Numerical and Experimental Study on Pressure Relief Mechanism of Roof Blasting Along Gob-Side Roadway

Comments 1: The study considers one particular mining condition, please discuss how the results of the study can be applied to other mining conditions or industries beyond gob-side roadways.

Response 1:

Thank you for your constructive feedback. In response to your suggestion, we have expanded the manuscript to discuss the broader applications of the proposed blasting technique, particularly in the areas of open-cut inclined roof-breaking blasting and advanced presplitting blasting in the working face. These additions aim to enhance the understanding of how the techniques can be adapted to different mining conditions.

(1) Open-cut Inclined Roof-Breaking Blasting:

The roof-breaking blasting process is initiated in the open-hole area of the working face, under the condition of a hard roof. Initially, the goaf roof is supported on both sides: one side by solid coal and the other by front solid coal. As mining progresses, a suspended roof forms, but the hard roof is resistant to collapse under the dead weight stress. As the suspended roof area increases in size, it may suddenly collapse, with the high static load caused by mine earthquakes potentially exceeding the critical load, triggering a rock burst. To mitigate this risk, roof-breaking blasting is conducted in the inclined direction of the working face, causing the roof to collapse early in short-distance mining. This process effectively transforms the roof from a two-side support system to a one-side support system, thereby reducing both the static load on the coal body and the dynamic load resulting from the initial roof collapse.

(2) Advanced Presplitting Blasting in the Working Face:

When the first caving lag is formed in the working face, the goaf roof stratum enters the periodic caving stage. The rock strata in the goaf change from two side support to one side support, and the coal body in the working face produces stress concentration and energy concentration under the action of compression and clamping of the roof overhanging and bending. When the stress reaches the strength limit of the coal body, the coal body cracks and the energy is released. The vibration caused by the energy re-lease will make the overhanging bending roof vibrate and rebound, causing the roof to break, thus inducing rock burst. With the advance of the working surface, the area of the suspended roof increases gradually, and the tensile stress of the roof increases continuously. When the limit of the strength of the roof is reached, the hard roof will spontaneously break and collapse. At the moment when the hard roof is broken, the vibration compression of the hard roof produces a large dynamic load, and the impact load of the coal body at the working face is carried out at a high loading rate. The large vertical pressure and lateral driving force cause the coal body to break and expand rapidly, resulting in outward lateral movement. In order to reduce the influence of hard roof on the impact risk of the working face, pre-splitting blasting can be applied to the roof strata before the working face is mined.

These additions provide a more comprehensive view of how the roof-breaking blasting technique can be adapted to various mining scenarios. We believe that these expanded applications enhance the practical relevance of our study, demonstrating its potential to improve safety and operational efficiency in different types of mining environments.

Thank you again for your valuable comments, and we hope these revisions meet your expectations.

Comments 2: Add more clarity on the boundary conditions and assumptions in the numerical simulation to make the methodology reproducible. How the scaling ratios chosen the physical model affect the results?

Response 2:

Thank you for your valuable suggestions. In response to your comment, we have revised the manuscript to provide a more detailed explanation of the boundary conditions and assumptions used in the numerical simulation to enhance reproducibility.

We have explicitly described the boundary conditions applied in the UDEC numerical model. These details are now included in section 4.2 and marked in red of the revised manuscript.

Besides, we have added a more detailed discussion on the impact of the scaling ratios used in the physical model and how they influence the results. Specifically, we have analyzed how geometric, stress, and material property scaling factors affect the interpretation of the experimental outcomes. This discussion can now be found in section 5.1 and marked in red of the revised manuscript.

We believe these modifications improve the transparency and reproducibility of our methodology, addressing the reviewer’s concerns. Thank you again for your insightful feedback.

Comments 3: Discuss the limitations of the simulation results, such as how well they correlate with field data or potential variability due to geological differences. you might need to use comparative graphs to highlight changes in stress and displacement before and after blasting, making it easier for readers to visualize the findings.

Response 3:

Thank you for your insightful comment. We appreciate the suggestion to discuss the limitations of the simulation results and the potential variability due to geological differences.

We have expanded the discussion on the limitations of the simulation results in section 7.2 of the manuscript. Besides, we agree that comparative graphs highlighting changes in stress and displacement before and after blasting would be a useful tool for readers to visualize the findings. In response to your suggestion, In conjunction with Comments 5, we have optimized Figures 5 to 7. This will help illustrate the effectiveness of the roof blasting technique in real-world applications.

Thank you again for your constructive feedback. We are confident that these revisions improve the clarity of our results and the overall quality of the manuscript.

Comments 4: Elaborate on the cost-effectiveness and feasibility of implementing the proposed blasting technique in real-world scenarios.

Response 4:

Thank you for raising this important point. We agree that the cost-effectiveness and feasibility of implementing the proposed roof blasting technique in real-world mining operations are crucial considerations for its broader application. We have expanded the discussion on these aspects in the revised manuscript to provide a more detailed and specific analysis in section 7.3 of the manuscript.

Thank you again for your constructive feedback. We are confident that these revisions improve the clarity of our results and the overall quality of the manuscript.

Comments 5: Improve the readability of the diagrams (e.g., Figures 5–7) by labeling critical stress zones or deformation areas directly on the plots.

Response 5:

Thank you for your valuable suggestion regarding the need to modify the figures. In response to your comment, we have revised the relevant figures to better illustrate the changes in stress and displacement before and after blasting, as well as to enhance the overall clarity and visual representation of the results.

Specifically, the revised figures clearly describe the stress and displacement variations at different points along the roadway. These adjustments were made to improve the visualization of the effects of blasting on stress distribution and to make the differences before and after blasting more easily understandable.

We hope these changes address your concern and enhance the presentation of the data. The revised figures can be found in (Figure 5-7) of the updated manuscript. Thank you again for your constructive feedback. We are confident that these revisions improve the clarity of our results and the overall quality of the manuscript.

Comments 1: The unit of shear modulus in the table looks incorrect; it should be GPa. Please verify and correct it.

Response 1:

Thank you for pointing this problem out. We completely agree with this comment. Therefore, we have changed the unit of the shear modulus from MPa to GPa in Table 2. We appreciate your careful review and attention to detail. Thank you for helping improve the accuracy of our manuscript.

Comments 2: It is unusual that the tensile strength of sandstone or siltstone is similar to that of coal and even other lithologies.

Response 2:

Thank you for your insightful comment. We understand the concern regarding the similar tensile strength values of sandstone, siltstone, and coal, which may appear unusual at first glance. However, it is important to note that the tensile strength values were calibrated based on the specific geological and mechanical characteristics of the materials within the study area.

The calibration process used to define these parameters took into account the particular conditions of the local geology, including the mineral composition, porosity, and the stress regime experienced by the rock formations. In some cases, sandstone, siltstone, and coal can exhibit similar tensile strength values under specific conditions due to factors such as stress history, compaction, and the way these materials interact with each other in the context of the mining environment.

We appreciate your attention to this detail, and we have checked it out. Thank you for your careful review and we believe these corrections improve the transparency and accuracy of our manuscript.

Comments 3: More information is needed regarding the parameter s and the models applied for joint contacts.

Response 3:

Thank you for your valuable comments. While reviewing the manuscript, we realised that there was an omission regarding the parameters and the models. To address this issue, we have now added the Bulk Modulus of the lithologies in Table 2. The corrected information can be found in Table 2.

Thank you for your careful review and we believe these corrections improve the transparency and accuracy of our manuscript.

Comments 4: Additional details on the joint and discontinuity conditions used in the models would be beneficial.

Response 4:

Thank you for your valuable suggestion. We acknowledge that additional details such as normal stiffness and tangential stiffness on the joint and discontinuity conditions used in the models could provide further insights into the modeling process. However, due to time constraints in the revision process, we were unable to include this additional information in Table 2 at this stage.

We appreciate the relevance of this point and will certainly consider incorporating more detailed discussions on joint and discontinuity conditions in our future studies. These factors will be carefully addressed to enhance the accuracy and realism of the models.

Thank you again for your constructive feedback.

Comments 5: While it is known that stress concentration in longwall mining occurs at a certain distance from the roadway to the unexcavated area, Figure 5 suggests that the stress concentration is on the gob. Given that the roof has collapsed, stress release would be expected—please clarify this contradiction.

Response 5:

Thank you for your insightful comments. We understand the concern about the unusual phenomenon that stress concentration on the gob. While reviewing the manuscript, we realized that we made a mistake. It needs to be clarified that the stress concentrations mainly occur at some distance from the roadway to the unexcavated area and along the coal pillar next to the gob-side roadway, not in the gob. The confusion may have been caused by unclear labelling during diagram preparation. We have now modified the Figure 5 to make it more clear.

Thank you for your close attention to this detail and we hope the revised version could meet your expectations. The corrected figure is now available at Figure 5.

Comments 6: How is fragmentation due to blasting simulated? Please provide clear details on how joint conditions were modified to account for blasting.

Response 6:

Thank you for your insightful comments. We have realized that Our description of the process of fragmentation due to blasting simulation was not sufficiently clear, and we have therefore added a relevant description to the modelling process in section 4.1 of the manuscript. The process of simulating the roof cutting by blasting based on the following steps involved determining the location, dip angle, thickness, length, and other parameters of the structural plane, setting up the structural plane and meshing the model in advance, and assigning relatively low joint parameters to make it prone to sliding or cracking under the influence of stresses.

Thank you for pointing this out, we sincerely hope the revised version could meet your expectations.

Comments 7: It should be noted that roof displacement on the gob side is not always a negative outcome; rather, stress concentration can be more catastrophic. Therefore, the focus should be on stress release rather than displacement.

Response 7:

Thank you for your insightful comment. We agree that stress concentration is a critical factor and that it should be a primary focus when assessing the effectiveness of roof blasting. Stress release is indeed a key aspect of ensuring the stability of the roadway and preventing catastrophic failures.

However, we would like to emphasize that displacement, serves as an important indicator of the strength of the surrounding rock and the potential for dynamic hazards such as rock bursts. Displacement provides valuable information about the degree of rock deformation and can help assess the likelihood of instability in the surrounding strata.

In future studies, we plan to consider both stress release and displacement to offer a more comprehensive analysis of the effects of roof blasting. By combining these two aspects, we aim to analyze the pressure relief effect from multiple angles, providing a more complete understanding of its impact on mine safety and stability.

Thank you again for your constructive feedback, and we look forward to addressing this more thoroughly in future research.

Comments 8: Comment 5 is also evident in Figure 6. Please explain why stress is shown as zero at a distance of 0.

Response 8:

Thank you for your thoughtful comment regarding the zero stress value shown at a distance of 0 in Figure 6. Upon reviewing the simulation process, we identified that the issue arose from an error in the placement of measurement points during the data extraction phase. The stress value at a distance of 0 should not have been shown as zero, both from a theoretical perspective and in terms of numerical simulation principles.

We have re-extracted the data with corrected measurement point placement, ensuring that the stress values are properly captured at the relevant locations. The updated data has been used to correct Figure 6, and the revised figure now accurately represents the stress distribution along the roadway.

Thank you for pointing this out, and we hope the revised version could meet your expectations.

Comments 9: Is a reduction of 3.54 MPa in stress significant under real mining conditions? Please elaborate.

Response 9:

Thank you for your valuable insights and inquiries regarding the significance of reducing stress by 3.54 MPa under actual mining conditions. We have realized that we need to elaborate the role of stress reduction of 3.54 MPa. Actually, this reduction plays a critical role in enhancing the safety and stability of mining operations.

The reduction of stress by 3.54 MPa significantly lowers the critical stress threshold of the rock mass. When the stress level drops below a certain threshold, the likelihood of rock bursts decreases considerably. This is particularly important in high-stress mining environments, where rock bursts pose a major safety hazard. The reduction of this magnitude of stress directly contributes to the stabilization of the mine, ensuring that the stress in the coal seam and surrounding rock mass does not exceed levels that could lead to catastrophic events.

From the perspective of static and dynamic load interactions, the reduction of stress plays a crucial role. Dynamic loads refer to stress changes induced by instantaneous events such as mining, blasting, or excavation, whereas static loads are generated by the weight of overlying strata and other long-term forces. In environments with high static loads, even slight changes in dynamic loads can trigger rock bursts. By reducing the static stress by 3.54 MPa, the likelihood of sudden failure induced by dynamic loads is significantly diminished.

In conclusion, the reduction of stress by 3.54 MPa under actual mining conditions is highly significant, as it effectively lowers the critical stress level, mitigates the risk of rock bursts, and enhances overall mining safety. This reduction alleviates the interaction between static and dynamic loads, leading to a more stable and safer mining environment. Brief elaboration of the theme topic is provided in section 4.3 and marked in red of our manuscript. Once again, we appreciate your insightful comments and hope this explanation clarifies the importance of reducing stress by 3.54 MPa under actual mining conditions.

Thank you for pointing this out, and we hope the revised version could meet your expectations.

Comments 1: Line 28 and 29; some keywords have been already used in the title of your manuscript. Please change them into different ones (to avoid the keywords repetition with the words used in the title).

Response 1:

Thank you for your comment. We acknowledge the concern regarding the repetition of keywords in lines 28 and 29, which are also used in the title of the manuscript. In response, we have revised the wording in these lines to avoid keyword repetition, replacing them with alternative terms that maintain the clarity and focus of the content.

We appreciate your attention to detail, and we believe this change enhances the readability of the manuscript. The updated version can be found in [lines 28 and 29] of the revised manuscript. Thank you again for your valuable feedback..

Comments 2: Equation (1) – (3); the units are missing.

Response 2:

Thank you for your valuable input. We reviewed equations (1) - (3) and found that units were indeed missing. We have now added the proper units for the introduction of the terms of these equations to ensure clarity and consistency with the rest of the manuscript.

The revised equations with the correct units are now included in lines 186, 187 and 198 of the updated manuscript! Thank you again for pointing this out, we believe this revision improves the accuracy of the manuscript.

Comments 3: There are no reactions shown in Figure 3 at point A as well as moment M₁.

Response 3:

Thank you for your insightful comment. We have reviewed Figure 3 and acknowledge that the reactions at point A and moment M1 were not shown in the original version. This omission was due to an oversight during the figure preparation. In response, we have now updated Figure 3 to include the missing reactions at point A and moment M₁, ensuring that all relevant forces and moments are properly represented.

Thank you again for your valuable feedback, and we hope this revision addresses your concern.

Comments 4: Equation (5); why is there no concentrated force P1 shown in the model (Figure 3)?

Response 4:

Thank you for your careful review. Upon re-examining Equation (5) and Figure 3, we realized that there was a mix-up between P₁ and P₀ during the equation-writing process.

 The correct formulation of the equation does not include P₁, which was mistakenly referenced.

To address this, we have carefully corrected all instances where this mix-up occurred, ensuring consistency between the equations and the model representation in Figure 3. The revised manuscript now accurately reflects the correct force notation, and the issue has been fully rectified.

We appreciate your attention to detail, and we believe this revision improves the clarity and accuracy of our work. Thank you again for your valuable feedback..

Comments 5: Figure 4; the X and Z axis should be introduced and shown in this figure. The load should be also marked.

Response 5:

Thank you for your valuable comment. We agree that the X and Z axes should be clearly shown in Figure 4 to improve the clarity of the diagram. Additionally, we have added the load marking to the figure as requested. These modifications ensure that the figure accurately represents the necessary information and enhances the understanding of the model.

The updated version of Figure 4, now including the X and Z axes and the load marking, is included in the revised manuscript. Thank you again for your helpful feedback.

Comments 6: Table 2 and 3; if the parameters of the coal and rock mass are taken from the literature, then the literature reference should be given.

Response 6:

Thank you for your insightful comment. The parameters for the coal and rock mass in Table 2 were obtained through a calibration process, not directly taken from the literature. For Table 3, the parameters were calculated using mix ratio formulas based on similar simulation materials. Due to space constraints, we did not provide a detailed description of these processes in the manuscript.

We appreciate your reminder about the need for citations. In future revisions, we will ensure that relevant literature is properly referenced, and we really hank you again your valuable feedback.

Comments 7: There is no information about the type of the finite element, number of nodes in an element, number of degrees of freedom in a single node, total number of elements, total number of nodes, what kind of mesh did the authors used, size of the mesh, and so on.

Response 7:

Thank you for your insightful comment. We would like to clarify that the model used in our study is based on the discrete element method (DEM), rather than the finite element method (FEM). However, we acknowledge the importance of providing more details regarding the mesh configuration used in our simulations.

To address this, we have added relevant information about the mesh division, element size, and related modeling details in section 4.2 of the revised manuscript. These additions, highlighted in red, further enhance the transparency of our methodology and improve the clarity of the model setup.

We appreciate your constructive feedback and believe that these modifications contribute to a better understanding of the study. Thank you again for your valuable suggestions.

Comments 8: There is no information about the numerical method which was applied in order to count the displacements and stresses presented in this manuscript.

Response 8:

Thank you for your valuable comment. We apologize for the lack of clarity regarding the numerical method used to calculate the displacements and stresses presented in the manuscript. To clarify, the stress calculations were based on monitoring the stress on the solid coal side along the gob-side roadway. Displacement values were obtained by monitoring measurement points set up around the roadway. For the sake of maintaining the manuscript’s overall flow and coherence, we had initially omitted detailed explanations of the monitoring and numerical methods.

We appreciate your helpful feedback, and we have now included this information in the revised manuscript to ensure full transparency. Thank you again for your attention to detail, and we hope this additional information improves the clarity of the study.