Study on Efficient Utilization Technology of Coal Pillar Based on Gob-Side Entry Driving in a Coal Mine with Great Depth and High Production

Round 1

Reviewer 1 Report

The main purpose of the manuscript is to analyse the basic mechanical structure of coal pillar and the characteristics of its weakening failure, based on theoretical calculations and numerical simulations. Moreover, through the analysis, the “instability critical line” and the distribution law of dangerous zones inside the coal pillar structure are obtained.

This is a very interesting topic of efficient utilization technology of coal resources under difficult mining conditions in relation to sustainability issues. From the overall presentation it is concluded that an interesting research work has been done.

However, from a mining engineering point of view, I think that the research questions as well as the original contribution of the work, comparing to other previous works are not adequately presented. I would expect a comparison of the proposed analysis with previous related research work, placing more emphasis on the new information and the original contribution of the work.

The authors should place more emphasis on (a) the research questions, (b) the coupling between the theoretical and the experimental analysis and (c) the original approach to the analysis of the problem.

Additional comments and recommendations for the improvement of the manuscript:

 

General notes

Ø  Please leave a space between numerical values and unit symbols

Ø  All the equation symbols must be defined within the manuscript.

 

Abstract

[Lines 12-13] “To improve the utilization…circular economy”. This sentence needs to be revised.

 

1. Introduction

General note: In this section, a more critical review is needed.

[Line 28] “Previous researches have revealed…”. A reference is required here.

 

2. The force transmission evolution of coal pillar structure

[Line 90] “To determine the location of the fracture line of main roof”. The title should be revised.

[Lines 99-100] “Therefore, the study of strata… coal pillar stability”. A better explanation is needed here.

[Fig. 1] Is it a sketch map? An explanation of Fig. 1 is required.

[Line 113] “According to the maximum bending moment theory…”. A reference is required here.

[Line 128] The units of the parameters should be mentioned.

[Line 138] “The basic geological parameters of 221_up01 gob-side entry in Shilawusu mine…”. A description of the mine is necessary in this section.

[Lines 139 & 140] Please define “rock A” and “rock B”.

[Lines 138-145] “The basic geological parameters...”. Are all these geological parameters?

[Line 144 and Fig. 2] Parameter x should be explained

[Eq. 2] Equation 2 should be checked and revised (are all limits the same?)

[Eq. 3 and Line 189] “f_m” or “f_M”? All the equation symbols must be written in the same way both in the equations and within the text.

 

3. The failure law analysis of coal pillar structure

[Fig. 7] A better explanation of Fig. 7 is required.

[Line 248] “According to previous studies…”. A reference is missing here.

[Fig. 9] Is it “X-Displacement”? A better explanation of Fig. 9 is required.

[Line 268] “According to the statistics…”. What do the authors mean by this sentence?

[Fig. 12] Is it “Z-Displacement”? Please explain “a”, “b” and “k”.

 

4. Model estimation and interpretation of research results

[Lines 320-321] “Shilawusu mine producing 10 million tons a year is located in Ordos City, Inner Mongolia, China”. A reference is needed here.

[Figs 13, 16 & 18] A scale is missing.

 

5. Conclusion

General note: In this section, the original contribution of the research has to be presented by focusing on the research results based on the research questions.

Author Response

We appreciate the time and efforts by the editor and reviewers in reviewing this manuscript. We have addressed the issues indicated in the review comments, and hope that the revised version can meet the journal publication requirements. The authors’ response to reviewers and editor’s comments and list of changes are as follows.

Comment 1

The main purpose of the manuscript is to analyse the basic mechanical structure of coal pillar and the characteristics of its weakening failure, based on theoretical calculations and numerical simulations. Moreover, through the analysis, the “instability critical line” and the distribution law of dangerous zones inside the coal pillar structure are obtained.

This is a very interesting topic of efficient utilization technology of coal resources under difficult mining conditions in relation to sustainability issues. From the overall presentation it is concluded that an interesting research work has been done.

However, from a mining engineering point of view, I think that the research questions as well as the original contribution of the work, comparing to other previous works are not adequately presented. I would expect a comparison of the proposed analysis with previous related research work, placing more emphasis on the new information and the original contribution of the work.

The authors should place more emphasis on (a) the research questions, (b) the coupling between the theoretical and the experimental analysis and (c) the original approach to the analysis of the problem.

Response

Thanks for your suggestions! I have compared the previous researches with my present research, through which I find that the previous researches mainly focused on the characteristics of stress distribution in coal pillar structure body. In contrast, rare efforts were made to study its mechanical structure, macro weakening crack initiation, and progressive failure mechanism. Here, FLAC3D was used to simulate the excavation process. In total, eight groups of model calculation were designed and conducted, the excavation distance being 30 m，35 m，40 m，45 m，50 m，55 m，60 m，and 70 m respectively. Then the progressive weakening failure mechanism of coal pillars was studied according to field stress monitoring and peephole images of coal pillar side. A whole set of efficient utilization technology for coal pillar were formed accordingly.

Comment 2

General notes

Ø  Please leave a space between numerical values and unit symbols

Ø  All the equation symbols must be defined within the manuscript.

Response

Thanks for your kind suggestions. I have checked and rectified the format accordingly and supplemented the definition of equation symbols “M_x” and “F”.

Comment 3

[Lines 12-13] “To improve the utilization…circular economy”. This sentence needs to be revised.

Response

Thank you very much for your kind suggestions. I have revised this sentence to “Improving the utilization of non-renewable resources takes a crucial position in circular economy” according to your comment.

Comment 4

General note: In this section, a more critical review is needed.

[Line 28] “Previous researches have revealed…”. A reference is required here.

Response

Thanks for your kind suggestions. I fully agree to your viewpoint and suggestions. Accordingly, I have added the analysis and comparison of the previous researches to my paper to strengthen its wholeness. Besides, corresponding references have also been added to “Previous researches have revealed…”.

Comment 5

[Line 90] “To determine the location of the fracture line of main roof”. The title should be revised.

Response

Thanks for your kind suggestions. I have revised “To determine the location of the fracture line of main roof” to “Main roof fracture line location”.

Comment 6

[Lines 99-100] “Therefore, the study of strata… coal pillar stability”. A better explanation is needed here.

Response

Thanks for your kind suggestions. Accordingly, I have revised this sentence to “Therefore, the location of fracture structure in the overlying strata above the narrow-coal-pillar roadway driving along gob is an essential prerequisite for the study of stress transmission in coal pillar structure.”

Comment 7

[Fig. 1] Is it a sketch map? An explanation of Fig. 1 is required.

Response

Thanks for your question and advice. Figure 1 is a Sketch of typical stope structure with narrow coal pillars. I have made an explanation to it which goes like this: … which is shown in Figure 1. Judging from whether key blocks can from form hinge structure in overlying strata, the stress structure above coal pillar can be divided into two types. One is the twisting force exerted upon coal pillar structure due to the slide and deflection of roof key strata, as is shown in Figure 1 (a); the other refers to the vertical stress exerted upon coal pillar structure by overlying key strata because roof key strata cannot form corresponding working structure, as is shown in Figure 1 (b). Therefore, the location of fracture structure in the overlying strata above the narrow-coal-pillar roadway driving along gob is an essential prerequisite for the study of stress transmission in coal pillar structure.

Comment 8

[Line 113] “According to the maximum bending moment theory…”. A reference is required here.

[Line 128] The units of the parameters should be mentioned

Response

Thank you very much for your kind suggestions. Accordingly, I have added the reference to “According to the maximum bending moment theory…” and the units of parameters like M₀, Q₀ and N.

Comment 9

[Line 138] “The basic geological parameters of 221up01 gob-side entry in Shilawusu mine…”. A description of the mine is necessary in this section.

Response

Thanks for your kind suggestions. I have added the related description of the mine and added corresponding references.

Comment 10

[Lines 139 & 140] Please define “rock A” and “rock B”.

Response

Thanks for your kind suggestions. “rock A” and “rock B” have been marked out in Figure 1.

Comment 11

[Lines 138-145] “The basic geological parameters...”. Are all these geological parameters?

Response

Thanks for your question and advice. In my paper, the basic geological parameters refer to the following  the thickness of main roof (h), the width of main roof (b), the length of fractured rock (L), unit weight of main roof strata( γ), cushion coefficient (k), load intensity on the overhanging part of fractured rock (q). all the rest parameters are obtained through calculation by the equations of Line 78-79.

Comment 12

[Line 144 and Fig. 2] Parameter x should be explained

Response

Thanks for your kind suggestions. I have added a definition of parameter x in the paper--- “x denotes the distance between the starting point of clamping and maximum bending moment”(Line 176-177).

Comment 13

[Eq. 2] Equation 2 should be checked and revised (are all limits the same?)

Response

Thanks for your question! I agree with your viewpoint. The limits of Equation 2 vary in accordance with the directions of forces. Therefore, revision has been made to Equation 2 and the symbols of forces in the same direction are unified.

Comment 14

[Eq. 3 and Line 189] “f_m” or “f_M”? All the equation symbols must be written in the same way both in the equations and within the text.

Response

Thanks for your kind suggestions. “f_m” is a spelling mistake which has been revised.

Comment 15

3. The failure law analysis of coal pillar structure

[Fig. 7] A better explanation of Fig. 7 is required

Response

Thanks for your kind suggestions. Figure 7 has been renamed as “Model of gob-side entry driving”; meanwhile an explanation is added to Figure 7 which goes as follows, “As can be seen from the model, the height of coal seam is 5.0m, and the roof is composed of medium-fine sandstone and the floor sandy mudstone.”

Comment 16

[Line 248] “According to previous studies…”. A reference is missing here.

Response

Thanks for your kind suggestions. The corresponding reference has been added up.

Comment 17

[Fig. 9] Is it “X-Displacement”? A better explanation of Fig. 9 is required.

Response

Thanks for your question and suggestion. Figure 9 is a Diagram of the relationship between X-displacement of coal pillar structure and the roadway; meanwhile an explanation is added to Figure 9 which goes as follows, “As it is shown in Figure 9, displacement boundary marks off the area where X-displacement is zero. The displacement above the displacement boundary is set as positive which directs towards the roadway; the displacement below the boundary is set as negative which directs towards the gob. On both sides of displacement change line, displacement direction deflects at some points in coal pillar structure. Point A denotes the first point where displacement direction deflects; CD represents the roadway width”.

Comment 18

[Line 268] “According to the statistics…”. What do the authors mean by this sentence?

Response

Thanks for your question and suggestion. “According to the statistics of roadway driving from 30 m to 70 m, the displacement change lags behind the head-on by 5-10 m which approximately equals to two working days’ driving amount in Shilawusu mine”. This sentence means that it takes two working days for the displacement inside coal pillar structure body to develop according to the driving speed in Shilawusu mine.

Comment 19

[Fig. 12] Is it “Z-Displacement”? Please explain “a”, “b” and “k”.

Response

Thanks for your question and suggestion. Figure 12 is a Fitting function diagram of Z-displacement, whose basic form is y=ke^ax+b. In this equation, a, b, k are all basic parameters.

Comment 20

4. Model estimation and interpretation of research results

[Lines 320-321] “Shilawusu mine producing 10 million tons a year is located in Ordos City, Inner Mongolia, China”. A reference is needed here.

[Figs 13, 16 & 18] A scale is missing.

Response

Thanks for your kind suggestions. References have been added and Figure 13. 16 &18 have been revised as suggested.

Comment 21

5. Conclusion

General note: In this section, the original contribution of the research has to be presented by focusing on the research results based on the research questions.

Response

Thanks for your kind suggestions. I have revised the conclusion (the fourth point). “A whole set of efficient, hence sustainable, utilization technology for coal pillar are formed accordingly.”.

Author Response File: Author Response.pdf

Reviewer 2 Report

L12  To improve → Improving

L13   widely applied in coal mines in China → widely applied in coal mines in China, such as in the Shilawusu mine (Ordos City, Inner Mongolia), here considered as a case study.

L18.    failure. Through the analysis, the “instability critical line” and the distribution law of dangerous → failure, providing theoretical reference for efficient recovery of coal resources.

Lines 18-24.    Through the analysis, the “instability critical line” and the distribution law of dangerous 18 zones inside the coal pillar structure were obtained, providing theoretical reference for efficient 19 recovery of coal resources. In terms of the coal pillar support condition of 221up 01working face in 20 Shilawusu mine, the parameter requirement “high-strength anchorage and special reinforcement 21 to the dangerous zone” was raised upon the gob-side entry driving, which has been proved to be 22 highly efficient in coal pillar utilization in the case of gob-side entry driving. This study offers 23 valuable reference for the sustainable development of mines with similar mining conditions → In general, results of this study can be helpful in pursuing the efficient, hence sustainable, development of mines with Gob-side entry driving technology.

List of Symbols is well done.

Please note that Sustainability is aimed at a wide audience. In this respect, it would also be very useful for potential readers an essential glossary of the coal-mining technical terms most used in this paper, as the following example.

Essential Glossary:

Abutment - The weight of the rocks above a narrow roadway is transferred to the solid coal along the sides, which act as abutments of the arch of strata spanning the roadway.

(...)

Beam - Bar or straight girder used to support a span of roof between two support props or walls.

(...)

Borehole : A hole bored or drilled in the earth, such as an exploratory well

(...)

Bolt load........

(....)

Gob - The part of the mine from which the coal has been removed 

 (..)

Goaf -A space of the coal mine filled up with refuse.

 (...)

Seam - A stratum or bed of coal.

(.....)

Line 44. AHP 
: Please explain 

Lines 52 - 54. Zhang et.al 
 [10-11] deduced the stress-strain relationship of coal pillar under collaborative deformation 
condition by establishing a simplified collaborative deformation mechanics model of rock stratum 
 and coal pillar. 


Please clarify the reasoning that "the stress-strain relationship of coal pillar......" can be deduced by a model? It seems to me more reasonable that "the model (by Zhang et al.) is useful to deduce the stress-strain relationships......"

Lines 59-60. and optimized the methods for EDG and coal pillar yield through numerical 
 calculation and field practice   → and optimized numerical modeling for advanced geotechnical analyses (e.g. DEC, Itasca Ltd.) 
for coal pillar yield

Line 61.  UDEC →  Universal Distinct Element Code (UDEC)

Line 72. were studies as well 
→ were studied as well

Line 84. On this basis, 
  → Here,

Line 89  2. The force transmission evolution of coal pillar structure  → Stress state analysis of coal pillar structure

Line 90.  2.1. To determine the location of the fracture line of main roof  →  Main roof fracture line location

Line 100.  In other words, to determine the location of the fracture line    →  In other words, the location of the fracture line

Line 112. The sketch map of stope structure model with narrow coal pillars  →  Sketch of typical stope structure with narrow coal pillars

It should be opportune to add a legend related to numbers overwritten on the different patterns. E.g. 1) Solid coal; 2) Roof block above the solid coal; 3) Goaf.....

Line 117: primary rock  → bedrock

Lines 131-137  131 .......L represents length of fractured rock (m); γ denotes unit weight of mainroof strata (km/m3) and k refers to cushion coefficient (MPa); q represents load intensity on the overhanging part of fractured rock (kN/m); Q‘ denotes weight and load of fractured strata (kN), and M0=0, Q0=Q、after the fracture of main roof; N‘denotes horizontal force caused by the rotation offractured rock structure (kN); ΔS refers to vertical displacement difference between the two ends of fractured rock (m); L denotes length of fractured rock in main roof (m), which is approximately equal to the field measurement result of periodic weighting length (L=22 m).  → Symbols as in the List on page 1 (L=22).

Line 147     Fig.2 The trend chart of....    →   Fig.2. Trend chart of.......   

Line 163. is asymmetric, to be specific, high in the middle and low at the two sides → is higher in the middle section than at both sides

Fig. 5 The sketch map of.....   → Sketch map of.....    

Fig. 6 The diagram.....   →  Fig. 6 Diagram........

Line 202    3.1. The establishment of numerical model    → Setting up the numerical model 

Line 211  Fig.7 The numerical calculation model of gob-side entry driving   →  Model of gob-side entry driving

Line 216  3.2. The stress characteristics of coal pillar in the excavation process of gob-side entry driving     →  Stress characteristics of coal pillar in the excavation process of gob-side entry driving

Fig. 8 The three-dimensional diagram of stress distribution characteristics of gob-side entry driving   →   Fig. 8    3D diagram of stress distribution characteristics of gob-side entry driving

Line 247    3.3. The distribution characteristics and law of dangerous zones  →  3.3. Distribution characteristics and law of dangerous zones

Fig. 9: Delete "The" in captions 

Line 277   delete "The"

Figure 11: see comment on Fig. 9

Line 318   4. The field practice of gob-side entry driving → Field practice of gob-side entry driving

Lines 324-326   The roof is middle-fine sandstone (with an average thickness of 13.78 m); the floor is sandy mudstone (with an average thickness of 2.67 m).   Add geological Ref.

Line 330   4.2. The field support condition  →   4.2 Field support conditions

Line 332  into three types:  → into three different segments

Line 358 As is shown in Figure 15,  →  As it is shown in Figure 15,

Line 364 As is indicated in the Figure   →   As is indicated in Figure 15

Line 407   1) A mechanical structure model of coal pillar structure  →  A numericalmodel of coal pillar structure

Line 441. A whole set of efficient utilization technology → A whole set of efficient, hence sustainable, utilization technology

Author Response

We appreciate the time and efforts by the editor and reviewers in reviewing this manuscript. We have addressed the issues indicated in the review comments, and hope that the revised version can meet the journal publication requirements. The authors’ response to reviewers and editor’s comments and list of changes are as follows.

Comments 1

L12 To improve → Improving

L13 widely applied in coal mines in China → widely applied in coal mines in China, such as in the Shilawusu mine (Ordos City, Inner Mongolia), here considered as a case study.

L18. failure. Through the analysis, the “instability critical line” and the distribution law of dangerous → failure, providing theoretical reference for efficient recovery of coal resources.

Lines 18-24. Through the analysis, the “instability critical line” and the distribution law of dangerous 18 zones inside the coal pillar structure were obtained, providing theoretical reference for efficient 19 recovery of coal resources. In terms of the coal pillar support condition of 221up 01working face in 20 Shilawusu mine, the parameter requirement “high-strength anchorage and special reinforcement 21 to the dangerous zone” was raised upon the gob-side entry driving, which has been proved to be 22 highly efficient in coal pillar utilization in the case of gob-side entry driving. This study offers 23 valuable reference for the sustainable development of mines with similar mining conditions → In general, results of this study can be helpful in pursuing the efficient, hence sustainable, development of mines with Gob-side entry driving technology.

List of Symbols is well done.

Please note that Sustainability is aimed at a wide audience. In this respect, it would also be very useful for potential readers an essential glossary of the coal-mining technical terms most used in this paper, as the following example.

Essential Glossary:

Abutment - The weight of the rocks above a narrow roadway is transferred to the solid coal along the sides, which act as abutments of the arch of strata spanning the roadway.

(...)

Beam - Bar or straight girder used to support a span of roof between two support props or walls.

(...)

Borehole : A hole bored or drilled in the earth, such as an exploratory well

(...)

Bolt load........

(....)

Gob - The part of the mine from which the coal has been removed

(..)

Goaf -A space of the coal mine filled up with refuse.

(...)

Seam - A stratum or bed of coal.

(.....)

Lines 59-60. and optimized the methods for EDG and coal pillar yield through numerical calculation and field practice → and optimized numerical modeling for advanced geotechnical analyses (e.g. DEC, Itasca Ltd.) for coal pillar yield

Line 61. UDEC → Universal Distinct Element Code (UDEC)

Line 72. were studies as well → were studied as well

Line 84. On this basis, → Here,

Line 89 2. The force transmission evolution of coal pillar structure → Stress state analysis of coal pillar structure

Line 90. 2.1. To determine the location of the fracture line of main roof → Main roof fracture line location

Line 100. In other words, to determine the location of the fracture line → In other words, the location of the fracture line

Line 117: primary rock → bedrock

Lines 131-137 131 .......L represents length of fractured rock (m); γ denotes unit weight of main roof strata (km/m3) and k refers to cushion coefficient (MPa); q represents load intensity on the overhanging part of fractured rock (kN/m); Q‘ denotes weight and load of fractured strata (kN), and M0=0, Q₀=Q′, after the fracture of main roof; N‘ denotes horizontal force caused by the rotation of fractured rock structure (kN); ΔS refers to vertical displacement difference between the two ends of fractured rock (m); L denotes length of fractured rock in main roof (m), which is approximately equal to the field measurement result of periodic weighting length (L=22 m). → Symbols as in the List on page 1 (L=22).

Line 147 Fig.2 The trend chart of.... → Fig.2. Trend chart of....... 

Line 163. is asymmetric, to be specific, high in the middle and low at the two sides → is higher in the middle section than at both sides

Fig. 5 The sketch map of..... → Sketch map of..... 

Fig. 6 The diagram..... → Fig. 6 Diagram........

Line 202 3.1. The establishment of numerical model → Setting up the numerical model 

Line 211 Fig.7 The numerical calculation model of gob-side entry driving → Model of gob-side entry driving

Line 216 3.2. The stress characteristics of coal pillar in the excavation process of gob-side entry driving → Stress characteristics of coal pillar in the excavation process of gob-side entry driving

Fig. 8 The three-dimensional diagram of stress distribution characteristics of gob-side entry driving → Fig. 8 3D diagram of stress distribution characteristics of gob-side entry driving

Line 247 3.3. The distribution characteristics and law of dangerous zones → 3.3. Distribution characteristics and law of dangerous zones

Fig. 9: Delete "The" in captions 

Line 277 delete "The"

Figure 11: see comment on Fig. 9

Line 318 4. The field practice of gob-side entry driving → Field practice of gob-side entry driving

Line 330 4.2. The field support condition → 4.2 Field support conditions

Line 332 into three types: → into three different segments

Line 358 As is shown in Figure 15, → As it is shown in Figure 15,

Line 364 As is indicated in the Figure → As is indicated in Figure 15

Line 407 1) A mechanical structure model of coal pillar structure → A numerical model of coal pillar structure

Line 441. A whole set of efficient utilization technology → A whole set of efficient, hence sustainable, utilization technology

Response

Thank you very much for these very important comments and kind suggestions for helping us improve the revised manuscript. I have revised the paper as suggested.

Comments 2

Line 44. AHP : Please explain

Response

Thanks for your question. AHP means “method of Analytic Hierarchy Process”.

Comments 3

Lines 52 - 54. Zhang et.al [10-11] deduced the stress-strain relationship of coal pillar under collaborative deformation condition by establishing a simplified collaborative deformation mechanics model of rock stratum and coal pillar.

Please clarify the reasoning that "the stress-strain relationship of coal pillar......" can be deduced by a model? It seems to me more reasonable that "the model (by Zhang et al.) is useful to deduce the stress-strain relationships......"

Response

Thanks for your question and suggestion. Zhang et.al established an ultra-thick stratum—coal pillar collaborative deformation model to analyze the vertical deformation mechanism of coal pillars. From their analysis they obtained the corresponding stress-strain relationship. But their analysis mainly focused on the compression deformation of intensive load (F) and gravity sedimentation of concentrated force (G); the torsional deflecting force is hardly interpreted in the analytical process.

Comments 4

Line 112. The sketch map of stope structure model with narrow coal pillars → Sketch of typical stope structure with narrow coal pillars

It should be opportune to add a legend related to numbers overwritten on the different patterns. E.g. 1) Solid coal; 2) Roof block above the solid coal; 3) Goaf.....

Response

Thanks for your kind suggestions. I have revised the paper as suggested. A legend is added to the Figure and the goaf, key block A, key block B, and solid coal have been marked out

Comments 5

Lines 324-326 The roof is middle-fine sandstone (with an average thickness of 13.78 m); the floor is sandy mudstone (with an average thickness of 2.67 m). Add geological Ref.

Response

Thanks for your kind suggestions. Corresponding references have been added accordingly. 

Author Response File: Author Response.pdf