FEM Modelling of Thin Weak Layers in Slope Stability Analysis

Round 1

Reviewer 1 Report

This paper attempted to deal with a parametric analysis of three different 2D numerical case studies (both ideal and real) of unstable or potentially unstable slopes containing a thin soft band  (or weak layer). The type of mesh elements, the mesh density, the geometry, thickness and outcrop shape were considered. The attempted topic is interesting and fits the scope of the journal. The reviewers have two main concerns:

1. The selection of input parameters of the numerical models should be carefully checked. The authors aim to conduct a parametric analysis by comparing the values of FS under different conditions. However, the input parameters have a significant effect on the FS. In Table 1, for case LLM, the cohesion of the landslides deposit is about 0, the reviewer wonders why the value of cohesion was  0. So, the reviewer strongly recommends the authors also conduct a sensitivity analysis of the input parameter on the FS.

2. The authors should clarify how the authors decide on the sliding zone. The FS is calculated based on the automatically deciding sliding zone or based on specified slideing zones. The location of the slides zone will also have a significant effect on the FS. 

The English is acceptable, however, the authors wrote many long sentences that confuse the readers. I suggest the authors to split sentences.

Author Response

This paper attempted to deal with a parametric analysis of three different 2D numerical case studies (both ideal and real) of unstable or potentially unstable slopes containing a thin soft band  (or weak layer). The type of mesh elements, the mesh density, the geometry, thickness and outcrop shape were considered. The attempted topic is interesting and fits the scope of the journal. The reviewers have two main concerns:

1. The selection of input parameters of the numerical models should be carefully checked. The authors aim to conduct a parametric analysis by comparing the values of FS under different conditions. However, the input parameters have a significant effect on the FS. In Table 1, for case LLM, the cohesion of the landslides deposit is about 0, the reviewer wonders why the value of cohesion was  0. So, the reviewer strongly recommends the authors also conduct a sensitivity analysis of the input parameter on the FS.

Reply: In this paper, the parametric analysis regards only numerical and geometrical parameters; instead, mechanical parameters of the materials have been kept constant. The author is aware that a sensitivity analysis on selected mechanical parameters would be very useful, but it is beyond the scope of this paper. It has been clearly declared in the introduction. As regards the LLM, where cohesion is assumed equal to 0kPa, it corresponds to laboratory and field investigations, besides consideration on the history of that soil deposit, as reported in Cevasco et al. (2018).

Cevasco, A.; Termini, F.; Valentino, R.; Meisina, C.; Bonì, R.; Bordoni, M.; Chella, G.P.; De Vita, P. Residual mechanisms and kinematics of the relict Lemeglio coastal landslide (Liguria, northwestern Italy). Geomorphology 2018, 320, 64-81, doi: 10.1016/j.geomorph.2018.08.010.

2. The authors should clarify how the authors decide on the sliding zone. The FS is calculated based on the automatically deciding sliding zone or based on specified slideing zones. The location of the slides zone will also have a significant effect on the FS. 

Reply: Since the Finite Element Model is used in this study, the final result in terms of Critical Shear Strength Reduction Factor is not exactly corresponding to the Factor of Safety, which instead is typical for Limit Equilibrium Analysis. In FEM analyses the sliding zone is not specified nor determined a-priori.

3. The English is acceptable, however, the authors wrote many long sentences that confuse the readers. I suggest the authors to split sentences.

Reply: long sentences have been split and English has been revised throughout the text

Author Response File: Author Response.pdf

Reviewer 2 Report

The problem of the proper choice of mesh type in the case of thin layers is very important especially when we want to obtain good quality results in a reasonable portion of the time. The problem of the proper choice of mesh type in the case of thin layers is very important especially when we want to obtain good quality results in a reasonable portion of the time. The proposed parametric analysis is quite interesting. Doing such an analysis based on the models from the literature was a good idea. In general, the article is well written. The words are properly chosen, the English language is quite clear and the text is good to read.

But almost all the figures used in the manuscript should be improved. I tried to make comments on each figure in the manuscript but the remarks apply to practically every drawing. Below I listed some of them:

1. The fonts on the figures (axes, color bars, legends, labels) should have the same type as in the text. They should be visible and have a size of main text fonts or one- to two-point smaller.
2. If you use axes ( x and y) to show the size of the model, please make these visible - the axes lines should be thicker and not exceed the model size. Maybe the better idea will be to show only the scale bar on each of the figures rather than the axes.
3. The resolution of the figures in many cases is too small. I understand that the images are exported from the RS2 but you must do something to make it better.

4. If you want to show the boundary condition signs on the borders of the model they should be redrawn. They are drawn too densely. Please draw only a few signs (the triangles mean a blockade of the move in both directions)  (maybe 4 - 10 depending on the width of the borders). Please consider if there is a need to show these boundary condition signs. You do not mention anything about them in the text.

5. Please reorganize the color bars on each of the Figures where you show them: remove the information about the min and max (stage) values - it is unnecessary;  I understand that these are the screenshots from the program RS2 but for the purpose of proper presentation of the results it should have better resolution and readability. The fronts should have a bigger size and the type should be the same as in the main text. Remove the black rectangles around the color bars.

All other remarks I enclosed in the comments on the manuscript.

After making these improvements in the Figures and some small changes in the text I recommend the article to be published.

Comments for author File: Comments.pdf

Author Response

The problem of the proper choice of mesh type in the case of thin layers is very important especially when we want to obtain good quality results in a reasonable portion of the time. The problem of the proper choice of mesh type in the case of thin layers is very important especially when we want to obtain good quality results in a reasonable portion of the time. The proposed parametric analysis is quite interesting. Doing such an analysis based on the models from the literature was a good idea. In general, the article is well written. The words are properly chosen, the English language is quite clear and the text is good to read.

But almost all the figures used in the manuscript should be improved. I tried to make comments on each figure in the manuscript but the remarks apply to practically every drawing.

Reply: Thanks for the observation. All figures have been modified and improved.

Below I listed some of them:

1. The fonts on the figures (axes, color bars, legends, labels) should have the same type as in the text. They should be visible and have a size of main text fonts or one- to two-point smaller.

Reply: Axes have been removed; scale bars have been added in each figure; color bars have been modified; legends and labels have been added.

2. If you use axes ( x and y) to show the size of the model, please make these visible - the axes lines should be thicker and not exceed the model size. Maybe the better idea will be to show only the scale bar on each of the figures rather than the axes.

Reply: The scale bar has been used instead of axes.

3. The resolution of the figures in many cases is too small. I understand that the images are exported from the RS2 but you must do something to make it better.

Reply: Resolution of bad figures has been increased.

4. If you want to show the boundary condition signs on the borders of the model they should be redrawn. They are drawn too densely. Please draw only a few signs (the triangles mean a blockade of the move in both directions)  (maybe 4 - 10 depending on the width of the borders). Please consider if there is a need to show these boundary condition signs. You do not mention anything about them in the text.

Reply: The boundary conditions have been removed. The following sentence has been added on page 4: “In all the models analysed, displacements in the horizontal and vertical directions at the base and sides of the calculation domain were prevented, while nodes on the slope surface were left unblocked.”

5. Please reorganize the color bars on each of the Figures where you show them: remove the information about the min and max (stage) values - it is unnecessary;  I understand that these are the screenshots from the program RS2 but for the purpose of proper presentation of the results it should have better resolution and readability. The fronts should have a bigger size and the type should be the same as in the main text. Remove the black rectangles around the color bars.

Reply: the color bars have been modified. Information about the min and max (stage) values have been removed. It was impossible to modify the fonts of the color bars, but they have been enlarged. The black rectangles have been removed.

All other remarks I enclosed in the comments on the manuscript.

After making these improvements in the Figures and some small changes in the text I recommend the article to be published.

Thank you very much for your very useful comments and suggestions.

Author Response File: Author Response.pdf