The Characteristics and Seepage Stability Analysis of Toppling-Sliding Failure under Rainfall
Round 1
Reviewer 1 Report
The manuscript is written clearly and easy to read but major revisions are needed. So, in order to help the authors with some suggestions for improving their manuscript, I have provided a short non-all-inclusive list of general comments that I believe need to be carefully addressed and modifications made accordingly.
The authors should spend more words. In the introduction, the authors should improve the state of the art with some more recent contributions available on the topic. I suggest some papers to be added in lines 67 – 71: Landslide, spreading, deep-seated gravitational deformation: Analysis, examples, problems, and Proposals; Interaction between Perched Epikarst Aquifer and Unsaturated Soil Cover in the Initiation of Shallow Landslides in Pyroclastic Soils; Characterization of the groundwater response to rainfall on a hillslope with fractured bedrock by creep deformation and its implication for the generation of deep-seated landslides on Mt. Wanitsuka, Kyushu Island; Characterizing groundwater flow in a translational rock landslide of southwestern China; Prediction of shallow landslides in pyroclastic-covered slopes by coupled modeling of unsaturated and saturated groundwater flow. Nevertheless, more references are needed (international relevance should enhance).
In figure 10 I saw the terms “evapotranspiration” and "surface runoff". Does the model include them? How did you consider them? Can you add some information about?
The paper is not structured sufficiently and clearly. There is not a paragraph on “results and discussion”. Please add it to better keep the attention of the readers.
Add some information about the calibration of model parameters.
Explain the hypothesis of the factor of safety that you considered, especially regarding the sliding surface.
The authors should discuss also the geographic uncertainty of rainfall data (if can be significant).
Finally, in conclusion section, the limitations and recommendations of this research should be highlighted.
Best
Author Response
We would like to thank you, most sincerely, for all the effort and expertise that you have contributed to reviewing. Your insightful comments help us to improve this work greatly. Detailed responses to your comments are given below.
M1. The authors should spend more words. In the introduction, the authors should improve the state of the art with some more recent contributions available on the topic. I suggest some papers to be added in lines 67-71: Landslide, spreading, deep-seated gravitational deformation: Analysis, examples, problems, and Proposals; Interaction between Perched Epikarst Aquifer and Unsaturated Soil Cover in the Initiation of Shallow Landslides in Pyroclastic Soils; Characterization of the groundwater response to rainfall on a hillslope with fractured bedrock by creep deformation and its implication for the generation of deep-seated landslides on Mt. Wanitsuka, Kyushu Island; Characterizing groundwater flow in a translational rock landslide of southwestern China; Prediction of shallow landslides in pyroclastic-covered slopes by coupled modeling of unsaturated and saturated groundwater flow. Nevertheless, more references are needed (international relevance should enhance).
Thanks for your sharing. Relevant references have been added to the paper.
M2. In figure 10 I saw the terms “evapotranspiration” and "surface runoff". Does the model include them? How did you consider them? Can you add some information about?
The present study was focused on investigating the effect of infiltration of rainfall on the effect on the stability of toppling–sliding slope. Both evaporation and run-off which have a significant effort on the infiltration were not considered due to the lack of an effective method. However, when the precipitation is larger than infiltration, the surface runoff will generate resulting in water pressure head on the ground surface. The water pressure head is taken to include in the model.
Relevant information has been added in the “Conclusion” paragraph.
M3. The paper is not structured sufficiently and clearly. There is not a paragraph on “results and discussion”. Please add it to better keep the attention of the readers.
The “Discussion” paragraph has been added to the paper. Please refer to lines 340-366 (i.e., Based on the detailed investigation of the geological and deformation characteristics, The present study provides insight into the deformation and failure mechanism of a large–scale deep–seated toppling in Nandongzi Village, Pingquan City, Hebei Province. Combined with the monitoring data and numerical analysis, the stability of Nandongzi toppling is now highly sensitive to rainfall. When the rainfall intensity exceeds 220 mm/day (50 years return period storm), the factor of safety will fall below 1.05 and the failure may be triggered. The strength and physical properties of rock and soil material in different zones were determined by combining with the standard test in the laboratory and the trial–and–error "back analysis" according to the real local and overall stability, as a means to improve the model accuracy in predicting the factor of safety of the Nandongzi toppling under different design rainstorms of return periods. Because toppling–falling zone is mainly composed of quartz sandstone with loose–cataclastic structure, the VWC functions adopted in the present paper were for a gravel material included in SEEP/W with different saturated water contents for different zones. The SWCCs are then estimated using the Van Genuchten method with different Ksat. However, the hydraulic characteristics of block-gravel soil are very different from gravel soil. And the Van Genuchten method may not be suitable to estimate the SWCCs in different zones of Nandongzi toppling. In addition, the geographic uncertainty of rainfall data should be considered in the future. The relationship between the real landslide displacement and precipitation based on long-term multi-dimensional monitoring will help to reveal the formation mechanism, development process, and sensitive factors of such landslides. In summary, more works and studies are necessary for the characteristic and adequate stability analysis of Nandongzi toppling, e.g., i. the exploration adit, which is the best method to reveal inter structure of toppling and provide the opportunity for in-situ test in other zones, should be operated; ii. the site-specific hydraulic properties in different zones should be directly established from the test in situ or laboratory; etc.).
M4. Add some information about the calibration of model parameters.
The strength and physical properties of rock and soil material in different zones were determined by combining with the standard test in the laboratory and the trial–and–error "back analysis" according to the real local and overall stability, as a means to improve the model accuracy in predicting the factor of safety of the Nandongzi toppling under different design rainstorms of return periods. The value of K in the toppling-falling zone was obtained from the double-ring infiltration tests. However, the double-ring infiltration cannot be conducted in other zones due to the lack of exploration adit and the materials composed of rock mass which cannot operate the test. Therefore, the value of K in the other zones are estimated from the value of k in the toppling-falling and differential zonation characteristics of the toppled rock masses.
Therefore, the model is suitable to a certain extent. Revelant information has been added to the “Model setup” and “Discussion” paragraphs.
M5. Explain the hypothesis of the factor of safety that you considered, especially regarding the sliding surface.
SLOPE/W is a slope stability program that uses limit equilibrium analyses to determine the factor of safety. The factor of safety was obtained by using Bishop's simplified method, with a modified Mohr–Coulomb failure criterion to allow for shear strength variation due to rainfall infiltration through the toppling–falling zone.
The sliding surface in the model auto search by the Geostudio software, which is coincide with the locations of grayish white–white silty clay revealed by borehole drillings at the bottom of the toppling–falling zone.
M6. The authors should discuss also the geographic uncertainty of rainfall data (if can be significant).
The geographic uncertainty of rainfall data has been added in the “Discussion” paragraph.
M8. Finally, in conclusion section, the limitations and recommendations of this research should be highlighted.
The limitations and recommendations have been added in the “Discussion” paragraph.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
Minor comments:
1. The novelty of this study should be discussed clearly.
2. More discussion about Fig. 4 is needed to be added to the paper.
3. More details about numerical models are required.
4. How to obtain Figs. 13-14?
5. Please check the paper about rock slope: 10.1007/s40515-020-00137-4
6. How did the authors obtain the values of c and phi?
7. How did the authors get the value of k?
8. More discussion is needed.
Author Response
We greatly appreciate your taking time out of your busy schedule to reviewing. Your valuable comments are great helpful to improve the manuscript. Detailed responses to your comments are given below.
M1. The novelty of this study should be discussed clearly.
The main novelty of the present study is that we constructed an engineering geology model to describe the toppling–sliding under rainfall, and revealed the crucial factor affecting seepage stability of toppling–sliding slope under rainfall. Relevant information has been rewritten in the revision.
M2. More discussion about Fig. 4 is needed to be added to the paper.
Lithological sections have been added in the Figure. 4 and more discussion has been added to the paper. Please refer to lines 127-130 (i.e., Based on the varying rock quality designation (RQD) revealed by borehole drilling (Figure 4), the degree of fragmentation and weathering rate is gradually getting higher in going from the inner to the outer section.).
M3. More details about numerical models are required.
We have added more details about numerical models. Please refer to lines 228-235, 254-272, and etc.
M4. How to obtain Figs. 13-14?
The detailed information about Figures. 13-14 have been added. Please refer to lines 254-262 (i.e., SEEP/W includes a set of sample volumetric water content (VWC) functions for soils of varying textures. The VWC functions adopted here are for a gravel material with saturated water contents of 0.2, 0.25, and 0.3 for toppling-sliding zone, strong toppling zone, and slight toppling zone, respectively (Figure 13). Based on the double-ring infiltration tests operated in the toppling-falling zone and differential zonation characteristics of the toppled rock masses, the soil–water characteristic curves (SWCC) are then estimated using the Van Genuchten method [33], with the saturated hydraulic conductivity (Ksat) specified as 1x10-4 m/s, 1x10-5 m/s, and 1x10-6 m/s for top-pling-sliding zone, strong toppling zone, and slight toppling zone, respectively (Figure 14)).
M5. Please check the paper about rock slope: 10.1007/s40515-020-00137-4
Thanks for your sharing.
M6. How did the authors obtain the values of c and phi?
The values of c and phi were determined by standard testing methods in the laboratory, together with the trial–and–error "back analysis". Relevant information has been rewritten in the paper. Please refer to lines 262-272 (i.e., Based on the standard testing methods consisting of direct shear test and unconfined compression test operated in the laboratory, the strength and physical properties of rock and soil material in different zones were determined through the trial–and–error "back analysis". The trial–and–error "back analysis" was performed in terms of rainfall monitoring data, i.e., the maximum daily rainfall of 67.4 mm. We applied a flux (Q) with 7.8×10–7 m/s at the boundary for 24 hours (Figure 15). The properties listed in Table 3 gave the best modelled results (i.e., the overall factor of safety is 1.229 (stable state) but the local factor of safety is 0.959 (unstable state)), which best reproduced the stability state from the field survey and monitoring.).
M7. How did the authors get the value of k?
The value of K in the toppling-falling zone was obtained from the double-ring infiltration tests. However, the double-ring infiltration cannot be conducted in other zones due to the lack of exploration adit and the materials composed of rock mass which can not operate the test. Therefore, the value of K in the other zones are estimated from the value of k in the toppling-falling and differential zonation characteristics of the toppled rock masses. Relevant information has been rewritten in the paper. Please refer to lines 254-262 (i.e., SEEP/W includes a set of sample volumetric water content (VWC) functions for soils of varying textures. The VWC functions adopted here are for a gravel material with saturated water contents of 0.2, 0.25, and 0.3 for toppling-sliding zone, strong toppling zone, and slight toppling zone, respectively (Figure 13). Based on the double-ring in-filtration tests operated in the toppling-falling zone and differential zonation characteristics of the toppled rock masses, the soil–water characteristic curves (SWCC) are then estimated using the Van Genuchten method [33], with the saturated hydraulic conductivity (Ksat) specified as 1x10-4 m/s, 1x10-5 m/s, and 1x10-6 m/s for toppling-sliding zone, strong toppling zone, and slight toppling zone, respectively (Figure 14)).
M8. More discussion is needed.
The “Discussion” paragraph has been added to the paper.
Author Response File: Author Response.pdf
Reviewer 3 Report
The objectives of this study are to construct an engineering geological model to describe the toppling–sliding under rainfall and to demonstrate the sensitivity of the factors to permeability characteristics and rainfall intensities.
It is of some significance for geo-hazards early warning.
For lacking some aspects in the model, the manuscript should be reconsidered after major revision.
Please see the comments in the attached PDF file.
Comments for author File: Comments.pdf
Author Response
We very much appreciate that you read our original manuscript carefully and your insightful and very detailed comments. We have carefully considered the comments and have revised the manuscript accordingly. We hope that the revised manuscript is acceptable. Detailed responses to your comments are given below.
M1. of->on
Have revised.
M2. inducing -> triggering
Have revised.
M3. geology->geological?
Have revised.
M4. The climatic and hydrological characteristics of the study area are needed to be introduced in section 2.
The climatic and hydrological characteristics of the study area have been added to the paper. Please refer to lines 110-119 (i.e., The study area belongs to the warm temperate continental monsoon climate with four distinct seasons, i.e., windy springs, rainy summers, mild autumns, cold and dry winters. The average annual precipitation is 529.1 mm with a maximum daily rainfall of 64.6 mm. The total precipitation in summer accounts for more than 80% of the an-nual precipitation. The annual average temperature in the study area is 7.4°C. The highest temperature of 42.1°C was recorded in July, and the lowest temperature of -31.4 was recorded in January. The main groundwater type is Quaternary loose rock pore phreatic water and weathering bedrock fissure water. The groundwater level is highly dependent on the precipitation, thus the highest level is always observed from June to August. The groundwater mainly flows from south to north).
M5. Legends for blue lines are needed to be shown in Figure 1.
Legends for blue lines have been added to Figure 1.
M6. Please give the formula of the model showing the relationships between landslide displacement and its sensitive factors in different zones, such as Topping-falling zone, strong toppling zone etc.
Sorry, the present study combines the SLOPE/W and SEEP/W modules in the GeoStudio software to reveal the seepage law and factors affecting the seepage stability of toppling–sliding slopes under rainfall according to the factor of safety. The displacement field and plastic zone were not calculated through the SIGMA/W module was not used. In addition, displacement monitoring was just conducted on the toppling-falling zone. Therefore, we cannot give the relationships between landslide displacement and its sensitive factors in different zones.
M7. Could you please give lith sections in these 6 drills to provide evidence for model validation?
The lithological sections have been added to Figure 4.
M8. Validation on the model in toppling-falling zone, strong toppling zone, and slight toppling zone should be performed using field data and then give the accuracy of this model.
Relevant information has been added in the “discussion” paragraph. The strength and physical properties of rock and soil material in different zones were determined by combining with the standard test in the laboratory and the trial–and–error "back analysis" according to the real local and overall stability, as a means to improve the model accuracy in predicting the factor of safety of the Nandongzi toppling under different design rainstorms of return periods. However, the more works and studies are necessary for the characteristic and adequate stability analysis of Nandongzi toppling, e.g., i. the exploration adit, which is the best method to reveal inter structure of toppling and provide the opportunity for in-situ test in other zones, should be operated; ii. the site-specific hydraulic properties in different zones should be directly established from the test in situ or laboratory; etc.
M9. The characteristics of stability model including its validation and accuracy should also be concluded in section 5.
The characteristics of the stability model including its validation and accuracy have been discussed in the “discussion” paragraph. The strength and physical properties of rock and soil material in different zones were determined by combining with the standard test in the laboratory and the trial–and–error "back analysis" according to the real local and overall stability, as a means to improve the model accuracy in predicting the factor of safety of the Nandongzi toppling under different design rainstorms of return periods. However, the more works and studies are necessary for the characteristic and adequately stability analysis of Nandongzi toppling, e.g., i. the exploration adit, which is the best method to reveal inter structure of toppling and provide the opportunity for in-situ test in other zones, should be operated; ii. the site-specific hydraulic properties in different zones should be directly established from the test in situ or laboratory; etc.
Please refer to lines 340-366 (i.e., Based on the detailed investigation of the geological and deformation characteristics, The present study provides insight into the deformation and failure mechanism of a large–scale deep–seated toppling in Nandongzi Village, Pingquan City, Hebei Province. Combined with the monitoring data and numerical analysis, the stability of Nandongzi toppling is now highly sensitive to rainfall. When the rainfall intensity exceeds 220 mm/day (50 years return period storm), the factor of safety will fall below 1.05 and the failure may be triggered. The strength and physical properties of rock and soil material in different zones were determined by combining with the standard test in the laboratory and the trial–and–error "back analysis" according to the real local and overall stability, as a means to improve the model accuracy in predicting the factor of safety of the Nandongzi toppling under different design rainstorms of return periods. Because toppling–falling zone is mainly composed of quartz sandstone with loose–cataclastic structure, the VWC functions adopted in the present paper were for a gravel material included in SEEP/W with different saturated water contents for different zones. The SWCCs are then estimated using the Van Genuchten method with different Ksat. However, the hydraulic characteristics of block-gravel soil are very different from gravel soil. And the Van Genuchten method may not be suitable to estimate the SWCCs in different zones of Nandongzi toppling. In addition, the geographic uncertainty of rainfall data should be considered in the future. The relationship between the real landslide displacement and precipitation based on long-term multi-dimensional monitoring will help to reveal the formation mechanism, development process, and sensitive factors of such landslides. In summary, more works and studies are necessary for the characteristic and adequate stability analysis of Nandongzi toppling, e.g., i. the exploration adit, which is the best method to reveal inter structure of toppling and provide the opportunity for in-situ test in other zones, should be operated; ii. the site-specific hydraulic properties in different zones should be directly established from the test in situ or laboratory; etc.).
Author Response File: Author Response.pdf
Round 2
Reviewer 1 Report
The authors answered well to my comments. This paper can be accepted for the publication in such journal
Author Response
We would like to thank you, most sincerely, for all the effort and expertise that you have contributed to reviewing.
Reviewer 3 Report
For following reasons, this manuscript should be reconsidered after major revision.
1. There are some errors in figure 4(a):"According to figure 11, there is no toppling-sliding zone on the B06 top layers . How can you get it in this figure ?
2. Accuracy of this model is not given yet.
3. Please see more comments in the attached PDF file.
Comments for author File: Comments.pdf
Author Response
We would like to thank you, most sincerely, for all the effort and expertise that you have contributed to reviewing. After two rounds of review, your insightful comments help us to improve the manuscript greatly. Especially, thank you for the valuable suggestion for validating the numerical model. In addition, your careful and detailed changes to the language made the manuscript flow better. We have followed all of the changes to the text. We hope that the revision is acceptable. Detailed responses to your comments are given below.
M1. Please give lithology for different legends.
The lithology has been added for different legends.
M2. According to figure 11, there is no toppling-sliding zone on the B06 top layers. How can you get it in this figure?
Sorry, that is our serious mistake. The B01 in Figure 4 was drawn into the other borehole. In addition, we also made a mistake with the number of boreholes.
The mistake has been corrected. Thanks for your kindly remind.
PS> We conducted 11 boreholes with depths of 18.3–67.4 m in 2020 in the Nandongzi toppling. However, the present paper just includes 6 boreholes along the geological section, which are useful to set up the numerical model.
M3. Before this figure, I cannot find B01-B06 in any section or plan figures. Please move this figure after the figure 11.
The locations of B01-B06 have been added to the relief map, which has been moved before this figure.
M4. How can you valid your model through this section? Please explain it in details.
The model was set up based on the soil/rock structure, the degree of fragmentation and weathering rate revealed by the drilling, and deformation or failure characteristics revealed by the outcrop in different locations.
In addition, we have discussed the limitation of the present paper in the “discussion” paragraph, i.e., the exploration adit, which is the best method to reveal inter structure of toppling and provide the opportunity for in-situ test in other zones, should be operated to provide stronger evidence for Nandongzi slope structure and stability.
M5: Add "(e)" at the end of this sentence.
Have added.
M6. Accuracy of this model is not given yet. I suggest you can valid the model by comparing the model predicted displacements with the measured ones on the top layer as shown in Figure 9. Then, the accuracy/residual of the model can be calculated. As you know, without model validation, we cannot determine its correction and thus cannot accept your current version.
Thanks for giving a constructive suggestion to valid the model. We have added SIGMA/W module in the revision.
According to the rainfall monitoring data, we applied a flux (Q) with 7.8×10–7 m/s at the boundary for 24 hours in SEEP/W. The pore–water pressure profile computed from the seepage analysis using SEEP/W has been integrated into slope stability analysis using SLOPE/W and deformation analysis using SIGMA/W. The maximum deformation displacement is approximately 0.85 m near the toe of the slope. The deformation displacement range from the toe to the middle of slope is 0.8 m to 0.85 m which almost coincides with the G02 monitoring data (i.e., 0.73 m), and the deformation displacement is 0.43 m at where the G04 monitor was installed (i.e., ). In addition to the overall factor of safety is 1.229 (stable state, Figure 15a) but the local factor of safety is 0.959 (unstable state, Figure 15b), the results best reproduced the real stability state and deformation displacement from the field survey and monitoring. Therefore, the model accuracy in predicting the factor of safety of the Nandongzi toppling under different design rainstorms of return periods is valid and reliable.
M7. Delete "the effect on".
Have deleted.
Author Response File: Author Response.pdf
Round 3
Reviewer 3 Report
1. Please explain clearly what are the "local factor of safety" and "the overall factor of safety" referred to in Figure 15?
2. Why are their conclusions are different (the overall factor of safety is stable, while local factor of safety is unstable) ?
3. For their conclusions are different, how can you draw the conclusion that "the model aaccuracy in predicting the factor of safety of the Nandongzi toppling under different design rainstorms of return periods is valid and reliable." ?
Author Response
We would like to thank you, most sincerely, for all the effort and expertise that you have contributed to reviewing again. After three rounds of review, your insightful comments help us to improve the manuscript greatly. Detailed responses to your comments are given below.
Please explain clearly what are the "local factor of safety" and "the overall factor of safety" referred to in Figure 15?
Why are their conclusions are different (the overall factor of safety is stable, while local factor of safety is unstable)?
For their conclusions are different, how can you draw the conclusion that "the model aaccuracy in predicting the factor of safety of the Nandongzi toppling under different design rainstorms of return periods is valid and reliable." ?
A secondary sliding zone developed within the Nandongzi toppling. In the secondary sliding zone, the steep tension fractures at the back edge have almost connected with shear fissures along the bottom sliding surface and lateral scarps. The overall sliding have occurred in the secondary sliding zone during the early rainfall events. On the other hand, for the whole Nandongzi toppling, the surface ruptures highly developed in the back edge, but the surface ruptures have not connected with each other. Therefore, the whole Nandongzi toppling is in a stable state, but the secondary sliding zone is in an unstable state, i.e., the overall factor of safety should be larger than 1, but the local factor of safety should be less than 1 under the maximum daily rainfall according to monitoring data.
The relevant information has been added to the revision.
Author Response File: Author Response.pdf
Round 4
Reviewer 3 Report
This research is of some significance for geo-hazards early warning.
(1)As you can see that there are some grammar errors in the attached file, this manuscript should be revised and proofed by a native Englishman.
(2)There are some errors in your manuscript, please see them in the attached PDF file and correct them throughout the whole manuscript.
For these above reasons, I suggest a minor revision to this manuscript.
Comments for author File: Comments.pdf
Author Response
We would like to thank you, most sincerely, for all the effort and expertise that you have contributed to reviewing again. After fourth rounds of review, your insightful comments help us to improve the manuscript greatly. The revision has been edited for proper English language, grammar, punctuation, spelling, and overall style by one or more of the highly qualified native English speaking editors at Dr. Transpro. Please see certificate in the attached file. And, detailed responses to your comments are given below.
M1: According to the legends of figure15, Figure 15a is "the local factor of safety", not "the overall factor of safety" presented here and Figure 15b is the the "overall factor of safety", not the "local factor of safety" here. Please check your whole paper avoiding these errors.
Have checked the whole paper.
M2. In your response letter, you say "the local factor of safety is the secondary sliding zone developed within the Nandongzi toppling". You should give a plan map in the article showing different sliding zones, such as the first sliding zone, the second, ..., etc. to explain your opinion.
Have added different sliding zones in Figure 4.
M3. Furthermore, once a part of the landslide deforms, it should be considered as unstable and dangerous. Please correct it.
Have deleted the inappropriate descriptions.
M4. This sentence lacks subject.
Have revised, i.e., Figure 16. A secondary sliding zone developed within the Nandongzi toppling showing the connection of the (a) steep tension fractures at the rear edge of the secondary sliding zone, (b) shear fissures along the bottom sliding surface and (c) lateral scarps.
M5. delete "nan"? what's the meaning of this sentence?
Have revised.
The rainfall will change with geography, but we just have one rainfall monitor in the study area. Therefore, the geographic uncertainty of rainfall data can not considered in the paper. However, it should be considered in the future, because the rainfall intensity have significant influence on slope stability.
Author Response File: Author Response.pdf
