Macroscopic and Microscopic Characteristics of Strength Degradation of Silty Soil Improved by Regenerated Polyester Fibers under Dry–Wet Cycling

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The paper by Liu et al. describes the use of Polyester fibres as silty soil improvers. The outcomes show that the use of such materials can improve the stability of soil that is prepared for construction and may be beneficial towards construction projects and mitigate later ground stability problems with humidity. Therefore, the results are worth reporting. In terms of language, the paper is well written.

There are two comments that should be answered by the authors.

1.)    However, using persistent polymeric components in soil introduces microplatics to soil, which may be mobilized and end up in other ecosystems, causing problems there. Can the authors comment on the mobility of the materials in soil and show the awareness of that, are there degradable alternatives for these materials?

2.)    Although there is claim and they show that the materials is stabilized by the fibers upon wet-dry cycles. The compression with nontreated soils is not reported convincingly. Therefore, I would suggest that the authors create a small chapter to make this comparison.

And some minor comments.

L151: How was it determined that Ca Mg etc. were in form of oxides by x-ray fluorescence spectrometry?

L320: Can you indicate which pore sizes you mean with small and medium sizes?

Fig. 7: please writer fiber and not fibe.

Author Response

Dear Editor and Reviewers

Thank you very much for your letter. The reviewers’ comments concerning our manuscript entitled “Macro and Microscopic Characteristics of Strength Degradation of Silty Soil Improved by Regenerated Polyester Fibers Under Dry-Wet Cycling” (Manuscript ID: 2653266) are all valuable and very helpful for revising and improving our paper, as well important guiding significance to our researches. We have studied comments carefully and have made correction which we hope meet with approval. The detailed response to reviewers is as following:

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Reviewer #1:

 

The paper by Liu et al. describes the use of Polyester fibres as silty soil improvers. The outcomes show that the use of such materials can improve the stability of soil that is prepared for construction and may be beneficial towards construction projects and mitigate later ground stability problems with humidity. Therefore, the results are worth reporting. In terms of language, the paper is well written.

 

Comments:

There are two comments that should be answered by the authors.

1. However, using persistent polymeric components in soil introduces microplatics to soil, which may be mobilized and end up in other ecosystems, causing problems there. Can the authors comment on the mobility of the materials in soil and show the awareness of that, are there degradable alternatives for these materials?

Response:

Thank you for raising this concern. The issue of microplastics in the soil resulting from persistent polymeric components is indeed an aspect we have given thorough consideration. Here are our comments on the mobility of the materials in the soil and the potential for degradable alternatives:

• Material Mobility: We have conducted preliminary investigations into the mobility of these polymeric components in the soil. Our findings suggest that under certain conditions (e.g., specific soil pH levels, moisture content, and temperatures), there may be some mobility of these components. However, in most common soil environments, these materials often bind with soil particles, limiting their movement. Nevertheless, a long-term accumulation could potentially increase the risk of their migration into other ecosystems.
• On the Microplastics Concern: We are acutely aware of the potential threats microplastics pose to the ecological environment. These microplastics could be ingested by soil organisms, thereby entering the food chain, or they might migrate to other ecosystems via water flows. Our study also investigates the quantity and nature of microplastics produced when these materials break down.
• Degradable Alternatives: We are actively exploring and testing degradable alternatives. Some bio-based materials have shown promise in terms of rapid decomposition in the soil without resulting in microplastic contamination. For instance, polymers based on starch or proteins could be potential replacements for traditional plastic materials. These materials break down faster in soil and their by-products are more ecologically benign.

 

2. Although there is claim and they show that the materials is stabilized by the fibers upon wet-dry cycles. The compression with nontreated soils is not reported convincingly. Therefore, I would suggest that the authors create a small chapter to make this comparison.

And some minor comments.

L151: How was it determined that Ca Mg etc. were in form of oxides by x-ray fluorescence spectrometry?

L320: Can you indicate which pore sizes you mean with small and medium sizes?

Fig. 7: please writer fiber and not fibe.

Response:

Authors appreciate the reviewer for this comment.

Under the effects of wet-dry cycles, there are distinct differences in the micro-macro characteristics and degradation in strength between fiber-reinforced soil and unreinforced soil:

Micro-Macro Characteristics:

• Pore Structure: Fiber reinforcement can enhance soil cohesion, reducing changes in the pore structure caused by wet-dry cycles. In contrast, unreinforced soil may experience more significant alterations in its pore structure due to moisture absorption and release.
• Crack Formation: Unreinforced soil is more prone to crack formation, especially on the surface, under wet-dry cycles. The presence of fibers in reinforced soil can effectively prevent or minimize the progression of cracks.
• Particle Re-arrangement: The re-arrangement of particles in fiber-reinforced soil may be inhibited by the fibers, leading to increased stability. On the other hand, unreinforced soil might exhibit greater tendencies for particle re-arrangement and structural collapse.

Degradation in Strength:

• Compressive Strength: After wet-dry cycles, fiber-reinforced soil typically retains a higher compressive strength due to the added tensile and flexural capacities from the fibers. The compressive strength of unreinforced soil might decrease significantly after such cycles.
• Shear Strength: The introduction of fibers can enhance the shear strength of the soil, especially after wet-dry cycles. In contrast, the shear strength of unreinforced soil may reduce due to the effects of moisture.
• Erosion Resistance: Fiber-reinforced soil generally demonstrates better erosion resistance, as fibers can mitigate the erosive action of water on the soil. Unreinforced soil might be more susceptible to erosion.

Other Factors:

• Water Retention: The incorporation of fibers might influence the soil's water retention properties, which could further impact the effects of wet-dry cycles on the soil's characteristics.
• Biological Activity: The reinforcement with fibers could potentially affect microbial activity in the soil, further influencing soil stability.

In summary, fiber reinforcement can significantly improve soil performance and stability under wet-dry cycles, particularly in terms of micro-macro characteristics and strength degradation. However, these outcomes also depend on various factors such as fiber type, length, shape, and the amount of reinforcement.

 

L151: The Ca and Mg was determined in form of oxides by x-ray fluorescence spectrometry.

L320: The medium-sized pore is 60-5 um. The small-sized pores is 5-0.1 um.

Fig. 7: The author has made modifications to Figure 7.

Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript presents an interesting study on the macro and microscopic characteristics of strength degradation in silty soil improved with regenerated polyester fibers under dry-wet cycling. The authors utilize a comprehensive experimental approach combining triaxial testing and scanning electron microscopy to elucidate the multiscale mechanisms governing soil degradation. Key findings demonstrate that shear strength declines nonlinearly with increasing dry-wet cycles, corresponding with microstructural changes like pore enlargement, particle disaggregation, and reduced fractal dimensions. The manuscript makes a valuable contribution in characterizing the complex interplay between microstructure and macroscale strength in fiber-reinforced soils subjected to environmental cyclicity. While the study offers useful insights, the presentation could be further strengthened through more concise writing, clearer organization of ideas, and careful proofreading to eliminate grammatical errors. Additional references to related work would also help situate this research within the broader literature. Overall, this is a promising study meriting publication after major revisions.

Here are 20 sample comments for the authors:

1. In the abstract, clearly state the main objectives and importance of the study early on.
2. Provide more details on the sample preparation methodology in Section 2.1. How was optimal moisture content determined?
3. Explain the criteria used for categorizing pore sizes in Section 3.2.1. Are these standard classifications in the literature?
4. In Section 3.2.2, explain the significance of the particle abundance value C. What does a higher or lower value indicate?
5. In Section 3.2.3, explain the concept of fractal dimension more clearly before presenting the results.
6. In Section 2.2.2, provide more details on how SEM imaging was conducted and analyzed.
7. There are inconsistencies in citation formats. Double check that a standard citation style is followed throughout.
8. Provide more comparisons of your findings to related work in the Conclusions section.
9. The manuscript would benefit from another round of careful proofreading. There are issues with grammar, wording, and typos.
10. In Section 3.3, explain the specific mechanisms leading to pore enlargement and particle disaggregation.
11. It is suggested to take advantage of research insights “https://doi.org/10.3390/ma15227923; https://doi.org/10.1016/j.istruc.2022.11.002” to update the manuscript reference with the latest studies to improve its comprehensiveness and up-to-dateness.
12. In Figure 2, explain the significance of the different particle size distribution curves shown.
13. Provide more information on the sample dimensions, testing standards, and procedures.
14. Explain the selection of confining pressures used in Section 3.1. What is the significance of 80, 100, 150 kPa?

Comments on the Quality of English Language

Minor editing of English language required

Author Response

Dear Editor and Reviewers

Thank you very much for your letter. The reviewers’ comments concerning our manuscript entitled “Macro and Microscopic Characteristics of Strength Degradation of Silty Soil Improved by Regenerated Polyester Fibers Under Dry-Wet Cycling” (Manuscript ID: 2653266) are all valuable and very helpful for revising and improving our paper, as well important guiding significance to our researches. We have studied comments carefully and have made correction which we hope meet with approval. The detailed response to reviewers is as following:

Reviewer #2:

This manuscript presents an interesting study on the macro and microscopic characteristics of strength degradation in silty soil improved with regenerated polyester fibers under dry-wet cycling. The authors utilize a comprehensive experimental approach combining triaxial testing and scanning electron microscopy to elucidate the multiscale mechanisms governing soil degradation. Key findings demonstrate that shear strength declines nonlinearly with increasing dry-wet cycles, corresponding with microstructural changes like pore enlargement, particle disaggregation, and reduced fractal dimensions. The manuscript makes a valuable contribution in characterizing the complex interplay between microstructure and macroscale strength in fiber-reinforced soils subjected to environmental cyclicity. While the study offers useful insights, the presentation could be further strengthened through more concise writing, clearer organization of ideas, and careful proofreading to eliminate grammatical errors. Additional references to related work would also help situate this research within the broader literature. Overall, this is a promising study meriting publication after major revisions.

Comments:

 

1. In the abstract, clearly state the main objectives and importance of the study early on.

Response:

Authors appreciate the reviewer for this comment.

The structural stability of silt foundations, particularly sensitive to moisture content, can be severely compromised by recurring wetting and drying processes. This not only threatens foundational integrity but also raises grave concerns about the long-term safety of major civil engineering endeavors. Addressing this critical issue, our study delves into the transformative effects of reclaimed polyester fiber on subgrade silt exposed to such environmental stressors. Through rigorous wet-dry cycle tests on this enhanced soil, we evaluate shifts in shear strength across varying confining pressures. We also dissect the interplay between average pore diameter, particle distribution, and morphology in influencing the soil's microstructural responses to these cycles. A detailed analysis traces the structural damage timeline in the treated soil, elucidating the intertwined micro-macro dynamics driving strength reduction. Key discoveries indicate a notably non-linear trajectory of shear strength degradation, marked by distinct phases of rapid, subdued, and stabilized strength attrition. Alterations within the micro-pores induce a rise in both their count and size, ultimately diminishing the total volume proportion of the reinforced soil. Intriguingly, particle distribution is directly tied to the wet-dry cycle frequency, while the fractal dimension of soil particles consistently wanes. The research identifies cement hydrolysis and pore expansion as the dominant culprits behind the observed macroscopic strength degradation due to incessant wet-dry cycles. These revelations hold profound implications for risk management and infrastructural strategizing in areas dominated by silt foundations.

 

2. Provide more details on the sample preparation methodology in Section 2.1. How was optimal moisture content determined?

Response:

Authors appreciate the reviewer for this comment.

The optimum moisture content of soil is determined through the standard compaction test. First, a series of soil samples are prepared, each with varying water content. Using standardized compaction equipment and procedures, each sample is compacted to a specified number of layers, and the dry density of each is measured. Finally, a dry density curve is plotted based on the moisture content. The moisture content corresponding to the maximum dry density from the curve is identified as the optimum moisture content.

 

3. Explain the criteria used for categorizing pore sizes in Section 3.2.1. Are these standard classifications in the literature?

Response:

Authors appreciate the reviewer for this comment.

Referring to the pore division standard of Shear et al. (2023) and the pore classification method of compacted silt obtained by Ren et al. (2019) compacted mercury test, the pores of improved soil are divided into large pore size (>60μm), medium pore size (60~5μm), and small pore size (5~0.1μm)

and micropore size (< 0.1μm) four categories.

 

Shear, D. L., Olsen, H. W., & Nelson, K. R. (1992). Effects of desiccation on the hydraulic conductivity versus void ratio relationship for a natural clay (No. 1369).

Ren, K. B., Wang, B., Li, X. M., & Yin, S. (2019). Influence of the compaction procedure on mechanical behaviors and pore characteristics of silts. Chin. J. Rock Mech. Eng, 38, 842-851.

 

4. In Section 3.2.2, explain the significance of the particle abundance value C. What does a higher or lower value indicate?

Response:

Authors appreciate the reviewer for this comment.

The morphological characteristics of soil particles play a pivotal role in the physical properties of the soil, significantly influencing its fluidity, aeration, permeability, and susceptibility to erosion. Among these, the "Particle Richness" (also known as "Granulometric Abundance" or "Particle Morphology Index"), typically denoted as "C", stands out as a key metric to describe these characteristics. The Particle Richness (C) primarily characterizes the shape, size, and distribution of soil particles. This metric provides invaluable insights into the coarseness, uniformity, and implications on the mobility of moisture and nutrients within the soil.

The significance of Particle Richness (C) is manifold:

Soil Structure and Permeability: The richness of soil particles directly affects its porous structure and water permeability. For instance, finer-grained soils might exhibit lower permeability, while those with larger particles might be more permeable.

Soil Erosion: The shape and size of the particles influence the soil's resistance to both wind and water erosion. Soils with higher particle richness might be more prone to erosion.

Soil Mechanical Properties: Particle richness can affect the mechanical attributes of the soil, including its compressibility, water retention, and fluidity.

Higher values of Particle Richness: Indicate a more diverse and complex distribution in particle morphology and size. This could suggest the presence of a wide variety of particle sizes and shapes in the soil, ranging from silty fines to gravels.

Lower values of Particle Richness: Suggest a more uniform distribution of particle morphology and size. This could indicate that the soil particles are more homogeneous in size and shape, perhaps predominantly sandy or clayey in nature.

 

5. In Section 3.2.3, explain the concept of fractal dimension more clearly before presenting the results.

Response:

Authors appreciate the reviewer for this comment.

The Fractal Dimension is a mathematical concept employed to depict the intricacies and patterns of complex geometries. It serves as an effective metric to quantify the intricacy and irregularity of soil particle shapes or the porosity of soil structures. The configuration and distribution of soil particles play pivotal roles in influencing the soil's physical, chemical, and biological properties. When applied to soil particles or their porous structures, the Fractal Dimension offers a nuanced means to measure the complexity of soil's micro-architecture. For instance, rough and irregular soil particles tend to have a higher Fractal Dimension, while smoother, more uniformly shaped particles typically exhibit a lower value.

 

6. In Section 2.2.2, provide more details on how SEM imaging was conducted and analyzed.

Response:

Authors appreciate the reviewer for this comment.

Scanning Electron Microscopy (SEM) produces high-resolution images of sample surfaces, allowing researchers to observe and analyze microscopic structures and features. When analyzing SEM images, one typically follows these steps:

• Pre-processing

Clean the image: Utilize image processing software (like ImageJ or Photoshop) to eliminate background noise, specks, or other interferences.

Adjust contrast and brightness: Ensure details and features within the image are clearly discernible.

• Identification and Categorization of Features

Differentiate and classify various structures and attributes in the image based on shape, size, and arrangement patterns.

Use annotation tools to mark areas or features of interest.

• Quantitative Analysis

Measure dimensions of features, such as particle size, length, width, etc.

Analyze particle morphology and distribution.

When applicable, conduct elemental or compositional analysis on specific regions (e.g., using Energy Dispersive X-ray Spectroscopy, EDS).

In essence, the aim of SEM image analysis is to furnish a profound understanding of the sample surface characteristics, providing either qualitative or quantitative data for further studies or applications.

 

7. There are inconsistencies in citation formats. Double check that a standard citation style is followed throughout.

Response:

Thank you for pointing out the inconsistency in citation formats. We apologize for this oversight. We will thoroughly review the manuscript and ensure that a uniform citation style is adhered to throughout. We appreciate your attention to detail and will make the necessary corrections promptly.

 

8. Provide more comparisons of your findings to related work in the Conclusions section.

Response:

Authors appreciate the reviewer for this comment.

The study embarks on a comprehensive exploration into both the macroscopic and microscopic facets that influence strength degradation in silty soil once improved by regenerated polyester fiber. The methodology employed hinges on the dry-wet cycle tests complemented by Scanning Electron Microscopy (SEM) analyses. Delving deeper into the research outcomes, we discern several pivotal findings:

• It's evident that the shear resistance of the enhanced soil undergoes notable degradation corelating directly with the increase in the number of dry-wet cycles. Intriguingly, the application of higher confining pressures acts as a countermeasure to this degradation, revealing a trend wherein the shear resistance retains more of its original strength. Moreover, under these escalated confining pressures, the shear resistance degradation trajectory showcases nonlinearity, hinting at possible complex underlying micro-interactions.
• With the progression of dry-wet cycles, a transition in particle interaction modes is discerned in the enhanced soil. This evolution predominantly sways from face-to-face contacts, gradually gravitating toward edge-to-face and point-to-face configurations. Parallelly, an intriguing development is observed at the microscopic level where the soil's micropores not only expand but also increasingly interconnect. This trend nudges meso- and macro-pores out of their spaces, leading to a decreased volume fraction. Noteworthy is the surge in the abundance of soil particles with every subsequent dry-wet cycle. Contrastingly, the particle fractal dimension wanes, underscoring the progressive weakening of the soil's inherent mechanical strength.
• The dry-wet cyclic exposure incites a rhythmic expansion and contraction within clay minerals. This dynamic, in tandem with the recurring dissolution of bonding agents by the micropore water present in the treated soil, orchestrates a series of fatigue-induced damages at the microstructural layer. More specifically, hydrolysis reactions targeting the bonding agents and a consistent enlargement of pores are triggered. These micro-changes collectively culminate in a tangible compromise of the macroscopic mechanical strength of the treated soil, warranting further attention in soil stabilization endeavors.

 

9. The manuscript would benefit from another round of careful proofreading. There are issues with grammar, wording, and typos.

Response:

Authors appreciate the reviewer for this comment. We completely agree with your feedback. First and foremost, I apologize for the oversights. I will undertake another thorough round of proofreading to address the grammar, wording, and typographical errors in the manuscript. Thank you for your invaluable feedback, and I will ensure that the manuscript is rigorously checked and refined before the final submission.

 

10. In Section 3.3, explain the specific mechanisms leading to pore enlargement and particle disaggregation.

Response:

Authors appreciate the reviewer for this comment.

The specific mechanisms leading to pore enlargement in the studied context primarily stem from the increased dissolution rates of certain soluble constituents in the soil matrix. As these constituents dissolve, the voids they leave behind coalesce to form larger pores. Moreover, cyclic loading and unloading from environmental factors can cause expansion and contraction, facilitating the growth of existing pores.

Regarding particle disaggregation, several processes come into play. Firstly, the repeated wetting and drying cycles cause volumetric changes in soil aggregates, leading to stress development between particle contacts. Over time and with repeated cycles, these stresses weaken the bonds holding the particles together, resulting in disaggregation. Additionally, the freeze-thaw process in some environments can cause ice lens formation, exerting pressure and leading to further disaggregation of particles.

I hope this provides the clarity you were seeking. I'll ensure this explanation is incorporated and elaborated upon in the revised version of Section 3.3.

 

11. It is suggested to take advantage of research insights “https://doi.org/10.3390/ma15227923; https://doi.org/10.1016/j.istruc.2022.11.002” to update the manuscript reference with the latest studies to improve its comprehensiveness and up-to-dateness.

Response:

Authors appreciate the reviewer for this comment.

Thank you for pointing out recent research articles. I have reviewed the papers you provided through the given link and I agree that they provide valuable insights related to our research. I will integrate the findings of these articles into our manuscript to ensure that our references are comprehensive and up-to-date. I appreciate your feedback, which will definitely help improve the quality and relevance of our work.

 

12. In Figure 2, explain the significance of the different particle size distribution curves shown.

Response:

Authors appreciate the reviewer for this comment.

The particle size distribution curve is a common analytical tool in soil engineering, used to describe and classify the distribution of particle sizes within a soil sample. This curve provides quantitative insights into how soil particles are distributed, which is vital for understanding various physical properties and behavioral characteristics of the soil. The main functions of the particle size distribution curve include:

• Soil Classification: The distribution curve aids in determining the type of soil, such as sand, silt, or clay. This is crucial for the design and construction of soil engineering projects like roadbeds, dams, and other civil engineering structures.
• Permeability: Understanding particle size distribution can help estimate the soil's permeability. For instance, soils with smaller particles generally have lower permeability, while those with larger particles exhibit better permeability.
• Compressibility and Strength: The particle distribution influences the soil's compressibility and shear strength. An ideal mixture of particles can achieve superior compaction and strength.
• Erosion Susceptibility: Knowledge of particle size distribution also assists in evaluating the soil's susceptibility to erosion.
• Pore Structure: The distribution curve offers insights into the soil's pore structure, which is crucial for understanding its permeability, strength, and other properties.

In essence, the particle size distribution curve provides engineers and researchers with a tool to understand the fundamental characteristics of soil in a quantitative and graphical manner, allowing for better prediction and management of soil behavior.

 

13. Provide more information on the sample dimensions, testing standards, and procedures.

Response:

Authors appreciate the reviewer for this comment.

According to the testing standards [55], the soil samples were meticulously pre-pared to ensure accurate testing results. Initially, they were dried at a consistent temperature of 105°C to remove any moisture. Once dried, the samples were crushed using a mechanical crusher to break down larger particles and then sieved through a 2-mm mesh to maintain a uniform particle size distribution. This meticulous sieving process ensures that only particles smaller than 2 mm were utilized in subsequent procedures. After sieving, the soil was layered in a controlled manner, and moisture was incrementally added to achieve the optimal moisture content, ensuring that the soil had the best consistency for testing. This was followed by a 24-hour settling period, allowing the moisture to evenly disperse throughout the sample, guaranteeing uniformity. To enhance the soil's properties and meet the research objectives, a specialized mixture of regenerated polyester fibers, quicklime, fly ash, and gypsum was prepared. Each ingredient was measured and mixed in specific proportions, as dictated by the research protocol, to achieve the desired characteristics of the modified soil. Once the mixture was ready, standard triaxial specimens were fabricated. These specimens, measuring 39.1 mm in diameter and 80 mm in height, were shaped using a static pressure molding technique. This process was executed with precision to ensure that each specimen met the target dry density specifications, ensuring consistent and reproducible test results. Understanding the importance of moisture distribution within the specimen, a final conditioning step was introduced. Before subjecting the specimens to the wet-dry cycle testing, they were hermetically sealed to prevent external moisture intrusion. They were then stored in a climate-controlled chamber, maintaining a temperature of 20°C and a relative humidity of 98%, for 48 hours. This conditioning ensured that water was uniformly distributed throughout the specimen, creating an ideal environment for the subsequent wet-dry cycle testing.

 

14. Explain the selection of confining pressures used in Section 3.1. What is the significance of 80, 100, 150 kPa?

Response:

Authors appreciate the reviewer for this comment.

These values were selected to simulate the range of in-situ stresses that the soil is likely to experience in real-world conditions. Our preliminary site investigations indicated that these values were representative of the depths and loading conditions the soil would be exposed to. Our choice was also informed by relevant literature in the field. Past research involving similar soil types and conditions have frequently utilized these pressure ranges, making it valuable for comparative analysis.

In summary, the selected confining pressures of 80, 100, and 150 kPa offer a balanced representation of real-world conditions, align with previous research, and allow for a holistic understanding of the soil's behavior under various stress states.

 

Author Response File: Author Response.docx

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Okay for publication