Effect of Matric Suction on Shear Strength and Elastic Modulus of Unsaturated Soil in Reconstituted and Undisturbed Samples

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This study examines the effects of matric suction on shear strength and elastic modulus in unsaturated silty soil using filter paper tests and unconfined compression experiments. While the research addresses an important topic in unsaturated soil mechanics, it lacks methodological innovation and deeper mechanistic analysis. The reliance on traditional techniques (e.g., filter paper method for suction measurement, standard unconfined compression tests) limits the ability to capture microstructural dynamics, such as particle rearrangement under suction changes, which are critical for understanding soil behavior. The adoption of the Fredlund and Xing unimodal SWCC model is conventional, yet the study does not discuss its suitability for the soil’s potential bimodal pore structure, as hinted by the scatter in undisturbed sample data (Figure 11). The analysis of hysteresis effects remains descriptive rather than quantitative, and the small sample size (n=3 per condition) undermines the statistical reliability of trends like the nonlinear shear strength increase at low suction (Figure 15a). Additionally, the study neglects environmental factors such as temperature variability and soil chemical composition, which could significantly influence matric suction and mechanical properties.

Major comments are summarized as:

1. The filter paper method’s accuracy decreases at high suction (>500 kPa, Table 8). How would using pressure plate apparatus or TDR sensors improve the reliability of SWCC data for unsaturated zones with high suction?
2. Given the soil’s heterogeneous stratigraphy (e.g., clay lenses at 4–8 m, Table 7), why was a unimodal SWCC model preferred over bimodal models that account for dual pore systems?
3. The study mentions hysteresis in SWCC but does not compare wetting/drying curves for reconstituted vs. undisturbed samples. How do these cycles affect the elastic modulus values reported in Table 11?
4. Without SEM or CT scans, how can the study differentiate between suction-induced strength gains from capillary forces vs. soil fabric effects in undisturbed samples (Figure 18d)?
5. With only three replicates per suction level, how robust are the correlations between matric suction and shear strength (e.g., R²=0.905 in Figure 15a)? Could sampling variability skew the results?
6. Quito’s high-altitude climate experiences significant temperature fluctuations. How might thermal expansion/contraction alter matric suction measurements and soil stiffness?
7. The study ignores soil pH, cation content, or organic matter. Could these factors influence the adsorption of water and thus the SWCC shape?
8. The focus on static tests neglects cyclic loading (e.g., traffic or seismic forces). How would repeated loading affect the stress-strain behavior observed in Figure 13?
9. The water table is assumed >10 m, but how were seasonal variations in rainfall and their impact on suction at different depths (e.g., 2 m vs. 8 m, Table 9) addressed?
10. The Fredlund and Xing model parameters (Table 10) are derived from reconstituted samples. Can they accurately predict SWCC for undisturbed soils with higher clay content (e.g., LL=54 at 2 m, Table 7)?
11. The linear increase in elastic modulus with suction (Figure 17a) conflicts with Lu’s model, which predicts nonlinear hardening. What experimental conditions explain this discrepancy?
12. Reconstituted samples use soil finer than 2 mm, but natural deposits may include coarser fractions. How would gravel content affect suction retention and compressive strength?
13. The 7-day equilibration period does not account for soil aging or microbial activity. How might prolonged exposure to suction alter the soil’s microstructural stability?
14. How do the shear strength values (e.g., 80.82 kPa at 4 m, Table 13) compare to regional studies on similar soils, and what role does soil mineralogy play in these differences?
15. Given the site-specific nature of the data, what validation steps are needed to apply the proposed empirical models to other unsaturated silty soils with different geologic histories?

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The study employs a systematic approach to suction measurement, unconfined compression tests, SWCC modelling, and compares reconstituted and undisturbed samples alongside a relevant literature review.

However, the study relies on the filter paper method for suction measurement, which is indirect and lacks real-time suction control during mechanical testing. For unsaturated soils, suction-controlled triaxial or direct shear tests are mandatory to isolate the effects of matric suction on shear strength and stiffness. Thus, all the data are ill-determined.

Since the suction cannot be controlled during loading, it can lead to pore-water redistribution and unreliable measurements of ϕb. Furthermore, UCS assumes undrained conditions; however, unsaturated soils experience coupled hydro-mechanical processes (e.g., air/water phase changes) under load. The paper does not address how these interactions affect the results.

Also, appropriate tests would need to be conducted.

 

 

 

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

This paper investigates the influence of matric suction on shear strength and elastic modulus in unsaturated soils, analyzing the mechanical properties of both reconstituted and undisturbed samples through experimental methods. The study demonstrates a certain degree of innovation and practical application value, particularly in the assessment of the mechanical properties of unsaturated soils. However, there are several areas in the experimental design, data analysis, and textual narration that require improvement. The authors are advised to make revisions based on the following suggestions:
(1) In the introduction, further emphasize the innovative aspects of the study, especially in terms of experimental methods and data analysis. Comparisons with previous research should be made to clearly highlight the contributions and uniqueness of this paper.
(2) The depth and breadth of the experimental data analysis need to be enhanced, particularly in the interpretation of data within the high-suction range.
(3) There is a relative scarcity of experimental data in the high-suction range, and the explanation of soil behavior under high-suction conditions is insufficient. This may affect the understanding of the mechanical properties of unsaturated soils under extreme conditions.
(4) Although the SWCC model by Fredlund and Xing was utilized, there is a lack of detailed discussion on the model's goodness of fit and applicability.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

After carefully reviewing the revised manuscript, I am satisfied with the improvements made by the authors. The responses to the previous reviewers' comments are comprehensive and have effectively addressed the concerns raised. Overall, the paper now meets the high standards of the journal. I recommend that it be accepted for publication.

Reviewer 2 Report

Comments and Suggestions for Authors

I have no further comments