Process of overburden failure in steeply inclined multi-seam mining: insights from physical modelling

Ground surface damage caused by steeply inclined coal seam mining is widely distributed in China, but there is little research on the failure process and movement mechanism of strata induced by steeply inclined multi-seam mining. In this paper, a physical model test is carried out to study the failure process and movement mechanism of overburden in steeply inclined multi-seam stepwise mining. The results show that at the initial stage, the main failure of the rock mass is the small-scale collapse at the initial cut and the roof (stability stage of the rock mass). After the roof is exposed over a certain range, the rock mass in the downhill direction slips into the goaf and gradually destroys the interburdens of the goaf, similar to the displacement effect of dominoes (severe failure stage of the rock mass). When the structure of the goaf fails, the overburden subsides, causing extensive damage to the ground surface. The surface damage directly above the goaf is mainly caused by serious subsidence deformation, while the surface damage in the downhill direction is dominated by cracks.

3. Did the mining sequences in the physical model simulate the real mining sequence at the mine? 4. The coal seams thickness that you mentioned at the mine are 3.55m, 2.5m, 2.7m and 2.2m respectively, however, the coal seam thickness didn't look proportional in the physical model (# 1 and #2 apparently looks thicker than #3 and #4), please explain. 5. Please provide more explanation for the concept "Start-up vertical height of mining collapse". Since you have the measured numbers, please provide figures to show the measurement on the model for each seam. In addition, please explain how the "Start-up vertical height of mining collapse" will impact the real mining. 6. The paper mentioned "the displacement angle of the steeply inclined coal seam" in the physical model. Please explain what's the purpose to measure this angle and how this "angle" will impact the real mining. 7. This physical model is just one case with a specific geological setting (seam thickness, interburden, overburden and inclined angle etc.), what's the limitation of this model? Will different geological setting show different results, e.g. failure process and surface deformation, in the physical model and how?
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Dear Dr Qin
On behalf of the Editors, we are pleased to inform you that your Manuscript RSOS-210275 "Process of overburden failure in steeply inclined multi-seam mining: insights from physical modeling" has been accepted for publication in Royal Society Open Science subject to minor revision in accordance with the referees' reports. Please find the referees' comments along with any feedback from the Editors below my signature.
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Please note article processing charges apply to papers accepted for publication in Royal Society Open Science (https://royalsocietypublishing.org/rsos/charges). Charges will also apply to papers transferred to the journal from other Royal Society Publishing journals, as well as papers submitted as part of our collaboration with the Royal Society of Chemistry (https://royalsocietypublishing.org/rsos/chemistry). Fee waivers are available but must be requested when you submit your revision (https://royalsocietypublishing.org/rsos/waivers). Comments to the Author(s) I really enjoyed this paper. it is unusual nowadays to see a physical model study. However, they can provide a very clear depiction of the failure mechanics, as is the case in this study. My comments come under two heads. On the technical side, I think the paper would benefit from a more detailed discussion of the similitude analysis that was employed in to scale the model material properties. Also, it seems to me that Table 1 must be presenting the material properties that the author's were trying for. I can't believe that they were truly able to match the friction angle to one-tenth of a degree for example. Table 1 should also present the actual strengths and other properties that were actually achieved. Also, what was the thickness of the model (the thickness in the lateral direction, along the strike of the coal seams)? And what kind of material (if any) was used to constrain the model in the lateral direction?
My other comments have to do with the English translation. The translation is smooth and readily understandable for the most part. But some technical terms are unclear. for example, I think that the "open off cut" could be better translated as the "initial cut." A "small scope collapse" is probably a "small scale collapse," and the "direct roof" is the "immediate roof." The "start up vertical height of direct roof collapse" (see figure 3) is a very confusing term. I think you mean the vertical distance between the initial cut and face when the first collapse of the immediate roof occurs, but I am not sure. You should probably illustrate what you mean with a sketch. Lastly, the word "dehiscence" is a medical term that most English speakers have never heard of (I had to look it up). I think a better term might be "fracture zone." That term has the advantage that it fits with the well-known zones of overburden deformation above full extraction mining: the caving zone, the fracture zone, and the continuous deformation zone (see Prof. Syd Peng's work for example.) Reviewer: 2 Comments to the Author(s) 1. What's the ratio of the model (1:100 or something else), please explain? 2. Did the physical mode simulate real overburden at the mine? If you simulated the coal seams at "Balitang mine", please provide the actual overburden and the simulated overburden in the physical model. 3. Did the mining sequences in the physical model simulate the real mining sequence at the mine? 4. The coal seams thickness that you mentioned at the mine are 3.55m, 2.5m, 2.7m and 2.2m respectively, however, the coal seam thickness didn't look proportional in the physical model (# 1 and #2 apparently looks thicker than #3 and #4), please explain. 5. Please provide more explanation for the concept "Start-up vertical height of mining collapse". Since you have the measured numbers, please provide figures to show the measurement on the model for each seam. In addition, please explain how the "Start-up vertical height of mining collapse" will impact the real mining. 6. The paper mentioned "the displacement angle of the steeply inclined coal seam" in the physical model. Please explain what's the purpose to measure this angle and how this "angle" will impact the real mining. 7. This physical model is just one case with a specific geological setting (seam thickness, interburden, overburden and inclined angle etc.), what's the limitation of this model? Will different geological setting show different results, e.g. failure process and surface deformation, in the physical model and how? ===PREPARING YOUR MANUSCRIPT=== Your revised paper should include the changes requested by the referees and Editors of your manuscript. You should provide two versions of this manuscript and both versions must be provided in an editable format: one version identifying all the changes that have been made (for instance, in coloured highlight, in bold text, or tracked changes); a 'clean' version of the new manuscript that incorporates the changes made, but does not highlight them. This version will be used for typesetting. Please ensure that any equations included in the paper are editable text and not embedded images.
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See Appendix A.
Decision letter (RSOS-210275.R1) We hope you are keeping well at this difficult and unusual time. We continue to value your support of the journal in these challenging circumstances. If Royal Society Open Science can assist you at all, please don't hesitate to let us know at the email address below.
Dear Dr Qin, I am pleased to inform you that your manuscript entitled "Process of Overburden Failure in Steeply Inclined Multi-seam Mining: Insights from Physical Modeling" is now accepted for publication in Royal Society Open Science.
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On behalf of the Editors of Royal Society Open Science, thank you for your support of the journal and we look forward to your continued contributions to Royal Society Open Science. My other comments have to do with the English translation. The translation is smooth and readily understandable for the most part. But some technical terms are unclear. For example, I think that the "open off cut" could be better translated as the "initial cut." A "small scope collapse" is probably a "small scale collapse," and the "direct roof" is the "immediate roof."

Response:
Thanks for your careful review and suggestions.
(1) The "open-off cut" has been replaced with "initial cut" in full-text.
(2) The "small scope collapse" has been replaced with "small scale collapse" in Page 11.
(3) The "direct roof" has been replaced with "immediate roof" in full-text.

Comment # 3:
The "start up vertical height of direct roof collapse" (see figure 3) is a very confusing term. I think you mean the vertical distance between the initial cut and face when the first collapse of the immediate roof occurs, but I am not sure. You should probably illustrate what you mean with a sketch.

Response:
Thanks for your careful review. We have illustrated what we mean with a sketch and added it into Fig.3 as follow:

Comment # 4:
Lastly, the word "dehiscence" is a medical term that most English speakers have never heard of (I had to look it up). I think a better term might be "fracture zone." That term has the advantage that it fits with the well-known zones of overburden deformation above full extraction mining: the caving zone, the fracture zone, and the continuous deformation zone (see Prof. Syd Peng's work for example.)

Response:
Your suggestion is very responsible, and the authors are very appreciate.
However, we think the caving zone and the fracture zone are exclusive words used to describe deformation of overburden when a horizontal or gently inclined coal seam is being mined. In steeply inclined coal seam mining, these two exclusive terms may not fit well for the deformation characteristics are different with mining a horizontal or gently inclined coal seam.
We have redefined the dehiscence area as cracking area to avoid this misunderstanding and correspond with 'C'in Fig.5, as shown in Fig.6 and Page 15.

Comment # 1:
What's the ratio of the model (1:100 or something else), please explain?

Response:
The geometric similarity of the model is 250 which is stated in Page 4. Besides, in the revised manuscript, we have added detailed information of other similarity constants into Page 4.

Comment # 2:
Did the physical mode simulate real overburden at the mine? If you simulated the coal seams at "Balitang mine", please provide the actual overburden and the simulated overburden in the physical model.

Response:
Yes. According to the real geological model, we reduced it according to the similarity ratio and established the similarity model. Because this paper focuses on the process of rock failure in the process of model test, the real model is not introduced too much. Relevant lithology and parameter information are listed in Table 1 and figured in Fig.1.

Comment # 3:
Did the mining sequences in the physical model simulate the real mining sequence at the mine?

Response:
Yes. The mining sequences were introduced in Fig.1.
The coal seams thickness that you mentioned at the mine are 3.55m, 2.5m, 2.7m and 2.2m respectively, however, the coal seam thickness didn't look proportional in the physical model (# 1 and #2 apparently looks thicker than #3 and #4), please explain.

Response:
Thanks for your careful review. We have checked the figure of physical model carefully, and no mistake were found. Since the geometric similarity of the model is 250, the thickness difference between #1 and #4 coal seam is only about 5mm in physical model. We think this is maybe a visual error.

Comment # 5:
Please provide more explanation for the concept "Start-up vertical height of mining collapse". Since you have the measured numbers, please provide figures to show the measurement on the model for each seam. In addition, please explain how the "Start-up vertical height of mining collapse" will impact the real mining.

Response:
Thanks for your careful review. We have illustrated what we mean with a sketch and added it into Fig.3 as follow: Fig. 3. Start-up vertical height of immediate roof collapse in each coal seam And how the "Start-up vertical height of mining collapse" will impact the real mining, we explain as follows: It can be seen that in steeply inclined multi seam mining, the more backward the mining is, the larger the initial caving step distance of coal seam roof is. But in practice, the mechanical properties of rock mass should also be paid more attentions to prevent obvious mine pressure disasters.
The sentence have been added into revised manuscript in Page 12 with red words.