Experimental Research into the Evolution of Permeability in a Broken Coal Mass under Cyclic Loading and Unloading Conditions
Round 1
Reviewer 1 Report
The topic is very interesting, the experiments well organized and documented. The ms is very well compiled, written in very good English; it focuses to the point. There are, however, some improvements necessary before acceptance.
1. Chapter 2 comprises both methodology and results mixed. I recommend to remove all the results from this chapter and to add to the RESULTS chapter.
2. Chapter 3 contains similarly results and some analysis based on the results. I recommend deteriorating here to the results only. All the included analysis of the diagrams has to be moved to the discussion chapter.
3. Table 1, in the caption: the authors wrote ‘samples’ but there is one sample only. Additionally, they have to explain in the caption the abbreviations used in the Table 1. The same is valid for Table 2 too. P is used here as specific gravity whereas further downwards is used for pressure, which is confusing.
4. The authors have to explain why the used the 15.1-18.2 mm fraction of the coal.
I encourage the authors to proceed with the corrections the soonest and to re-submit an improved version of the paper.
Author Response
Dear Editors and Reviewers,
We appreciate your time in reviewing our paper and providing us your feedback on our work entitled “Experimental Research into the Evolution of Permeability in a Broken Coal Mass under Cyclic Loading and Unloading Conditions” (ID:applsci-426187). The comments provided were valuable and helpful to revise and improve our paper, which has improved our paper significantly and highlights our research better. We have addressed all the comments carefully, based on which the contents have been updated, which should now meet the journal's expectations. The main corrections in the paper and our response to the reviewer's comments are as follows.
Responds to the reviewer’s comments:
Reviewer #1:
The topic is very interesting, the experiments well organized and documented. The ms is very well compiled, written in very good English; it focuses to the point. There are, however, some improvements necessary before acceptance.
1. Chapter 2 comprises both methodology and results mixed. I recommend to remove all the results from this chapter and to add to the RESULTS chapter.
Response: Many thanks for your suggestion. I have removed all the results from Chapter 2 and add them to the Results chapter (Chapter 3). Please check it.
2. Chapter 3 contains similarly results and some analysis based on the results. I recommend deteriorating here to the results only. All the included analysis of the diagrams has to be moved to the discussion chapter.
Response: Many thanks for your suggestion. I have removed all the analysis results from Chapter 3 and add them to Chapter 4 (Discussion chapter). Namely, Chapter 3.2 has changed to Chapter 4.1, and Chapter 3.3 has changed to Chapter 4.2. In addition, the Chapter Numbers and Figure Numbers of other parts of the paper have been changed accordingly. Please check it.
3. Table 1, in the caption: the authors wrote ‘samples’ but there is one sample only. Additionally, they have to explain in the caption the abbreviations used in the Table 1. The same is valid for Table 2 too. P is used here as specific gravity whereas further downwards is used for pressure, which is confusing.
Response: Many thanks for your suggestion. Although only coal sample with single particle size (15.1-18.2mm) was studied in this paper, three groups of coal samples were selected for seepage test, and the average result of the three groups of coal samples was the test data. So the“samples” in Table 1 was used. Additionally, the abbreviations used in the Table 1 and 2 have been explained. Please check it. Finally, The P in Table 1 is a spelling mistake, which is actually ρ, representing the actual density of the coal sample. It has been corrected, please check it.
4. The authors have to explain why the used the 15.1-18.2 mm fraction of the coal.
Response: The permeability characteristics of a broken coal mass exert significance on spontaneous combustion of coal in goaf. However, goaf is an enclosed space with complex and unknown environments and therefore it is extremely dangerous to measure directly the permeability of broken coal masses in goaf. In current research, the permeability of coal masses in goaf is generally indirectly evaluated by establishing the prediction model for permeability of coal masses in goaf based on theoretical analysis, numerical simulation, and laboratory trials. In laboratory test, particle size of broken coal sample is generally determined according to geometric similarity ratio. The size of broken coal in goaf varies from several meters to dozens of meters, and the size of goaf varies from dozens to hundreds of meters. The coal sample is placed in the colloid sleeve of the laboratory test equipment, and the size of the colloid sleeve is 50×100mm, according to the size similarity ratio law, the size of the coal sample in the colloid sleeve should vary from a few millimeters to dozens of millimeters. Therefore, to sum up, 15.1-18.2mm coal sample was selected in the laboratory.
We are very grateful for the valuable and insightful comment, which is very favorable for the promotion and application of our research.
We once again appreciate the time taken by the editors and reviewers to review our work and suggest their thoughts and feedback. We hope that the correction will meet your expectations and would be subsequently approved.
Once again, thank you very much for your comments and suggestions!
We look forward to hearing from you.
Reviewer 2 Report
Conclusions presented in the abstract (1-4) are obvious without any measurements.
After reading the abstract no one will read rest of the paper. In this paper valuable information are fitting functions. This should be included into the abstract.
Line 22-23: “ the loss of permeability … was gradually reduced” Maybe: permeability was gradually increased?
Fig.6. x axises are not readable please correct it.
Fig.6 y axises why on the left hand side and right hand side we have (mass and percentage) the same values? Is this correct?
Author Response
Dear Editors and Reviewers,
We appreciate your time in reviewing our paper and providing us your feedback on our work entitled “Experimental Research into the Evolution of Permeability in a Broken Coal Mass under Cyclic Loading and Unloading Conditions” (ID:applsci-426187). The comments provided were valuable and helpful to revise and improve our paper, which has improved our paper significantly and highlights our research better. We have addressed all the comments carefully, based on which the contents have been updated, which should now meet the journal's expectations. The main corrections in the paper and our response to the reviewer's comments are as follows.
Responds to the reviewer’s comments:
Reviewer #2:
(1) Conclusions presented in the abstract (1-4) are obvious without any measurements.
Response: Thank you for the feedback. In fact, conclusions (1-4) presented in the abstract are obtained by a combination of the seepage test results of broken coal and theoretical analysis. The details are as follows: (1) Conclusion 1 is obtained, which is based on the experimental results in Figs. 4, 11, 12, 14 and 15. (2) Conclusion 2 is obtained, which is based on experiment analysis results in Table 3 and Figs. 8, 9. (3) Conclusion 3 is obtained, which is based on experiment analysis results in Figs. 5 and 7. (4) Conclusion 4 is obtained, which is based on experiment analysis results in Fig. 13. I hope you can be satisfied with my explanation. If you still have any questions, please contact me.
(2) After reading the abstract no one will read rest of the paper. In this paper valuable information are fitting functions. This should be included into the abstract.
Response: Thank you for the feedback. Abstract is the gist of the paper, after you read the abstract of the paper, you should have a general understanding on this paper, this is also the basic purpose of the abstract. Whether you want to continue reading the content of the paper depends on whether you are interested in the paper. As you said, the fitting formulas in this paper are indeed valuable, but they are not suitable for writing in the abstract. The reasons are as follows: on the one hand, there are too many fitting formulas in the paper, which are generally not included in the abstract due to limited by the length of the abstract; On the other hand, the abstract mainly introduces the background, what methods are adopted, what conclusions are drawn and other contents, generally does not involve the specific formula. I hope you can be satisfied with my explanation. If you still have any questions, please contact me.
(3) Line 22-23: “the loss of permeability … was gradually reduced” Maybe: permeability was gradually increased?
Response: Thank you for the feedback. Line 22-23: “the loss of permeability … was gradually reduced” is obtained, which is based on experiment analysis results in Table 3 and Figs. 8, 9. Therefore, with increased cyclic loading and unloading, the loss of permeability of broken coal samples and stress sensitivity was gradually reduced instead of gradually increasing. I hope you can be satisfied with my explanation. If you still have any questions, please contact me.
(4) Fig.6 x axises are not readable please correct it.
Response: Thank you for the feedback. X axis of Fig.6 (Fig.5 in revised manuscript) is “Particle size”, the unit is “mm”. It is divided into 7 particle size ranges, which are 15.1-18.2, 10.2-15.1, 6.8-10.2, 4.6-6.8, 2.8-4.6, 1-2.8, ＜1, respectively. Please check it.
(5) Fig.6 y axises why on the left hand side and right hand side we have (mass and percentage) the same values? Is this correct?
Response: Thank you for the feedback. As shown in Fig. 6 (Fig.5 in revised manuscript), Y axis of Fig.6 (Fig.5 in revised manuscript) on the left hand side is the mass of the coal samples with different particle sizes, and Y axis of Fig.6 (Fig.5 in revised manuscript) on the right hand side is the mass percentage of the coal samples with different particle sizes. Since the total mass of the coal sample is equal to 100g, the mass of each particle size and the mass percentage of each particle size should be equal in value after the test. Therefore, Y axis of Fig.6 (Fig.5 in revised manuscript) on the left hand side and right hand side are (mass and mass percentage) the same values. So it is correct. I hope you can be satisfied with my explanation. If you still have any questions, please contact me.
We are very grateful for the valuable and insightful comment, which is very favorable for the promotion and application of our research.
We once again appreciate the time taken by the editors and reviewers to review our work and suggest their thoughts and feedback. We hope that the correction will meet your expectations and would be subsequently approved.
Once again, thank you very much for your comments and suggestions!
We look forward to hearing from you.
Author Response File: 
Author Response.pdf
Reviewer 3 Report
This is a good paper, however, I would like to have further discussion about the following statements in the conclusion.
(5) Rearrangement, breakage, and compressive deformation of coal particles in the loading stage 1 can cause a significant decrease in the porosity of coal samples, thus resulting in a decreased permeability thereof. In the unloading stage, only the loss of permeability resulting from particle deformation could be recovered.
(6) The permeability of broken coal sample decreased with increasing strain and increased with the increasing of porosity of the broken coal samples. The fitting curves of between permeability and strain, porosity are cubic function and power function respectively. Besides, the permeability of broken coal sample under cyclic loading and unloading conditions is determined by its porosity, which conforms to the cubic law.
Author Response
Dear Editors and Reviewers,
We appreciate your time in reviewing our paper and providing us your feedback on our work entitled “Experimental Research into the Evolution of Permeability in a Broken Coal Mass under Cyclic Loading and Unloading Conditions” (ID:applsci-426187). The comments provided were valuable and helpful to revise and improve our paper, which has improved our paper significantly and highlights our research better. We have addressed all the comments carefully, based on which the contents have been updated, which should now meet the journal's expectations. The main corrections in the paper and our response to the reviewer's comments are as follows.
Responds to the reviewer’s comments:
Reviewer #2:
This is a good paper, however, I would like to have further discussion about the following statements in the conclusion.
(5) Rearrangement, breakage, and compressive deformation of coal particles in the loading stage 1 can cause a significant decrease in the porosity of coal samples, thus resulting in a decreased permeability thereof. In the unloading stage, only the loss of permeability resulting from particle deformation could be recovered.
Response: As shown in Fig. 13, the reasons why the porosity of broken coal samples reduced in the loading stage involves three things: (1) the structural re-arrangement of broken coal particles caused a significant reduction in porosity: in the early stage of loading of broken coal masses, there was no cohesion between particles to therefore cause slippage and rotation of particles. The particle structures were rearranged resulting in the significant reduction in porosity, which was the primary reason why broken coal masses were compacted and deformed. (2) Broken coal samples were crushed further and then the broken fine particles filled the void space, resulting in the further reduction of the porosity of the coal samples. (3) The compressive deformation between particles reduced the porosity: the compressive deformation is actually elastic deformation, which can be recovered in the unloading process. The first two factors caused a significant reduction in the porosity of coal samples, so they were the main reasons causing the reduction of porosity, and this portion of porosity of coal samples cannot be recovered during stress unloading. The reduction of porosity induced by the third factor can be recovered in the unloading process, which was the reason why the porosity of coal samples increased in the unloading stage, however, the influence of the third factor on the porosity was much less significant than those of the first two factors. In addition, the permeability of broken coal sample under cyclic loading and unloading conditions is determined by its porosity, which conforms to the cubic law. The relationship between permeability and porosity is shown in formula 9, That is, the change of porosity is proportional to the change of permeability. In conclusion, conclusion (5) is obtained.
The corresponding part of the paper has been modified, please check it.
(6) The permeability of broken coal sample decreased with increasing strain and increased with the increasing of porosity of the broken coal samples. The fitting curves of between permeability and strain, porosity are cubic function and power function respectively. Besides, the permeability of broken coal sample under cyclic loading and unloading conditions is determined by its porosity, which conforms to the cubic law.
Response: The permeability of broken coal sample decreased with increasing strain and increased with the increasing of porosity of the broken coal samples. This is based on the results of the seepage test, as shown in Fig. 14 and 15. The fitting curves of between permeability and strain, porosity are cubic function and power function respectively. This is also based on the results of the seepage test, as shown in Fig. 14 and 15.
As shown in Fig. 14 and 15, under the same strain or porosity, the permeability of the broken coal sample at the loading stage in each loading and unloading stage is the same as that at the unloading stage. This indicates that as long as the porosity of the broken coal sample is same during each loading and unloading (formula 8 shows that strain and porosity are equivalent), the permeability of the broken coal sample is same, independent of the stress state of the broken coal sample at that time. For example, it can be seen from Figs. 11 and 12 that in each loading and unloading process, when the strain or porosity at the loading stage is equal to the strain or porosity at the unloading stage, the effective stress at the loading stage is greater than the effective stress at the unloading stage. At this time, the stress states of the broken coal samples at the loading stage and unloading stage are different, but the permeability is the same. Therefore, the permeability of broken coal sample under cyclic loading and unloading conditions is determined by its porosity, which conforms to the cubic law.
We are very grateful for the valuable and insightful comment, which is very favorable for the promotion and application of our research.
We once again appreciate the time taken by the editors and reviewers to review our work and suggest their thoughts and feedback. We hope that the correction will meet your expectations and would be subsequently approved.
Once again, thank you very much for your comments and suggestions!
We look forward to hearing from you.
Round 2
Reviewer 2 Report
Fig.6. x axises are not readable please correct it.
I still do not like an abstract. The statements are too obvious or difficult to read.:
line 21-22: With increased cyclic loading.... the loss of permeabillity of broken coal samples ...was gradually reduced"
Some sentences are out of logic.:
line 17-18.: during the cyclic loading... the fitting curves of between permeability ..."
etc.
Author Response
Dear Editors and Reviewers,
We appreciate your time in reviewing our paper and providing us your feedback on our work entitled “Experimental Research into the Evolution of Permeability in a Broken Coal Mass under Cyclic Loading and Unloading Conditions” (ID:applsci-426187). The comments provided were valuable and helpful to revise and improve our paper, which has improved our paper significantly and highlights our research better. We have addressed all the comments carefully, based on which the contents have been updated, which should now meet the journal's expectations. The main corrections in the paper and our response to the reviewer's comments are as follows.
Responds to the reviewer’s comments:
Reviewer #2:
(1) Fig.6 x axises are not readable please correct it.
Response: Thank you for the feedback. X axis of Fig.6 has been revised. Please check it.
(2) I still do not like an abstract. The statements are too obvious or difficult to read:
Line 21-22: With increased cyclic loading.... the loss of permeability of broken coal samples ...was gradually reduced"
Some sentences are out of logic:
Line 17-18: during the cyclic loading... the fitting curves of between permeability ..."etc.
Response: Thank you for the feedback. As you said, I read the abstract carefully and found it was not very good. Therefore, I have carefully revised the abstract, please check it. I hope you will be satisfied with the modifications. Line 17-18 and Line 21-22 have been also revised, please check it.
We are very grateful for the valuable and insightful comment, which is very favorable for the promotion and application of our research.
We once again appreciate the time taken by the editors and reviewers to review our work and suggest their thoughts and feedback. We hope that the correction will meet your expectations and would be subsequently approved.
Once again, thank you very much for your comments and suggestions!
We look forward to hearing from you.
Author Response File: Author Response.pdf
