a lot of advantages such as less investment, short operating period, and quick returns. Particularly, a set of short-wall mining equipment is only 20% of a set of long-wall mining fully mechanized mining equipment in terms of cost. To be specific, based on the block caving mining technology, the gangues are filled in the gob as the backfill body after the mining of a block, and the next block is simultaneously mined. Accordingly, both the temporal continuity and spatial independence of mining and backfilling during the whole operating process can be ensured; i.e., SBBM can be regarded as a high-efficiency backfilling mining method characterized by mining/backfilling concurrence. The SBBM technique can greatly enhance the recovery ratio of the mine and prolong the mine’s service life. Meanwhile, the filling of gangues that are piled up on the surface in the gob can reduce the damages to the environment and the enterprise’s investment in environmental protection.
Figure
Illustration of the short-wall block backfill mining (SBBM) system.
In the SBBM working face, four branch roadways were arranged for constituting a mining block, and the protective pillar was set between the two blocks. In each block, the pillars were extracted in retreat from top to bottom. During the recovery of coal pillars, the temporal coal pillar, with a width in the range 0.5–1.5 m, was arranged between the wings. The length of the wing was no greater than 11 m, and the width was about 3 m. The intersection angle between the wing and the branch roadway was about 45°. After the recovery of a block, the gangues were filled into the wing, the branch roadway, and the joint roadway from top to bottom. The temporal coal pillars and the fire dams can act as a barrier to ensure that the filling ratio satisfies the design requirements.
SBBM technology mainly includes mining and backfilling. With regard to the mining technology, four track walking hydraulic supports were used and cooperated with the coal cutter for ensuring the safety in the coal cutting and loading. In more detail, four track walking hydraulic supports were divided into two sets: one set (the first two supports) was arranged in the branch roadway, and the other set (the latter two supports) was arranged in the joint roadway between the two adjacent branch roadways. Both the branch roadway and the joint roadway were about 5 m in width. With regard to the backfilling technology, two track walking hydraulic supports were arranged in the branch roadway; in cooperation with the waste throwing machine and the pushdozer, the gangues were filled in the gob and were in full contact with the roof. Accordingly, the gangues can serve as the permanent carrier and cooperated with the protective coal pillars between the blocks for supporting the overlying strata. Figure
Arrangement in the SBBM working face and key equipment.
According to SBBM technological characteristics, this study calculated the damage height of the overlying strata in the working face after mining and backfilling to determine the height of the water flowing fracture zone in the final overlying strata formed in the SBBM. As shown in Figure
Basic environment state for mechanical analysis of SBBM stope.
The elastic foundation coefficients of the backfill body, the coal seam, and
As shown in Figure
When
Established mechanical model of the
According to Winkle’s assumption for elastic foundations [
Based on the elastic foundation beam theory, the deflection of the rock beam, denoted as
Therefore, by assuming the prearranged protective coal pillars in the SBBM working face, the backfill body and (
By setting the characteristic coefficients
The rotation angle, bending moment, and shearing force of any cross section in the
The elastic foundation coefficient of the backfill body and the backfill ratio satisfy the following expression [
Because of the symmetrical model structure and load, the deflection curve and the bending-moment curve of the rock beam are symmetrical, and the rotation-angle curve and the shearing-force curve are anti-symmetrical; accordingly, the rotation angle and shearing force at the symmetrical point equaled to 0. Furthermore, according to the qualitative analysis results of the semi-infinite long beam as shown in Figure
The continuity condition can then be described as follows. The deflection, bending moment, rotation angle, and shearing force at the connection point among the protective coal pillar, the gob, and the entity coal equaled to each other.
By substituting the boundary conditions and continuity condition into (
Figure
Established mechanical model of the
According to the elastic foundation beam theory, the general formula of the deflection curve of the
The boundary conditions of the model can be described as
Similarly, the continuity condition can be described as follows. The deflection, bending moment, rotation angle, and shearing force at the connection point among the protective coal pillar, the gob, and the entity coal equaled to each other.
Similarly, by substituting the boundary conditions and continuity condition into (
According to mechanics of materials, the maximum tensile stress of the rock beam can be calculated as
According to the first strength theory, the fracturing of the rock beam should satisfy the following expression:
If (
The detailed engineering parameters and the mechanical parameters of various strata, as displayed in Figures
Currently, the height of the water flowing fracture zone was mainly predicted according to the related formula in the
According to the mechanical parameters of various strata in Figure
As listed in Table
Contrastive analysis of the calculated and measured height of water flowing fracture zone.
Height of the water flowing fracture zone | Measured results of hole F | Prediction results according to the Regulations | Calculation results based on the mechanical model |
26.8 m | 48.7 m | 27.0 m |
For preventing and controlling the loss of water resources after the mining in the working face, the
Relationship between the thickness of the protective layer and the height of the water flowing fracture zone.
According to the provisions in the
By substituting the related data into (
This study proposed the SBBM method for the recovery of corner coal pillars, industrial square pillars, and irregular blocks beneath the aquifer and simultaneously addressed the problem of massive stacking of gangues on the earth’s surface. In addition, by measuring the loss of washing fluid, core damage analysis, and drilling TV imaging detection, the height of the water flowing fracture zone in the SBBM working face was determined as 26.8 m. Based on the technological characteristics of SBBM and the theory of elastic foundation beam, a mechanical model for the calculation of the height of the water flowing fracture zone in the overlying strata using SBBM was established. The calculated height of the water flowing fracture zone according to the mechanical mode (27.0 m) almost equals to the measured value (26.8 m), thus verifying the accuracy of the proposed mechanical model. The distance between the coal seam and the aquifer was 95 m, which far exceeds the allowable minimum thickness of the protective coal pillar (42 m). Therefore, it can be concluded that the aquifer was not affected by mining activities, and simultaneously the mining upper limit of coal seam as well as the recovery ratio of coal can be enhanced in the SBBM. The calculated height of the water flowing fracture zone according to the prediction formula in the
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare have no conflict of interest.
This study was supported jointly by the National Key Basic Research Program of China (2015CB251600), the National Natural Science Foundation of China (51874284), and Fundamental Research Funds for the Central Universities (2018ZDPY04).