Examining Shear Behavior in Sandy Gravel Interfaces: The Role of Relative Density and Material Interactions

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The title, "Influence of Relative Density and Interface Materials on Shear Behavior of Sandy Gravel-Structure Interface," accurately reflects the study’s scope. However, it could be rephrased to enhance its impact and attract a broader readership. A potential alternative is: "Examining Shear Behavior in Sandy Gravel Interfaces: The Role of Relative Density and Material Interactions."

Abstract

The abstract provides a concise overview of the study’s objectives, methodology, and findings. However, it could better emphasize the novelty and broader implications of the research. For instance:

• The introduction of relative density into the Mohr-Coulomb shear strength formula should be highlighted as a key innovation and its practical implications elaborated.
• A stronger statement of the study's contribution to addressing gaps in the literature on sandy gravel-structure interfaces would enhance clarity and appeal.

Introduction

The introduction comprehensively reviews the existing literature and situates the study within the context of geotechnical engineering. However:

• The research gap and the significance of addressing it should be more explicitly stated.
• While the references are relevant, the introduction could benefit from integrating more global perspectives to extend its relevance beyond the specific regional context.

Methodology

The experimental methodology is described in detail, which is commendable. However:

• The rationale for selecting specific relative densities (30%, 50%, 70%, and 90%) and normal pressures (100–400 kPa) should be justified more explicitly.
• The frequent occurrence of formatting errors, such as "Error! Reference source not found," detracts from the credibility and clarity of the methodology. These must be corrected to ensure the replicability of the study.
• Visual elements (e.g., Figure 4) could be refined for better resolution and interpretability.

Results and Discussion

The results are presented with clarity, supported by appropriate graphs and tables. The discussions are technical and well-structured. Nonetheless:

• The implications of the findings for engineering applications, such as photovoltaic (PV) pile foundations in sandy gravel terrains, should be discussed in greater detail.
• The comparative analysis of shear behavior across interfaces (steel vs. concrete) is insightful but could be expanded to provide more practical recommendations for material selection.
• The transition from strain hardening to strain softening with increasing relative density is an intriguing finding that warrants further exploration, particularly regarding its implications for interface stability.

Conclusions

The conclusions effectively summarize the findings but lack depth in discussing future research directions and broader implications. Consider:

• Providing specific recommendations for practitioners and researchers.
• Highlighting how the new formulation of the Mohr-Coulomb shear strength equation can be applied in real-world scenarios.

Language and Presentation

• The manuscript generally adheres to high academic standards; however, some sentences could be restructured for better readability and flow. For instance:
• Replace "the shear curve changes from strain hardening to strain softening" with "the shear behavior transitions from strain hardening to strain softening."
• Avoid repetitive phrasing, particularly in sections discussing relative density effects.
• Ensure all figures, tables, and equations are cross-referenced accurately to eliminate formatting issues.

The manuscript presents a well-structured study with significant contributions to understanding the shear behavior of sandy gravel-structure interfaces. However, addressing the points above will substantially enhance its academic rigor, practical relevance, and overall readability. I recommend major revisions to ensure the manuscript meets the highest scholarly standards.

Author Response

Comments 1: The title, "Influence of Relative Density and Interface Materials on Shear Behavior of Sandy Gravel-Structure Interface," accurately reflects the study’s scope. However, it could be rephrased to enhance its impact and attract a broader readership. A potential alternative is: "Examining Shear Behavior in Sandy Gravel Interfaces: The Role of Relative Density and Material Interactions."

Response 1: Thank the reviewer for the comment. We have made the modifications in the manuscript based on your suggestion.

 

Comments 2: The abstract provides a concise overview of the study’s objectives, methodology, and findings. However, it could better emphasize the novelty and broader implications of the research. For instance:

The introduction of relative density into the Mohr-Coulomb shear strength formula should be highlighted as a key innovation and its practical implications elaborated.

A stronger statement of the study's contribution to addressing gaps in the literature on sandy gravel-structure interfaces would enhance clarity and appeal.

Response 2: Thank the reviewer for the comment. We have made the modifications in the manuscript based on your suggestion, please see lines 15-33.

 

Comments 3: The introduction comprehensively reviews the existing literature and situates the study within the context of geotechnical engineering. However:

The research gap and the significance of addressing it should be more explicitly stated.

While the references are relevant, the introduction could benefit from integrating more global perspectives to extend its relevance beyond the specific regional context.

Response 3: Thank the reviewer for the comment. We have made the modifications in the manuscript based on your suggestion, please see lines 88-97.

 

 

Comments 4: The experimental methodology is described in detail, which is commendable. However:

The rationale for selecting specific relative densities (30%, 50%, 70%, and 90%) and normal pressures (100–400 kPa) should be justified more explicitly.

Response 4: Thank the reviewer for the comment. We have provided supplementary explanations in the Test Methods section. The relative density of the sandy gravel stratum at the site was 0.7. Considering that the construction may disturb the stratum leading to a decrease in relative density or compaction leading to an increase in relative density, four relative densities of 0.3, 0.5, 0.7, and 0.9 were selected for this study. Considering the stress state at different stratigraphic depths and under different loading conditions (wind, snow, etc.), the range of normal stresses chosen in this paper is from 100 kPa to 400 kPa. The relevant content has been supplemented in the manuscript, please see lines 128-133.

 

Comments 5: The frequent occurrence of formatting errors, such as "Error! Reference source not found," detracts from the credibility and clarity of the methodology. These must be corrected to ensure the replicability of the study.

Response 5: Thank the reviewer for the comment. It may be that the manuscript we submitted did not meet the format requirements of the journal, and the editorial department re-formatted our manuscript, which resulted in errors in the citations of figures and tables. We apologize for not discovering this issue promptly and causing you inconvenience. We have now corrected all the errors.

 

Comments 6: Visual elements (e.g., Figure 4) could be refined for better resolution and interpretability.

Response 6: Thank the reviewer for the comment. We have revised Figure 4 based on your suggestions, and we have also improved the resolution of the other figures. The specific changes can be found in the revised manuscript.

 

Comments 7: The results are presented with clarity, supported by appropriate graphs and tables. The discussions are technical and well-structured. Nonetheless:

The implications of the findings for engineering applications, such as photovoltaic (PV) pile foundations in sandy gravel terrains, should be discussed in greater detail.

Response 7: Thank the reviewer for the comment. We have added relevant content to the manuscript based on your suggestion, please see lines 317-322.

 

 

Comments 8: The comparative analysis of shear behavior across interfaces (steel vs. concrete) is insightful but could be expanded to provide more practical recommendations for material selection.

Response 8: Thank the reviewer for the comment. We have added relevant content to the manuscript based on your suggestion, please see lines 222-224.

 

Comments 9: The transition from strain hardening to strain softening with increasing relative density is an intriguing finding that warrants further exploration, particularly regarding its implications for interface stability.

Response 9: Thank the reviewer for the comment. This may be attributed to the fact that the porosity of the specimen is high when the relative density is low, resulting in internal changes of the specimen during shear pre-dominantly characterized by pore compression. As the density increases, the porosity of the specimen decreases, leading to an increased resistance to particle movement during shear. Consequently, particle rolling gradually becomes the dominant mechanism. The relevant content has been supplemented in the manuscript, please see lines 163-168.

 

Comments 10: The conclusions effectively summarize the findings but lack depth in discussing future research directions and broader implications. Consider:

Providing specific recommendations for practitioners and researchers.

Response 10: Thank the reviewer for the comment. Response: Thank the reviewer for the comment. We have added relevant content to the manuscript based on your suggestion, please see lines 385-391.

 

Comments 11: Highlighting how the new formulation of the Mohr-Coulomb shear strength equation can be applied in real-world scenarios.

Response 11: Thank the reviewer for the comment. Response: Thank the reviewer for the comment. We have added relevant content to the manuscript based on your suggestion, please see lines 385-387.

 

Comments 12: The manuscript generally adheres to high academic standards; however, some sentences could be restructured for better readability and flow. For instance:

Replace "the shear curve changes from strain hardening to strain softening" with "the shear behavior transitions from strain hardening to strain softening."

Avoid repetitive phrasing, particularly in sections discussing relative density effects.

Ensure all figures, tables, and equations are cross-referenced accurately to eliminate formatting issues.

Response 12: Thank the reviewer for the comment. We have made modifications to the corresponding sections based on your suggestions, please see lines 21-22, lines 261-264, line 271, line 273.

Reviewer 2 Report

Comments and Suggestions for Authors

The manuscript investigates the impact of relative density and interface materials (steel and concrete) on the shear behavior of sandy gravel-structure interfaces through large-scale direct shear tests. The study examines stress-strain relationships, volume changes, and shear strength, integrating relative density into the Mohr-Coulomb formula for enhanced practical application. Key findings include distinct strain-hardening and strain-softening behaviors and significant variations in interface friction angles and adhesion. However, major revisions are needed. The manuscript also requires improved linkage to practical applications and validation of proposed models for broader generalizability.

1. The 40% similarity identified in the iThenticate report suggests a significant overlap with existing sources. While some similarity may be acceptable due to technical language or properly cited references, the high percentage warrants a detailed review to ensure all matched content is appropriately paraphrased or cited to avoid potential issues of plagiarism.
2. Frequent occurrences of "Error! Reference source not found." in place of references detract significantly from the manuscript's readability and professionalism. This issue must be resolved to maintain credibility.
3. Annotations or callouts highlighting critical points (e.g., peaks, transitions between shrinkage and dilatancy) are necessary for better understanding.
4. Equations in the manuscript (e.g., those related to the Mohr-Coulomb model and relative density relationships) lack intermediate steps, assumptions, and validations. Clarify the derivation and ensure the applicability is fully justified.
5. Justify the selected range of relative densities (30%, 50%, 70%, and 90%). Are these representative of field conditions in PV power plants or general sandy gravel applications?
6. Provide further explanation of the normal pressures applied (100, 200, 300, and 400 kPa). What scenarios in geotechnical practice do these values represent?
7. Discuss whether potential confounding factors, such as particle shape or moisture content, were controlled during experimentation. These can significantly influence shear strength and volume change behavior.
8. While the study describes trends (e.g., shrinkage vs. dilatancy), the underlying mechanics are not well explained. For instance: Why does the steel interface primarily exhibit shrinkage, while the concrete interface shows occasional dilatancy at high densities? Relate these findings to physical mechanisms, such as interparticle locking, sliding friction, and interface roughness effects.
9. Explain why the steel interface's friction angle remains unaffected by relative density, while the concrete interface shows changes. The discussion lacks insight into differences in surface roughness, stiffness, or particle behavior during shear.
10. Address why the concrete interface experiences strain softening under specific conditions (90% relative density, 100 kPa normal pressure). Does this result align with previous literature or field observations?
11. The study focuses on specific materials and conditions (sandy gravel from Qinghai Province). Discuss the applicability of these results to other soil types, geographies, or engineering applications.
12. The linear relationships and parameters in the Mohr-Coulomb-based model need validation with independent datasets or field-scale studies.
13. Terms such as "strain hardening type" and "strain softening type" could be simplified to "strain hardening behavior" and "strain softening behavior" for clarity.
14. Correctly format in-text citations and ensure all references are listed in a consistent style.
15. There are instances of unclear phrasing (e.g., "the volume change curves show two stages"), which should be revised for conciseness and precision.
16. Strengthen the conclusion by summarizing how the findings advance the state of knowledge in sandy gravel-structure interaction studies. Provide practical design recommendations or future research directions, such as exploring other interface materials or conditions.

Comments on the Quality of English Language

The manuscript requires significant improvement in English language quality, particularly in grammar, clarity, and technical terminology.

Author Response

Comments 1: The manuscript investigates the impact of relative density and interface materials (steel and concrete) on the shear behavior of sandy gravel-structure interfaces through large-scale direct shear tests. The study examines stress-strain relationships, volume changes, and shear strength, integrating relative density into the Mohr-Coulomb formula for enhanced practical application. Key findings include distinct strain-hardening and strain-softening behaviors and significant variations in interface friction angles and adhesion. However, major revisions are needed. The manuscript also requires improved linkage to practical applications and validation of proposed models for broader generalizability.

Response 1: Thank the reviewer for the comment. We have made all the revisions to the manuscript based on your suggestions. For the specific details of the changes, please refer to the revised manuscript.

 

Comments 2: The 40% similarity identified in the iThenticate report suggests a significant overlap with existing sources. While some similarity may be acceptable due to technical language or properly cited references, the high percentage warrants a detailed review to ensure all matched content is appropriately paraphrased or cited to avoid potential issues of plagiarism.

Response 2: Thank the reviewer for the comment. We have made significant revisions to the introduction section, please see lines 44-87.

 

Comments 3: Frequent occurrences of "Error! Reference source not found." in place of references detract significantly from the manuscript's readability and professionalism. This issue must be resolved to maintain credibility.

Response 3: Thank the reviewer for the comment. It may be that the manuscript we submitted did not meet the format requirements of the journal, and the editorial department re-formatted our manuscript, which resulted in errors in the citations of figures and tables. We apologize for not discovering this issue promptly and causing you inconvenience. We have now corrected all the errors.

 

Comments 4: Annotations or callouts highlighting critical points (e.g., peaks, transitions between shrinkage and dilatancy) are necessary for better understanding.

Response 4: Thank the reviewer for the comment. We have revised the corresponding figures based on your suggestions. For details, please refer to Figures 5, 7, 8, and 13 in the revised manuscript.

 

Comments 5: Equations in the manuscript (e.g., those related to the Mohr-Coulomb model and relative density relationships) lack intermediate steps, assumptions, and validations. Clarify the derivation and ensure the applicability is fully justified.

Response 5: Thank the reviewer for the comment. Due to the existence of good linear relationships between shear strength and both relative density and normal pressure, a binary quadratic functional relationship between shear strength, relative density, and normal pressure is constructed, with the coefficients of the squared terms set to zero. In addition, we have validated the formula using data published by other scholars, and have provided supplementary explanations in the manuscript regarding this. The relevant content has been supplemented in the manuscript, please see lines 327-330, lines 348-358.

 

Comments 6: Justify the selected range of relative densities (30%, 50%, 70%, and 90%). Are these representative of field conditions in PV power plants or general sandy gravel applications?

Response 6: Thank the reviewer for the comment. The relative density of the sandy gravel stratum at the site was 0.7. Considering that the construction may disturb the stratum leading to a decrease in relative density or compaction leading to an increase in relative density, four relative densities of 0.3, 0.5, 0.7, and 0.9 were selected for this study. This range can encompass the conditions of most sandy gravel. The relevant content has been supplemented in the manuscript, please see lines 128-131.

 

Comments 7: Provide further explanation of the normal pressures applied (100, 200, 300, and 400 kPa). What scenarios in geotechnical practice do these values represent?

Response 7: Thank the reviewer for the comment. Considering the stress state at different stratigraphic depths and under different loading conditions (wind, snow, etc.), the range of normal stresses chosen in this paper is from 100 kPa to 400 kPa. The relevant content has been supplemented in the manuscript, please see lines 131-133.

 

Comments 8: Discuss whether potential confounding factors, such as particle shape or moisture content, were controlled during experimentation. These can significantly influence shear strength and volume change behavior.

Response 8: Thank the reviewer for the comment. All sandy gravels were mixed well and dried to a constant weight at 105℃ in preparation for the test, to avoid any influence on the results from factors such as particle gradation, particle shape, and moisture content. Additionally, the sheared sandy gravel was not reused, to prevent changes in grading caused by particle fragmentation that might interfere with the results. The relevant content has been supplemented in the manuscript, please see lines 108-114.

 

Comments 9: While the study describes trends (e.g., shrinkage vs. dilatancy), the underlying mechanics are not well explained. For instance: Why does the steel interface primarily exhibit shrinkage, while the concrete interface shows occasional dilatancy at high densities? Relate these findings to physical mechanisms, such as interparticle locking, sliding friction, and interface roughness effects.

Response 9: Thank the reviewer for the comment. This is due to the fact that the steel interface is extremely smooth, and during the shearing process, the movement of particles is mainly characterized by sliding. However, compared to the steel interface, the concrete interface exhibits a significantly greater relative roughness. At lower relative densities, the locking effect between particles and the concrete interface is minimal, and particle movement at the interface is primarily characterized by sliding. However, when the relative density reaches 90%, the locking effect between particles and the interface increases, and a normal pressure of 100kPa exerts relatively little vertical restraint on the particles. As the shear displacement increases, the specimen is compressed, leading to an increase in resistance to particle movement. This results in particles breaking through the constraint of the normal pressure and undergoing rolling, which leads to the occurrence of shear dilatancy behavior and the generation of a peak shear stress. As shearing continues, the volume of the specimen expands continuously, the density decreases, and the shear stress gradually diminishes accordingly. The relevant content has been supplemented in the manuscript, please see lines 197-209.

 

Comments 10: Explain why the steel interface's friction angle remains unaffected by relative density, while the concrete interface shows changes. The discussion lacks insight into differences in surface roughness, stiffness, or particle behavior during shear.

Response 10: Thank the reviewer for the comment. According to the research conducted by Dove and Frost, the friction coefficient of soil-structure interface is influenced by two frictional components: sliding and plowing. In this study, due to the significantly higher hardness of the steel board compared to that of the sandy gravel, changes in relative density do not cause variations in the plowing component during the shear process. However, the hardness difference between the concrete board and the sandy gravel is relatively small, and particle plowing increases with the increase in relative density, leading to an increase in the friction coefficient. The relevant content has been supplemented in the manuscript, please see lines 309-315.

References: Dove J E and Frost J D. Peak Friction Behavior of Smooth Geomembrane-Particle Interfaces[J]. Journal of Geotechnical and Geoenvironmental Engineering, 1999, 125(7): 544-555.

 

Comments 11: Address why the concrete interface experiences strain softening under specific conditions (90% relative density, 100 kPa normal pressure). Does this result align with previous literature or field observations?

Response 11: Thank the reviewer for the comment. This is because the concrete interface in this study has a relatively high roughness compared to a smooth steel interface. At lower relative densities, the locking effect between particles and the concrete interface is minimal, and particle movement at the interface is primarily characterized by sliding. However, when the relative density reaches 90%, the locking effect between particles and the interface increases, and a normal pressure of 100kPa exerts relatively little vertical restraint on the particles. As the shear displacement increases, the specimen is compressed, leading to an increase in resistance to particle movement. This results in particles breaking through the constraint of the normal pressure and undergoing rolling, which leads to the occurrence of shear dilatancy behavior and the generation of a peak shear stress. As shearing continues, the volume of the specimen expands continuously, the density decreases, and the shear stress gradually diminishes accordingly. This same phenomenon was present in the study by Janipour, the eighth reference of this paper. The relevant content has been supplemented in the manuscript, please see lines 197-209.

References: Janipour A K, Mousicand M, Bayat M. Study of interface shear strength between sand and concrete[J]. Arabian Journal of Geosciences, 2022, 15: 172.

 

Comments 12: The study focuses on specific materials and conditions (sandy gravel from Qinghai Province). Discuss the applicability of these results to other soil types, geographies, or engineering applications.

Response 12: Thank the reviewer for the comment. We have validated our results using experimental data from other scholars, and the results indicate that the formula has good applicability to sand in other regions. In our subsequent research work, we will investigate the applicability of this result to other types of soil, such as silt and clay. The relevant content has been supplemented in the manuscript, please see lines 348-358.

 

Comments 13: The linear relationships and parameters in the Mohr-Coulomb-based model need validation with independent datasets or field-scale studies.

Response 13: Thank the reviewer for the comment. We have validated our results using experimental data from other scholars, and the results indicate that the formula has good applicability. The relevant content has been supplemented in the manuscript, please see lines 348-358.

 

Comments 14: Terms such as "strain hardening type" and "strain softening type" could be simplified to "strain hardening behavior" and "strain softening behavior" for clarity.

Response 14: Thank the reviewer for the comment. We have made modifications to the corresponding sections based on your suggestions, please see line 161, line 162, line 170, line 172.

 

Comments 15: Correctly format in-text citations and ensure all references are listed in a consistent style.

Response 15: Thank the reviewer for the comment. We have corrected all formatting issues. For details, please refer to the revised manuscript.

 

Comments 16: There are instances of unclear phrasing (e.g., "the volume change curves show two stages"), which should be revised for conciseness and precision.

Response 16: Thank the reviewer for the comment. We have made the modifications in the manuscript based on your suggestion, please see line 255.

 

Comments 17: Strengthen the conclusion by summarizing how the findings advance the state of knowledge in sandy gravel-structure interaction studies. Provide practical design recommendations or future research directions, such as exploring other interface materials or conditions.

Response 17: Thank the reviewer for the comment. We have added relevant content to the manuscript based on your suggestion, please see lines 385-391.

Reviewer 3 Report

Comments and Suggestions for Authors

The authors chose sandy gravel as the object and carried out a series of tests to investigate the influence of relative density and interface materials on the shear behavior of sandy gravel-structure interface. This article has some engineering significance and scientific value, and aligns with the scope of the journal. However, the following suggestions are provided to further enhance the quality of the manuscript:

1.     Abstract needs to be revised.

2.     The material of the steel board and the proportion of the concrete board needed to be added.

3.     Figure 7 in line 156 needs to be changed to Fig. 7.

4.     According to line 187, it is recommended that the shear shrinkage in the name of Fig.9 be changed to the shrinkage value, and other figure names need to be checked as well.

5.     It is necessary to compare the completed work with other works on the same topic.

6.     The formula section needs to be corrected, including the format of the parameter description and the multiplication sign.

7.     Add the limitations of the present work and the future work in conclusions section.

Author Response

Comments 1: Abstract needs to be revised.

Response 1: Thank the reviewer for the comment. We have revised the abstract, please see lines 15-33.

 

Comments 2: The material of the steel board and the proportion of the concrete board needed to be added.

Response 2: Thank the reviewer for the comment. We have added relevant content to the manuscript based on your suggestion, please see lines 112-114.

 

Comments 3: Figure 7 in line 156 needs to be changed to Fig. 7.

Response 3: Thank the reviewer for the comment. We have corrected all formatting issues. For details, please refer to the revised manuscript.

 

Comments 4: According to line 187, it is recommended that the shear shrinkage in the name of Fig.9 be changed to the shrinkage value, and other figure names need to be checked as well.

Response 4: Thank the reviewer for the comment. We have made the modifications in the manuscript based on your suggestion, please see line 250, line 251, line 275, line 294.

 

Comments 5: It is necessary to compare the completed work with other works on the same topic.

Response 5: Thank the reviewer for the comment. We have added relevant content to the manuscript based on your suggestion, please see lines 197-209, lines 308-309, lines 348-355.

 

Comments 6: The formula section needs to be corrected, including the format of the parameter description and the multiplication sign.

Response 6: Thank the reviewer for the comment. We have made the modifications in the manuscript based on your suggestion, please see lines 331-332.

 

Comments 7: Add the limitations of the present work and the future work in conclusions section.

Response 7: Thank the reviewer for the comment. We have added relevant content to the manuscript based on your suggestion, please see lines 385-391.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript is now suitable for publication.

 

 

Author Response

We sincerely thank the reviewer for their invaluable suggestions, which have further enhanced the quality of our manuscript.

Reviewer 2 Report

Comments and Suggestions for Authors

 

The authors deserve commendation for significant revisions, including clarifying experimental methods and expanding discussions on shear behavior mechanisms. These efforts highlight their commitment to addressing reviewer concerns. However, major revisions are still required to strengthen the manuscript. Key improvements should include adding numerical results to enhance clarity, providing statistical validation for models, and emphasizing practical applications with specific recommendations for engineering practices. These enhancements will ensure broader impact and applicability.

1.       Revisit overlapping sections to ensure that all paraphrased content is technically accurate and does not dilute the meaning of the original text. Justify any remaining similarity due to technical terminology or widely cited references.

 

2. How were the coefficients in the Mohr-Coulomb formula determined? Were they validated against independent datasets or statistical models?
3. Can the proposed formula be generalized to other soil types, such as clay or silt, and how does it compare to existing models in geotechnical engineering?
4. Were confounding factors such as particle shape, roughness, or moisture content quantitatively controlled, and how might variations in these factors impact the results?
5. Why was the practical relevance of findings (e.g., for PV pile foundation design) not detailed further? Are there specific thresholds or design parameters derived from the study?
6. Is the dataset used for validation sufficiently diverse, or does it require further data from different geographical regions or soil conditions?
7. Add numerical findings to highlight key results (e.g., specific increases in cohesion, friction angles, or shear strength due to changes in relative density). Emphasize the practical implications more explicitly, such as how the formula can be applied to improve foundation design or predict soil behavior under construction-induced disturbances.
8. Clearly articulate how this study fills existing research gaps and link findings to practical engineering challenges.
9. Quantify the control of factors such as roughness, particle shape, and moisture content to ensure robustness of the results. Provide more details on how data were processed and analyzed to derive the relationships presented.
10. Include detailed numerical analysis of key findings, such as changes in interface friction angles, cohesion, or volume change behavior across different densities and pressures. Validate findings with statistical metrics (e.g., R² values, error margins) for fitted models to strengthen credibility. Expand the discussion on practical applications, providing actionable insights for engineers, such as specific design recommendations for PV pile foundations or similar structures.
11. Summarize key numerical results clearly and highlight their significance for advancing geotechnical engineering practices. Provide specific, practical recommendations (e.g., thresholds for relative density or normal pressure for pile foundation design) derived from the study’s findings. Suggest concrete next steps for research, such as testing the formula on other soil types or under additional conditions (e.g., temperature or moisture variations).

 

Comments on the Quality of English Language

The English could be improved to more clearly express the research.

Author Response

Comments 1: Revisit overlapping sections to ensure that all paraphrased content is technically accurate and does not dilute the meaning of the original text. Justify any remaining similarity due to technical terminology or widely cited references.

Response 1: We sincerely thank the reviewer for their valuable feedback. We have carefully refined the introduction section to ensure that the paraphrased content maintains technical accuracy without diluting the original meaning. The revised manuscript has achieved a significant reduction in repetition rates. According to the plagiarism check results from ithenticate, the repeated content primarily focuses on descriptions of other scholars' research in the introduction, followed by figure names and technical terminology such as “the shear stress-shear displacement curves”, “normal pressure”, “relative density”, “shear shrinkage”, “shear dilatancy”, “the angle of internal friction”, and so on.

 

Comments 2: How were the coefficients in the Mohr-Coulomb formula determined? Were they validated against independent datasets or statistical models?

Response 2: We sincerely thank the reviewer for their valuable feedback. These coefficients were obtained by fitting experimental data to the formulas provided in the manuscript using Origin software. The formula has been validated using independent data, please see lines 352-359. The coefficients for different types of soil can be obtained through relevant experiments.

 

Comments 3: Can the proposed formula be generalized to other soil types, such as clay or silt, and how does it compare to existing models in geotechnical engineering?

Response 3: We sincerely thank the reviewer for their valuable feedback. Relative density has rarely been used in clays, and the current study focuses on density. We suspect that the formula may not be applicable in clays due to the more complex factors affecting clays. There are fewer studies on the interfacial properties of silt, and we have not found any papers addressing the influence of relative density on the interface properties of silt. Therefore, the applicability of the formula in slit needs to be verified through relevant experiments. We will undertake these experiments and publish our findings in subsequent papers.

Compared to the existing models in geotechnical engineering, this formula establishes a quantitative relationship between relative density and the shear strength of gravels.

 

Comments 4: Were confounding factors such as particle shape, roughness, or moisture content quantitatively controlled, and how might variations in these factors impact the results?

Response 4: We sincerely thank the reviewer for their valuable feedback. To ensure that the particle shapes of all samples are as consistent as possible, the sandy gravel collected from the site is uniformly mixed before being distributed. All gravels were dried at 105°C to ensure that the moisture content was 0. Additionally, the sheared sandy gravel was not reused, to prevent changes in grading caused by particle fragmentation that might interfere with the results. Both steel board and concrete boards have smooth surfaces. We have added relevant content to the manuscript based on your suggestion, please see lines 108-115.

 

Comments 5: Why was the practical relevance of findings (e.g., for PV pile foundation design) not detailed further? Are there specific thresholds or design parameters derived from the study?

Response 5: We sincerely thank the reviewer for their valuable feedback. We have added relevant content to the manuscript based on your suggestion, please see lines 322-326, lines 389-393.

 

Comments 6: Is the dataset used for validation sufficiently diverse, or does it require further data from different geographical regions or soil conditions?

Response 6: We sincerely thank the reviewer for their valuable feedback. Due to the lack of relevant research on sandy gravel by other scholars, the formula in the manuscript has been validated using data from sand with different interfaces (sand interface and concrete interface) and different roughness levels (smooth and rough). However, due to the scarcity of relevant data on silt, further experiments are needed to determine its applicability in silt and other soils. We will conduct these experiments and publish our findings in subsequent papers.

 

Comments 7: Add numerical findings to highlight key results (e.g., specific increases in cohesion, friction angles, or shear strength due to changes in relative density). Emphasize the practical implications more explicitly, such as how the formula can be applied to improve foundation design or predict soil behavior under construction-induced disturbances.

Response 7: We sincerely thank the reviewer for their valuable feedback. As the relative density of sandy gravel increases, both the cohesion and the angle of internal friction increase significantly. For instance, when the relative density increases from 0.3 to 0.9, the cohesion increases from approximately 27.2 kPa to 43.6 kPa, and the angle of internal friction increases from 43.0° to 48.0° (Table 2). For the steel interface, the interface adhesion increases with relative density, while the interface friction angle remains relatively constant around 28.9° (Table 2). For the concrete interface, both the interface friction angle and interface adhesion increase with relative density. For example, the interface friction angle increases from 30.2° to 34.2°, and the interface adhesion increases from 6.4 kPa to 8.0 kPa as the relative density increases from 0.3 to 0.9 (Table 2). There is a strong linear relationship between the relative density and the shear strength of sandy gravel. The increase in shear strength with relative density is primarily due to the increase in both cohesion and the angle of internal friction (Figure 16). We have added relevant content to the manuscript based on your suggestion, please see lines 301-311.

The proposed formula that incorporates relative density into the Mohr-Coulomb shear strength formula can be used to evaluate the changes in mechanical properties of sandy gravel formations after they have been disturbed by factors such as construction. This information is crucial for designing foundations, particularly pile foundations, in sandy gravel geological conditions. By understanding how relative density affects the shear behavior of sandy gravel, construction methods can be optimized to minimize soil disturbances and maintain favorable soil conditions. This can lead to cost savings, improved construction efficiency, and enhanced structural performance. We have added relevant content to the manuscript, please see lines 322-326, lines 389-393.

 

Comments 8: Clearly articulate how this study fills existing research gaps and link findings to practical engineering challenges.

Response 8: We sincerely thank the reviewer for their valuable feedback. Current research on soil-structure interface properties mainly focuses on sand, clay, and silt, with little attention given to sandy gravel. In addition, most of the current research is conducting regularity descriptions rather than establishing quantitative relationships. During the construction of photovoltaic (PV) power stations, due to the small length of the PV pile foundations, factors such as construction can readily alter the density of the sandy gravel stratum, leading to changes in its mechanical properties and thus affecting the bearing capacity of the foundations. Furthermore, due to the large particle size of sandy gravel, conventional direct shear instruments cannot be used to conduct shear tests on it. This means that obtaining the mechanical properties of sandy gravel through experimental means is relatively difficult. Therefore, it is necessary to study the effect of relative density on the mechanical properties of sandy gravel and to establish a quantitative relationship between these factors. We have added relevant content to the manuscript, please see lines 88-100.

 

Comments 9: Quantify the control of factors such as roughness, particle shape, and moisture content to ensure robustness of the results. Provide more details on how data were processed and analyzed to derive the relationships presented.

Response 9: We sincerely thank the reviewer for their valuable feedback. To ensure that the particle shapes of all samples are as consistent as possible, the sandy gravel collected from the site is uniformly mixed before being distributed. All gravels were dried at 105°C to ensure that the moisture content was 0. Additionally, the sheared sandy gravel was not reused, to prevent changes in grading caused by particle fragmentation that might interfere with the results. Both steel board and concrete boards have smooth surfaces. We have added relevant content to the manuscript based on your suggestion, please see lines 108-115. The maximum axial displacement of the specimen during shearing is taken as the shrinkage value. The difference in axial displacement of the specimen from the maximum axial displacement to the end of the shear is taken as the dilatation value, the relevant content please lines 241-243. By plotting a scatter plot of relative density versus peak shear strength and fitting it to a line, it was found that they have a linear relationship, the relevant content please lines 329-331 and Figure 16.

 

Comments 10: Include detailed numerical analysis of key findings, such as changes in interface friction angles, cohesion, or volume change behavior across different densities and pressures. Validate findings with statistical metrics (e.g., R² values, error margins) for fitted models to strengthen credibility. Expand the discussion on practical applications, providing actionable insights for engineers, such as specific design recommendations for PV pile foundations or similar structures.

Response 10: We sincerely thank the reviewer for their valuable feedback. For the steel interface, the interface adhesion increases linearly with relative density (R² is 0.992), from 9.8 kPa to 22.0 kPa when the relative density increases from 0.3 to 0.9, while the interface friction angle remains relatively constant around 28.9°. For the concrete interface, both the interface friction angle and interface adhesion increase linearly with relative density, with R² being 0.985 and 0.973. To further strengthen the credibility of our results, we have validated the fitted models using statistical metrics such as R² values and error margins. As detailed in the manuscript, the R² values for the fitted shear strength formulas range from 0.992 to 0.997, indicating a high degree of goodness-of-fit. Additionally, we have validated our proposed formula (Equation 1) using experimental data from Janipour et al. [8], as shown in Figure 18 and Table 5. The resulting R² values (ranging from 0.998 to 0.999) confirm the applicability and accuracy of our formula not only to sandy gravel but also to sand, demonstrating its broader potential utility. We have added relevant content to the manuscript, please see lines 301-311.

We have expanded our discussion on practical applications to provide actionable insights for engineers. Specifically, we have highlighted the importance of considering the potential decrease in friction coefficient between pile sides and soil due to factors such as construction activities. This information is crucial for engineers designing pile foundations in sandy gravel geological conditions, as it helps them to assess the bearing capacity of pile foundations more accurately. Additionally, our findings on the effects of relative density and interface materials on shear behavior can guide engineers in selecting appropriate materials and construction methods to optimize the performance of PV pile foundations or similar structures. The relevant content has been supplemented in the manuscript, please see lines 322-326, lines 389-393.

 

Comments 11: Summarize key numerical results clearly and highlight their significance for advancing geotechnical engineering practices. Provide specific, practical recommendations (e.g., thresholds for relative density or normal pressure for pile foundation design) derived from the study’s findings. Suggest concrete next steps for research, such as testing the formula on other soil types or under additional conditions (e.g., temperature or moisture variations).

Response 11: We sincerely thank the reviewer for their valuable feedback. Below, we summarize the key numerical results, highlight their significance for advancing geotechnical engineering practices, provide specific practical recommendations derived from our study's findings, and suggest concrete next steps for future research.

Summary of Key Numerical Results and Their Significance

1.Increase in Cohesion and Angle of Internal Friction with Relative Density: The cohesion and angle of internal friction of sandy gravel both increase with increasing relative density. This finding is crucial for understanding the mechanical behavior of sandy gravel formations, particularly in engineering contexts where relative density may change due to factors such as construction activities.

2.Transition from Strain Hardening to Strain Softening Behavior: The shear stress-shear displacement curves transition from strain hardening to strain softening with increasing relative density. This transition provides important insights into the failure mechanisms of sandy gravel formations under shear loading.

3.Linear Increase in Shear Strength with Relative Density: The shear strength of sandy gravel increases linearly with increasing relative density. This linear relationship can be used to predict the shear strength of sandy gravel formations with varying relative densities.

4.Interface Shear Strength Characteristics: The interfacial shear strengths of sandy gravel with steel and concrete interfaces both increase linearly with increasing relative density. The interface friction angle increases with relative density for the concrete interface but remains constant for the steel interface. These findings are essential for the design of pile foundations and other structures in sandy gravel geological conditions.

Practical Recommendations Derived from the Study’s Findings

Construction can lead to loosening or compaction of the stratum, and long-term loading may also lead to changes in the relative density of the stratum, all of which need to be taken into account when designing pile foundations.

Next Steps for Research

To further advance our understanding of the shear behavior of sandy gravel and other soil types, we recommend the following concrete next steps for research:

1.Testing the Formula on Other Soil Types: The shear strength formula developed in this study should be tested on other soil types, such as clay and silt, to determine its applicability and limitations.

2.Additional Testing Conditions: Additional testing conditions, such as varying temperatures, roughness, and moisture contents, should be investigated to understand their effects on the shear behavior of sandy gravel.

3.Long-Term Performance Evaluation: Long-term monitoring and evaluation of pile foundations and other structures in sandy gravel geological conditions are needed to assess their performance over time and under varying load conditions.

Round 3

Reviewer 2 Report

Comments and Suggestions for Authors

The authors have addressed the reviewer comments in a thoughtful manner. They have provided clarifications and have made necessary adjustments in the manuscript. However, certain aspects can be further improved or clarified to enhance the robustness of the study. The response addresses specific queries but some clarifications and expansions could strengthen the credibility of the paper.

• The abstract does not contain numerical or statistical results, which are important for summarizing the key findings of the study. It would be beneficial to include some key numerical outcomes, such as R² values, changes in shear strength, or relative density.
•  The quality of most of the figures needs improvement. Consider revising them for better clarity and precision.
• The classification of the soil as "sandy gravel" is mentioned but not sufficiently explained. There is no clear mention of how the soil was categorized and which specific tests were performed to classify it as sandy gravel. This part is very important for the validity of the study and should be clearly outlined, including the procedures or standards followed in the classification.
• The authors have included R² values and validation against experimental data, which is a positive step. It would be beneficial to expand this by including error margins, testing conditions, and more context for the validation datasets used (e.g., geographic diversity). A more robust statistical analysis would strengthen the conclusions drawn from the data.
• The conclusion still includes descriptive paragraphs but lacks quantitative results or statistical metrics. Including key figures and statistics, such as the R² values, error margins, and the impact of relative density, would make the conclusion more impactful and evidence-based.

Comments on the Quality of English Language

The English could be improved to more clearly express the research.

Author Response

Comments 1: The abstract does not contain numerical or statistical results, which are important for summarizing the key findings of the study. It would be beneficial to include some key numerical outcomes, such as R² values, changes in shear strength, or relative density.

Response 1: We sincerely thank the reviewer for their valuable feedback. We have added relevant content to the abstract based on your suggestion, please see lines 20-32.

 

Comments 2: The quality of most of the figures needs improvement. Consider revising them for better clarity and precision.

Response 2: We sincerely thank the reviewer for their valuable feedback. We have redrawn all the figures in the manuscript in tif format, which is clearer than the previous png format. Since figures may lose clarity when inserted into a word file or converted to a pdf file, we uploaded a summary of all the figures in a zip package for viewing.

 

Comments 3: The classification of the soil as "sandy gravel" is mentioned but not sufficiently explained. There is no clear mention of how the soil was categorized and which specific tests were performed to classify it as sandy gravel. This part is very important for the validity of the study and should be clearly outlined, including the procedures or standards followed in the classification.

Response 3: We sincerely thank the reviewer for their valuable feedback. Sandy gravel is a common name for gravel with sand (ASTM D2487-17), which refers to the coarse-grained soil with its major proportion (mass content >50%) being gravel particles (of sizes from 4.75 to 75 mm) and its minor proportion being sand particles (of sizes from 0.075 to 4.75 mm). We determined the type of this soil by sieving. We have added relevant content to the manuscript, please see lines 86-89.

Reference: Deng Z Z, Wang G, Jin W, et al. Characteristics and quantification of fine particle loss in internally unstable sandy gravels induced by seepage flow[J]. Engineering Geology, 2023, 321: 107150.

 

Comments 4: The authors have included R² values and validation against experimental data, which is a positive step. It would be beneficial to expand this by including error margins, testing conditions, and more context for the validation datasets used (e.g., geographic diversity). A more robust statistical analysis would strengthen the conclusions drawn from the data.

Response 4: We sincerely thank the reviewer for their valuable feedback. We add to the manuscript an analysis of the errors and validation data on sand-gravel mixtures from the shores of the Caspian Sea, please see Table 4. The results show that the formula is well applicable to sand, sand-gravel mixtures, and sandy gravel in direct shear tests with an error margin within 4%. The applicability of this formula in other tests, such as the triaxial test, will be studied in our next step.

 

Comments 5: The conclusion still includes descriptive paragraphs but lacks quantitative results or statistical metrics. Including key figures and statistics, such as the R² values, error margins, and the impact of relative density, would make the conclusion more impactful and evidence-based.

Response 5: We sincerely thank the reviewer for their valuable feedback. We have added relevant content to the conclusion section based on your suggestion, please see lines 373-393.