Study on Overburden Movement Deformation and Roof Breakage Law of Under-Protective Steeply Inclined Coal Seam Mining

Round 1

Reviewer 1 Report

In this paper, based on the elastic thin plate theory, the basic roof deflection was calculated in the mining of deeply inclined protection layer, and the similar simulation test was carried out. The results are useful and interesting. However, there are also some problems. It could be accepted after modifications. The specific comments are as follows.

 

1. In the abstract, the scientific question of this work was absent. Authors are advised to highlight the innovative nature of this work.

 

2. In the introduction part, the overburden movement deformation and roof breakage law of steeply inclined coal seam mining are summarized in detail, but the research status of protection layer mining is not fully introduced.

 

3. In this manuscript, the thin plate theory is used to analyze the roof breakage law of protective layer mining under steep incline. Why do you choose this theory instead of other theories?

 

4. What is the help of the experimental results obtained by similar simulation experiment for protective steeply inclined coal seam mining?

 

5. The language of the manuscript needs to be improved.

 

6. The conclusion should be compressed to make it more concise.

 

Author Response

Thanks for the reviewer’s meritorious comments. The English language and style have been checked in the manuscript, please see the latest uploaded manuscript. And please check the bellowing for the point-by-point response to the reviewer’s comments:

Response to Reviewer 1 Comments

 

Thanks for your effort on our manuscript. We appreciated the reviewers’ effort and these comments drove us for a better understanding of our work. We address all of the reviewers’ comments point by point:

 

 

Point 1: In the abstract, the scientific question of this work was absent. Authors are advised to highlight the innovative nature of this work.

Response 1:

Thank you for your advice. It is true that the abstract of the original manuscript did not highlight the innovative nature. In view of this, we have strengthened the innovation nature and modified the abstract as follows:

 

The occurrence of the steeply inclined coal seam is extraordinary, and the coal body is seriously damaged by extrusion. The most steeply inclined coal seam is high gas or outburst coal seam, and protective layer mining is the safest and most effective measure for regional prevention of coal and gas outburst. Based on considering the coefficient of lateral pressure and vertical height of section, the deflection of the basic roof of the steeply inclined protective layer in a mine in western Henan, China, is calculated using the deflection calculation method of thin plate theory of elas-ticity. Using MATLAB to understand the deflection, the deflection curve is obtained. The law of rock movement and deformation in the mining process of the protective layer is studied by a sim-ilarity simulation experiment. The results show that, after mining, the roof mainly sinks slowly without large-scale collapse, and the largest rock strata movement is located in the upper part of the slope. Rock strata movement and fracture development can relieve the pressure of the pro-tected layer and provide a channel for gas migration and drainage. The mining conditions of the protected layer will not be destroyed, and mining this type of protected layer in this mine has bet-ter safety and feasibility. The conclusions of this study have a guiding and scientific significance for the control of surrounding rock and the layout of gas drainage boreholes of under-protective steeply inclined coal seam mining.

 

Point 2: In the introduction part, the overburden movement deformation and roof breakage law of steeply inclined coal seam mining are summarized in detail, but the research status of protection layer mining is not fully introduced.

Response 2:  

Thank you very much for this comment. We added about the research status of protection layer mining as follow:

 

In the “1. Introduction”

In 1933, France began to conduct experimental research on protective layer mining, where after Germany, Poland, and other countries also began applying this technology to prevent coal and gas outbursts[4][5]. Since the late 1950s, China has applied protective layer technology to prevent coal and gas outbursts in southwest China and achieved remarkable results[6][7]. Since 1998, Huainan Mining Group has conducted experimental research on exploiting protective layers under various geological conditions and achieved remarkable application effects. The gas control concept of "all that can be protected" has been formed in the Huainan mining area, which has been popularized and applied to high gas outburst mining areas in China[8][9].

 

References

4. Cheng, X.; Zhao, G.; Li, Y.; Meng, X.; Tu, Q.; Huang, S.; Qin, Z. Mining-Induced Pressure-Relief Mechanism of Coal-Rock Mass for Different Protective Layer Mining Modes. Advances in Materials Science and Engineering 2021, 2021, 1–15, doi:10.1155/2021/3598541.
5. Niu, Y.; Wang, E.; Li, Z.; Gao, F.; Zhang, Z.; Li, B.; Zhang, X. Identification of Coal and Gas Outburst-Hazardous Zones by Electric Potential Inversion During Mining Process in Deep Coal Seam. Rock Mechanics and Rock Engineering 2022, 55, 1–12, doi:10.1007/s00603-022-02804-z.
6. Yang, W.; Lin, B. quan; Qu, Y. an; Zhao, S.; Zhai, C.; Jia, L. li; Zhao, W. qiang Mechanism of Strata Deformation under Protective Seam and Its Application for Relieved Methane Control. International Journal of Coal Geology 2011, 85, 300–306, doi:10.1016/j.coal.2010.12.008.
7. Yin, G.; Li, M.; Wang, J.G.; Xu, J.; Li, W. Mechanical Behavior and Permeability Evolution of Gas Infiltrated Coals during Protective Layer Mining. International Journal of Rock Mechanics and Mining Sciences 2015, 80, 292–301, doi:10.1016/j.ijrmms.2015.08.022.
8. Chen, H.; Cheng, Y.; Ren, T.; Zhou, H.; Liu, Q. Permeability Distribution Characteristics of Protected Coal Seams during Unloading of the Coal Body. International Journal of Rock Mechanics and Mining Sciences 2014, 71, 105–116, doi:10.1016/j.ijrmms.2014.03.018.
9. Wang, H.; Cheng, Y.; Yuan, L. Gas Outburst Disasters and the Mining Technology of Key Protective Seam in Coal Seam Group in the Huainan Coalfield. Natural Hazards 2013, 67, 763–782, doi:10.1007/s11069-013-0602-5.

 

Point 3: In this manuscript, the thin plate theory is used to analyze the roof breakage law of protective layer mining under steep incline. Why do you choose this theory instead of other theories?

Response 3:  

In elasticity, a plate is defined as an object surrounded by two parallel planes and a cylindrical or prismatic surface perpendicular to the two parallel planes. Thin plate theory is a very mature theory in elasticity and is widely used. According to the mining situation of No.1-8 coal seam in the protective layer of a mine in western Henan, its basic roof fully conforms to the geometric conditions of elastic thin plate. In addition, its deflection must not be greater than the mining thick-ness of the coal seam, which is also in line with the premise of the thin plate bending small deflection theory.

 

Point 4: What is the help of the experimental results obtained by similar simulation experiment for protective steeply inclined coal seam mining?

Response 4:

In the engineering practice of protective layer mining, it is difficult to accurately measure the movement of each rock stratum.Through the similar simulation experiment, the field engineering can be simplified and simulated, and the movement and deformation of each rock layer can be observed intuitively.The experimental results have certain guiding significance for the control of surrounding rock and the layout of gas drainage boreholes.

 

Point 5: The language of the manuscript needs to be improved.

Response 5:  

Thank you very much for pointing out our shortcomings of language. We have highlighted all the language revisions in RED in the revised manuscript.

Such as：

1. Introduction

Coal seams with an occurrence angle of more than 45° are called steeply inclined coal seams, which are recognized as difficult to mine in the mining industry, and a considerable part of them are coal and gas outburst seams. Protective layer mining is considered to be the most effective and economic regional measure to prevent and control coal and gas outburst[1] [2][3]. In 1933, France began to conduct experimental research on protective layer mining, where after Germany, Poland, and other countries also began applying this technology to prevent coal and gas outbursts [4][5]. Since the late 1950s, China has applied protective layer technology to prevent coal and gas outbursts in southwest China and achieved remarkable results [6][7]. Since 1998, Huainan Mining Group has conducted experimental research on exploiting protective layers under various geological conditions and achieved remarkable application effects. The gas control concept of "all that can be protected" has been formed in the Huainan mining area, which has been popularized and applied to high gas outburst mining areas in China [8] [9].After mining the protective layer, the stress of the surrounding rock of the stope can be redistributed, and the overlying coal and rock mass of the stope has a pressure relief area. In a certain period, the stress of the roof of the goaf will be reduced within a specific range, resulting in the expansion and deformation of the coal stratum, which makes the coal stratum move towards the goaf, the unloading and swelling deformation of overlying coal and rock mass can be reflected by the displacement and deformation of roof coal and rock strata to a certain extent[10] [11]. Previous studies have shown that with the increase of coal seam dip angle, the tangential slip component along the rock layer increases, and the vertical pressure acting on the layer decreases so that the roof subsidence will gradually decrease[12] [13]. When mining steeply inclined coal seams, the gangue falling in the goaf cannot stay in place in most cases due to its weight but slides down along the coal seam[14]. The condition of sliding is tgα>f''(α is the dip angle of the coal seam, f is the dynamic friction coefficient of the caving gangue along the coal seam floor)If f'=0.6~0.7, when α>31~35°, the falling gangue will slip, and α the larger the sliding, the more intense it will occur[15] [16] [17] [18]. The condition of falling gangue does not simply depend on the dip angle of rock stratum but is related to the nature of roof rock stratum, mining height, and rock stratum thickness [19]. At the initial stage of roof caving, due to the small amount of caving gangue, it is not enough to fill the goaf, so the interior or upper part of the goaf is in a suspended state[20]. In this case, the section with a suspended roof will produce caving. It may gradually develop upward until the coal pillar above the ventilation roadway so that the gangue stored in the upper old goaf will also slide downward and fill the goaf of this working face. After the goaf is filled, when the working face continues to advance and cave, the rock movement characteristics in different parts of the goaf are different[21] [22] [23].

Based on the thin plate theory of elasticity, this paper focuses on analyzing the law of roof movement and deformation in mining steep protective layer. On this basis, through a similar simulation test, the movement and deformation law of the protective layer mining under the steep dip and close distance in a mine in western Henan is studied. Then it is analyzed whether mining this kind of protective layer destroys the mining conditions of the overlying protective layer and whether its safety and feasibility are guaranteed.

 

 

Point 6: The conclusion should be compressed to make it more concise.

Response 6:  

Thank you very much for this comment. We have modified the conclusion as follows:

 

(1) In the initial mining stage, before the roof collapses for the first time, there is no filling effect of the collapsed gangue, and the roof is in the elastic deformation stage. The deflection deformation is symmetrically distributed along the direction of the working face. The maximum deflection point along the working face is located slightly below the middle. In the normal mining stage, the maximum deflection point along the inclination of the working face is located slightly above the middle, which is less than the maximum deflection during the initial collapse. The reason why the maximum deflection position shifts upward along the slope is precisely due to the filling effect of the caving gangue.

(2) During the mining process of similar simulation experiments, the roof of the protective layer did not collapse on a large scale, and the roof is mainly sinking slowly. The maximum subsidence point of the overlying rock layers is basically on a straight line, and the maximum subsidence point is slightly above the middle of the slope. The experimental results of similar simulation is consistent with the roof force analysis results and deflection calculation in the normal mining stage.

(3) The steeply inclined coal seam has a large inclined angle of the coal seam so that the tangential component force along the rock layer increases. Furthermore, the vertical stress on the layer decreases, and the roof caving shape, and pressure step distance differ from the coal seam with a slight inclination angle.

(4) Affected by the coal seam inclined angle and the mining layer's thickness, the "vertical three-zone" division of the goaf is not apparent. The mining of this type of lower protective layer will not damage the mining conditions of the overlying protective layer. The development of fractures can provide a channel for the migration and extraction of pressure relief gas. The mining of this type of protective layer in this mine has good safety and feasibility.

 

 

Reviewer 2 Report

Thanks to the editor for inviting me to evaluate the article titled " Study on Overburden Movement Deformation and Roof Breakage Law of Under-protective Steeply Inclined Coal Seam Mining ". The manuscript has carried out a lot of basic experimental work and theoretical analysis. This topic will be of interest to most researchers in the related territory. However, there are some questions that the authors should answer:

 

1. The abstract needs to be modified and updated to display the research results of the article better.

 

2. In this manuscript, the section 2 is theoretical analysis of roof breaking regular of under-protective steeply inclined coal seam mining, and the section 3 is similar simulation experiment of overburden movement deformation of under protective steeply inclined coal seam mining. How are these two parts related?

 

3.How are the figure 9 and figure10 relevant? How was the original data for the curve in Figure 10 obtained？

 

4. How are the similar simulation results applied to the field engineering practice?

 

5. The conclusion is not a simple repetition of the experimental results but should be condensed and elevated.

 

6. The citation format of references should be uniform and standardized.

Author Response

Thanks for the reviewer’s meritorious comments.  And, please check the bellowing for the point-by-point response to the reviewer’s comments:

Response to Reviewer 2 Comments

 

Thank you very much for this comment and suggestion.

 

 

Point 1: The abstract needs to be modified and updated to display the research results of the article better.

Response 1:

Thank you for your advice. We have modified and updated the abstract as follows:

The occurrence of the steeply inclined coal seam is extraordinary, and the coal body is seriously damaged by extrusion. The most steeply inclined coal seam is high gas or outburst coal seam, and protective layer mining is the safest and most effective measure for regional prevention of coal and gas outburst. Based on considering the coefficient of lateral pressure and vertical height of section, the deflection of the basic roof of the steeply inclined protective layer in a mine in western Henan, China, is calculated using the deflection calculation method of thin plate theory of elas-ticity. Using MATLAB to understand the deflection, the deflection curve is obtained. The law of rock movement and deformation in the mining process of the protective layer is studied by a sim-ilarity simulation experiment. The results show that, after mining, the roof mainly sinks slowly without large-scale collapse, and the largest rock strata movement is located in the upper part of the slope. Rock strata movement and fracture development can relieve the pressure of the pro-tected layer and provide a channel for gas migration and drainage. The mining conditions of the protected layer will not be destroyed, and mining this type of protected layer in this mine has bet-ter safety and feasibility. The conclusions of this study have a guiding and scientific significance for the control of surrounding rock and the layout of gas drainage boreholes of under-protective steeply inclined coal seam mining.

 

Point 2: In this manuscript, the section 2 is theoretical analysis of roof breaking regular of under-protective steeply inclined coal seam mining, and the section 3 is similar simulation experiment of overburden movement deformation of under protective steeply inclined coal seam mining. How are these two parts related?

Response 2:

The theoretical study of section 2 can analyze the movement and deformation law of the mining strata in the steeply inclined protective layer from the perspectives of strike and inclination direction. The similar simulation experiment in section 3 can show the movement and deformation of each coal strata more intuitively. From the theoretical analysis and experimental results, the two parts have a good coincidence.Therefore, the two parts are mutually supportive.

 

Point 3:How are the figure 9 and figure10 relevant? How was the original data for the curve in Figure 10 obtained？

Response 3:

Part Ⅰ in Figure 9 is the original experimental image, and part Ⅱ in Figure 9 is the displacement comparison map of marker points.The original data in Figure 10 are derived from part Ⅱ of Figure 9, which is the displacement of marker points. And in order to present the experimental results better, we combine Figure. 9 and Figure. 10 together. After combined, the original experimental picture, coordinate comparison, coal and rock displacement can be clearly seen from left to right in the lateral direction. At the same time, in the longitudinal direction, there is a sharp contrast between the various stages of mining.

 

 

Point 4: How are the similar simulation results applied to the field engineering practice?

Response 4:

In the engineering practice of protective layer mining, it is difficult to accurately measure the movement of each rock stratum.Through the similar simulation experiment, the field engineering can be simplified and simulated, and the movement and deformation of each rock layer can be observed intuitively.The experimental results have certain guiding significance for the control of surrounding rock and the layout of gas drainage boreholes.

 

Point 5: The conclusion is not a simple repetition of the experimental results but should be condensed and elevated.

Response 5:

Thank you very much for this comment. We have modified the conclusion as follows:

 

(1) In the initial mining stage, before the roof collapses for the first time, there is no filling effect of the collapsed gangue, and the roof is in the elastic deformation stage. The deflection deformation is symmetrically distributed along the direction of the working face. The maximum deflection point along the working face is located slightly below the middle. In the normal mining stage, the maximum deflection point along the inclination of the working face is located slightly above the middle, which is less than the maximum deflection during the initial collapse. The reason why the maximum deflection position shifts upward along the slope is precisely due to the filling effect of the caving gangue.

(2) During the mining process of similar simulation experiments, the roof of the protective layer did not collapse on a large scale, and the roof is mainly sinking slowly. The maximum subsidence point of the overlying rock layers is basically on a straight line, and the maximum subsidence point is slightly above the middle of the slope. The experimental results of similar simulation is consistent with the roof force analysis results and deflection calculation in the normal mining stage.

(3) The steeply inclined coal seam has a large inclined angle of the coal seam so that the tangential component force along the rock layer increases. Furthermore, the vertical stress on the layer decreases, and the roof caving shape, and pressure step distance differ from the coal seam with a slight inclination angle.

(4) Affected by the coal seam inclined angle and the mining layer's thickness, the "vertical three-zone" division of the goaf is not apparent. The mining of this type of lower protective layer will not damage the mining conditions of the overlying protective layer. The development of fractures can provide a channel for the migration and extraction of pressure relief gas. The mining of this type of protective layer in this mine has good safety and feasibility.

 

 

Point 6: The citation format of references should be uniform and standardized.

Response 6:

We have checked each reference in all citation, and unified and standardized all of the citation and highlighted all the revisions in RED in the revised manuscript. In the process of revising the manuscript, several references were added and one of the references was repositioned. All references in the manuscript are cited in a format that is annotated according to ‘Sustainbility’ journal standards.

Such as：

In the “1. Introduction”

In 1933, France began to conduct experimental research on protective layer mining, where after Germany, Poland, and other countries also began applying this technology to prevent coal and gas outbursts[4][5]. Since the late 1950s, China has applied protective layer technology to prevent coal and gas outbursts in southwest China and achieved remarkable results[6][7]. Since 1998, Huainan Mining Group has conducted experimental research on exploiting protective layers under various geological conditions and achieved remarkable application effects. The gas control concept of "all that can be protected" has been formed in the Huainan mining area, which has been popularized and applied to high gas outburst mining areas in China[8][9].

 

References

 

4. Cheng, X.; Zhao, G.; Li, Y.; Meng, X.; Tu, Q.; Huang, S.; Qin, Z. Mining-Induced Pressure-Relief Mechanism of Coal-Rock Mass for Different Protective Layer Mining Modes. Advances in Materials Science and Engineering 2021, 2021, 1–15, doi:10.1155/2021/3598541.
5. Niu, Y.; Wang, E.; Li, Z.; Gao, F.; Zhang, Z.; Li, B.; Zhang, X. Identification of Coal and Gas Outburst-Hazardous Zones by Electric Potential Inversion During Mining Process in Deep Coal Seam. Rock Mechanics and Rock Engineering 2022, 55, 1–12, doi:10.1007/s00603-022-02804-z.
6. Yang, W.; Lin, B. quan; Qu, Y. an; Zhao, S.; Zhai, C.; Jia, L. li; Zhao, W. qiang Mechanism of Strata Deformation under Protective Seam and Its Application for Relieved Methane Control. International Journal of Coal Geology 2011, 85, 300–306, doi:10.1016/j.coal.2010.12.008.
7. Yin, G.; Li, M.; Wang, J.G.; Xu, J.; Li, W. Mechanical Behavior and Permeability Evolution of Gas Infiltrated Coals during Protective Layer Mining. International Journal of Rock Mechanics and Mining Sciences 2015, 80, 292–301, doi:10.1016/j.ijrmms.2015.08.022.
8. Chen, H.; Cheng, Y.; Ren, T.; Zhou, H.; Liu, Q. Permeability Distribution Characteristics of Protected Coal Seams during Unloading of the Coal Body. International Journal of Rock Mechanics and Mining Sciences 2014, 71, 105–116, doi:10.1016/j.ijrmms.2014.03.018.
9. Wang, H.; Cheng, Y.; Yuan, L. Gas Outburst Disasters and the Mining Technology of Key Protective Seam in Coal Seam Group in the Huainan Coalfield. Natural Hazards 2013, 67, 763–782, doi:10.1007/s11069-013-0602-5.

 

 

Reviewer 3 Report

The illustrations shown in figure 9 should be combined as they are of the same type and show only distinctive features, similarly, it is necessary to combine the parametric dependencies shown in figure 10 into single parametric dependencies and show only the difference.

Author Response

Thanks for the reviewer’s meritorious comments. The English language and style will be checked in the manuscript.

The revision to figure 9 and figure 10, Please see the attachment.

Author Response File: Author Response.pdf