Deformation Laws of Coal Mining-Affected Slopes in Loess Gully Area

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Review:

Deformation Laws of Coal Mining-affected Slopes in Loess 2 Gully Area

 

The authors proposed the use of a numerical model to study the deformation behavior of mining-induced slopes in Loess gully area. The study is interesting and tried to understand the mechanical behaviour of Loess type soils at a relatively large scale.

The methodology is correct and clear. The numerical results were compared to field measurements for validation.

 

Remarks:

• The scale was not provided in all figures. The reader cannot read clearly and have an idea on the real size of the research area shown in the figures. Adding a scale in figure 1 for example will help a lot.
• The three-dimensional numerical model in figure 2 is fuzzy and unclear. I could not relate it exactly to the topography provided in figure 1.
• In line 129 is mentioned Fig.1a to refer to panels 501, 502 and 503 but what I can see is mainly the map of the province and the rest is very small to read. It is suggested to, either, separate the pictures and use bigger sizes for more clarity.
• The authors introduced the model in section 3, but I could not see any mention of the constitutive relations for the soil or rock as a material. Was the used constitutive model elastic? Elasto-plastic?
• The graphs shown in figure 3 are very difficult to read, the fonts are too small. The authors are asked to replace with bigger and clearer figures. The same remark holds for the rest of the figures in the manuscript. In figures 10, 11 and 12 for example the curves are unreadable.
• In lines 492-493 it would be better to cite the figures of interest in addition to the text. The same holds for the rest of the paragraph.  

Comments for author File: Comments.pdf

Author Response

Comment 1:The scale was not provided in all figures.The reader cannot read clearly and have an idea on the real size of the research area shown in the figures.Adding a scale in figure 1 for example will help a lot.

Response 1:We sincerely thank you for pointing out the absence of scales in our figures.You are absolutely correct that without a scale bar,readers cannot properly interpret the real dimensions of the research area or the spatial relationships shown in the figures.To address this concern,we have revised all relevant figures as follows:In Figure 1,2,3,4,we have now added a clear scale bar (in metres).

Comment 2:The three-dimensional numerical model in figure 2 is fuzzy and unclear.I could not relate it exactly to the topography provided in figure 1.

Response 2:We sincerely thank you for pointing out the poor quality of Figure 2 and the lack of clear correspondence between the numerical model and the topography shown in Figure 1.We fully agree that a reader should be able to visually link the model geometry to the actual terrain,and that a fuzzy or poorly labelled figure undermines the credibility of the study.To address this issue,we have thoroughly revised Figure 2 and its caption,as detailed below.

• improvement of image resolution and clarity

The original Figure 2 was generated at a low screen-capture resolution,which made the mesh and boundary features difficult to distinguish.We have now re-rendered the three-dimensional model using the maximum capability of our numerical software,with a resolution of 600 dpi.The new figure clearly shows the surface topography,the coal seam,the excavation panels,and the slope-face geometry with sharp.The font sizes for labels and axes have been increased to ensure readability.

(2)Enhancement of the topographic correspondence

To directly address your concern about relating Figure 2 to Figure 1, we have added the working face,slopes boundary mining direction above the 3D discrete element model.

(3)Revision of the figure caption
The new caption for Figure 2 now explicitly states:“3D discrete element model of mining-induced overburden and loess strata at working faces 50205 and 50206,Zhen'er Coal Mine”.  

Comment 3:In line 129 is mentioned Fig.1a to refer to panels 501,502 and 503 but what I can see is mainly the map of the province and the rest is very small to read.It is suggested to,either, separate the pictures and use bigger sizes for more clarity.

Response 3:We sincerely thank you for pointing out the legibility issue with Figure 1a.You are absolutely correct that the original composite figure presented the provincial location map at a large scale,while the detailed study area showing panels 501,502,and 503 was too small to read. This indeed made it difficult to locate the specific panels referred to in Line 129-130.To fully address your concern,we have completely redesigned Figure 1.

 

Comment 4:The authors introduced the model in section 3,but I could not see any mention of the constitutive relations for the soil or rock as a material.Was the used constitutive model elastic? Elasto-plastic?

Response 4:Thank you for pointing out this oversight.In our model,we indeed adopted the Mohr-Coulomb elasto-plastic constitutive model to describe the mechanical behavior of the soil and rock materials.In the original manuscript, we only briefly mentioned the model setup in Section 3 without explicitly stating the constitutive relations,which may have caused confusion.In the revised manuscript,we will add a new paragraph in Section 3 to clearly describe:

The adopted constitutive model (Mohr-Coulomb elasto-plastic);The yield criterion and flow rule (non-associated flow rule with a dilation angle of 5°);The key material parameters (density, bulk modulus,shear modulus,cohesion, friction angle, tensile strength, etc.),and we will present them in table for each lithological unit.

All additions will be highlighted in the revised version(line 159-162).We appreciate your constructive comment, which helps improve the clarity and reproducibility of our work.

 

Comment 5:The graphs shown in figure 3 are very difficult to read,the fonts are too small.The authors are asked to replace with bigger and clearer figures.The same remark holds for the rest of the figures in the manuscript.In figures 10,11 and 12 for example the curves are unreadable.

Response 5:Thank you for pointing out this issue.We fully agree that the figures in the original manuscript were not clear enough,and we apologise for the inconvenience this caused during the review process.In response to your comment,we have taken the following actions:

All figures (including Figure 3,Figures 10-12,and all others) have been re-plotted and replaced with high-resolution versions.

Font sizes for axis labels,legends,and annotations have been significantly increased to improve readability.To enhance the resolution, we have divided Figure 3 into Figures 3 and 4.

For figures with multiple curves (e.g.,Figures 10-12),we have used distinctive markers and colours to make each curve clearly distinguishable.The original Figures 10-12 has been renumbered as Figures 11 to 13 in the revised manuscript.All revised figures have been exported in jpg format (＞300dpi) to ensure sharpness when printed or zoomed in.

We believe the revised figures are now clear and easy to read.All changes have been made in the revised manuscript and are highlighted for your convenience.

We thank you again for your constructive suggestion,which has significantly improved the visual quality of our paper.

Comment 6:In lines 492-493 it would be better to cite the figures of interest in addition to the text. The same holds for the rest of the paragraph.  

Response 6:Thank you for your valuable suggestion.We fully agree that citing the relevant figures alongside the text will greatly improve the clarity and traceability of our results.In the revised manuscript,we added explicit citations to the appropriate figures (e.g.,B1 slopes cited Figure 14a and Figure 14b) at lines 492-493(orignal manuscript,in revised manuscipt,line 523-554),where we describe the key observations.We believe this modification will make the presentation more self-contained and reader-friendly.The changes will be highlighted in the revised version for your easy reference.Thank you again for your constructive feedback.

 

All revisions are highlighted in Red in the re-submitted manuscript for your easy identification.

We believe these modifications substantially improve the clarity and scientific rigor of our study,and we hope that this response adequately addresses your valuable concern.We are grateful for your suggestion,which has undoubtedly strengthened our paper.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The article addresses the important issue of assessing slope deformation in loess gully areas affected by underground coal mining. The work is of interest because it combines 3DEC numerical modeling with fracture analysis based on orthophotos and considers several slopes differing in shape, position relative to the working faces, and morphometric characteristics. At the same time, some methodological aspects and interpretations of the results require clarification.

1. The validation of the numerical model is predominantly qualitative. The authors compare the calculated zones of increased displacement with the distribution of cracks identified from orthophotos. However, they do not provide a quantitative comparison between calculated and measured displacements. To improve the reliability of the study, it would be desirable to supplement the validation with field monitoring data or, at least, with a quantitative assessment of the spatial correspondence between the calculated zones of increased displacement and the observed fracture distribution.
2. The article should more clearly define which specific quantity is treated as deformation. Judging from the figures and tables, the analysis mainly refers to displacement magnitude. However, for assessing slope stability and crack formation, vertical and horizontal displacement components, as well as tensile and shear strains, are also important. In its current form, it is not always clear how directly the calculated quantity used in the study is related to the mechanisms of slope failure.
3. The reported temporal evolution of deformation requires additional methodological clarification. The model uses stepwise removal of coal seam blocks with equilibrium reached at each stage. Therefore, the calculation is essentially quasi-static. At the same time, the article presents deformation rates in mm/day and draws conclusions about the presence of a time lag. In this regard, the authors should explain how the calculation steps are related to real calendar time and whether it is appropriate to estimate the rate of deformation development on this basis. In addition, the conclusion regarding the presence of a time lag generally appears plausible for the considered mining and geological conditions. However, as currently presented, it is not fully demonstrated. The delayed surface response may be related not only to the time factor itself, but also to the spatial position of the working face relative to a particular slope. To confirm the existence of a time lag, it would be desirable to use actual time-series monitoring data or to provide a more detailed justification of the relationship between longwall advance, calculation stages, and the dates used in the analysis.
4. The geometry of the numerical model requires clarification. The article states that the model dimensions are 580 × 460 × 102 m, whereas the average burial depth of the coal seam within the considered longwall panels is about 138 m. In this regard, it is necessary to explain in more detail how the actual overburden thickness up to the surface is reproduced given this model height, how the absolute elevations are defined, and how the load from the overlying strata is taken into account. In addition, it would be desirable to further justify the dimensions of the computational domain and the adopted 20 m boundary coal pillar. Given the longwall length of 540 m, width of 200 m, and mining depth of approximately 90–130 m, the influence of the lateral boundaries may be significant. Therefore, the authors should demonstrate that the selected dimensions of the computational domain do not distort the formation of the subsidence basin or the distribution of slope deformation.
5. The modeling scheme for the mined-out area is presented in a somewhat simplified manner. The removal of the coal seam using the excavate command does not make it clear how the behavior of caved rocks, their bulking, subsequent compaction, residual bearing capacity, and interaction with the roof were taken into account in the calculation. For longwall panels using full-caving roof management, these processes can substantially affect the magnitude and pattern of subsidence. The authors should clarify how the formation of the caving zone and the supporting effect of the caved rocks were considered in the model.
6. There is an inaccuracy in the text related to the slopes crossing the working faces. The characteristics table indicates that slopes B1 and B5 cross the working faces, whereas in the discussion B2 is mentioned as an example of such a slope. This should be corrected, since this error may lead to an incorrect interpretation of the factors affecting deformation development.
7. The conclusions should be formulated more specifically and less generally. For example, the statement that coal mining increases slope deformation is rather self-evident. It would be more useful to formulate quantitative conclusions, including the ranges of calculated displacements, the characteristic locations of maximum deformation zones, the conditions under which the most intensive deformations occur, and the limitations of the applicability of the obtained results.

 

Author Response

Comment 1:The validation of the numerical model is predominantly qualitative.The authors compare the calculated zones of increased displacement with the distribution of cracks identified from orthophotos.However,they do not provide a quantitative comparison between calculated and measured displacements.To improve the reliability of the study,it would be desirable to supplement the validation with field monitoring data or,at least,with a quantitative assessment of the spatial correspondence between the calculated zones of increased displacement and the observed fracture distribution.

Response 1:We sincerely thank you for your insightful and constructive comment regarding the validation of our numerical model.You rightly pointed out that the current validation is predominantly qualitative,as we compared the calculated displacement zones with crack distributions from orthophotos without providing a quantitative comparison or field monitoring data.We fully agree that a more rigorous validation would significantly strengthen the reliability of our study.In response to this concern,we have taken the following steps in our revised manuscript.Due to the lack of on-site monitoring data and the steep terrain,which made field surveys extremely difficult,UAV survey method was employed for verification.We are sorry to have to mention that we have not performed a quantitative spatial correspondence analysis between the calculated high-displacement zones and the mapped fracture network,because the Quantitative data are lacking.However,we have redesigned Figure 13 by combining the final numerical simulation results and close-up photos into a single composite figure,so that readers can compare them more easily.Which supports the reliability of our simulation despite the lack of in-situ displacement time series.

Comment 2:The article should more clearly define which specific quantity is treated as deformation.Judging from the figures and tables,the analysis mainly refers to displacement magnitude.However,for assessing slope stability and crack formation,vertical and horizontal displacement components,as well as tensile and shear strains, are also important.In its current form, it is not always clear how directly the calculated quantity used in the study is related to the mechanisms of slope failure.

Response 2:We thank you for this important comment.We agree that the definition of ‘deformation’ and its direct link to slope failure mechanisms must be clarified.In our original manuscript,the term ‘deformation’ was used loosely to refer to the total displacement magnitude.To address your concern,we have made the following revisions:

(1)we have explicitly clarified that the primary ‘deformation’ quantity analyzed in this study is the total displacement magnitude. This clarification has been added in Section 4 (Line 212-214).

(2)More importantly,following your suggestion,we have strengthened the link between our calculated quantities and the slope failure mechanisms.We now also present and discuss the tensile and shear strain distributions in section 5.3(line 688-734) and add figure 15 to clarify this point.

Comment 3:The reported temporal evolution of deformation requires additional methodological clarification.The model uses stepwise removal of coal seam blocks with equilibrium reached at each stage.Therefore,the calculation is essentially quasi-static.At the same time, the article presents deformation rates in mm/day and draws conclusions about the presence of a time lag. In this regard,the authors should explain how the calculation steps are related to real calendar time and whether it is appropriate to estimate the rate of deformation development on this basis.In addition,the conclusion regarding the presence of a time lag generally appears plausible for the considered mining and geological conditions.However,as currently presented,it is not fully demonstrated.The delayed surface response may be related not only to the time factor itself,but also to the spatial position of the working face relative to a particular slope.To confirm the existence of a time lag,it would be desirable to use actual time-series monitoring data or to provide a more detailed justification of the relationship between longwall advance,calculation stages,and the dates used in the analysis.

Response 3:We thank you for raising this important methodological point.You are correct that our numerical scheme is quasi-static,with equilibrium reached after each stepwise removal of coal blocks,and that we subsequently report deformation rates in mm/day and discuss a time lag.We fully understand your concerns about the mapping between calculation steps and real calendar time,as well as the need to rigorously demonstrate the time-lag phenomenon.In the revised manuscript,we have clearly explained how the quasi-static calculation steps are linked to real calendar time.Specifically,each 10 m excavation step corresponds to a realistic mining duration based on the average daily advance rate of the longwall face (approximately 3 m/day in the 50206/50205working face of Zhener coalfield), resulting in a step duration of about 3.3days.The equilibrium state achieved at each step is regarded as representing the quasi‑static deformation at the corresponding calendar date.On this basis,the deformation rates reported in mm/day are estimated by dividing the incremental displacement between successive steps by the step duration. We have justified this approach by referencing widely adopted practices in mining subsidence numerical modeling and by demonstrating that the resulting time-settlement curves are consistent with typical longwall subsidence profiles.This clarification has been added in Section 4.2 (Lines 382-390).Regarding the existence of a time lag,we agree with the reviewer that our original demonstration was insufficient and that spatial position may contribute to the delayed surface response.To address this,we have substantially strengthened the argument in the revised manuscript.We have strengthened the analysis within the Discussion section 5.1 .Specifically,we now discuss in detail how the temporal evolution of deformation at the monitoring points,as shown by the calculated displacement-time curves in Section 4.2,reveals a persistent time lag even after accounting for the spatial offset between the working face and the slope.We attribute this lag mainly to the time-dependent compaction of caved rocks and stress redistribution.We also compare our findings with published time-settlement observations from similar geological and mining conditions, which supports the plausibility of the observed lag.

Comment 4:The geometry of the numerical model requires clarification. The article states that the model dimensions are 580×460×102m,whereas the average burial depth of the coal seam within the considered longwall panels is about 138 m.In this regard,it is necessary to explain in more detail how the actual overburden thickness up to the surface is reproduced given this model height, how the absolute elevations are defined, and how the load from the overlying strata is taken into account.In addition,it would be desirable to further justify the dimensions of the computational domain and the adopted 20 m boundary coal pillar.Given the longwall length of 540 m,width of 200 m,and mining depth of approximately 90-130m,the influence of the lateral boundaries may be significant.Therefore,the authors should demonstrate that the selected dimensions of the computational domain do not distort the formation of the subsidence basin or the distribution of slope deformation.

Response 4:We sincerely thank you for your thorough and valuable comment regarding the geometric setup of our numerical model.You have rightly pointed out that the model height (102 m) appears to be less than the reported average burial depth (138 m),and that the dimensions of the computational domain,together with the 20 m boundary pillar,need justification to rule out boundary effects.In response,we have carefully revised the manuscript and performed additional checks.we modified as ‘The horizontally layered model dimensions are 580 m×460 m×100 m.The loess layer model varies in height with the surface topography,but its width and length are consistent with those of the underlying horizontally layered model’(section3 line153-155).The fish programme in 3DEC is shown as the following screenshot.

 

Comment 5:The modeling scheme for the mined-out area is presented in a somewhat simplified manner.The removal of the coal seam using the excavate command does not make it clear how the behavior of caved rocks,their bulking,subsequent compaction,residual bearing capacity,and interaction with the roof were taken into account in the calculation.For longwall panels using full-caving roof management,these processes can substantially affect the magnitude and pattern of subsidence.The authors should clarify how the formation of the caving zone and the supporting effect of the caved rocks were considered in the model.

Response 5:We sincerely thank you for this critical and practically important comment.You are absolutely correct that the ‘excavate’ command in its simplest form implies a complete removal of the coal with no residual support,which does not explicitly capture the bulking,compaction, residual bearing capacity,and roof interaction of caved rocks in a full-caving longwall panel.We fully acknowledge that our original description was overly simplified.We apologize for the lack of clarity.In the revised manuscript,we have substantially clarified how the behavior of caved rocks and their supporting effect are simulated.Specifically,after the coal seam is excavated using the “excavate” command,the resulting void is filled with a caved rock material that obeys a double-yield constitutive model (or a strain-hardening model),which can capture the processes of bulking,compaction,and the gradual increase of residual bearing capacity.The bulking factor, compaction modulus,and friction angle are calibrated against empirical data for caved zones in longwall panels.The interaction between the caved rocks and the roof is simulated through the automatic contact and load transfer;as the roof converges,the caved material compacts and provides increasing support,which directly affects the overlying strata deformation and surface subsidence.We have added a detailed description of this modeling approach,together with relevant parameter tables and a verification example,in Section 3.3.3 (Lines 204-213).

 

Comment 6:There is an inaccuracy in the text related to the slopes crossing the working faces. The characteristics table indicates that slopes B1 and B5 cross the working faces,whereas in the discussion B2 is mentioned as an example of such a slope.This should be corrected,since this error may lead to an incorrect interpretation of the factors affecting deformation development.

Response 6:We sincerely thank you for carefully reading our manuscript and for pointing out this factual inconsistency.You are absolutely correct: the characteristic table (Table 1) lists slopes B1 and B5 as those crossing the working faces,whereas in the Discussion section we incorrectly cited B2 as an example.We apologise for this oversight,which could indeed mislead readers regarding the factors controlling deformation development.To address this error,we have thoroughly revised the manuscript in the Discussion section(line 662).

 

Comment 7:The conclusions should be formulated more specifically and less generally.For example,the statement that coal mining increases slope deformation is rather self-evident.It would be more useful to formulate quantitative conclusions, including the ranges of calculated displacements,the characteristic locations of maximum deformation zones,the conditions under which the most intensive deformations occur,and the limitations of the applicability of the obtained results.

Response 7:We sincerely thank you for this constructive and practical suggestion. You are absolutely right that our original conclusions were too general and that the statement ‘coal mining increases slope deformation’ is indeed self-evident and does not provide meaningful scientific insight. In response,we have thoroughly revised the Conclusions section to make it more specific, quantitative,and practically useful.

 

 

All revisions are highlighted in Red in the re-submitted manuscript for your easy identification.

We believe these modifications substantially improve the clarity and scientific rigor of our study,and we hope that this response adequately addresses your valuable concern.We are grateful for your suggestion,which has undoubtedly strengthened our paper.

 

Yours sincerely

Shiyue Fang

College of Geology and Environment,Xi’an University of Science and Technology

fangshiyue@xust.edu.cn

 

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors answered all the pending questions submitted by the reviewer.