Study on the Working Resistance of a Support under Shallowly Buried Gobs According to the Roof Structure during Periodic Weighting
Round 1
Reviewer 1 Report
The paper presents the problem of determination of powered roof support working resistance in panels located under shallowly buried gob (PSBG). The phenomenon of strong periodic weighting of support is explained in detail, depending on the structure formed by the surrounding rocks. The terms: equivalent main key stratum (EMKS), broken key stratum (BKS), broken main key stratum (BMKS) and broken inferior key stratum (BIKS) are defined.
It was distinguished 12 types of roof structure on PSBG depending on the breaking form of EMKS and BKS. The theory of determining the required working resistance of support is clearly presented for each type of roof structure considered. The theoretical studies are illustrated by two engineering examples of analysis of required working resistance
of support in panels with roof maintenance difficult and powered roof support failures.
The required working resistance of support was also analyzed using numerical simulations
of PSBG mining realized with FLAC2D software. Measures used to prevent roof collapse and damage to the support in the analysed panels are discussed.
Although the subject of the publication concerns a special type of workings, the presented method of determining the working resistance of support, due to detailed theoretical analyses and computer simulations, will be of broad interest to readers.
My attention was drawn to the attempt to correlate theoretical considerations of the BKS structure with the results of computer simulation of the operation in the 22,309 panel, presented as blue dashed lines in Figure 11. It is a pity that the figures were not commented on more widely. What is the Authors' view on the feasibility of numerical simulation of all 12 defined roof structure types on the PSBG?
The numerical simulation schemes analyzed in Sections 4.1.2 and 4.2.2 are only distinguished by the value of the yield load of the support. Therefore, I would like to suggest that in the titles of Figs: 12 (line 327), 13 (line 339), 14 (line 345), 19 (line 403), 20 (line 411) and 21 (line 416) to replace the term "simulation schemes" with " yield load of support ".
The discussion of the problem of horizontal displacement of the upper and lower wall (lines 329 - 337) requires more extensive explanation. Are you referring to the coal wall of panels in the #1-2 seam and the #2-2 seam? The distance between these seams, is 28.1 m (line 254), while Figure 13 shows that the maximum distance between monitoring points is about 7.5 m? In contrast, the mining height of the #22309 panel is 6.8 m (line 268). At which places in the numerical model were these monitoring points located? Similar considerations apply to the discussion of Figure 20.
Please verify that the description in Figure 17 is correct. The #1404 panel in the #4 seam is the research object, while the description of the figure indicates that the sample of the immediate roof of the #2 seam is being tested. The immediate roof of both seams is composed of sandy mudstone, so perhaps the description in Figure 17 is correct.
In Table 6, the Authors listed the basic parameters of the individual hydraulic props additionally installed in the #1404 panel. It would also be advisable to include in the article
a table containing the basic parameters of ZY18000/32/70D and ZY8500/20/42 powered roof supports installed in the panels under consideration.
Author Response
Dear Reviewer,
The authors greatly appreciate your kind suggestions and comments. As you pointed out, there are many problems that we did not consider or did not consider deeply enough. Therefore, your comments are essential to improving the quality of our manuscript. We have carefully revised the manuscript according to each of your requirements and suggestions. Additionally, we have provided a detailed description in the manuscript and the response report for the content that has not been completely modified according to your suggestions. We invited professional organizations to polish the manuscript to meet the publishing requirements. For convenience, we marked the revised part of the manuscript with the "Track Changes" function.
Hope to get your understanding and support. All authors thank you for your efforts to improve the quality of the manuscript.
The following explains are the response point-by-point.
Point 1. My attention was drawn to the attempt to correlate theoretical considerations of the BKS structure with the results of computer simulation of the operation in the 22309 panel, presented as blue dashed lines in Figure 11. It is a pity that the figures were not commented on more widely. What is the Authors' view on the feasibility of numerical simulation of all 12 defined roof structure types on the PSBG?
Response 1: I'm sorry that we didn't elaborate on this part in the previous manuscript. In fact, we simulate the breaking law of key stratum in overburden after coal seam is mined based on the gob compaction theory. The specific process is as follows: (1) The height of caving zone after coal seam is mined is calculated theoretically. (2) The excavation height of the model in the vertical direction is determined according to the thickness of the coal seam, the spacing between the coal seam and the key stratum, the height of the caving zone and the bulking factor. (3) The numerical model is excavated step by step, and the distance of each excavation is 5m. The constitutive model of caving strata in the gob is changed to double yield, and the mechanical parameters are determined according to Table 7. (4) The model after excavation is numerically calculated and the movement law of key stratum is observed. It is considered that the key stratum has been broken when the key stratum generates a penetrating shear-tensile fracture along the vertical direction. (5) Judge whether the breaking interval of key stratum in the model is consistent with the field monitoring data. If the two are inconsistent, the mechanical parameters of key stratum need to be adjusted. (6) The fracture morphology of key stratum in the overburden after mining of upper coal seam is obtained.
As you pointed out, FLAC2D is a finite element numerical simulation software, which has some limitations in simulating rock stratum movement. It can not truly simulate the 12 types of roof structures proposed in this paper. However, the above methods can still reflect the migration law of rock strata to a certain extent. In the future, we will continue to conduct in-depth research on the roof structure of shallow seams by using discrete element software. Hope to get your understanding and support.
Point 2. The numerical simulation schemes analyzed in Sections 4.1.2 and 4.2.2 are only distinguished by the value of the yield load of the support. Therefore, I would like to suggest that in the titles of Figs: 12 (line 327), 13 (line 339), 14 (line 345), 19 (line 403), 20 (line 411) and 21 (line 416) to replace the term "simulation schemes" with " yield load of support ".
Response 2: It’s a pity that we neglected this problem in the previous manuscript. We have modified the titles of the above figures according to your suggestions. (Pages 16, 17, 18, and 19)
Point 3. The discussion of the problem of horizontal displacement of the upper and lower wall (lines 329 - 337) requires more extensive explanation. Are you referring to the coal wall of panels in the #1-2 seam and the #2-2 seam? The distance between these seams, is 28.1 m (line 254), while Figure 13 shows that the maximum distance between monitoring points is about 7.5 m? In contrast, the mining height of the #22309 panel is 6.8 m (line 268). At which places in the numerical model were these monitoring points located? Similar considerations apply to the discussion of Figure 20.
Response 3: As you pointed out, it is easy for readers to be confused about Figures 13 and 20 in original manuscript. In fact, Figures 13 and 20 show the horizontal displacement curves of coal wall with different yield loads of the support in the #22309 and #1404 panels. In Figure 13, we arranged a measure line with a length of 7.5m along the floor of the #2-2 coal seam. And in Figure 20, we arranged a measure line with a length of 4 m along the floor of the #4 coal seam. Both measure lines are located on the surface of the coal wall. The mining heights of #22309 and #1404 panels are 6.8m and 3.3m respectively, so part of the two measure lines are in the roof. We have revised Figures 13 and 20 in the original manuscript to avoid confusion. (Pages 16 and 19)
Point 4. Please verify that the description in Figure 17 is correct. The #1404 panel in the #4 seam is the research object, while the description of the figure indicates that the sample of the immediate roof of the #2 seam is being tested. The immediate roof of both seams is composed of sandy mudstone, so perhaps the description in Figure 17 is correct.
Response 4: Thank you for your reminder. The relevant description in Figure 17 in the original manuscript is inappropriate due to our negligence. We have modified Figure 17 accordingly. (Page 14)
Point 5. In Table 6, the Authors listed the basic parameters of the individual hydraulic props additionally installed in the #1404 panel. It would also be advisable to include in the article a table containing the basic parameters of ZY18000/32/70D and ZY8500/20/42 powered roof supports installed in the panels under consideration.
Response 5: Thanks for your suggestion. We have added Tables 3 and 5 in Sections 4.1 and 4.2, respectively. Readers can understand the main technical parameters of the two types of supports through the above two tables. (Pages 12 and 13)
Once again, thank you very much for your comments and suggestions.
Best regards.
Yours sincerely,
Cheng Zhu
Guizhou University
Phone number: +86 17851143177.
Email address: [email protected]
Reviewer 2 Report
Dear Authors,
this article is of good scientific quality, relevant and practically oriented. In general, it deserves publishing after some corrections, wich look for me, however, as considerable ones. I am sure that the article will win if two notes are observed:
- There is a lack of Discussion Section, aimed at proving correctness of provisions given in the article, and enlightening the Authors' contribution to geomechanics.
- Most of the references point at the works of Chineese researchers, wich are no doubt make a significant contribution to mining science and geomechanics development. However, in this article the observation of the works of US, German, Russian, etc. researchers in geomechanics would give more comprehensive picture of the state-of-art.
I am sure, that the article will be fine after revising, and wish you good luck.
Author Response
Dear Reviewer,
The authors greatly appreciate your kind suggestions and comments. The comments are all valuable and helpful for revising and improving our paper. We have carefully revised the corresponding content in the manuscript according to your comments, and to facilitate your review of the revised content, we marked the revised part with the "Track Changes" function.
The point-by-point responses are as follows.
Point 1. There is a lack of Discussion Section, aimed at proving correctness of provisions given in the article, and enlightening the Authors' contribution to geomechanics.
Response 1: It’s a pity that we neglected this problem in the previous manuscript. We have added Discussion Section in Section 5.3 of the manuscript. The rationality and applicability of the method to determine the reasonable working resistance of the support based on the roof structure during periodic weighting is discussed. And some technical measures that can be taken to avoid support crushing and roof falling when the working resistance of the support is low in actual production are introduced. (Pages 19 and 20)
Point 2. Most of the references point at the works of Chinese researchers, wich are no doubt make a significant contribution to mining science and geomechanics development. However, in this article the observation of the works of US, German, Russian, etc. researchers in geomechanics would give more comprehensive picture of the state-of-art.
Response 2: All the authors agree with your suggestion. The Yushenfu Mining Area is one of the largest distribution areas of shallow seams in the world. Therefore, Chinese scholars have done a lot of research on the roof structure and rock pressure law of shallow seams. The most representative achievement is the key strata theory proposed by Academician Minggao Qian. In recent years, the key strata theory has been continuously developed and improved, and has been unanimously recognized by scholars at home and abroad. At present, scholars in other countries have relatively little research on this aspect.
The related research in this paper is mainly based on the key layer theory. Therefore, we refer to the research results of a large number of Chinese scholars. All authors sincerely hope to get your understanding and support.
Once again, thank you very much for your comments and suggestions.
Best regards.
Yours sincerely,
Cheng Zhu
Guizhou University
Phone number: +86 17851143177.
Email address: [email protected]
Round 2
Reviewer 2 Report
Dear Authors,
I wish you good luck!
