Overview of Retrogressive Landslide Risk Analysis in Sensitive Clay Slope

Round 1

Reviewer 1 Report

Dear Authors,

the proposed review is well written and the intention to show the (high or low) suitability of different methods to evaluate the risk analysis related to retrogressive landslides (as those involving sensitive clays) is clear.

However, this goal is not fully reached by Section 2.3 “Stability analysis: Analytical Methods” that seems a “cold” overview of well-known methods, without providing a real contribution. Given the declared objectives of the paper, such section should contain important considerations about the "intrinsic limits" of the limit equilibrium methods. In fact such methods, modelling the soil as a “rigid-perfect plastic” material, are of course inadequate to correctly evaluate the stability conditions of a slope susceptible to progressive failure that is accompanied by deformations occurring before the global failure. In particular, a problem about the representativeness of the peak strength parameters, that could highly overestimate the stability conditions, does exist. Particularly referring to London Clays, such topic was widely discussed by some very significant works in the 1960's and 1970's. For instance, Skempton (1964) and Burland et al. (1977) back-analyzed some landslides occurred in natural slopes, showing that the strength parameters mobilized at global failure could be very different at each point of the slipping surface, ranging from the residual to the peak value. In particular, Skempton (1964) defined the “residual factor” in order to quantify the magnitude of the progressive failure, while Burland et al. (1977) described the amount of observed post-failure displacements. As a consequence, such studies alerted the scientific community that “saved” the (very advantageous and widely used) equilibrium methods, but assigning to the assumed failure surface the critical values attained at large displacements as overall strength parameters.

Therefore, I do suggest to insert such crucial considerations, mentioning them in the Discussion Section too.

Again concerning Section 2.3, I also suggest to express Mohr-Coulomb criterion (Eq. 4) in the effective stress terms (as provided in Section 2.5) and not in the total stress terms.

 

Concerning Section 2.6, “Modelling of the Progressive Failure Mechanisms” can not disregard the pioneering considerations provided by Bjerrum (1977), who highlighted the role of plasticity and brittleness of soils on the occurrence of progressive failure.

 

The above suggested changes are also reported in the "Comments" tool of the attached annotated pdf file.

 

SUGGESTED REFERENCES

Bjerrum, L. (1967). Progressive failure in slopes of overconsolidated plastic clay and clay shales. ASCE, Journal of the Soil Mechanics and Foundations Division, Vol 93 (No. SM5), pp.2-49.

Burland, J.B., Longwortht, T.I., Moore, J.F.A. (1977). A Study of Ground Movement and Progressive Failure Caused by a Deep Excavation in Oxford Clay. Géotechnique, 27(4): 557-591.

Skempton, A.W., 1964. Long-term stability of clay slopes. Géotechnique, 14(2):77-101.

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.docx

Reviewer 2 Report

The paper “Overview of retrogressive landslide risk analysis in sensitive clay slopes” of Richer et al is an interesting, well written and clear review about the risk analysis of sensitive clay slopes. It is focused only on retrogressive slides.

I think that the paper can be accepted after minimal modification. Since this is a review, I suggest to improve the bibliography, especially in the introduction section. For example, add some more reference between lines 33 and 37, after “quick clay” at line 34 and after “0.4 kPa”. The same between lines 69 and 76. This is an introduction section of a review paper.

Line 48: that is underlined.

Figure 5 is from Fellenius? If yes, please cite in the caption. Please explain the variables of figure 5.

Author Response

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This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.

Round 1

Reviewer 1 Report

Dear author's

I have carefully read your review paper about the common methods for slope stability in sensitive clays. This is interesting to have such a review paper, but some key points are missing in the paper in this actual form. Major points are given below, but the attached pdf contains other points to consider in your revision.

First, one of the most important parameter necessary to conduct such analysis are the failure criterion, which will vary either the analysis is done in total stress or effective stress. Such a review should at least present the difference between both, and discuss them. An introduction about effective vs total stress analysis can be found in Duncan and Wright (Soil strength and slope stability, chapter 3).

In the analytical methods, you show formulas but do not explain any of them. In particular, you are only talking about forces, but actual methods such as MP do not only consider equilibrium of forces, but also equilibrium of moments. It is noted in the tables, but not explained in the text.

Also in the analytical methods section, you refer (p.5 L132) to groundwater level. The groundwater regime is one of the most important parameter to include in a slope stability analysis, and most of the time, it is not only the groundwater level that is important, but the whole regime as surpressure can be found, and this is very important to take into account to have a correct slope stability analysis (see e.g. Lefebvre, 2017, Sensitive clays of Eastern Canada: from geology to slope stability, Landslides in Sensitive clays, Advances in natural and technological hazards research)

You also mention (p.5 L135) that limit equilibrium methods can only be used to estimate the SF of the first slide. This is not true, as explained in the pdf. Reference are also given in the pdf. The same is also mentioned in L194-197 when you are writing on the SSR technique.

For the estimation of the retrogression distance, I suggest some recent references in the pdf, as how the retrogression distance is estimated in Quebec province and in Norway. Neither of them use the reference you give in the paper, as they were proved to not work.

Finally, in the discussion and in the conclusion, you refer to probability of occurence, where in the methods you only refer to deterministic methods (and not probabilistics). Probabilistic methods exists in the context of slope stability analysis, in order to refer to probability of occurence, they should be presented and discussed. These approach involves i.e. monte carlo simulations of slope stability analysis taking into account the incertainty in the input parameters. In this context, the reliability based approach is more and more used in practice.

Some other comments are provided as comments in the pdf file.

I with all the best to the author's on their review, this paper could become useful to practitioners if completed.

 

Some other key refenrence missing : 

Lefebvre, 1981, Fourth canadian geotechnical colloquium, Strength and slope stability in Canadian soft clay deposits, Canadian Geotechnical Journal

Alonso, E. 1976 Risk Analysis of slopes and implications to slopes in Canadian Sensitive clays, Géotechnique 26, #3

Tavenas, Trak and Leroueil, 1980, Remarks on the validity of stability analyses, Canadian geotechnical journal

Comments for author File: Comments.pdf

Author Response

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Reviewer 2 Report

Slope stability analysis in sensitive clays is a very important topic in geotechnical study with significant research works done and on-going. The work aims to present review of existing methods and to give recommendations for future developments. Aiming to deliver this is a formidable task that requires deep understanding of various approaches and this in turn requires the knowledge to be able to analyse with some of these methods if not all. I do not see this article.

The work presents very basic aspect of some of the approaches used in slope stability of sensitive clay and completely ignores other emerging approaches, i.e. Material point method, XFEM and similar approaches to solve regularization problems (mesh dependency) in conventional FEM methods. Even the way the methods are described has sometimes inaccuracies in the descriptions.  The way the word “probability” is used in the article is incorrect. Determination of probabilities by itself an important stability analyses method overlooked in the article. I do not see how field observation for tension crack is more conclusive than analytical methods for potential retrogression distance (flow slides occur very rapidly!). I could go on but the point is that there are several things that do not add up in the article and that lacks conciseness.

My suggestion for the authors, to contribute with the title they chose, is to do a more through literature study and be more concise in description of the various approaches. The authors could then supplement this by performing stability analysis with the various methods/approaches. Then these results can be evaluated to answer what and why one gets a specific answer with a given method. In this way, the authors could give a well-informed review as valuable contribution supported with illustration. It could also be smart to focus on selected few approaches (rather than all) and compare thoroughly based on a numerical illustration. I think this could then be a proper review work where the authors could demonstrate clear understanding of methods they review.  

Author Response

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Reviewer 3 Report

The review paper submitted to Geosciences concerns the research carried out on the numerical modelling strategies suitable to date for the prediction of the retrogressive landsliding in terms of both the probability of occurrence and of the potential retrogressive distance for spread or flowslide in sensitive clay.

It analyses different numerical strategies, trying to highlight both the strengths and the weaknesses of each numerical approach.

General observation:

I do not get the point of this paper. I have understood that the Authors tried to propose a review of all the numerical methods available for the prediction of the retrogressive landsliding in sensitive clay, but the result seems to me of poor quality.

A review paper should be inclusive, clear and should provide a comprehensive understanding of the issue of reference.

There several weak points in this review. I think this contribution is not clear, and as such, it is meaningless as a review paper. I would suggest the Authors to take inspiration from other review papers on similar issues, which have been published in the literature. For example, Elia et al. (2017), doi: https://doi.org/10.1144/qjegh2016-079, was a successful example of a review article, giving all the numerical strategies and the strengths and weaknesses of each method. This review paper was also very clear since the Authors gave one or more examples of case studies for each numerical strategy.

The examples should report at least the most important hypotheses and results in order to be effective.

I would suggest the Authors to write this contribution with a wider perspective, maybe taking inspiration from the paper I suggested, even if the landslide type of reference are completely different. But this may help the Authors in building a better and stronger scheme for the review paper.

The reference list is not particularly well written. I do not think the Authors followed the template on this. For some contribution, there are no details useful for looking for the paper.

There are some points in which the paper is not very well written. The level of the English should be improved. In particular, in the paper there are some typographical errors to correct.

Specific comments:

Line/Lines	Comment
25-28;	It is not clear why you say this (i.e. see general comments also)
29-31	No references for the sensitivity, even if there are several papers on this.
37-39	English should be improved
Figure 1	Is the figure an original product? Otherwise, the reference is missing in the figure caption.
57-62	English should be improved
63-67	The progressive failure process should be better explained. There is a huge scientific production on this issue.
93-155	I do not understand why you put this paragraph in the paper. It is not clear to me why to explain and revise the limit equilibrium analysis theory, and application. All the general part of this theory has already been discussed in several past papers in the scientific literature.
Eq. 2 and 3	It is not very nice that equation are given in the paper as pictures. This is not allowed in the template,  I think. More over, in the Eq. 3 there is something not going well with the equation. It seems there is a part which is missing. In fact, in the description of the equation, there is the cohesion (c’), which actually is not present in the picture of the equation.
125	“…and others.”. Please be more specific. This is a review article, and as such it should be everything clear.
Reference [25]	It is not possible to find this reference since in the references list, no details at all are given on this contribution.
157-160	English should be improved
165-167	In limit equilibrium analysis it can be possible to analyse several different slip surfaces, and as such, it can be possible to find the most unstable one, which would represent the “preferred” one. The main difference between FEM and LEM is that the strain and displacements fields are not computed in the LEM. This circumstance has several impacts. But the Authors did not mention it.
Eq. 4 and 5	Equations given as pictures.
194-195	Not clear.
Figure 6	The plot should be revisited. Specify which is the “stress” and which is the “displacement or strain” in the graph.
218-220	Not very clear. Should be better explained.

Author Response

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Round 2

Reviewer 2 Report

The authors came with changes and this shows the desire they have to push this work through to publish it. While this is positive, the work still needs to be thoroughly examined to be accepted on its merits. I have suggested comments in my earlier review. The authors have attempted to address some of them but I feel there is still a room for improvement.

To help the authors, I have tried to give comments as specific as possible. Sometimes looking at general picture and sometimes looking at specific sections. For me the paper is far from being accepted in current form and needs a complete restructure and supplemented with a through insight. I understand that the work is a review work, but it should still have its own value. The authors must ask themselves why they present this? What would a reader learn for this article?

I think it is important to be clear in evaluating various methods being reviewed with regard to their special applicability, suitability and implications. That way it becomes a well-informed review. I have also noticed that the authors could have looked into more literature to get more on the topics they touched. It is also important to be careful and concise when describing others work so that the presentation is a correct representation.

Still, out of the will to genuinely help the authors and my interest for this topic – I gave the following specific comments.

I do not think that the title is representative of the work presented or vice-versa. I mention some examples here.

• Not all stated as methods can be considered as “Common methods”- Probably only the LEM and FEM (SRF) methods.
• The topics touched upon are not only for “slope stability analysis”. The work mixes two conceptually different things. The first is slope stability analysis which is an analysis performed on a slope to determine its safety level or its potential for failure. The other is related to post-failure behaviour of slopes (after failures) and for sensitive clays this is looking at e.g. retrogression and runout distance.
• Nothing is specified if the work shall only look at static analysis (vs. dynamic stability analysis)

It is generally accepted that landslide characterization includes all stages of slope movement and these are referred to as pre- failure, onset of failure/failure stage, post-failure and reactivation stages (e.g. Leroueil 2001 ; Vaunat and Leroueil 2002 ). The authors could clarify the stages that they are looking at while presenting their methods. It is not correct to present section 2.1, 2.2, 2.4 and 2.5 under “materials and methods”. They fall into different characterization of landslide assessment and the sequence used by authors mixes thigs in unclear way. The structure and the way they are presented thus lacks logic. This is a major weakness in the structure of the work. Alternatively, the authors could focus on one stage/aspect and go in a better detail.

Slope stability analysis methods (Deterministic ones) can broadly be divided in to two 1) LEM and 2) I call them Numerical methods and they include approaches that takes into account stress and strain. Methods that fall into this category include FEM, FDM, DEM and so on. Other analysis methods include for e.g. ANN and Limit analyses methods. Post failure is another topic where common existing methods are mainly is based on empiricism and some semi-analytical approaches from fluid dynamics.

The authors should be critical of the figures, tables and equations/formula they chose to present. This must be evaluated based on to how much they contribute for their cause. Or consider better alternatives.

I found the discussion section to be highly fragmented and lacking logical flow. It also seems not to follow the works presented in the body of the work.

Most of the conclusion also displays the same characteristics as the discussion section! It is not easy to follow and not clear as well. I could state questionable sentence formulations here and there (also in discussion) but I consider this unnecessary.

Below are some selected specific comments

Line 30 – “post-failure soil strength” should be replaced by remoulded shear strength.

Line 98-99 – This is in contradiction with what is presented in Line 95-94.

Section 2.2 - stability number is mainly used to determine retrogression. This is only one empirical approach used in this regard. The authors overlooked other that fall under this category and can look at summary given in Thakur et al. (2013) - Characterization of Post-failure Movements of Landslides in Soft Sensitive Clays.

Line 92-94: “This paper reviews existing methods of stability analysis and their application to slopes in sensitive clays, and we formulate recommendations for the further development of stability analysis approaches.” This is not done! Neither the applicability of different methods for sensitive clays is discussed/evaluated nor recommendations is given for all the methods presented/mentioned in the work. For example in section 2.1 – no applicability is mentioned or recommendation…..

Line 99-100 “Landslide risk analysis relies mainly on the assessment of slope stability in homogeneous terrain” – This does not seem to be true or is not clear to understand. What is meant by homogeneous terrain ? The slope is what it is and it has to be assessed using an approach that suits it especially if a risk element is related to it.

Table 1 and table 2 – The information presented here is more or less available in text books or in several publications. The authors could have done a bit more here. As suggested in previous review, for e.g to do analysis on an idealized slope using all the methods and state the implications. (This can easily be done in several commercially available software).

Table 2 present assumptions used in the methods – but does not state the implication of these assumptions and simplifications in the resulting results (as promised in Line 92-94).

Table 2 – “Ordinary method of slices” should be replaced by “Ordinary method of slices/Fellinius method”

Janbu’s (1973) method (generalised procedure of slides) is overlooked in Table 2. In addition, Bishop and Janbu each have to methods – simplified and corrected/rigorious. See the book by Abramson et al, 2002.

Line 158-159 “the required simplifying assumptions decrease the accuracy of the results sometimes to an inappropriate level.” This needs to be specific for the specific methods – it is too qualitative and this is unnecessary. What is meant by this? What is the accuracy level within all the methods presented? What is the reference here for accurate result? FEM? What constitutes inappropriate level ?

Line 220 - NIFS method is not defined. The authors gave however more details on the MTMDET method. Still no evaluation of the various methods or recommendation is mentioned as stated in line 92-94. The authors could have presented these two approaches with a common sketch and both compared. I felt it was too much text that should have been supplemented with a figure/sketch. See for examples Strand et al. 2017 (Runout of landslides in sensitive clays)

Line 319 - The hardening soil model is not defined! It should be stated that it is a numerical model available in Plaxis….

Line 325-326 - “In a hardening soil model, the initial shear stress is input within a finite element

326 program, such as the BIFURC program developed by the Norwegian Geotechnical Institute [41,42].” I am not sure if the authors understood the work presented by Locat et al. (12). The reference made about that work here is not correct! Locat et al. used hardening soil model to only generate initial stress and use another inhouse FEM software BIFURC to do the rest of modelling of progressive failure. The earlier mention about hardening soil model (Line 319-320) is at best incorrect.