Destabilization and energy characteristics of coal pillar in roadway driving along gob based on rockburst risk assessment

Roadway driving along adjacent goafs is an effective method to develop the recovery rate of coal resources. However, rock burst triggered by dynamic destabilization of coal pillars poses a serious threat to safe and efficient mining, thereby significantly restricting the sustainable development of coal mines. In this study, from the perspectives of energy accumulation and dissipation, a mathematical model of coal pillars is established and the energy equilibrium relationship of the mechanical system is analysed. The rock burst mechanism of coal pillars in gob-side entry is obtained based on a fold catastrophe mathematical model. Results indicate that the rock burst triggered by the instability is a destabilization phenomenon. If the stiffness factor of the mechanical system is less than 1 and the external force is enough to lead coal pillars to the peak stress point, then rock burst disaster occurs. The engineering analysis and numerical simulation are conducted to study the rock burst in the gob-side entry that occurred in Xin'an coal mine. Stress release caused by mining can reduce the risk of rock burst to a certain extent. The amount of elastic energy released is 6.4512 × 107 J, which is close to the observation data and verifies the correctness and rationality of the research method. The research result provides a theoretical basis and technical guidance for rock burst prevention and control in roadway driving along adjacent goafs.

see Wu et al., 2019. Rock Mechanics and Rock Engineering) 8. In figure 14, the energy density at the two ends of goaf reach to 1.96×1016 J. This value is so large. Is it reasonable? Please further analyze and discuss. 9. I didn't see the roadway driving along the goaf in the numerical model. Therefore, how does the author compare the theoretical results with the observation result (Line 14 to 16 of P15)? It is questionable. 10. The topic of paper is energy characteristics of coal pillar in gob-side entry. But, in the section of numerical simulation, the authors focuses on the analysis the stress evolution of coal seam mining rather than roadway driving along goaf. The numerical results need to be more closely related to the topic of the article.
Decision letter (RSOS-190094.R0) 17-May-2019 Dear Dr Xue, The editors assigned to your paper ("Destabilitation and energy characteristics of coal pillar in roadway driving along gob based on rockburst risk assessment") have now received comments from reviewers. We would like you to revise your paper in accordance with the referee and Associate Editor suggestions which can be found below (not including confidential reports to the Editor). Please note this decision does not guarantee eventual acceptance.
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Comments to Author:
Reviewers' Comments to Author: Reviewer: 1 Comments to the Author(s) See attached file.

Reviewer: 2
Comments to the Author(s) The authors present a theoretical analysis of the energy accumulation and dissipation of coal pillar in gob-side entry. The numerical simulation and case study are conducted to verify the rationality of the research method. I have a few major comments as follows: 1. In section 2.2, the author used two mathematical models, named "Weibull distribution law" and "damage theory of micro-statistics". What is the basis for this ? A theoretical model established needs to be based on a large number of experiments. The authors need to justify that these two models can truly describe the strain softening characteristics of coal samples. Otherwise, the results derived from such a non-consistent model are thus questionable. 2. What is "quasi-static deformation". Please explain it in detail. 3. How to determine the position of the points j and s in the coal pillar instability stage ( Figure 3)? 4. The authors need to justify the reasonability of the fold catastrophe model compared to other previous studies. 5. In line 35 of P11, what is the criterion for judging the coal seam's burst tendency? 6. Page 13. Correct: Figures 11 and 12. 7. In the numerical modeling, I didn't see any results about crack propagation. How does the author come to this conclusion that "Many cracks in coal seam are development, ……"? (Please see Wu et al., 2019. Rock Mechanics and Rock Engineering) 8. In figure 14, the energy density at the two ends of goaf reach to 1.96×1016 J. This value is so large. Is it reasonable? Please further analyze and discuss. 9. I didn't see the roadway driving along the goaf in the numerical model. Therefore, how does the author compare the theoretical results with the observation result (Line 14 to 16 of P15)? It is questionable. 10. The topic of paper is energy characteristics of coal pillar in gob-side entry. But, in the section of numerical simulation, the authors focuses on the analysis the stress evolution of coal seam mining rather than roadway driving along goaf. The numerical results need to be more closely related to the topic of the article.

Review form: Reviewer 2
Is the manuscript scientifically sound in its present form? Yes

Recommendation?
Accept as is

Comments to the Author(s)
The revised paper provides a better explanation of the energy characteristics of coal pillar in roadway driving along gob and answers the all questions. The revised manuscript has also improved the clarity of presentation and language. This reviewer recommends its acceptance for publication.

18-Jun-2019
Dear Dr Xue, I am pleased to inform you that your manuscript entitled "Destabilitation and energy characteristics of coal pillar in roadway driving along gob based on rockburst risk assessment" is now accepted for publication in Royal Society Open Science.
You can expect to receive a proof of your article in the near future. Please contact the editorial office (openscience_proofs@royalsociety.org and openscience@royalsociety.org) to let us know if you are likely to be away from e-mail contact. Due to rapid publication and an extremely tight schedule, if comments are not received, your paper may experience a delay in publication.
Royal Society Open Science operates under a continuous publication model (http://bit.ly/cpFAQ). Your article will be published straight into the next open issue and this will be the final version of the paper. As such, it can be cited immediately by other researchers. As the issue version of your paper will be the only version to be published I would advise you to check your proofs thoroughly as changes cannot be made once the paper is published. Comments to the Author(s) The revised paper provides a better explanation of the energy characteristics of coal pillar in roadway driving along gob and answers the all questions. The revised manuscript has also improved the clarity of presentation and language. This reviewer recommends its acceptance for publication.
This manuscript presents an investigation on the mechanism of rockbursts in the context of gob-side entry roadway driving, which is of great interest to coal mining engineers. It is an important topic of rock mechanics and rock engineering and very interesting for the readers of the journal. This article is very innovative and worth publishing. However, a concern with the manuscript is the explanation of results. The numerical analysis of the results should be further described. Therefore, I recommend publication of this manuscript after the minor revision. Specific comments and suggestions for the improvement of the manuscript are noted below.
1. Roadway driving along adjacent goafs is an effective method to develop the recovery rate of coal resources. The wide of coal pillar is important, the authors do not discuss. This part should be replenished in the revised version. 2. We usually describe the dispersion of rock materials by Weibull distribution, normal distribution or other distribution. In section 2.2, is it valid to use Weibull distribution to describe the F-u relationship of coal? The author should elaborate on this part. 3. In section 4.1, what are those parameters for? 4. In section 4.2, rock burst index is a very effective index to predict rock burst tendency. How are the dynamic damage time, elastic energy index and impact energy index defined and measured? 5. In section 4.4, the variation of stress abutment distance ahead of the working face (from 10 m at 50 m advance to 40 m at 100 m advance) should not be expected, in that the model which is homogeneous should yield similar volumes of failure zones around the working face. 6. The theory proposed is reasonable and innovative. Therefore, I recommend the use of theoretical models in numerical models and the authors can strengthen of the connection with the theoretical part. 7. Coal and gas outburst and rock burst are the most serious dynamic disasters in coal mining. In shallow mining, most of the dynamic disasters are single coal and gas outburst or rock burst, and the interaction and interaction are not significant. With the increase of mining depth, the interaction between the two kinds of disasters began to appear, resulting in coal and gas outburst, rock burst and two kinds of disasters coexisting and compounding each other. Both of them are unstable sudden dynamic disasters, and their triggering mechanism can be explained by energy theory. Whether the author tries to combine them? 8. In the process of mining, the effects of soft coal and hard coal seem to be very different, and their risk of rock burst is different. For the model of Fig. 2a, does the author consider the influence of strength on rock burst of coal body? 9. The mining stress path is a process in which the vertical stress increases and the horizontal stress decreases during the mining process, which is inconsistent with the conventional stress loading path. Therefore, can Figure 3 reflect the effect of mining process on rock stress and strain? I suggest that the author explain this matter more clearly in order to provide better guidance and help for mining.

Summary of amendments
Re: "Destabilitation and energy characteristics of coal pillar in roadway driving along gob based on rockburst risk assessment"

For Royal Society Open Science
Manuscript ID: RSOS-190094 Many thanks for reviewers and editor's constructive comments. These comments and suggestions have been carefully incorporated into the revised manuscript. The point-to-point replies for the comments are summarized as below:

Reviewer #1:
This manuscript presents an investigation on the mechanism of rockbursts in the context of gob-side entry roadway driving, which is of great interest to coal mining engineers. It is an important topic of rock mechanics and rock engineering and very interesting for the readers of the journal. This article is very innovative and worth publishing. However, a concern with the manuscript is the explanation of results. The numerical analysis of the results should be further described.
Therefore, I recommend publication of this manuscript after the minor revision.
Specific comments and suggestions for the improvement of the manuscript are noted below.
(1) Roadway driving along adjacent goafs is an effective method to develop the recovery rate of coal resources. The wide of coal pillar is important, the authors do not discuss. This part should be replenished in the revised version. have been done to clarify that "Weibull distribution law" and "damage theory of micro-statistics" are suitable for the dispersion and strain softening characteristics of of rock materials. On this basis, it is valid to use "Weibull distribution law" and "damage theory of micro-statistics" to describe the F-u relationship of coal, and the load-displacement relationship of coal mass could be expressed as where represents the initial stiffness factor, represents the maximum displacement, and represents the kin exponent. Reference: [1] Karcinovic D. (1982): Statistial aspects of the continuous damage theory.  9. The mining stress path is a process in which the vertical stress increases and the horizontal stress decreases during the mining process, which is inconsistent with the conventional stress loading path. Therefore, can Figure 3 reflect the effect of mining process on rock stress and strain? I suggest that the author explain this matter more clearly in order to provide better guidance and help for mining.
A: Thank you for this suggestion. In mining practice, the vertical stress increases and the horizontal stress decreases in the front of mining face, which is different from the conventional stress loading path. This stress state of coal has an important influence roadway roof coal pillar on the mechanical response of coal, more importantly, the magnitude and location of peak stress of abutment pressure should be researched systematically. In Figure 3, the change rule of vertical stress (especially the characteristic of peak stress of abutment pressure) can be reflected in the manuscript. On this basis, better guidance and help for mining, such as reasonable development layout and protecting seam mining technology, are adopted in engineering practice to reduce the high stress concentration in the coal seam and prevent high load from leading the coal pillar to the peak stress point. Water injection in coal seam is also employed to reduce the strain-softening stiffness factor of the coal pillar in the post-peak stage and improve the stiffness factor of a double-block system in rock burst prevention. This manuscript was revised with reference to this suggestion.  (7): The authors present a theoretical analysis of the energy accumulation and dissipation of coal pillar in gob-side entry. The numerical simulation and case study are conducted to verify the rationality of the research method. I have a few major comments as follows: 1. In section 2.2, the author used two mathematical models, named "Weibull distribution law" and "damage theory of micro-statistics". What is the basis for this? A theoretical model established needs to be based on a large number of experiments.
The authors need to justify that these two models can truly describe the strain softening characteristics of coal samples. Otherwise, the results derived from such a non-consistent model are thus questionable.
where represents the initial stiffness factor, represents the maximum displacement, and represents the kin exponent. Reference: [1] Karcinovic D (1982): Statistial aspects of the continuous damage theory.
[2]Pan Y; Wang ZQ (2004)   The fold catastrophe model is one of seven types of catastrophe mathematical model.
According to the fold catastrophe theory, the control variable a and state variable   A: Thank you for this suggestion. The burst tendency is an inherent property of coal, x  and the criterion for judging the coal seam's burst tendency is "classification and laboratory test method on bursting liability of coal (GB/T 25217.2-2010)". To be more specific, there are four indices of burst tendency (dynamic damage time, uniaxial compressive strength, elastic energy index and impact energy index), which can be obtained in mechanical test of bursting tendency of coal samples, and these all play an important role in evaluating rock burst hazards in coal seams. In this article, the dynamic damage time is 46 ms (strong), the elastic energy index is 6.79 (strong), the impact energy index is 5.14 (strong), and the uniaxial compressive strength is 14.0 MPa (strong), according to the bursting tendency classification of coal sample, shown in table 1. Therefore, the comprehensive evaluation results of burst tendency indicate that the coal samples show a strong impact tendency. This manuscript was revised with reference to this suggestion. A: Thank you for this suggestion. The conclusion that "Many cracks in coal seam are developed, ……" is obtained in mining practice, rather than in numerical simulation.

DT DT DT
This manuscript was revised with reference to this suggestion.
8. In figure 14, the energy density at the two ends of goaf reach to 1.96×10 16 J. This value is so large. Is it reasonable? Please further analyze and discuss.
A: Thank you for this suggestion. In this article, the unit of energy density in figure 14 is J/mm 3 , so the energy density at the two ends of goaf reaches to 1.96×10 16 J/mm 3 , namely 1.96×10 7 J/m 3 . This manuscript was revised with reference to this suggestion.
9. I didn't see the roadway driving along the goaf in the numerical model. Therefore, how does the author compare the theoretical results with the observation result (Line 14 to 16 of P15)? It is questionable.
A: Thank you for this suggestion. In this article, the numerical simulation focuses on the analysis the stress evolution of coal seam mining rather than roadway driving along goaf, and theoretical results is compared with the observation result in mining practice.
To be more precise, in mining practice, a rock burst induced by coal pillar in the gob-side entry, located in frontal abutment pressure region of No. 3306 working face, occurred in Xin'an coal mine. The earthquake observation station records that the rock burst intensity is equivalent to an earthquake of 2.3 Richter magnitude (in section 4.1). Meanwhile, in theoretical analysis, The intensity of this rock burst, in which the energy released is 6.4512×10 7 J, is equivalent to an earthquake of 2.1 Richter magnitude (in section 4.6). Therefore, the theoretical result is close to the observation result in mining practice. This manuscript was revised with reference to this suggestion.
10. The topic of paper is energy characteristics of coal pillar in gob-side entry. But, in the section of numerical simulation, the authors focuses on the analysis the stress evolution of coal seam mining rather than roadway driving along goaf. The numerical results need to be more closely related to the topic of the article.
A: Thank you for this suggestion. In mining practice, the width of coal pillar in gob-side entry is approximately 4 m to 10 m, and the length of working face is approximately 200 m to 250 m; thus, confined to size discrepancy, the stress evolution of coal seam mining is researched carefully by numerical simulation in this paper.
Besides, the better understanding of stress and energy evolution of coal seam mining is beneficial for researching energy characteristics of coal pillar in gob-side entry. The more closely research related to the topic of the article would be done in follow-on work.