Migration of Water and Sand Inrush through the Mining-Induced Caving Zone: Insights from Model Test and Numerical Analysis

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript investigating the transport process of sand particles through mining-induced caving zones under different conditions by using the laboratory test and numerical simulation. The results are helpful for understanding the mechanism and behavior of water and sand mixture inrush hazards. Overall, the manuscript highlights a valuable topic, well written, without major grammar and typing errors, and the subject matter is more relevant to the journal. I suggest it can be published after minor revisions if the following comments can be well considered: 

1. The introduction must be informative for the readers, concise and to the point with focus on the novelty of the work.

2. In general, clay is playing the role in the water and sand mixture inrush hazards, why did the authors use only sand. In addition, please provide more details on the ways for selecting grain size.

3. The authors have used the packing material instead of the real broken rock mass. What if the caving zone is not completely porous sediments. there may be a big difference in the porosity and permeability properties. Please clarify this in the manuscript.

4. The porosity is a very important factor influencing the sand flow speed in the PFC model. How did the authors consider this factor?

5. In the paper, the author mentioned that the decrease in sand flow speed is due to the continuous accumulation of sand particles. As the number of sand particles increases, will there be clogging phenomenon that affects the sand flow speed?

6. The authors should add the discussion, and the discussion must be strengthened by citing more relevant literature. and the results obtained should be compared with those of previous studies.

7. Whether the experimental results or simulation results have been combined with the actual project, and what is the effect?

8. The clarity of Fig. 1 is too low, please replace a figure with a higher clarity.

9. In section 4.2, when introducing the model parameters, the parameter verification method and verification results are not reflected, and whether the parameters taken are accurate.

10. In section 4.3, Table 4 is not found “The value in Table 4 shows that as the height of the caving zone increases, the speed of sand flow decreases…”.

 

11. In section 4.3, the numerical simulation results in which the sand particle size are 0.5 mm,1-2 mm, and 2 mm are not introduced, and this part of the results is compared with the laboratory test results in section 3.2 because the simulation parameter verification results are not given in the previous article, it is impossible to determine whether the simulation results can replace the experimental results and whether this comparison method is reasonable

Comments on the Quality of English Language

Minor editing of English language required

Author Response

Responses to Reviewer#1

The manuscript investigating the transport process of sand particles through mining-induced caving zones under different conditions by using the laboratory test and numerical simulation. The results are helpful for understanding the mechanism and behavior of water and sand mixture inrush hazards. Overall, the manuscript highlights a valuable topic, well written, without major grammar and typing errors, and the subject matter is more relevant to the journal. I suggest it can be published after minor revisions if the following comments can be well considered:

1. The introduction must be informative for the readers, concise and to the point with focus on the novelty of the work.

Response: Thanks for the valuable comments. We have rewritten the introduction as suggested by the reviewer. And the novelty of the work has been added in the introduction. All the changes are marked on red in the revised manuscript.

2. In general, clay is playing the role in the water and sand mixture inrush hazards, why did the authors use only sand. In addition, please provide more details on the ways for selecting grain size.

Response: As the reviewer’s point that the clay is playing the role in the sand water mixture inrush. However, researches show that the possibility of water and sand mixture inrush weakens as the clay content in unconsolidated layers increases. The particle size of aeolian sand in the western China coal mines usually ranges from 0.075-2 mm, which is considered a fine to medium grained sand with minor clay content. Moreover, the clay content in the sand grains of unconsolidated layers from coal mines in eastern China, where sand and water inrush is a common geohazard, does not exceed 10% by weight of the total sand grains. To reduce uncertainties that can considerably complicate the analysis of the experimental data, the influence of clay contents was not considered in the current study. We have mentioned this problem in the section 2.2, and we will consider this problem in the future. All the changes are marked in red in the revised manuscript.

In the revised manuscript, we have now added more details to explain the reasons why we select the grain size at Section 2.2. All the changes are marked in the revised manuscript.

3. The authors have used the packing material instead of the real broken rock mass. What if the caving zone is not completely porous sediments. there may be a big difference in the porosity and permeability properties. Please clarify this in the manuscript.

Response: The data show that the porosity of the caving zone is up to 0.2-0.45. In order to reduce the difference in porosity between the ideal fractured rock mass and the real fractured rock mass, the porosity of the porous medium in this study is approximately 0.4-0.46. In addition, compared with the above sand layer, the permeability for the rock mass and glass beads are quite high, which would cause large volumes of water and sand flow.

In the revised manuscript, we have now added more details in the section 4.4 to explain the different between the real rock mass with glass beads. All the changes are marked in the revised manuscript.

4. The porosity is a very important factor influencing the sand flow speed in the PFC model. How did the authors consider this factor?

Response: We agree with the reviewer’s statement on the porosity. The research of Zhang et al. (2020) shows that the porosity of the caving zone is up to 0.45. The porosity of the porous medium in this study is approximately 0.4. Therefore, the porosity used in this simulation is about 0.15.

5. In the paper, the author mentioned that the decrease in sand flow speed is due to the continuous accumulation of sand particles. As the number of sand particles increases, will there be clogging phenomenon that affects the sand flow speed?

Response: Clogging is important to prevent the sand water mixture flow inrush. The deposition and accumulation of fine sand particles can block the voids and pores in the caving zone, and further help to prevent the sand water mixture inrush. For example, there was a surface subsidence on the ground surface, while water and sand inrush did not occur in the panels (Zhang and Hou 2005). It indicated a clogging of sand above or in the caving zone. The occurrence of clogging phenomenon depends mainly on the size of the sand particles and porous media pores. When the sand particles are small, they can easily migrate through a porous medium. When the sand particles are large, the probability of sand particles being captured significantly increases as the number of sand particles increases. Previous studies shows that when the fine particles will be deposited in the porous medium when the ratio of glass beads to sand particles by diameter is less than 15 (Jung et al. 2018; Kerimov et al. 2018; Racha Medjda et al. 2020). In this study, the diameter ratio of glass beads to sand particles ranged from 14 to 83, and no clogging phenomenon was observed.

6. The authors should add the discussion, and the discussion must be strengthened by citing more relevant literature. and the results obtained should be compared with those of previous studies.

Response: We have added the section 4.4 Discussions as suggested by the reviewer.

7. Whether the experimental results or simulation results have been combined with the actual project, and what is the effect?

Response: The experimental work is not intended to simulate any specific project. The findings from this research would provide additional insight into the treatment of water and sand mixture inrush in mining, we have added the application in the section 4.4 Discussion. All the changes are marked on red in the revised manuscript.

8. The clarity of Fig. 1 is too low, please replace a figure with a higher clarity.

Response: Figure 1 have been replaced with a higher clarity figure.

9. In section 4.2, when introducing the model parameters, the parameter verification method and verification results are not reflected, and whether the parameters taken are accurate.

Response: In the revised manuscript, we have now added more details in the section 4.2 to explain how to determine microscopic parameter parameters, and how to verify the simulation the model. All the changes are marked in the revised manuscript.

10. In section 4.3, Table 4 is not found “The value in Table 4 shows that as the height of the caving zone increases, the speed of sand flow decreases…”.

Response: The error has been changed. Table 4 have been changed to Table 3.

11. In section 4.3, the numerical simulation results in which the sand particle size are 0.5 mm,1-2 mm, and 2 mm are not introduced, and this part of the results is compared with the laboratory test results in section 3.2 because the simulation parameter verification results are not given in the previous article, it is impossible to determine whether the simulation results can replace the experimental results and whether this comparison method is reasonable.

Response: In numerical simulations, microscopic parameters cannot be directly obtained from macro laboratory experiments, as simulation parameters are often based on idealized or simplified assumptions. Therefore, we adjusted the parameters by comparing the simulation results with experimental data through the trial and error method to ensure the reliability of the simulation results. By referring to the microscopic parameter of Li et al. (2011), we determined the parameters by constant adjustment. Table 3 in revised manuscript summarizes a comparison of sand flow speed between an experiment and the corresponding numerical simulation. The calculated value of sand flow speed using PFC agrees well with the experimental value, which implies the appropriateness of microscopic parameter.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

 

1.     Please ensure consistency between the text references to “Fig. 1” and the figure title “Figure 1”, and apply the same modification to all subsequent figures. Please maintain uniformity in the presentation of figures throughout the document.

 

2.     In the introduction, the authors mention that “more than 90% of China’s coal mining areas are covered by loose aquifers of the Neogene and Quaternary systems, which formed a unique binary geological structure of loose layers and bedrock”, and provide relevant case studies and research by Chinese scholars on this phenomenon. It would be beneficial to address whether similar issues occur in coal mines in other countries worldwide. Is the significance of this research limited to China, or can it be more broadly applied on a global scale? I recommend that the authors include necessary additions in the introduction to highlight the broader impact and applicability of this study.

 

3.     Please address the grammatical issues in the sentence located on lines 170-171: “1) Installation of t experimental setup, Fix the model box on the bracket to ensure that the device is stable and vertical.”

 

4.     Please ensure the correct expression of units, particularly the proper use of superscripts.

 

5.     On lines 203-204, please revise the sentence “where the spheres have a diameter of the is 21 mm and the sand particles range in size from 0.1 to 0.5 mm” for clarity.

 

6.     The authors reference “Figures 3a and 3b” on line 212; however, I only found Figure 3 and could not locate Figures 3a and 3b in the manuscript. Please confirm whether there are missing figures or if there is a discrepancy in the text.Perhaps the authors intended to refer to a comparison between Figure 4a and Figure 4b?

 

7.     In Section 3.1, the authors demonstrate how the caving zone height affects sand flow speed and porosity variation through experimental results. However, the conclusion that “the speed of sand flow mainly depends on the pore characteristics of the caving zone” is drawn solely based on the observation that “the trend in porosity variation with respect to the height of the caving zone is basically consistent with the trend observed in the sand flow speed changing with the height of the caving zone”. This reasoning lacks logical rigor. While the trends in porosity and sand flow speed may be similar, this does not directly prove a causal relationship between the two. Besides pore characteristics, other factors also significantly influence sand flow speed. For example, when sand flows through a higher caving zone, it encounters more broken rock fragments, which create substantial physical obstacles and make the flow path more complex, thus impeding the flow and reducing the speed. Additionally, friction within the sand flow increases with distance. As sand particles flow from a higher elevation, the mutual friction among particles intensifies, further slowing down the flow. Only by controlling these factors can a more accurate assessment of the impact of porosity on sand flow speed be achieved. It is recommended that the authors revise this section to include necessary explanations and ensure the logical rigor of the study.

 

 

8.     On lines 250-251, the statement “In Fig. 6, yellow particles represent sand particles, green particles represent broken rock mass, and sand particle sizes are set to 1 mm and 5 mm, respectively” raises several questions

 

(a) Are the sizes of 1 mm and 5 mm referring to the radius or the diameter of the sand particles?

(b)  Does “green particles represent broken rock mass” refer to the green spheres with a diameter of 25 mm shown in the image? If so, it would be clearer to specify that the green spheres represent the broken rock mass directly when describing the sphere sizes in lines 247-249. If this is not the case, please revise the description to ensure that the model is clearly understood by readers.

(c)   Does “sand particle sizes are set to 1 mm and 5 mm” mean that the model on the left side of Figure 6 uses 1 mm sand particles and the model on the right uses 5 mm sand particles, or does it mean that both models have simulations using both 1 mm and 5 mm sand particles? The current phrasing is ambiguous; please clarify the meaning of these sizes to facilitate reader comprehension.

(d)  The authors mention that “sand particle sizes are set to 1 mm and 5 mm” here. Is this for comparing the impact of different particle sizes? In the subsequent results analysis, I did not find any results related to sand particles of 5 mm. I only found the comparison of particle sizes of 1 mm and 0.75 mm in Figure 8. If I have overlooked this section, could the authors please indicate its specific location? If this content is indeed absent, please include the relevant results.

 

 

9. On line 316, the reference to “Fig. 8 shows the force chain…” may need correction. Should this be "Figure 9" instead? Please verify and make the necessary adjustments to ensure accurate referencing

 

 

 

Comments on the Quality of English Language

The manuscript contains multiple grammatical issues that affect readability and comprehension. It is recommended that these be carefully revised to ensure the accuracy of the expressions.

Author Response

Responses to Reviewer#2

1. Please ensure consistency between the text references to “Fig. 1” and the figure title “Figure 1”, and apply the same modification to all subsequent figures. Please maintain uniformity in the presentation of figures throughout the document.

Response: “Fig.” have been changed to “Figure ” in the revised manuscript.

2. In the introduction, the authors mention that “more than 90% of China’s coal mining areas are covered by loose aquifers of the Neogene and Quaternary systems, which formed a unique binary geological structure of loose layers and bedrock”, and provide relevant case studies and research by Chinese scholars on this phenomenon. It would be beneficial to address whether similar issues occur in coal mines in other countries worldwide. Is the significance of this research limited to China, or can it be more broadly applied on a global scale? I recommend that the authors include necessary additions in the introduction to highlight the broader impact and applicability of this study.

Response: In the revised manuscript, this sentence has been deleted. The occurrence of water and sand inrush geohazards in coal mining is not frequently reported in other countries. However, water and sand inrush encountered during tunneling has been studied. We have added some relevant researches conducted by scholars from other countries on water and sand inrush, and have emphasized the novelty of our work in the introduction. Additionally, we have added Section 4.4 to discuss the applicability of this study.

3. Please address the grammatical issues in the sentence located on lines 170-171: “1) Installation of t experimental setup, Fix the model box on the bracket to ensure that the device is stable and vertical.”

Response: thanks. We have corrected this sentence in the revised manuscript. This sentence was changed to “Installation of the experimental setup. Firstly, fix the model box onto the bracket to guarantee that the device remains stable and vertically aligned.”

4. Please ensure the correct expression of units, particularly the proper use of superscripts.

Response: We much appreciate the reviewer’s careful review. In the manuscript, we double checked the expression of units and corrected these errors.

5. On lines 203-204, please revise the sentence “where the spheres have a diameter of the is 21 mm and the sand particles range in size from 0.1 to 0.5 mm” for clarity.

Response: This sentence has been deleted in revised manuscript. The previous sentence have been change to “Figure 4a is a schematic diagram of the relationship between the speed of sand flow and the height of the caving zone for (D, d) = (21 mm, 0.1–0.5 mm).” In the revised manuscript, "D" was used to represent the diameter of glass beads and "d" was used to represent the particle size of sand grains. We have added this explanation in the section 2.2.

6. The authors reference “Figures 3a and 3b” on line 212; however, I only found Figure 3 and could not locate Figures 3a and 3b in the manuscript. Please confirm whether there are missing figures or if there is a discrepancy in the text. Perhaps the authors intended to refer to a comparison between Figure 4a and Figure 4b?

Response: We thank the reviewer for pointing out this error. Table 4 have been changed to Table 3.

7. In Section 3.1, the authors demonstrate how the caving zone height affects sand flow speed and porosity variation through experimental results. However, the conclusion that “the speed of sand flow mainly depends on the pore characteristics of the caving zone” is drawn solely based on the observation that “the trend in porosity variation with respect to the height of the caving zone is basically consistent with the trend observed in the sand flow speed changing with the height of the caving zone”. This reasoning lacks logical rigor. While the trends in porosity and sand flow speed may be similar, this does not directly prove a causal relationship between the two. Besides pore characteristics, other factors also significantly influence sand flow speed. For example, when sand flows through a higher caving zone, it encounters more broken rock fragments, which create substantial physical obstacles and make the flow path more complex, thus impeding the flow and reducing the speed. Additionally, friction within the sand flow increases with distance. As sand particles flow from a higher elevation, the mutual friction among particles intensifies, further slowing down the flow. Only by controlling these factors can a more accurate assessment of the impact of porosity on sand flow speed be achieved. It is recommended that the authors revise this section to include necessary explanations and ensure the logical rigor of the study.

Response: We appreciate the reviewer's insightful comments and helpful suggestions. We have deleted the statement regarding the influence of porosity on sand flow speed from Section 3.1. As the reviewer points out, there are many factors that significantly influence sand flow speed, and we have added Section 4.4 to discuss the influence of other factors on the sand flow speed.

The results in the study reveal that the sand flow speed decreases as the height of the caving zone increases for the same particles size. That maybe because of the major difference in pores geometry and distribution between the ideal and real caving zone. The pores in the idealized caving zone, regarded as a homogeneous porous medium, are generally continuous, while the actual caving zone, which is full of fragmented rock mass, contains a lot of disconnected pores and dead-end pores. The geometry of dead-end pores resembles stagnant pockets, which favors the deposition and accumulation of sand particles in these zones (Biswas and Kartha, 2019). In addition, the disconnected pores can re-strain the movement of sand particles, and consequently, sand particles only migrate through well-interconnected pores, which is likely to lead to a preferential flow path-way. However, the uniform pore distribution resulting from the single size of the glass bead prevents the formation of significantly preferential flow pathways. Furthermore, the absence of an immobile domain within the rock mass is likely to diminish the sand flow speed during water and sand inrush. The water and sand inrush usually contains large volumes of sand with a high solid content, which can be regarded as dense rapid flow. According to Hong et al. (2022), the increase in friction and viscosity in dense flows directly leads to a decrease in mobility, causing pore blockages that further restrict the movement. Furthermore, research conducted by Fjaestad and Tomac (2019) has demonstrated that the mutual interactions between particles have a substantial impact on the flow and transport of particles within dense flows. However, the mechanism of this behavior remains unidentified, which will be the focus of our future research.

 

8.On lines 250-251, the statement “In Fig. 6, yellow particles represent sand particles, green particles represent broken rock mass, and sand particle sizes are set to 1 mm and 5 mm, respectively” raises several questions

(a) Are the sizes of 1 mm and 5 mm referring to the radius or the diameter of the sand particles?

(b) Does “green particles represent broken rock mass” refer to the green spheres with a diameter of 25 mm shown in the image? If so, it would be clearer to specify that the green spheres represent the broken rock mass directly when describing the sphere sizes in lines 247-249. If this is not the case, please revise the description to ensure that the model is clearly understood by readers.

(c) Does “sand particle sizes are set to 1 mm and 5 mm” mean that the model on the left side of Figure 6 uses 1 mm sand particles and the model on the right uses 5 mm sand particles, or does it mean that both models have simulations using both 1 mm and 5 mm sand particles? The current phrasing is ambiguous; please clarify the meaning of these sizes to facilitate reader comprehension.

(d) The authors mention that “sand particle sizes are set to 1 mm and 5 mm” here. Is this for comparing the impact of different particle sizes? In the subsequent results analysis, I did not find any results related to sand particles of 5 mm. I only found the comparison of particle sizes of 1 mm and 0.75 mm in Figure 8. If I have overlooked this section, could the authors please indicate its specific location? If this content is indeed absent, please include the relevant results.

Response: In the revised manuscript, this sentence has been changed to “Figure 6 shows the generated numerical model, in which green spheres with a diameter of 25 mm represent glass beads in the caving zone, and the heights of the caving zone are 25 × 1 mm and 25 × 5 mm, respectively. The yellow particles in Figure 6 represent sand particles. In this study, the diameters of sand particle were selected as 2 mm, 1 mm, and 0.75 mm, respectively.”

 

9.On line 316, the reference to “Fig. 8 shows the force chain…” may need correction. Should this be "Figure 9" instead? Please verify and make the necessary adjustments to ensure accurate referencing.

Response: We thank the reviewer for pointing out this error. Fig.8 has been changed to Figure 9.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript has been improved, and it might be accepted in the present form.