Stability evaluation and potential failure process of rock slopes characterized by non-persistent fractures

Zhang, Wen; Wang, Jia; Xu, Peihua; Lou, Junqing; Shan, Bo; Wang, Fengyan; Cao, Chen; Chen, Xiaoxue; Que, Jinsheng

doi:https://doi.org/10.5194/nhess-20-2921-2020

Articles | Volume 20, issue 11

https://doi.org/10.5194/nhess-20-2921-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

https://doi.org/10.5194/nhess-20-2921-2020

© Author(s) 2020. This work is distributed under
the Creative Commons Attribution 4.0 License.

Articles | Volume 20, issue 11

Research article

|

05 Nov 2020

Research article |

| 05 Nov 2020

Stability evaluation and potential failure process of rock slopes characterized by non-persistent fractures

Wen Zhang, Jia Wang, Peihua Xu, Junqing Lou, Bo Shan, Fengyan Wang, Chen Cao, Xiaoxue Chen, and Jinsheng Que

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Final revised paper (published on 05 Nov 2020)
Preprint (discussion started on 30 Mar 2020)

Interactive discussion

Status: closed

AC: Author comment | RC: Referee comment | SC: Short comment | EC: Editor comment

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RC1: 'Comment', Anonymous Referee #1, 16 May 2020
- AC1: 'Response to Referee #1', Chen Cao, 02 Jul 2020
RC2: 'Review of manuscript: "Stability evaluation and potential failure process of rock slopes characterized by non-persistent fractures"', Anonymous Referee #2, 25 May 2020
- AC2: 'Response to Referee #2', Chen Cao, 02 Jul 2020

Peer-review completion

AR: Author's response | RR: Referee report | ED: Editor decision

ED: Reconsider after major revisions (further review by editor and referees) (07 Jul 2020) by Filippo Catani

dear authors

on the basis of the reports of two peer-reviewers, complemented by my own revision of your manuscript, I consider that the content of your research may be of interest for the journal reader and may represent a suitable publication in NHESS should you be ready to incorporate some major revisions.
Please carefully look at all the comments and remarks made by the two referees and reply to them, one by one. Where required, please modify the original manuscript and provide a new amended copy along with the document with replies to the reviews.
In particular, I am concerned about the main issue that is raised by Reviewer #2 on the model adopted to represent the rock mass and the fracture network. I agree with the reviewer that such DFN representation, despite being suitable in terms of conceptual model representation, does not seem suitable for the representation of the actual rock mass, where the role of the small fracture sub-network cannot be overlooked without compromising the entire numerical description of the whole.
Also, please consider the remarks of reviewer #1, with specific reference to the one related to the choice of the slope for the numerical analysis, which is quite stable and not prone to failure. That leaves no room to variability in slope behaviour and seems to underestimate the scope of the work. Again, this issue could be related to the previous one as raised by reviewer #2.
Please also consider all remaining remarks and minor requested made by the two referees, as in the attached material.
I am confident that, after considering all the previous points, your manuscript will be in a form which will be of great interest for the journal readers.

I wish you productive work and I thank you again for submitting to NHESS,
my best regards
FC

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AR by Chen Cao on behalf of the Authors (30 Jul 2020) Author's response Manuscript

ED: Referee Nomination & Report Request started (11 Aug 2020) by Filippo Catani

RR by Anonymous Referee #2 (24 Aug 2020)

Suggestions for revision or reasons for rejection

In this reviewed version of the manuscript, most of the points raised in the previous review have been addressed. The paper still requires some polishing here and there (including in writing quality, see point-by-point comments), but overall clarity improved. There is still one point of concern, again related to the estimation of the strength of the equivalent rock mass, which does not sound very solid. Specifically, the Authors used a failure criterion originally developed for non-persistent discontinuities, rather than rock masses. Importantly, however, in the presented approach the equivalent rock mass strength is dependent on the DFN already included in the model, and not the smaller discontinuities that were not included in the DFN. This means that although the factor of safety is lower, as expected, because the intact material is weaker than in the previous simulation, the mechanical properties responsible for such a decrease seem rather arbitrary. A more solid method would comprise the estimation of the GSI (excluding the fractures included in the DFN) to derive the equivalent shear strength parameters for the rock mass between larger discontinuities. In this reviewer’s opinion this point cannot be overlooked, and it is critical that the Authors address it adequately. Also, some tables appear not to have been updated with the new properties used for the simulations.

In the following section, some recommended changes are outlined.
Lines 63-64: please revise this sentence, as it seems that only model is investigated (i.e., Authors state that “a single-slope model” is analyzed). And, if that is actually the case, it is unclear how the runout analysis was undertaken for all the DFN realizations.
Line 71: please change “became” with “develop”
Line 72: I suggest using “Understanding whether rock slides will happen requires adequate/careful geological investigations”. Simply stating “calculating and evaluating” leaves the reader wondering what should be calculated and evaluated.
Line 74: I recommend using “by a low-mountain terrain, characterized by higher elevation to the north than to the south”
Lines 84-85: probably bedding planes and weak interlayers do not provide useful information with regard to tectonic stability. Also, I assume that this statement is backed by historic seismicity data, which is probably more indicative than the presence of faults and faults.
Line 101: Note that the K value is not a constant. I recommend changing in “The computed Fisher value (K) for the various sets……imply an high dispersion…”.
Lines 103-105: the Authors state the different “possibilities” that may result in bias, but do not explain how such bias is generated (ie, the window is can be smaller than the discontinuities, and the actual trace length cannot be measured). It is fair to say that this is basic knowledge, however, the Authors write that “the measures trace length bias occurs due to…”. Therefore, the reader would expect a more detailed explanation is to be expected in the following.
Line 115: change “from the exposed surface” to “across the exposed surface”.
Line 133: unclear what “fracture data” refers to – perhaps are the Authors referring to “fracture intensity”?
Lines 136-139: this part remains unclear. “We intersected the fractures….along the dip direction”. Did the Authors virtually extend the fractures across the section, to identify the points where these extended fractures would intersect one another (if this is correct, the term “exposed surface” is ambiguous)? The subsequent procedure is still not clear – I suggest including a conceptual figure to enhance clarity.
Lines 159-160: I suggest the Authors to refer to the “trial-and-error method” to describe the procedure of repeatedly changing particle size to achieve the best compromise between runtime and detail.
Line 181: In the present form, there is a mixture between actual geological processes (progressive strength deterioration) and the numerical approach used to compute the factor of safety (the SSR). I recommend the Authors to modify this sentence, either changing the brackets with “which can be investigated using the shear strength reduction method“ or stating that “the rock mass failure is generally SIMULATED/INVESTIGATED by reducing the shear strength”.
Line 188: Units, ° and MPa, should be included after the values. Please correct “filed” in “field”.
Lines 183-193: The procedure for computing the linear persistence and the rock mass strength is unclear. The Authors use the equation from Shang et al., 2018 (which is used to estimate the strength of non-persistent discontinuities with co-planar rock bridges) in order to evaluate the strength of the equivalent rock mass. More precisely, they are effectively estimating the shear strength along the rupture surface. Authors refer to cohesion and friction angle of “intact rock mass”. This should be simply “intact rock”, as the equivalent rock mass strength is being estimated. In any case, the equation simply involves a balance between the strength of intact rock and fractures, as a function of the linear persistence of the latter. In other words, a persistence of 50% implies that the strength is given for a 50% by the fractures, and for the other 50% by the intact rock. Here is the problem: in the equation, the Authors used the strength (cohesion and friction angle) of the intact rock and fractures to compute an equivalent rock mass strength. As such, the value obtained is a rough estimation of the strength of an equivalent continuum rock mass constituted by DFN + intact rock. Then, this strength is assigned to the intact material in the model, while leaving the DFN in place. It is true that this approach results in a decrease in the FoS of the slope, simulating, in a way, the presence of a fractured rock mass between DFN fractures. On the other hand, such a decrease in strength is not based on actual field data (ie, small fractures), but only on the value of linear persistence, which essentially depends on the DFN. A more sensible approach would be an estimation of the GSI of the rock mass between the large fractures, in order to estimate its strength, and assign it to the intact material.
Line 202: please remove “by” from “by this way”.
Line 203: please change “equivalent parameters” with “microparameters”.
Line 205: please change “generate” with “simulate”.
Figure 1: I recommend simply using a North arrow (just with the N) and state the strike in the caption. Otherwise it is unclear what exactly the 200 stands for.
Figure 2: many fractures are trimmed by the window. Check if long vertical fractures are included in the DFN.
Figure 3: please include the number of poles plotted in each stereonet
Tables 2-3: the micro-parameter values were not updated, with respect with the previous version of the manuscript, to reflect the use of the equivalent rock mass.

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ED: Publish subject to minor revisions (review by editor) (24 Aug 2020) by Filippo Catani

AR by Chen Cao on behalf of the Authors (02 Sep 2020) Author's response Manuscript

ED: Publish as is (28 Sep 2020) by Filippo Catani

AR by Chen Cao on behalf of the Authors (30 Sep 2020) Author's response

Short summary

Slope failure is extremely common in mountainous areas. Therefore, the stability and potential failure of slopes must be analysed accurately. For most fractured rock slopes, the aforementioned analyses are considerably challenging. This study aims to propose a comprehensive approach that combines three well-established methods to conduct the aformentioned analyses. Finally, the critical slip surface, factor of safety, and accumulation distance are selected for safety assurance in slope analysis.