This short, concise and well-written paper presents some useful mechanical data for deformation of the Opalinus clay, a potential host rock for nuclear waste, and interprets the data in terms of deformation mechanisms. The paper is well constructed and straight forward to read, the results are useful and the interpretations convincing.

One of the only significant problems in the paper is the use of the term “ductile”. In rock mechanics, brittle and ductile were defined in a very specific way, depending on the amount of strain before failure. Heard (1960) defined deformation as brittle if fracture formation was preceded by less than 3% permanent strain.  Griggs & Handin (1960) defined ductility as a material’s ability to withstand more than 5% permanent strain before failure.  This paper does not seem to be using ductile in these terms, since failure is occurring at < 1.5% strain, which would be brittle in the original definitions. Given that the paper is based on rock mechanics, the original definitions would seem to be relevant.

The paper uses brittle and ductile in a variety of ways: plastic is also in the mix. It would be good to straighten out the intended use of the terminology early in the paper. It is also surprising that the classic papers by Griggs, Handin etc that identified brittle and ductile behaviour in the 1960’s are not cited.

The large values of Poisson’s ratio in table 2 are vastly in excess of the maximum of 0.5 for an elastic solid. That is possible with highly dilatant materials, but it is worth a comment about this in the text, and possibly some comparison with other similar materials in the literature. Poisson’s ratio as used in this way is far from an elastic constant for materials. It turns out that there are even problems with the definition of Poisson’s ratio when it is less than 0.5 (Dong, L., Xu, H., Fan, P., & Wu, Z. (2021). On the Experimental Determination of Poisson’s Ratio for Intact Rocks and Its Variation as Deformation Develops. Advances in Civil Engineering, 2021. https://doi.org/10.1155/2021/8843056).

The description of the grain and pore alignment in the shear zones in lines 182-185 is oversimplified and partly contradictory to the interpretation that grains may have a P foliation: this would not be parallel to the shear zone boundary, and indeed such grains can be seen in Fig. 9.

Finally, the paper did mention the possible use of Opalinus clay as a host rock for nuclear waste. It would really round this paper off if a paragraph in the discussion could consider the implication of these results for waste disposal. Presumably one important additional consideration would be to what extent temperature is likely to change the results presented here.

These and other comments are on the annotated pdf.

T. G. Blenkinsop

General Comments

“Failure mode transition in Opalinus Clay: a hydro-mechanical and microstructural perspective” by Winhausen et al is a clearly written and thorough investigation into the interplay between different modes of failure at varying confining stresses within a clay rich shale with potential as a nuclear waste repository. To my knowledge an investigation into this transition of failure modes over this pressure range in a clay-rich material is novel, and the combination of discussions of the macroscopic failure modes and underlying deformation processes works well together. The data being limited to 4 experiments is a little sparse, but is completely understandable given the 100+ hour duration of the sample saturation stage.

Specific Comments

My only significant suggestion is that the paper might benefit from some discussion of the influence of the heterogeneous sample material on the Skempton B-checks used to determine the sample saturation. I was not previously aware of the method of B-checks, and having read around it a little, the relationship between B and saturation seems to assume a constant bulk modulus, porosity and permeability in the matrix? In Sections 2.1 and 4.2, the authors draw attention to the heterogeneity of the material presented here, and so I wonder if the authors could comment on how confident they are certain that the Skempton B-check is confirming the full saturation of the sample, or just the saturation of the more porous components? If there was a heterogeneous saturation state within the sample due to lower porosity/permeability regions (presumably with different bulk moduli), would this be likely to affect any of the deformation processes discussed in Section 4.2?

It might also be worth putting the range of confining pressures in the abstract, as I see this as being quite a large differentiator between this study and the earlier papers of Amann et al.

Beyond that, I found the paper clear and enjoyable, and the conclusions well supported.

Technical Corrections

I think there is a rogue “a” on line 294, and it should read something like “ … failure on the microscale is less dilatant, forming a broader…”