Out-fashioned goaf is the protective structure for mining the upper residual coal, and its stability is the core problem in mining the upper residual coal. According to the upward mining demand for No. 5 coal seam above the out-fashioned goaf in Baizi Coal Mine, a new method is proposed to determine the upward mining safety. According to the analysis of the actual situation of the mine, the coal pillar and suspended roof in the out-fashioned goaf are taken as the objects. Furthermore, a “coal pillar-suspended roof” system model based on the variable load induced by abutment pressure of upper coal seam mining is established. After the mechanical model was solved, the parameter acquisition method of the model was established. The basic parameters of Baizi Coal Mine were considered to determine the feasibility of mining residual coal above out-fashioned goaf. And the effects of variable load on the coal pillar and suspended roof stability were analyzed. The results show that the upper No. 5 coal seam in Baizi Coal Mine can be mined safely. Compared to the traditional method, which simplifies all the upper loads to uniform loads, the new method is safer. The system stability of the suspended roof and coal pillar is influenced by “
The amount of residual coal exceeds 1.2 Gt [
Many researchers analyzed the integrity of the upper coal seam and its surrounding rock stability, proposing corresponding methods for determining the feasibility of upward mining. These methods mainly include the rational analytical method, the “three-zone” method, mathematical statistics method, and the method of equilibrium surrounding rock. Their determination formulas and application conditions are shown in Table
Stability determination methods of the upward mining of longwall goaf.
Methods | Calculation formula and application condition |
---|---|
Rational analytical method [ | |
Mathematical statistics method [ | |
The “three-zone” method [ | Caving zone 1: |
Caving zone 2: | |
Fracture zone 1: | |
(here (a) strong and hard: | |
Fracture zone 2: | |
(here | |
The method of equilibrium surrounding rock [ | |
(here |
These methods are proposed after statistically analyzing the field measured data, which are derived from longwall mining conditions. But, the out-fashioned mining differentiates from the longwall mining. In the out-fashioned goaf, compared with the longwall goaf, coal pillars with irregular size and shape were left to support the roof, which results in a limited roof caving or even no caving. The upper coal seam and its surrounding rock are integral and stable. So, if these methods are adopted to determine the feasibility of the mining upper residual coal seam above the out-fashioned goaf, plentiful coal resources will be wasted due to limitations on the applicability of these methods.
Analyzing the stability of the system composed of coal pillars and suspended roofs in the out-fashioned goaf is preferred to determine the feasibility of mining the upper residual coal seam above the out-fashioned goaf. A large number of scholars have been studying these relative issues. The residual coal pillar and its roof in the lower out-fashioned goaf were simplified to the mechanical model of beam and plate by using elastic theory. In the reference, the suspended roof was simplified as a simply supported beam and clamped beam [
Furthermore, the influence of abutment pressure on the upper coal seam has been considered in upward mining. Li et al. [
In the above studies, beam or slab models were built to study the stability of the system consisting of suspended roofs and coal pillars. And abutment pressure induced by upper coal seam mining was simplified to static additional load. The system stability of the “suspended roof and coal pillar” is a criterion determining whether the upper coal seam could be mined.
These research studies solve the problem of upward mining to some extent and promote the recovery and utilization of upper coal resources above the out-fashioned goaf [
In order to ensure the safety of upper residual coal above the out-fashioned goaf, a method for determining the feasibility of upward mining considering upward mining effects was proposed first. The method is used to judge the feasibility of upward mining by judging the stability of the out-fashioned goaf. Then, taking the coal pillar and suspended roof in out-fashioned goaf as an integral object, a stability mechanical model focusing on the variable load induced by abutment pressure of upper coal seam mining was established. And the parameter acquisition method of the model was established. Thirdly, the basic parameters of Baizi Coal Mine were considered to determine the feasibility of upward mining residual coal above the out-fashioned goaf. And the effects of variable load on the coal pillar and suspended roof stability were analyzed.
As shown in Figure
Location plan of Baizi Coal Mine.
As shown in Figure
Bar chart of coal seam geology of the coal seam.
The physical and mechanical parameters of the rock strata of the roof and floor are essential for determining the feasibility of ascending mining. The elasticity, volume weight, cohesion, and tensile strength of lithology are acquired in Table
Physical and mechanical parameters of lithology.
Lithology | Elasticity modulus, | Volume weight, | Cohesion, | Tensile strength, | Friction angle, |
---|---|---|---|---|---|
No. 5 coal seam | 5.1 | 14.00 | 3.0 | 1.2 | 30 |
Charcoal mudstone | 10.3 | 24.83 | 5.5 | 4.1 | 32 |
Mudstone | 12.4 | 23.69 | 3.5 | 3.5 | 35 |
Siltite | 14.5 | 25.34 | 12.2 | 5.1 | 38 |
Charcoal mudstone | 17.1 | 24.63 | 3.6 | 3.2 | 32 |
No. 8 coal seam | 5.5 | 14.30 | 2.8 | 1.3 | 29 |
For historical reasons, No. 8 coal seam has been mined using the reamer-pillar coal mining method, which is one of the out-fashioned mining methods. The 4 m wide coal was mined, and 4 m wide coal pillars were left in No. 8 coal seam with this method. But the coal pillar is irregular in size and shape (Figure
Mining engineering plan schematic map.
Briefly, the method is to judge the feasibility of upward mining by judging the stability of lower out-fashioned goafs consisting of the coal pillar and their suspended roof. The stability of the coal pillar and suspended roof is determined by using the theoretical analysis method, based on structural mechanics and elastic theory [
Figure
Diagrammatic figure and simplified mechanical model.
The model was solved by using mechanics of materials and structural mechanics [
The structure and load of the model are symmetrical when only overburden load acts. Half of the model was used to simplify the calculation and solve the mechanical model. The model before and after simplification is shown in Figure
Model simplification bearing only overburden load.
The simultaneous equation (
According to the force balance in the vertical direction, utilizing the above results, the axial force of the coal pillar
As shown in Figure
Model simplification bearing only varied triangular load.
The simultaneous equations (
Then, the methods of moment distribution are utilized to obtain the torque equilibrium equation to solve the moments of coal and the suspended roof:
The simultaneous equations (
As shown in Figure
So far, the mechanical equation of the coal pillar and the suspended roof in the situation of bearing variable triangular load is obtained. According to equations (
In order to determine the stability of the model, the maximal axial stress and shearing stress should be calculated [
The stress of axial and shearing is correlated with the moment of the beam, and the detailed relationship is shown in formula (
The shearing stress of the coal pillar has little influence on the strength stability of the out-fashioned goaf. So when we utilized the abovementioned results of the system stability of the coal pillar and the suspended roof, the effect of shearing stress on the coal pillar can be ignored.
The basic parameters of the determination model are acquired by engineering geologic reports, operational rules, and rock mechanical tests. In addition, the suspended roof, load, and compression strength of the coal pillar are determined by the following methods [
In the system model, the suspended roof is a key rock stratum. The suspended roof is the main structure that bears overburden load and abutment load. According to the theory of the key rock stratum [
The loads of overburden and abutment pressure are the key parameters [
The compression strength of the coal pillar correlates with uniaxial compressive strength and coal pillar size [
Furthermore, the strength of the coal pillar in the field is calculated using the relationship of uniaxial strength of the critical cube and coal pillar [
The study data of Baizi Coal Mine [
Detailed values of parameters in Baizi Coal Mine.
3.72 | 10.68 | 2.4 | 4 | 4 | 4 | 6.7 | 4 | 133 | 5.3 | 14 | 5.5 |
The key rock stratum of No. 8 coal seam, the load of overburden and abutment pressure, and uniaxial compression strength of the coal pillar are vital for the feasibility determination of mining upper coal seam. So, the vital parameters of the suspended roof, load, and coal pillar are solved in the following.
Firstly, combining the geological data in Table
Furthermore, the vertical pressure data of No. 5 coal seam floor, measured by stress sensor, are collected and analyzed. We find that the pressure of No. 5 coal seam floor is equal when the first pressure is approaching; the vertical pressure
Finally, the compression strength of the coal pillar correlates with uniaxial compressive strength and coal pillar size. The test specimens are round; specimen diameter is
The maximal shearing stress of the suspended roof is
The varying values of “
The relationship between stresses and “
As shown in Figure
As shown in Figure
In general, the compressive strength of coal and rock mass is vital in all strength [
From practical ascending mining engineering project cases [
According to formulas (
Shearing stress of the suspended roof with “
Axial stress of the suspended roof with “
Axial stress of the residual coal pillar with “
As shown in Figure
In practical mining engineering projects, when the length of the suspended roof gets wider, the system strength stability will be destroyed more easily. And the shearing stress of the suspended roof increases with the increase of the varied load range on the suspended roof. Based on the abovementioned analysis results, engineers in coal mine can calculate the maximal length and the varied load range to determine whether the suspended roof can be fractured.
The relationship of axial stress of the suspended roof with “
The relationship curves of axial stress of the coal pillar with “
Furthermore, when all values of “
With all other conditions being equal, we changed the value of variable load
The relationship of the stress ratio
Figure
For cases of varying ratio of
A method for determining the feasibility of upward mining residual coal above the out-fashioned goaf was studied, and the following conclusions were obtained: Variable additional load is mainly considered to simulate the influence of abutment pressure caused by upper coal seam mining. On this basis, the suspended roof and coal pillar in the lower out-fashioned goaf are taken as objects to establish the mechanical model. The mechanical model solution and parameter acquisition method were alsoestablished. The practical engineering data of Baizi Coal Mine were utilized to analyze the relationship of stress with “ Compared with the system without variable load, the ultimate stresses obtained by the system model considering the variable load are much higher. Therefore, compared with this method, the traditional method, which simplifies upper loads to uniform loads, leaves a greater safety factor to ensure the safe mining of the upper coal seam above the out-fashioned goaf. Through the analyses of determining formula of key parameters, the system stability of the suspended roof and coal pillar is influenced by “ Under the action of variable load caused by No. 5 coal seam mining in Baizi Coal Mine, the suspended roof and coal pillar of the lower out-fashioned goaf remain stable, which is consistent with the results of engineering practice. This study provides a reference for the mining of coal resources under similar conditions.
The method was validated by the data provided in Ref. [
The authors declare that they have no conflicts of interest.
This work was funded by the Joint Research Fund under cooperative agreement between the National Natural Science Foundation of China (NSFC) and Funds for Coal-Based Low-Carbon Technology of Shanxi (No. U1710258), Shanxi Province Applied Basic Research Program (No. 201801D221342), the National Natural Science Foundation of China (Nos. 51422404 and 51574172), Science and Technology Innovation Project of Shanxi Colleges and Universities (2019L0181), Program for Key Research Project of Shanxi Province in the Field of Social Development (No. 201803D31044), Yan’an Industry Key Core Technology Innovation Project (2019ZCGY-025), Program for “1331” Key Team of Scientific and Technology Innovation in Shanxi Province, Program for the Excellent Innovation Team of Higher Learning Institutions of Shanxi Province, Program for the Leading Talents of Emerging Industry of Shanxi Province, and Shanxi Province Technology Innovation Program for the Key Team.