Verification of the Fredlund (2019) Unsaturated Shear Strength Function

Round 1

Reviewer 1 Report

This paper represents an important contribution to making unsaturated soil mechanics accessible to practicing geotechnical engineers.  Comparisons to data are quite useful for evaluation of the approach to estimation of shear strength of soils, including limitations and sensitivity to SWCC parameters - and where some additional data and research may be needed to improve the method.  Following are some suggestions for edits: (1) line 97, define c' as effective stress parameter from saturated tests; (2) line 157-Give more details of how to obtain S₀ the "near saturation" degree of saturation. This determination seems a bit subjective, and also the value of suction "near saturation" can vary widely. (3)Line 193- Fig. 3, use log(suction, kPa); (4) Line 139- consider a discussion that AEV is not precisely fixed, but is a function of net normal stress, yet for shear strength estimations use of AEV determined under light confinement appears to be adequate for use in practice. Maybe give a reference if there is one; (5) Line 227 - sentence is incomplete, add "is" between suction and based; (6) Line 389, Fig 10.  The ξ  term should be labeled on this plot, it's not the same of the residual/AEV ratio, is it? Should an arrow and label with ξ  on it be added to the figure? Similarly, can ξ be shown more clearly, perhaps with a label, on Fig.11? Section 4.1, containing Figs 10 and 11, was, in general, difficult to follow, requiring reread.

Author Response

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Reviewer 2 Report

The Authors present comprehensive works related to the determination of the unsaturated shear strength of soil based on the SWCC data in term of degree of saturation. It is very excellent study to indicate the importance of the variables of SWCC (i.e. air-entry value and residual suction) to be determined from degree of saturation of SWCC. The manuscript is well written and it is presented logically and structurally well. The only concern of the study is related to the comparison between the study by the Authors with the recent studies on the determination of the unsaturated shear strength from SWCC which have been published within this ten year (2010-2020). The Authors only review literatures based on year 2010 and older than that. The details of my comments are shown below:

1. The Authors are required to compare the works done in this paper with the recent works by: Zhai, Q., Rahardjo, H., Satyanaga, A., Dai1, G. (2019). “Estimation of unsaturated shear strength from soil-water characteristic curve”, Acta Geotechnica, Vol. 14, pp. 1977-1990. 
   Zhai, Q., Rahardjo, H., Satyanaga, A. and Dai, G. (2020) “Estimation of tensile strength of sandy soil from soil-water characteristic curve” Acta Geotechnica. Vol 15, pp. 3371-3381. What is the originality of the study by the Authors as compared to the previous studies by Zhai et al.
2. Please add more recent literature related to the determination of the unsaturated shear strength. For example:
   Satyanaga, A. and Rahardjo, H. (2019). “Unsaturated Shear Strength of Soil with Bimodal Soil-water Characteristic Curve”. Geotechnique. September 2019. Vol. 69, No. 9, pp. 828-832.
3. In Section 3.4, the Authors did parametric studies to evaluate the significance of best fitting parameters a, n, m. What are the importance of this study as compared with the previous studies by:
   Zhai, Q. and H. Rahardjo (2013), "Quantification of Uncertainties in Soil–water Characteristic Curve associated with Fitting Parameters", Journal of Engineering Geology, June, Vol 163, pp. 144-152.  
4. Previous studies have also been conducted on the uncertainty of in using SWCC for estimating the unsaturated shear strength. Please review Zhai, Q., Rahardjo, H., Satyanaga, A., Dai, G. and Du, Y. (2020) “Effect of the uncertainty in soil-water characteristic curve on the estimated shear strength of unsaturated soil”, Journal of Zhejiang University-SCIENCE A, April, Vol 21, No 4, pp. 317-330. The Authors are required to verify the proposed results from their studies by comparing with the study by Zhai et al (2020)
5. Where is the conclusion of the paper? 

Author Response

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Reviewer 3 Report

In this manuscript, the authors did an extensive literature review regarding the Fredlund 2019 unsaturated shear strength function. The Fredlund proposition is described in detail, and the logical structure of the manuscript allows not to get lost in a vast amount of information. During this review, I have a few times changed my mind to advise to divide this article into two parts, but finally, in my opinion, the presented work can be published as a whole. Nevertheless, by writing that, I would like to stress that the same opinion may have other readers.

The manuscript deals with an important matter which is verification. In geotechnical engineering, the amount of data and its analysis resulted in plenty of equations; for example, as the authors stated, there are around 50 equations related to the undrained shear strength, and this amount refers only to the literature written in English. In my opinion, it is about a time to verify usefulness not only by own experiments but also by using the data gathered from the literature.

The authors entitled their article “Verification of the Fredlund…” and the article's structure shows that verification is not a primary goal of this manuscript. Consider changing the title to account for a Fredlund 2019 and S-SWCC introduction or extend the verification of the Fredlund 2019 proposition by more soils. Since the manuscript has 27 pages, I would recommend rewriting the title.

On Line 100 two times „angle”, consider to put space between “φ” and “b”

Finally, I would recommend accepting this manuscript after minor changes which allow the authors to consider what I have suggested above.

Author Response

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Author Response File: Author Response.docx