Multi-Scale Mechanisms for Permeability Evolution in Remolded Fault Gouge: From Mineral-Particle Migration to Pore Structure

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The manuscript investigates the multi-scale mechanisms that govern permeability evolution in remolded fault gouge through an integrated program of in-situ field testing, laboratory permeability measurements, and microstructural analyses. The topic is technically relevant and of clear engineering value. However, substantial revision is required to meet publication standards.

1. The proposed remolded sample preparation method that combines mineral self-cementation with directional compaction is promising, but its novelty relative to prior approaches has not been clearly articulated. Please add a comparative discussion that positions this technique against existing methods and highlights its advantages and specific contributions.

2. The manuscript does not include statistical significance testing to substantiate the reported differences between in-situ and remolded samples (for example, Table 4 and Table 5). Please report uncertainty consistently, including error bars or confidence intervals, the number of replicates, and appropriate significance tests. Reporting effect sizes will also strengthen the evidence.

3. Table 5 compares mechanical parameters between in-situ and remolded samples, including cohesion and friction angle for the in-situ material. The procedure used to obtain in-situ shear strength is not stated. Please clarify whether these values come from field shear tests, back-analysis of field data, or previously published studies, and provide the test conditions and analysis steps.

4. Allowing the remolded sample to self-cement is an interesting idea. The methods mention adding water during mixing followed by air-drying for stability. Please specify the curing protocol, including duration and environmental conditions such as temperature and humidity, and indicate whether any chemical additives were used to promote cementation. A brief explanation of the intended mechanism would improve transparency.

5. Ensure consistent units and terminology throughout. For example, the table heading “Destory hydraulic gradient if”appears to be a typographical error and should be corrected to “Destroy hydraulic gradient” or, more clearly, “Failure hydraulic gradient.” Use the symbol ν (nu) for the Poisson ratio rather than μ to avoid confusion.

6. Several typographical errors require correction. For example, the section heading “Introduciton” should be “Introduction,” the caption of Figure 1c contains a repeated phrase “shows shows,” the caption of Figure 7 uses “curces” instead of “curves,” and Table 4 uses “Destory,” which should be “Destroy” or preferably “Failure.” A thorough proofread is recommended.

7. Please add one or two sentences in the Discussion or Conclusions that address the generalizability of the findings. While this study focuses on a compressional fault gouge from a single site, the identified mechanisms such as cement dissolution and pore collapse may be relevant to other fault zones with similar mineralogy. Briefly note limitations, for example potential differences in other lithologies or sample-size effects, to delineate the scope of applicability.

Author Response

Reviewer 1

The manuscript investigates the multi-scale mechanisms that govern permeability evolution in remolded fault gouge through an integrated program of in-situ field testing, laboratory permeability measurements, and microstructural analyses. The topic is technically relevant and of clear engineering value. However, substantial revision is required to meet publication standards.

1. The proposed remolded sample preparation method that combines mineral self-cementation with directional compaction is promising, but its novelty relative to prior approaches has not been clearly articulated. Please add a comparative discussion that positions this technique against existing methods and highlights its advantages and specific contributions.

A: We thank the reviewer for the constructive feedback. In the revised manuscript, we have added a comparative discussion that positions our method against traditional compaction techniques. Specifically, for the preparation of remolded samples from weak fault gouge and similar materials, mainstream methods mainly fall into two categories, both with significant limitations:

Traditional compaction methods (e.g., impact compaction, static compaction): These methods compact materials using external mechanical energy to simulate in-situ compaction. However, they only control macroscopic density and cannot recover the natural fabric anisotropy commonly found in fault zones (such as particle alignment and lamination-like structures). Moreover, these methods completely overlook the contribution of geological processes, such as mineral hydrolysis, precipitation, and cementation, which contribute to the strength of the rock mass. Furthermore, high-intensity impact or static pressure can lead to particle breakage, altering the original gradation and mechanical properties.

Binder simulation methods: These methods add external binders (such as cement, resin, or gypsum) to simulate the strength of the rock mass. However, the chemical properties, mechanisms, and long-term behaviors of these binders differ fundamentally from those of natural mineral cementation (such as silica, calcite, etc.), leading to a lack of chemical authenticity. Additionally, the distribution of binders within the pore space is uneven, leading to the formation of non-natural “cement clusters,” which further affect the mechanical and seepage responses of the sample.

To more clearly highlight the advantages of our approach, we have added Table 1, which compares these methods and emphasizes the strengths of our technique in terms of structural fidelity, chemical authenticity, and representativeness for hydraulic and mechanical behavior.

Table 1. List of remold sample preparation methods in fault zone.

Methods	Traditional compaction method	Binder simulation method	Our method
Definition	External mechanical energy to simulate in-situ compaction	External binders to simulate the strength of the rock mass	Mineral self-cementation combined with directional compac-tion
Advantages	Easy operation	High bonding strength	Restoration of stress conditions
Shortcomings	a. Anisotropic irreversibility b. Absence of geological processes c. Mechanical crushing alters particle size distribution	a. Differences in cementation b. Bonding distribution non-uniformity	a. complex sample preparation process
References	Ma[22,24]; Gui[23]; Huang[25]	Yu[18,20]; Ma[19]	/

 

2. The manuscript does not include statistical significance testing to substantiate the reported differences between in-situ and remolded samples (for example, Table 4 and Table 5). Please report uncertainty consistently, including error bars or confidence intervals, the number of replicates, and appropriate significance tests. Reporting effect sizes will also strengthen the evidence.

A: We sincerely thank the reviewer for this valuable feedback. We fully agree that statistical significance testing is important for substantiating the reported differences between in-situ and remolded samples, and we appreciate the suggestion to report uncertainty consistently.

Regarding the in-situ permeability tests, the challenge we face is the limited dataset, which is why laboratory-scale permeability tests are typically used as a substitute in such studies. In the revised manuscript, we have clarified the differences in field conditions and specific geological settings, particularly highlighting the scale effect that may influence the results. We also emphasize that the limited data available from field permeability testing makes it difficult to perform comprehensive statistical analysis.

For the laboratory permeability tests, we have included a more detailed uncertainty analysis, referencing our previously published work on laboratory permeability testing to validate the accuracy of the results. As the manuscript has just been accepted, we are currently unable to provide additional data beyond what is presented in this study.

We hope that this explanation provides clarity regarding the data limitations and the need for the substitution of field data with laboratory tests.

 

Fig.1 Differential permeability of remold samples in fault zone. (a) Particle size distribution; (b) Relationship between permeability coefficient and fine-grained content.

 

3. Table 5 compares mechanical parameters between in-situ and remolded samples, including cohesion and friction angle for the in-situ material. The procedure used to obtain in-situ shear strength is not stated. Please clarify whether these values come from field shear tests, back-analysis of field data, or previously published studies, and provide the test conditions and analysis steps.

A: We thank the reviewer for pointing this out. In the revised manuscript, we have clarified the procedure used to obtain the in-situ shear strength values, specifically the cohesion (c) and internal friction angle (φ).

The values for cohesion and friction angle of the in-situ material were determined through field shear tests conducted in accordance with the GB/T 50266-2013 Standard for Rock Testing in Engineering Geology. The procedure is as follows:

Sample Preparation: In-situ shear test specimens were carefully prepared from fault gouge exposures in trench or adit (50 cm × 50 cm × 30 cm). To preserve the original structure, the specimens were carefully exposed and separated to ensure that the test specimen remained within the predetermined shear plane.

Shear Testing: After the specimen was prepared, a normal stress (σ_n) was applied and stabilized. A hydraulic jack was used to apply horizontal shear force during a slow and steady loading process until complete failure of the specimen. Displacements in both the normal and shear directions were monitored using digital micrometers.

Repetition and Analysis: Multiple tests were conducted under different normal stresses (σ_n) to obtain peak shear strength (τ_f) values. The data was then plotted on a τ_f - σ_n curve (Mohr-Coulomb failure envelope), where the intercept represents cohesion (c) and the slope gives the tangent of the internal friction angle (φ). Regression analysis was performed to derive the final values for c and φ.

 

4. Allowing the remolded sample to self-cement is an interesting idea. The methods mention adding water during mixing followed by air-drying for stability. Please specify the curing protocol, including duration and environmental conditions such as temperature and humidity, and indicate whether any chemical additives were used to promote cementation. A brief explanation of the intended mechanism would improve transparency.

A: We thank the reviewer for the helpful suggestion. In the revised manuscript, we have provided a more detailed explanation of the self-cementation process, as follows:

During the sample preparation, only distilled water was used to mix and stir the materials, and no external cementing agents were added. After mixing, the samples were compacted into shape and then subjected to a 7-day controlled self-cementation process. This process was conducted in a constant-temperature environment (25 ± 2°C), and the relative humidity was gradually reduced from 95% to 50% through a programmed procedure, simulating natural diagenetic conditions.

The purpose of this method is to activate the material's own physicochemical activity, relying on mechanisms such as dissolution-recrystallization of soluble salts and hydration-stabilization of clay minerals. These processes result in the formation of self-generated cementation between particles, which significantly enhances the structural strength of the remolded samples.

 

5. Ensure consistent units and terminology throughout. For example, the table heading “Destory hydraulic gradient if”appears to be a typographical error and should be corrected to “Destroy hydraulic gradient” or, more clearly, “Failure hydraulic gradient.” Use the symbol ν (nu) for the Poisson ratio rather than μ to avoid confusion.

A: We thank the reviewer for pointing out the typographical errors. We have carefully reviewed the manuscript and made the necessary corrections to ensure consistency in terminology and units. Specifically:

• The phrase "Destory hydraulic gradient if" in Table 4 has been corrected to "Failure hydraulic gradient" for clarity and consistency with the terminology used throughout the manuscript.
• The symbol for Poisson's ratio has been corrected to ν (nu) instead of μ (mu) to avoid confusion with other variables.
• Other typographical errors, including the heading “Introduciton” (which has been corrected to “Introduction”) and the repeated phrase "shows shows" in Figure 1c, have also been addressed.

These corrections have been made throughout the manuscript to improve its clarity and readability.

 

6. Several typographical errors require correction. For example, the section heading “Introduciton” should be “Introduction,” the caption of Figure 1c contains a repeated phrase “shows shows,” the caption of Figure 7 uses “curces” instead of “curves,” and Table 4 uses “Destory,” which should be “Destroy” or preferably “Failure.” A thorough proofread is recommended.

A: We appreciate the reviewer’s attention to detail. We have thoroughly proofread the manuscript and corrected the typographical errors as pointed out:

• The heading "Introduciton" has been corrected to "Introduction".
• The repeated phrase "shows shows" in the caption of Figure 1c has been corrected to a single "shows".
• The term "curces" in Figure 7 has been corrected to "curves".
• The term "Destory" in Table 4 has been corrected to "Failure".

These and other minor typographical errors have been fixed, and the manuscript has been carefully proofread to ensure consistency and accuracy throughout.

 

7. Please add one or two sentences in the Discussion or Conclusions that address the generalizability of the findings. While this study focuses on a compressional fault gouge from a single site, the identified mechanisms such as cement dissolution and pore collapse may be relevant to other fault zones with similar mineralogy. Briefly note limitations, for example potential differences in other lithologies or sample-size effects, to delineate the scope of applicability.

A: We thank the reviewer for the valuable suggestion. In the revised manuscript, we have added a brief discussion in the Conclusions section addressing the generalizability of our findings. While this study focuses on a compressional fault gouge from a single site, the mechanisms identified—such as cement dissolution and pore collapse—are likely relevant to other fault zones with similar mineralogy.

However, we also acknowledge that the relative importance of these mechanisms may vary depending on lithology (e.g., content of rigid particles) and structural context. We mention these variations to delineate the scope of applicability of our findings.

The revised Conclusions section now includes the following statements:

"The findings of this study go beyond the specific fault gouge investigated, as the identified mechanisms such as cement dissolution and pore collapse are likely applicable to other fault zones with similar mineral compositions. However, these mechanisms’ relative significance may differ with lithological variations, such as rigid particle content or structural differences, which limits the generalizability of the conclusions. Future studies could investigate these effects across different geological settings."

This addition aims to highlight the broader applicability of the study's findings while acknowledging potential limitations.

 

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript investigates the critical issue of permeability differences between in-situ and remolded fault gouge samples, a significant source of uncertainty in geotechnical parameterization for dam foundations. The study provides valuable insights into why laboratory tests often underestimate in-situ permeability and overestimate shear strength. The research is well-structured, the methodology is comprehensive, and the topic is highly relevant to rock mechanics and geotechnical engineering. However, there are several limitations in this paper. My main comments are as follows:

1. The proposed "mineral self-cementation combined with directional compaction" method is a key claim of novelty. However, the manuscript lacks a direct and quantitative comparison demonstrating how this method better replicates the in-situ mechanical and hydraulic properties compared to traditional compaction methods.
2. The discussion attributes this to "artificially enhanced cohesion" but does not provide a satisfactory micro-mechanical explanation. The analysis should delve deeper into how the remolding process (e.g., particle rearrangement, destruction of natural fabric, and the purported "self-cementation") leads to such a dramatic overestimation of cohesion.
3. While NMR and SEM-EDS analyses are presented, the connection between the pore size distribution data and the permeability values remains qualitative. 
4. The term "destory hydraulic gradient" in Table 4 is a typo and should be "failure hydraulic gradient" or "destruction hydraulic gradient" for consistency with the text.
5. The literature review is adequate but could be better integrated with the broader context of fault zone hydrogeology and permeability .
6. Ensure consistent use of italics for variables throughout the text and in equations.

Comments on the Quality of English Language

N/A

Author Response

Reviewer 2

This manuscript investigates the critical issue of permeability differences between in-situ and remolded fault gouge samples, a significant source of uncertainty in geotechnical parameterization for dam foundations. The study provides valuable insights into why laboratory tests often underestimate in-situ permeability and overestimate shear strength. The research is well-structured, the methodology is comprehensive, and the topic is highly relevant to rock mechanics and geotechnical engineering. However, there are several limitations in this paper. My main comments are as follows:

 

1. The proposed "mineral self-cementation combined with directional compaction" method is a key claim of novelty. However, the manuscript lacks a direct and quantitative comparison demonstrating how this method better replicates the in-situ mechanical and hydraulic properties compared to traditional compaction methods.

A: We thank the reviewer for the constructive feedback. In the revised manuscript, we have added a comparative discussion that positions our method against traditional compaction techniques. Specifically, for the preparation of remolded samples from weak fault gouge and similar materials, mainstream methods mainly fall into two categories, both with significant limitations:

Traditional compaction methods (e.g., impact compaction, static compaction): These methods compact materials using external mechanical energy to simulate in-situ compaction. However, they only control macroscopic density and cannot recover the natural fabric anisotropy commonly found in fault zones (such as particle alignment and lamination-like structures). Moreover, these methods completely overlook the contribution of geological processes, such as mineral hydrolysis, precipitation, and cementation, which contribute to the strength of the rock mass. Furthermore, high-intensity impact or static pressure can lead to particle breakage, altering the original gradation and mechanical properties.

Binder simulation methods: These methods add external binders (such as cement, resin, or gypsum) to simulate the strength of the rock mass. However, the chemical properties, mechanisms, and long-term behaviors of these binders differ fundamentally from those of natural mineral cementation (such as silica, calcite, etc.), leading to a lack of chemical authenticity. Additionally, the distribution of binders within the pore space is uneven, leading to the formation of non-natural “cement clusters,” which further affect the mechanical and seepage responses of the sample.

To more clearly highlight the advantages of our approach, we have added Table 1, which compares these methods and emphasizes the strengths of our technique in terms of structural fidelity, chemical authenticity, and representativeness for hydraulic and mechanical behavior.

Table 1. List of remold sample preparation methods in fault zone.

Methods	Traditional compaction method	Binder simulation method	Our method
Definition	External mechanical energy to simulate in-situ compaction	External binders to simulate the strength of the rock mass	Mineral self-cementation combined with directional compac-tion
Advantages	Easy operation	High bonding strength	Restoration of stress conditions
Shortcomings	a. Anisotropic irreversibility b. Absence of geological processes c. Mechanical crushing alters particle size distribution	a. Differences in cementation b. Bonding distribution non-uniformity	a. complex sample preparation process
References	Ma[22,24]; Gui[23]; Huang[25]	Yu[18,20]; Ma[19]	/

 

2. The discussion attributes this to "artificially enhanced cohesion" but does not provide a satisfactory micro-mechanical explanation. The analysis should delve deeper into how the remolding process (e.g., particle rearrangement, destruction of natural fabric, and the purported "self-cementation") leads to such a dramatic overestimation of cohesion.

A: We thank the reviewer for this insightful comment. We agree that a more detailed micro-mechanical explanation is necessary to clarify how the remolding process leads to the observed overestimation of cohesion. In the revised manuscript, we have expanded the discussion to provide a deeper analysis of the mechanisms at play.

The artificially enhanced cohesion observed in the remolded samples can be attributed to the combination of two factors: (1) fabric reorganization during the remolding process, and (2) self-cementation that occurs during the controlled curing phase.

• Fabric reorganization: During the remolding process, particles are rearranged into a more uniform and compact configuration, which improves the inter-particle contact and increases mechanical interlocking. This reorganization results in enhanced cohesion due to the optimized particle arrangement and minimized void spaces. While this improves cohesion, it does not necessarily represent the natural cohesion found in in-situ fault zones, where the cohesion is influenced by the original mineralogical structure and weak cementation.
• Self-cementation: The self-cementation process (activated during the curing phase) leads to the formation of new cement bonds between particles, especially through dissolution-recrystallization of soluble salts and hydration of clay minerals. This process enhances the overall cohesion of the sample. However, the cementation created in the lab is more uniform and strong than the natural weak cementation in the fault zone. In nature, the cohesion is typically weaker and more localized, controlled by natural mineral cementation (e.g., silica, calcite precipitates), which is often irregular and heterogeneous.

These two factors, fabric reorganization and self-cementation, contribute to a drastic overestimation of cohesion when compared to the in-situ material. The overestimation occurs because the remolded samples have a more uniform, densely packed structure, and stronger cement bonds, both of which do not fully replicate the natural heterogeneity and weaker cementation of in-situ fault gouge.

Section 4.2:

“Reconstitution significantly improved the mechanical properties of fault breccia ma-terials by altering the soil structure, increasing density, and optimizing intergranular contact. The remolding process enhances cohesion through three primary mechanisms: particle rearrangement, destruction of the natural fabric, and mineral self-cementation. The enhanced cohesion observed in remolded samples results primarily from increased particle interlocking and bonding during compaction and self-cementation, processes ab-sent in the natural, weakly bonded fault gouge structure.”

 

3. While NMR and SEM-EDS analyses are presented, the connection between the pore size distribution data and the permeability values remains qualitative.

A: We thank the reviewer for raising this important point. We fully agree that while the NMR and SEM-EDS analyses provide valuable insights into the pore structure, the connection between the pore size distribution and permeability values remains qualitative at this stage.

As the reviewer pointed out, permeability modeling is a classic research challenge, and our study primarily aims to investigate the differences between in-situ and remolded samples, with a focus on understanding the underlying mechanisms. In this preliminary study, we have analyzed the self-cementation mechanism but have not yet developed a full quantitative relationship between pore structure and permeability.

We plan to develop a permeability model once we gain a deeper understanding of the mechanisms involved. This would allow us to quantitatively relate the pore size distribution to the permeability values. In fact, we are actively exploring the influence of pore structure on permeability evolution in our ongoing research. As mentioned in our previous work, we have investigated the relationship between porosity and permeability evolution using NMR coupling tests in carbonate rocks. Two related studies that we have conducted include:

Wang, H.-M.; Zhou, Q.; Sheng, J.-C.; Luo, Y.-L.; Liu, J.; Liu, X.X. Effect of long-term infiltration on porosity-permeability evolution in carbonate rocks: An online NMR coupling penetration test[J]. J. Hydrol., 2023, 617, 129029.

Wang, H.-M.; Tian, J.-L.; Lin, J.-X.; Liu, J.; Maharjan, N.; Sheng, J.-C. NMR investigation on pore-scale stress sensitivity during pore fluid pumping cycles: Implication for geological fluid storage[J]. Int. J. Hydrogen Energy, 2025, 120, 1-12.

These studies provide a foundation for further developing a quantitative permeability model based on pore structure. We intend to extend this research in future work to refine the understanding of permeability evolution in remolded fault gouge and other geological materials.

 

4. The term "destory hydraulic gradient" in Table 4 is a typo and should be "failure hydraulic gradient" or "destruction hydraulic gradient" for consistency with the text.

A: We thank the reviewer for pointing out this typographical error. We have corrected the term "destory hydraulic gradient" in Table 4 to "failure hydraulic gradient" for consistency with the terminology used throughout the manuscript. This change has been made to improve clarity and ensure consistency with the terminology in the text.

 

5. The literature review is adequate but could be better integrated with the broader context of fault zone hydrogeology and permeability.

A: We sincerely thank the reviewer for the valuable feedback. As the manuscript focuses on comparing the differences between remolded and in-situ samples, the Introduction primarily addresses the sample preparation differences rather than the detailed results of in-situ permeability tests and the associated hydrogeological conditions.

We agree with the reviewer that a broader integration of the hydrogeological context would strengthen the manuscript. In response, we have expanded the in-situ permeability results section to include a more detailed discussion of the hydrogeological conditions at the study site. This includes factors such as the fault zone characteristics, geological formation, and how these influence the permeability of the fault gouge in the in-situ conditions.

"The F2 fault exhibits a width of approximately 20 cm, a dip angle of 75–80°, and an exposed elevation of approximately 2070 m. This fault is located 105 m below the reservoir's normal storage level of 2175 m. Consequently, sample width was determined based on the fault breccia zone width and fault plane dip angle to ensure that samples accurately reflected in-situ geological conditions."

 

6. Ensure consistent use of italics for variables throughout the text and in equations.

A: We thank the reviewer for this important suggestion. We have thoroughly checked the manuscript to ensure consistent use of italics for variables throughout the text and in the equations. All variables, including physical parameters such as cohesion (c), friction angle (φ), permeability (k), Poisson's ratio (ν), and others, have been properly italicized to adhere to standard scientific writing conventions.

This change has been applied consistently across the manuscript, including in the equations and tables.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

I recommend acceptance​ of this manuscript now.