Surface Subsidence Response to Safety Pillar Width Between Reactor Cavities in the Underground Gasification of Thin Coal Seams

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Dear Authors,

 

Thank you for submitting your manuscript titled "Surface subsidence responses on width of safety pillar between reactor cavities during underground gasification of thin coal seam" for consideration in Sustainability. Your study provides valuable insights into the optimization of safety pillar widths in underground coal gasification (UCG) and its implications for surface subsidence and rock stability. The use of finite element analysis in ANSYS software to model the stress-strain state and temperature distribution in surrounding rocks is commendable.

 

Major concerns:

 

1. While the numerical simulations are robust, the paper lacks experimental validation of the results. Incorporating field data or laboratory experiments would strengthen the findings and provide more confidence in the conclusions.
2. Although the paper mentions environmental hazards such as flooding and water pollution, it does not delve deeply into how these risks can be mitigated beyond optimizing pillar width. A more detailed discussion on environmental risk management strategies would be beneficial.
3. The paper assumes that the thermal effects on rock properties are negligible beyond a certain distance from the reactor. While this simplification may be reasonable for the scope of the study, it could be further justified with additional references or sensitivity analysis.
4. The study focuses on a specific coal seam thickness (1.05-1.35 m) and depth (392-465 m). While this is useful for the given conditions, the findings may not be directly applicable to other geological settings. Expanding the study to include a range of coal seam thicknesses and depths would enhance its generalizability.
5. The paper does not compare its findings with previous studies or alternative methods for pillar design. A comparative analysis would help contextualize the results and highlight the novelty of the approach.
6. A sensitivity analysis could be conducted to assess the impact of varying thermal and mechanical properties on the results. This would help identify which parameters have the most significant influence on surface subsidence and pillar stability.
7. It is widely-recognized that the rock discontinuities (rock joints) play an important role in the rock mass stability. Therefore, it is necessary to consider the influence of the rock discontinuities with different scale during the numerical simulation. In your discussion of rock stability and the characterization of rock mass behavior, I would like to suggest that you consider citing the following article, which could provide additional context and support for your findings:"Rock discontinuities characterization from large-scale point clouds using a point-based deep learning method". This article explores advanced methods for characterizing rock discontinuities, which could be relevant to your discussion of rock mass behavior and the stability of overburden rocks during UCG. The use of deep learning techniques for rock characterization aligns well with the modern approaches to geomechanical modeling and could enhance the methodological robustness of your study. Including this reference could strengthen your manuscript by demonstrating awareness of cutting-edge techniques in rock mass characterization and their potential applications in UCG-related research.

Minor concerns:

1. Figure 1: The caption is incomplete: "(a) strata histogram; (b) layout of gasification panel" lacks context (e.g., units for depth in Figure 1a, and the location of strata histogram in Figure 1a).The resolution of the strata histogram (Figure 1a) is low, making text (e.g., "Quaternary sediments") difficult to read. Color coding or hatching for different rock types would improve clarity.
2. Figure 4:The legend for vertical displacement (mm) is missing in subfigures (a, b, c). Readers must infer values from the text description. The term "prestress stage" (Step 1) is not clearly defined in the caption or text, which may confuse non-specialists.
3. Figure 5:The x-axis label ("Distance, m") is too generic. Specify whether it refers to horizontal distance from the panel center. Error bars or confidence intervals for monitoring data would strengthen the validation claim.
4. Figure 7:The caption does not specify the units for displacement (assumed to be mm, but not stated). The vertical exaggeration (1:20 scale) is mentioned but not visually annotated, which could mislead readers.
5. Figure 11:The temperature scale (0–1050°C in Figure 11a) lacks tick marks, making precise interpretation difficult. Figure 11b (stress distribution) uses a color gradient but does not explicitly state whether values are compressive or tensile.
6. Figure 15:The y-axis label ("Minimum principal stress, MPa") should clarify that negative values indicate compression. The legend for pillar widths (3.75m, 8.5m, etc.) is missing; readers must cross-reference Table 2.
7. Table 1:Units for parameters like G12, G23, and G31 are listed as "GPa" but formatted inconsistently (e.g., "0.59" vs. "2.95" without units in the table body). The term "Quaternary sediments" lacks a footnote explaining its composition or relevance to the model.
8. Table 3 :The "Goaf" row lists "rubble pyrometamorphic rock" but lacks parameters for cohesion and friction angle, which are essential for numerical stability. The "Thermal conductivity" column mixes units (W/m/K vs. W/m·K), causing inconsistency.
9. Table 4:The column "Linear" (likely thermal expansion coefficient) has unclear units (e.g., "1.6 10 s" should be 1.6×10−5/°C). Missing footnotes to explain abbreviations (e.g., "CP" for specific heat capacity).
10. Grammar and Language Issues, Example:Example: "The lack of significant oil and gas reserves and substantial coal reserves determine the great potential of underground gasification." Correction: "The lack of significant oil and gas reserves, combined with substantial coal reserves, determines the great potential of underground gasification."
11. Inconsistent Terminology:"CRIP method" is sometimes written as "Controlled Retracting Injection Points" and other times as "Controlled Retraction Injection Point." Standardize to one term (e.g., "Controlled Retraction Injection Point").
12. Missing Articles:Example: *"Coal’s share of total global energy generation remained around 36% [6]." Correction: "Coal’s share of the total global energy generation remained around 36% [6]."
13. Punctuation Errors:Example: *"However, in the case of safety pillars destruction there is a high risk of crack evolution in overburden rock." Correction: "However, in the case of the safety pillars' destruction, there is a high risk..." 
14. Passive Voice Overuse:Example: *"The numerical experiment algorithm applied in the work can be used to optimize the pillar’s width..." Improvement: "The algorithm applied in this study enables optimization of pillar width..." (Active voice enhances clarity).
15. Redundancy:Example: *"The final calibration of the model was performed by comparing the results of the numerical analysis of the subsidence in pessimistic scenario with the results of monitoring the subsidence above the longwall." Simplification: "The model was calibrated by comparing simulated subsidence in the pessimistic scenario with longwall monitoring data."

 

Thank you for considering this suggestion. I look forward to seeing the revised version of your manuscript.

Author Response

1. Summary

Thank you very much for taking the time to review this manuscript. We appreciate the constructive suggestions, which have been valuable and helpful for revising and improving our paper. We have seriously considered all the questions/comments from Reviewers #1, and revised the manuscript as possible as we can, in which the new analysis is highlighted in the text. The part modified according to the reviewer 's Comments is highlighted in red.

Comments1. While the numerical simulations are robust, the paper lacks experimental validation of the results. Incorporating field data or laboratory experiments would strengthen the findings and provide more confidence in the conclusions.

Response 1. Thank you for your suggestion. I agree with the importance of experimental validation of the simulation results. The results of the surface subsidence calculation in the model were compared with the results of subsidence monitoring over the southern longwall of L1 seam of Kotlyarevska mine in a pessimistic scenario (please look at figure 5). As for the experimental validation of the results, unfortunately there is no possibility of such confirmation in the current circumstances. The article is aimed at increasing sustainability after the end of the war in Ukraine. The authors are interested in field monitoring and will conduct such studies at the earliest opportunity.

Comments2. Although the paper mentions environmental hazards such as flooding and water pollution, it does not delve deeply into how these risks can be mitigated beyond optimizing pillar width. A more detailed discussion on environmental risk management strategies would be beneficial.

Response 2. Thank you very much for this important suggestion. The discussion on environmental risk management strategies was added in introduction section and highlighted in red.

« Today, there are three main strategies for preventing environmental hazards over goaf: optimizing pillar sizes, filling the goaf, and overburden rock grouting. Goaf filling requires a significant complication in the organization of work and is not implemented today in underground gasification. The grouting reinforcement construction is only used as a backup (in case of an accident) plan due to its time consumption and high cost. The simplest and cheapest way is to optimize pillar sizes. With optimal pillar sizes, two important conditions are achieved. First, overburden rock retains its bearing capacity and the fracture zone does not reach the aquifers and the surface. This avoids the negative environmental consequences of underground gasification and avoids the risks of destruction for surface infrastructure and groundwater inflow. Second, coal losses in pillars are minimal, and gasification efficiency is high. Therefore, this study adopted the strategy of optimizing pillar width.»

Comments3. The paper assumes that the thermal effects on rock properties are negligible beyond a certain distance from the reactor. While this simplification may be reasonable for the scope of the study, it could be further justified with additional references or sensitivity analysis

Response 3. Thank you for pointing this out. The revised content was added in section 4.1, and highlighted in red. “The heat-affected zone was located near the reactor cavity in the surrounding rocks, as established by previous studies of Otto and Kempka [55], Wang et al. [59], Xin et al. [60], Sarhosis et al. [61], and also evident from the temperature distribution in the model in Figure 11a.”

Comments4. The study focuses on a specific coal seam thickness (1.05-1.35 m) and depth (392-465 m). While this is useful for the given conditions, the findings may not be directly applicable to other geological settings. Expanding the study to include a range of coal seam thicknesses and depths would enhance its generalizability

Response 4. Thank you for your suggestion. Your comment is very important to us. In this study we focused on a coal seam thickness of 1.05-1.35 m and depth of 392-465 m. However, the results of the study, according to the authors, are not limited to these conditions. The 3 main results of the study (conclusion section):

- Surface subsidence and rock movement above gasification cavity are in the pre-peak limits in the case of saving safety pillar’s bearing capacity.

- The significant influence of the gasification process on the stability of the surrounding rocks around previously extracted cavities has not been established, since the size of heat-affected zone of the UCG reactor is less than the thickness of the coal seam. At the gasification stage there is a high risk of coal seam failure in side walls of the UCG reactor.

-To avoid the hazard of overburden rock and surface destructions, caused by irreversible deformations, with minimal coal losses, the width of the pillars should be optimized.

Coal seam's thickness and depth have no impact on these conclusions. As for the pillar’s width, the numerical experiment algorithm, described in the paper, can be used to optimize it in any mining and geological conditions. But first of all, of course, for thin coal seams, which is indicated in the title of the paper.

Comments5. The paper does not compare its findings with previous studies or alternative methods for pillar design. A comparative analysis would help contextualize the results and highlight the novelty of the approach.

Response 5. Thank you for pointing this out. The revised content was added in section 4.2.2, and highlighted in red.

«The stability of pillars between UCG reactors for thin coal seam conditions has not been studied earlier, which highlights the novelty of the approach. However, the most closely related studies consider:

- surface subsidence at a constant pillar width of 15 m with a seam thickness of 15 m by Jiang et al. [32];

- stability evaluation method of gasification coal pillar at a constant pillar width of 16 m  with a seam thickness of 5.0 m by Tang et al. [66];

- the influence of the the wall curvature (different arch depth ratios) on the reduction of the useful pillar's width by Xu et al. [33], Li et al. [34].»

 Comments6. A sensitivity analysis could be conducted to assess the impact of varying thermal and mechanical properties on the results. This would help identify which parameters have the most significant influence on surface subsidence and pillar stability.

Response 6. Thank you for your suggestion. Your comment is very important to us. In this study, we did not set the task of conducting sensitivity analysis. Thermal and mechanical properties were adopted in accordance with the lithology. In further studies, such analysis will be carried out.

Comments7. It is widely-recognized that the rock discontinuities (rock joints) play an important role in the rock mass stability. Therefore, it is necessary to consider the influence of the rock discontinuities with different scale during the numerical simulation. In your discussion of rock stability and the characterization of rock mass behavior, I would like to suggest that you consider citing the following article, which could provide additional context and support for your findings:"Rock discontinuities characterization from large-scale point clouds using a point-based deep learning method". This article explores advanced methods for characterizing rock discontinuities, which could be relevant to your discussion of rock mass behavior and the stability of overburden rocks during UCG. The use of deep learning techniques for rock characterization aligns well with the modern approaches to geomechanical modeling and could enhance the methodological robustness of your study. Including this reference could strengthen your manuscript by demonstrating awareness of cutting-edge techniques in rock mass characterization and their potential applications in UCG-related research.

Response 7. Thank you for your suggestion. I agree with your comment. The revised content was added in section 4.2.2, and highlighted in red.

The reference to the article that you have kindly recommended was added to literature.

“It is widely-recognized that the rock discontinuities (rock joints) play an important role in the rock mass stability and affect the behavior of rock masses. At the same time, understanding the characteristics of rock discontinuities plays a huge role in predicting stability [67]. However, the influence of rock joints was not considered in this study."

Comments8. Figure 1: The caption is incomplete: "(a) strata histogram; (b) layout of gasification panel" lacks context (e.g., units for depth in Figure 1a, and the location of strata histogram in Figure 1a).The resolution of the strata histogram (Figure 1a) is low, making text (e.g., "Quaternary sediments") difficult to read. Color coding or hatching for different rock types would improve clarity.

Response 8. Thank you for your suggestion. Figure 1 was revised.

Comments9. Figure 4:The legend for vertical displacement (mm) is missing in subfigures (a, b, c). Readers must infer values from the text description. The term "prestress stage" (Step 1) is not clearly defined in the caption or text, which may confuse non-specialists.

Response 9. Thank you for your suggestion. The caption of Figure 4 was revised. Vertical displacement (m) evolution in sequence of simulation process by pessimistic scenario

The term "prestress stage" was added in section 3.1 and highlighted in red.

«Step 1 (prestress stage): Loading the model with gravity, and writing the values of stresses and displacements in all nodes of the model to a file using the “Inistate” script in Ansys Parametric Design Language.

Step 2 (initial stress-strain stage): Reading the recorded data from file and recalculating the model for zeroing displacement. In this case, a situation of the initial stress-strain state of the strata was obtained.

Step 3 (post-gasification stage): Analyse of the stress-strain state of model after coal gasification and formation of caved zones.»

Comments10. Figure 5:The x-axis label ("Distance, m") is too generic. Specify whether it refers to horizontal distance from the panel center. Error bars or confidence intervals for monitoring data would strengthen the validation claim.

Response 10. Thank you for your suggestion. Figure 5 was revised.

Comments11.Figure 7:The caption does not specify the units for displacement (assumed to be mm, but not stated). The vertical exaggeration (1:20 scale) is mentioned but not visually annotated, which could mislead readers.

Response 11. Thank you for your suggestion. The caption of Figure 7 was revised

Figure 7: Vertical displacement (m) distribution: (а) scenario #1; (b) scenario #2. Vertical displacement exaggeration 20:1.

Comments12.Figure 11:The temperature scale (0–1050°C in Figure 11a) lacks tick marks, making precise interpretation difficult. Figure 11b (stress distribution) uses a color gradient but does not explicitly state whether values are compressive or tensile.

Response 12. Thank you for your suggestion. The caption of Figure 11 was revised Figure 11: Temperature (°C) of rock mass (a) and minimum principal stress (Pa) (b) distributions.

The following text was added and highlighted in red: «Stresses with a minus sign are compressive».

Comments13. Figure 15:The y-axis label ("Minimum principal stress, MPa") should clarify that negative values indicate compression. The legend for pillar widths (3.75m, 8.5m, etc.) is missing; readers must cross-reference Table 2.

Response 13. Thank you for your suggestion. The caption of Figure 15 was revised Figure 15: Minimum principal stress (MPa) in coal pillar along the monitoring line E-E1. Negative stress values indicate compression.

The legend for different pillar widths (3.75m, 8.5m, etc.) in Figure 15 is the same for all pillar widths.

Comments14. Table 1:Units for parameters like G12, G23, and G31 are listed as "GPa" but formatted inconsistently (e.g., "0.59" vs. "2.95" without units in the table body). The term "Quaternary sediments" lacks a footnote explaining its composition or relevance to the model.

Response 14. Thank you for your suggestion. G12, G23, and G31 in the table are correct. An orthotropic model  with different properties across and along the bedding was used to simulate the behavior of rocks.

Line 211. Quaternary sediments are represented by clays and loams with a thickness of 10-20 to 50 m, distributed almost everywhere.

Comments15. Table 3 :The "Goaf" row lists "rubble pyrometamorphic rock" but lacks parameters for cohesion and friction angle, which are essential for numerical stability. The "Thermal conductivity" column mixes units (W/m/K vs. W/m·K), causing inconsistency.

Response 15. Thank you for pointing this out. The rubble pyrometamorphic rock was modelled as a compacted elastic material. Therefore, the cohesion and friction angle values are not specified in the table. Thermal conductivity unit was revised (?) (W·m⁻¹·K⁻¹)

Comments16. Table 4:The column "Linear" (likely thermal expansion coefficient) has unclear units (e. g., "1.6 10 s" should be 1.6×10−5/°C). Missing footnotes to explain abbreviations (e.g., "CP" for specific heat capacity).

Response 16. Agree. Thank you for pointing this out. Linear thermal expansion coefficient has unit (α) (K⁻¹). The values in table were revised: 1.6×10^-5, 1.0×10^-5, 5×10^-6.

Comments17. Grammar and Language Issues, Example:Example: "The lack of significant oil and gas reserves and substantial coal reserves determine the great potential of underground gasification." Correction: "The lack of significant oil and gas reserves, combined with substantial coal reserves, determines the great potential of underground gasification."

Response 17. Thank you for your suggestion. Agree. Revised.

Comments18. Inconsistent Terminology:"CRIP method" is sometimes written as "Controlled Retracting Injection Points" and other times as "Controlled Retraction Injection Point." Standardize to one term (e.g., "Controlled Retraction Injection Point").

Response 18. Thank you for your suggestion. Agree. Revised.

Comments19. Missing Articles:Example: *"Coal’s share of total global energy generation remained around 36% [6]." Correction: "Coal’s share of the total global energy generation remained around 36% [6]."

Response 19. Thank you for your suggestion. Agree. Revised.

Comments20. Punctuation Errors:Example: *"However, in the case of safety pillars destruction there is a high risk of crack evolution in overburden rock." Correction: "However, in the case of the safety pillars' destruction, there is a high risk..." 

Response 20. Thank you for your suggestion. Agree. Revised.

Comments21.Passive Voice Overuse:Example: *"The numerical experiment algorithm applied in the work can be used to optimize the pillar’s width..." Improvement: "The algorithm applied in this study enables optimization of pillar width..." (Active voice enhances clarity).

Response 21. Thank you for your suggestion. Agree. Revised.

Comments22.Redundancy:Example: *"The final calibration of the model was performed by comparing the results of the numerical analysis of the subsidence in pessimistic scenario with the results of monitoring the subsidence above the longwall." Simplification: "The model was calibrated by comparing simulated subsidence in the pessimistic scenario with longwall monitoring data."

Response 22. Thank you for your suggestion. Agree. Revised. The model was calibrated by comparing simulated subsidence in the pessimistic scenario with the monitoring data of the subsidence above the longwall.

Reviewer 2 Report

Comments and Suggestions for Authors

Surface subsidence responses on width of safety pillar between reactor cavities during underground signification of thin coal seam

sustainability-3503234-review

This manuscript investigated the underground gasification of thin coal seam with parallel CRIP method. The influence of pillar parameters on the stress-strain state, temperature distribution in surrounding rocks and surface subsidence have been discussed by using ANSYS. There are some major defects in the paper. My comments are as follows:

1. Avoid using the general terms without prior definition in the Abstract.
2. The Author mentioned Ukraine warmany times in the manuscript. Given that this is a scientific and technological paper, to ensure the objectivity and professionalism of the research, please avoid excessive involvement in political factors, and instead focus on in-depth exploration of technological issues.
3. The geological and engineering conditions serve as the foundation for numerical simulation. In the manuscript, detailed introduction to the thickness and lithological characteristics of the rock stratashould be provide in the Section 2, as well as the hydrogeological conditions of the mine. In addition, it is crucial to clarify whether there are aquifers, fault structures, and other geological factors within the mine, as these are the key foundations that determine the accuracy of numerical simulation.
4. Scenario #1, #2 and pessimistic scenarioare of great importancein this manuscript, but the author only shows these three scenarios in Figure.1 without providing specific descriptions. Why the author considered these three scenarios
5. The Parallel CRIP method is only shown in Figure 2. However, the author has not provided a detailed description of this method, nor explained the reasons for adopting it.
6. The selection of rock layer parameters is crucial in numerical simulation.what rock layers do group #1 and group #2 in Figure 3 (write Figure 1 in themanuscript) represent, respectively?
7. Authors mentioned in the page 7 that “The final calibration of the model was performed by comparing the results of the numerical analysis of the subsidence in pessimistic scenario with the results of monitoringthe subsidence above the longwall.”, What are the sources of these monitoring data? And how do you ensure the accuracy of the monitoring data?
8. To facilitate a better understanding for readers, the color scale in Figure 4, 9, 11,12 should have units.
9. Is there any actual data to support the numerical results? Some field measured data should be added to verify your numerical results.
10. Discussions should also include some comparison with some existing works done elsewhere. Have similar method and study performed before by other researchers? What are your new contributions?

 

Author Response

1. Summary

Thank you very much for taking the time to review this manuscript. We appreciate the constructive suggestions, which have been valuable and helpful for revising and improving our paper. We have seriously considered all the questions/comments from Reviewers #2, and revised the manuscript as possible as we can, in which the new analysis is highlighted in the text. The part modified according to the reviewer 's Comments is highlighted in red.

Comments1. Avoid using the general terms without prior definition in the Abstract.

Response 1. Thank you for your suggestion. Agree. Revised.

In the Abstract CRIP has been replaced by Controlled Retraction Injection Points.

Comments2.The Author mentioned Ukraine war many times in the manuscript. Given that this is a scientific and technological paper, to ensure the objectivity and professionalism of the research, please avoid excessive involvement in political factors, and instead focus on in-depth exploration of technological issues.

Response 2. Thank you for your suggestion. Revised. The number of mentions of the war in Ukraine has been decreased (in Line 46, Line 176, Line 178).

Comments3. The geological and engineering conditions serve as the foundation for numerical simulation. In the manuscript, detailed introduction to the thickness and lithological characteristics of the rock strata should be provide in the Section 2, as well as the hydrogeological conditions of the mine. In addition, it is crucial to clarify whether there are aquifers, fault structures, and other geological factors within the mine, as these are the key foundations that determine the accuracy of numerical simulation.

Response 3. Thank you for this important suggestion. The geological characteristics of the rock strata were added in section 2 and highlighted in red.

The hydrogeological conditions in the mine field are favorable. Water inflows into the working faces during operation did not exceed 3 cubic meters per hour. There is no danger of water breakthrough from aquifers. Safety pillars are located near large geological faults. Coal gasification in such areas is not recommended. Therefore, the article does not take into account the influence of fault structures and other geological factors on the subsidence.

The coal-bearing strata are represented mainly by layers of sandstone, sandy mudstones, mudstones of different thickness. The average uniaxial compressive strength of sandstones is 50-60 MPa, sandy mudstones – 35-40 MPa, mudstones – 18-28 MPa. Figure 1a shows the lithological column of Carboniferous rocks. For the numerical simulation in this study the lithological column was simplified. The rock strata were combined into groups. The groups differed in the proportion of sandstones in the considered strata. Even visually it is clear that in group 4 the proportion of sandstones is greater than in group 3. The inclusion criterion for the strata group was the average weighted compressive strength. If the strength of a rock lithotype differed by more than 20% from the average strength of the group, this lithotype was included in a new group. In this case, layers with a thickness of more than 1.0 m were taken into account. After that the grouping process was repeated.

Comments4. Scenario #1, #2 and pessimistic scenarioare of great importancein this manuscript, but the author only shows these three scenarios in Figure.1 without providing specific descriptions. Why the author considered these three scenarios

Response 4. Thank you for pointing this out. The revised content was added in section 2.1, and highlighted in red.

The article examines three UCG scenarios. In the scenario #1, the safety pillar width was 15 m, in the scenario #2 – 3.75 m (Figure 1b). In this way, the smallest and the largest values of the rational range of the pillar width were modeled [25-29]. In the scenario #1, the gasification panel had 7 cavities and 6 pillars (70% of coal extraction), in the scenario #2 – 9 cavities and 8 pillars (90% of coal extraction). Both of these scenarios assume stability saving of the coal pillar. Also, the pessimistic scenario with failure of safety pillars between reactor cavities was simulated. In this case, the surface subsidence responses to the accident was studied.

Comments5.The Parallel CRIP method is only shown in Figure 2. However, the author has not provided a detailed description of this method, nor explained the reasons for adopting it.

Response 5. Thank you for pointing this out. The description of CRIP method was added in section 2.2, and highlighted in red.

Linear CRIP involves a single injection well and a single production well. The produced gas is collected through the production well. Parallel CRIP involves multiple injection wells and multiple production wells. The injection wells are drilled parallel to the coal seam. The injection wells are then retracted at a controlled rate, and the coal is gasified as the combustion front moves upward [38]. The producer gas is collected through the production well (figure 2). Comparison of linear CRIP and parallel CRIP technologies by Seifi et al. [37] and Lozynskyi et al. [38] showed higher efficiency of synthesis gas produced by parallel CRIP. In addition, the Parallel CRIP method is characterized by more stable gas production levels and increased coal utilization rates. This corresponds to the concept of a sustainable and efficient world energy system.

Comments6. The selection of rock layer parameters is crucial in numerical simulation. what rock layers do group #1 and group #2 in Figure 3 (write Figure 1 in the manuscript) represent, respectively?

Response 6. Thank you for this important suggestion. The geological characteristics of the rock strata were added in section 2 and highlighted in red.

The procedure of layers grouping for numerical simulation has been described in more detail in section 2.1 and has been illustrated in Fig. 1a

Figure 1a shows the lithological column of Carboniferous rocks. For the numerical simulation in this study the lithological column was simplified. The rock strata were combined into groups. The groups differed in the proportion of sandstones in the considered strata. Even visually it is clear that in group 4 the proportion of sandstones is greater than in group 3. The inclusion criterion for the strata group was the average weighted compressive strength. If the strength of a rock lithotype differed by more than 20% from the average strength of the group, this lithotype was included in a new group. In this case, layers with a thickness of more than 1.0 m were taken into account.

Comments7. Authors mentioned in the page 7 that “The final calibration of the model was performed by comparing the results of the numerical analysis of the subsidence in pessimistic scenario with the results of monitoringthe subsidence above the longwall.”, What are the sources of these monitoring data? And how do you ensure the accuracy of the monitoring data?

Response 7. Thank you for this important suggestion. The explanation was added in page 10 and highlighted in red.

The basic regularities of the subsidence are sufficiently well-studied and they are the basis of relevant normative documents that take into account regional character. For the conditions of the Ukrainian Donbas, the parameters of the subsidence through are calculated in accordance with the DSTU 101.00159226.001-2003 “Rules of undermining Earth surface objects”. This makes it is possible to calculate subsidence outside fault zones and aquifers taking into account the error of measuring instruments with an accuracy of up to 5%.

Comments8. To facilitate a better understanding for readers, the color scale in Figure 4, 9, 11,12 should have units.

Response 8. Thank you for pointing this out. The captions of Fig. 4, 9, 11,12 were revised

Figure 4: Vertical displacement (m) evolution in sequence of simulation process by pessimistic scenario: (a) Step 1 (prestress stage); (b) Step 2 (initial stress-strain stage); (c) Step 3 (post-gasification stage).

Figure 9: Minimum principal stress (Pa) distribution: (а) scenario #1; (b) scenario #2. Negative stress values indicate compression.

Figure 11: Temperature (°C) of rock mass (a) and minimum principal stress (Pa) (b) distributions.

Figure 12: The distribution of rock mass temperature (°C) (a), minimum principal stress (Pa) (b), and minimum principal strain (c).

Comments9. Is there any actual data to support the numerical results? Some field measured data should be added to verify your numerical results.

Response 9. Thank you for your suggestion. I agree with the importance of experimental validation of the simulation results. But, unfortunately, there is no possibility of such confirmation in the current circumstances. The article is aimed at increasing sustainability after the end of the war in Ukraine (sorry, but this is not a politic, this is real terrible life). The authors are interested in field monitoring and will conduct such studies at the earliest opportunity.

Comments10. Discussions should also include some comparison with some existing works done elsewhere. Have similar method and study performed before by other researchers? What are your new contributions?

Response 10. Thank you for pointing this out. The revised content was added in section 4.2.2, and highlighted in red.

«The stability of pillars between UCG reactors for thin coal seam conditions has not been studied earlier, which highlights the novelty of the approach. However, the most closely related studies consider:

- surface subsidence at a constant pillar width of 15 m with a seam thickness of 15 m by Jiang et al. [32];

- stability evaluation method of gasification coal pillar at a constant pillar width of 16 m with a seam thickness of 5.0 m by Tang et al. [65];

- the influence of the wall curvature (different arch depth ratios) on the reduction of the useful pillar's width by Xu et al. [33], Li et al. [34]».

Reviewer 3 Report

Comments and Suggestions for Authors

In this paper, the influence of pillar parameters on surface subsidence, considering the non-rectangular shape of the pillar and the presence of voids above the UCG reactor in the immediate roof, was investigated using the finite element method in ANSYS software to optimize the safety pillar width to maintain rock stability and minimize coal losses. The Kotlyarevska mine, located in the southwestern part of the Pokrovsk region, Donbas, Ukraine, was selected as the target case.

The structure of the manuscript is well-organized, and the need for this investigation is clearly described, based on a review of the current problems and relevant literature on the topic. The methodologies are well explained, and the figures and tables presented are necessary and clearly explained. The references are relevant, up-to-date, and accessible.

In conclusion, the study captures key elements of underground coal gasification, such as surface subsidence, safety pillar width, reactor cavities, and underground gasification in thin coal seams. It makes valuable contributions to underground coal gasification technology, particularly in assessing geohazards such as the risk of ground surface subsidence and Overburden rock instability commonly associated with underground gasification. It is therefore recommended that the paper incorporate minor revisions.

Comments to the Authors:

1. Although the language of the manuscript is generally understandable, some improvements are necessary.
2. Modify the title to: "Surface Subsidence Response to Safety Pillar Width Between Reactor Cavities in Underground Gasification of Thin Coal Seams."
3. Figures should be numbered consistently. Figure 1 on line 236 should be referred to as Fig. 3.
4. The criteria for grouping the rock mass modeled in Fig. 3a were not clearly defined. What rock types were modeled in Fig. 3a? Were the rock masses representative of the Kotlyarevska mine?
5. Limitations and Future Research: The article should clearly address potential future research directions.

Comments on the Quality of English Language

Although the language of the manuscript is generally understandable, some improvements are necessary.

Author Response

1. Summary

Thank you very much for taking the time to review this manuscript. We appreciate the constructive suggestions, which have been valuable and helpful for revising and improving our paper. We have seriously considered all the questions/comments from Reviewers #3, and revised the manuscript as possible as we can, in which the new analysis is highlighted in the text. The part modified according to the reviewer 's Comments is highlighted in red.

Comments1. Although the language of the manuscript is generally understandable, some improvements are necessary.

Response 1. Thank you for pointing this out. The language of the article has been thoroughly improved. For example:

Grammar and Language Issues. Example: "The lack of significant oil and gas reserves and substantial coal reserves determine the great potential of underground gasification." Correction: "The lack of significant oil and gas reserves, combined with substantial coal reserves, determines the great potential of underground gasification."

Inconsistent Terminology:"CRIP method" is sometimes written as "Controlled Retracting Injection Points" and other times as "Controlled Retraction Injection Point." Standardize to one term -"Controlled Retraction Injection Point".

Missing Articles: Example: "Coal’s share of total global energy generation remained around 36% [6]." Correction: "Coal’s share of the total global energy generation remained around 36% [6]."

Punctuation Errors:Example: "However, in the case of safety pillars destruction there is a high risk of crack evolution in overburden rock." Correction: "However, in the case of the safety pillars' destruction, there is a high risk..." 

Passive Voice Overuse:Example: "The numerical experiment algorithm applied in the work can be used to optimize the pillar’s width..." Improvement: "The algorithm applied in this study enables optimization of pillar width..."

Redundancy:Example: "The final calibration of the model was performed by comparing the results of the numerical analysis of the subsidence in pessimistic scenario with the results of monitoring the subsidence above the longwall." Simplification: The model was calibrated by comparing simulated subsidence in the pessimistic scenario with the monitoring data of the subsidence above the longwall.

Comments2. Modify the title to: "Surface Subsidence Response to Safety Pillar Width Between Reactor Cavities in Underground Gasification of Thin Coal Seams."

Thank you for your suggestion. Agree. Revised.

Comments3. Figures should be numbered consistently. Figure 1 on line 236 should be referred to as Fig. 3.

Response 3. Thank you for pointing this out. Agree. Revised.

Comments4.The criteria for grouping the rock mass modeled in Fig. 3a were not clearly defined. What rock types were modeled in Fig. 3a? Were the rock masses representative of the Kotlyarevska mine?

Response 4. Thank you for this important suggestion. The procedure of layers grouping for numerical simulation has been described in more detail in section 2.1 and has been illustrated in Fig. 1a

Figure 1a shows the lithological column of Carboniferous rocks. For the numerical simulation in this study the lithological column was simplified. The rock strata were combined into groups. The groups differed in the proportion of sandstones in the considered strata. Even visually it is clear that in group 4 the proportion of sandstones is greater than in group 3. The inclusion criterion for the strata group was the average weighted compressive strength. If the strength of a rock lithotype differed by more than 20% from the average strength of the group, this lithotype was included in a new group. In this case, layers with a thickness of more than 1.0 m were taken into account.

Comments5. Limitations and Future Research: The article should clearly address potential future research directions.

Response 5. Thank you for this important suggestion. The Limitations and Future Research section was added in paper, and highlighted in red.

6. Limitations and Future Research.

There are a number of limitations in this study. Firstly, unfortunately, this study does not contain the experimental validation of the results, since there is no possibility of such confirmation in the current circumstances. The article is aimed at increasing sustainability after achieving a just and sustainable peace in Ukraine. Secondly, this study does not experimentally confirm the insignificant size (less than the coal seam thickness) of the heat-affected zone obtained by numerical simulation. The authors found confirmation in the studies of Otto and Kempka [55], Wang et al. [59], Xin et al. [60], Sarhosis et al. [61]. In the future, the authors plan to conduct a sensitivity analysis of thermal extent for the rocks of the coal-bearing strata of the Ukrainian Donbass in laboratory conditions. Thirdly, the authors used the results of studies by Zhang et. al [56], Wu et al [57], Sygała et al [58], Otto and Kempka [55] to determine the temperature-dependent properties of rock for the numerical model. Further research by the authors will be devoted to determining the influence of temperature on the mechanical properties of rocks of coal-bearing strata of the Ukrainian Donbass.

Reviewer 4 Report

Comments and Suggestions for Authors

Comments to Authors:

Thank you for submitting your work to Sustainability. I have the following comments and suggestions for improvement:

1. In Figure 4: The figure currently lacks a unit for the values presented. Please specify the appropriate unit in the caption or on the figure itself. Additionally, the meaning of positive and negative values is unclear—clarify what these represent. Lastly, define how pillar failure is identified in the simulation by including explicit criteria (e.g., stress thresholds, deformation limits) in the methods section.
2. Figure 1 requires more detail about the perspective shown. Indicate whether it depicts a cross-section, plan view, or another type of representation, and include a brief description in the caption for clarity.
3. Figure 9 is missing a unit.
4. In Figure 15, the y-axis label is ambiguous. Provide a clear description of what it represents.
5. The paper asserts that the heat-affected zone is small (less than the coal seam thickness) and does not significantly affect surrounding rock stability near previously extracted cavities. However, this claim relies heavily on simulation results without sufficient supporting field data or references. Strengthen this claim with additional simulation data (e.g., sensitivity analysis of thermal extent) or cite field observations from similar UCG projects. Discuss potential scenarios where thermal effects might become significant.
6. The author can add a little section discussing how pillar width affects operational outcomes, such as gasification efficiency, coal recovery rates, or risks to surface infrastructure and groundwater. Link findings to real-world UCG design considerations.
7. Given the high temperatures in UCG (up to 1000°C), neglecting temperature-dependent properties could overlook critical changes in rock behavior. Include a brief analysis or references specific to the Kotlyarevska mine’s rock types to support this assumption

 

Overall, addressing these points will improve the manuscript’s clarity, scientific rigor, and practical value. I look forward to reviewing the revised version. Thank you for your efforts.

Comments for author File: Comments.pdf

Author Response

1. Summary

Thank you very much for taking the time to review this manuscript. We appreciate the constructive suggestions, which have been valuable and helpful for revising and improving our paper. We have seriously considered all the questions/comments from Reviewers #4, and revised the manuscript as possible as we can, in which the new analysis is highlighted in the text. The part modified according to the reviewer 's Comments is highlighted in red.

Comments1. In Figure 4: The figure currently lacks a unit for the values presented. Please specify the appropriate unit in the caption or on the figure itself. Additionally, the meaning of positive and negative values is unclear—clarify what these represent. Lastly, define how pillar failure is identified in the simulation by including explicit criteria (e.g., stress thresholds, deformation limits) in the methods section.

Response 1. Thank you for your suggestion. The caption of Figure 4 was revised. The Legend of Figure 4 (minus sign) was revised.

The content “In the simulation, pillar failure is identified by the stress thresholds criteria” was added in page 19, and highlighted in red.

Comments2. Figure 1 requires more detail about the perspective shown. Indicate whether it depicts a cross-section, plan view, or another type of representation, and include a brief description in the caption for clarity.

Response 2. Thank you for pointing this out. Agree. Revised.

Comments3. Figure 9 is missing a unit.

Response 3. Thank you for your suggestion. The caption of Figure 9 was revised.

Comments4. In Figure 15, the y-axis label is ambiguous. Provide a clear description of what it represents.

Response 4. Thank you for your suggestion. The Figure 15 was revised. Stress scale was shifted.

Comments5. The paper asserts that the heat-affected zone is small (less than the coal seam thickness) and does not significantly affect surrounding rock stability near previously extracted cavities. However, this claim relies heavily on simulation results without sufficient supporting field data or references. Strengthen this claim with additional simulation data (e.g., sensitivity analysis of thermal extent) or cite field observations from similar UCG projects. Discuss potential scenarios where thermal effects might become significant.

Response 5. Thank you for pointing this out. The revised content was added in section 4.1, and highlighted in red. “The heat-affected zone was located near the reactor cavity in the surrounding rocks, as established by previous studies of Otto and Kempka [55], Wang et al. [59], Xin et al. [60], Sarhosis et al. [61]…”. This question has also been mentioned in the Limitations and Future Research section.

Comments6. The author can add a little section discussing how pillar width affects operational outcomes, such as gasification efficiency, coal recovery rates, or risks to surface infrastructure and groundwater. Link findings to real-world UCG design considerations.

Response 6. Thank you very much for this important suggestion. The discussion was added in introduction section and highlighted in red.

« Today, there are three main strategies for preventing environmental hazards over goaf: optimizing pillar sizes, filling the goaf, and overburden rock grouting. Goaf filling requires a significant complication in the organization of work and is not implemented today in underground gasification. The grouting reinforcement construction is only used as a backup (in case of an accident) plan due to its time consumption and high cost. The simplest and cheapest way is to optimize pillar sizes. With optimal pillar sizes, two important conditions are achieved. First, overburden rock retains its bearing capacity and the fracture zone does not reach the aquifers and the surface. This avoids the negative environmental consequences of underground gasification and avoids the risks of destruction for surface infrastructure and groundwater inflow. Second, coal losses in pillars are minimal, and gasification efficiency is high. Therefore, this study adopted the strategy of optimizing pillar width.»

Comments7. Given the high temperatures in UCG (up to 1000°C), neglecting temperature-dependent properties could overlook critical changes in rock behavior. Include a brief analysis or references specific to the Kotlyarevska mine’s rock types to support this assumption

Response 7. Thank you for your suggestion.

The authors want to emphasize that the properties of the roof and floor rocks in the heat-affected zone of the UCG reactor were calculated taking into account the studies [55, 56, 57, 58] (Table 4). And the properties of rocks beyond the affected zone were temperature-independent. Also, this issue has been mentioned in the Limitations and Future Research section.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

All of my concerns have been addressed by the authors.

Reviewer 2 Report

Comments and Suggestions for Authors

Accept in present form

Reviewer 4 Report

Comments and Suggestions for Authors

Thank you so much for figuring out all my comments. The revised manuscript has improved a lot and can be accepted for publication. Thanks.

Comments for author File: Comments.pdf