Development and Application of Methodology for Quantification of Overbreaks in Hard Rock Tunnel Construction
Round 1
Reviewer 1 Report
Nothing special to add. Accept in present form.
Author Response
Ljubljana, 06.01.2023
Subject: Reply to the comments of Reviewer 1
Dear Editor,
We would like to thank the reviewer on his support end encouragement for our research work.
Comment 1
Nothing special to add. Accept in present form.
Thank you very much for your kind words about our paper. We are grateful for the time and energy you expended on our behalf.
Author Response File: 
Author Response.docx
Reviewer 2 Report
In general, the overall methodology and presentation of the results has to be improved.
The introduction does not mention anything about smooth contour blasting. This has to be mentioned as part of the SoA in overbreak control.
L36: D&B is often applied as part of the NATM (New Austrian Tunnelling Method) in which the primary support of the tunnel is made predominantly of shotcrete lining and radial rock bolts.
This is not true, there are many other D&B excavations approach rather than the NATM. Mining tunnels rarely follow the NATM, that is primarily conceived for civil tunnels.
L56: In mining industry, in which functional role of secondary tunnel lining is rarely needed, the focus is on critical evaluation of the technological factors influencing blast damage to achieve better fragmentation and thus mining efficiency
Mining is rarely achieved by the tunnel itself, and tunneling is the way to access a mineral resource, not to properly mine it. Despite room and pillar, underground mining methods rely on caving or blasting of stopes for resource extraction. Thus, fragmentation control of mining tunnels improves tunneling efficiency, but not mining itself.
L168: The size of the blocks depends also on the spatial location of the joints shaping the block.
L182: What about the Q-index?. This a widespread rock mass classification method in tunneling. Also, from tunnel face mapping is often impossible to evaluate the persistence of joints.
L349: How is the threshold value calculated?
L531: This is only true assuming that the parameters have a normal distribution. Why are you assuming that all parameters have a normal distribution?
L613: Why is the overbreak measured on m2. Most of your context and discussion are based on wedges and volumes (in m3). This approach seems not consistent with the described problem.
Figure 11: The logarithmic fit is highly arguably. Also, RMR has a minimum rating of 3 for RQD < 25% and 5 for spacing < 60mm. This gives a minimum possible value of RMR of 8 before applying the factor for discontinuity orientation. Can you graph the overbreak against this last factor?
Author Response
Ljubljana, 08.1.2023
Subject: Reply to the comments of Reviewer 2
Dear Editor,
We would like to thank the reviewer on his/her valuable comments, which helped us to clarify and better formulate our manuscript. The changes in the manuscript are marked red.
We accepted all the comments of Reviewer 2 and made due changes as appropriate, which were added to the text. In the continuation we give detailed itemised answer to reviewer´s comments, as follows.
Comment 1
In general, the overall methodology and presentation of the results has to be improved.
Thank you for this general comment. We trust that after replying to all posed question and the comments of all reviewers the manuscript has improved so that methodology and presentation of the results is now clearer and more comprehensive. We are grateful for the insightful and important issues that reviewer highlighted and we think that his contribution made our manuscript more thorough and focused.
Comment 2
The introduction does not mention anything about smooth contour blasting. This has to be mentioned as part of the SoA in overbreak control.
We would like to thank reviewer for this valuable comment. A sentence has been added in Abstract:
It is widely accepted in engineering practise the overbreaks can be inevitable even that smooth contour blasting is applied.
Also, to highlight the issue, several sentences were added in Introduction, as follows:
Regularly, a smooth contour blasting technique [1]is used to reduce the damage of the rock mass surrounding the underground excavation. This technique involves drilling a number of closely spaced boreholes along the final excavation contour, in which low charge density-decoupled charges are installed. The detonation of the contour charges is triggered simultaneously after the detonation of the main charges located in the blastholes at the face of excavation with an aim to obtain as smooth contour as possible. Sometimes, presplitting blasting, in which the peripheral holes are detonated prior to main boreholes, is used with the same aim but typically lower efficiency [2].
With some exceptions [3], the evaluation of geological conditions, which are a key cause of the overbreak quantity are seldom evaluated in the context of the efficiency of the blasting [4].
Comment 3
L36: D&B is often applied as part of the NATM (New Austrian Tunnelling Method) in which the primary support of the tunnel is made predominantly of shotcrete lining and radial rock bolts.
This is not true, there are many other D&B excavations approach rather than the NATM. Mining tunnels rarely follow the NATM, that is primarily conceived for civil tunnels.
We agree with the reviewer that this sentence is not clear in terms of application of D&B in Mining and can be misleading, as clearly pointed out.
We changed this paragraph as follows:
Drill and blast excavation is readily used in hard rock tunnel excavation due to its economics and adaptability to changing rock mass conditions both in Mining and Civil Engineering. In Civil Engineering, drill and blast is often applied as part of the NATM (New Austrian Tunnelling Method) in which the primary support of the tunnel is made predominantly of shotcrete lining and radial rock bolts [1]. In the NATM, the purpose of installation of primary support is to achieve equilibrium in the interaction with rock mass so that ideally secondary lining can be constructed free of any additional external load.
Comment 4
L56: In mining industry, in which functional role of secondary tunnel lining is rarely needed, the focus is on critical evaluation of the technological factors influencing blast damage to achieve better fragmentation and thus mining efficiency
Mining is rarely achieved by the tunnel itself, and tunnelling is the way to access a mineral resource, not to properly mine it. Despite room and pillar, underground mining methods rely on caving or blasting of stopes for resource extraction. Thus, fragmentation control of mining tunnels improves tunnelling efficiency, but not mining itself.
We agree that the text is not clear and there is a need to clarify this issue. We changed the sentence appropriately:
In mining industry, in which functional role of secondary tunnel lining is rarely needed, the focus is on critical evaluation of the technological factors influencing blast damage to achieve better fragmentation and thus more efficient access to mineral resource [5].
Comment 6
L168: The size of the blocks depends also on the spatial location of the joints shaping the block.
We agree with reviewer that this need to be highlighted . We have added “spatial location of discontinuities” in the text (L168).
The joint families, present along the unsupported tunnel section, form wedges in the shape of tetrahedron, whose dimensions and positions depend on the orientation and spatial location of joints, and the orientation and geometry of the tunnel contour.
Comment 7
L182: What about the Q-index?. This a widespread rock mass classification method in tunnelling. Also, from tunnel face mapping is often impossible to evaluate the persistence of joints.
We could not use Q-system in our research as the geological mapping of the face of the excavation was based on RMR classification. We think that this is not deficiency to our work as Q-system doesn’t directly account for joint persistence while RMR does. Q system was therefore not subject of the research and consequently was not mentioned or elaborated.
We agree that Joint persistence is one of the most important parameters in structural analysis and that it is very difficult to evaluate this parameter from the face mapping, as it is mainly related to the joint trace length on the excavation face. However, we did some form of ¨back analyses¨ to evaluate joint persistence as we considered the existing failures along joints in the rock mass, which was part of the developed methodology.
We also believe that, based on the repeatability of the events and the evaluation of wedge failures, an experienced mapping geologist can properly assess this parameter in relatively consistent conditions, as it was in the particular case example.
Comment 8
L349: How is the threshold value calculated?
The full explanation is given in the section 3.3 (Quantitative analyses), as indicated in the text. We changed the sentence in the text to clarify this, as follows:
The determination of threshold value is explained in detail in Section 3.3. for the given application of the methodology on the case example presented below.
Comment 9
L531: This is only true assuming that the parameters have a normal distribution. Why are you assuming that all parameters have a normal distribution?
A large number of data is needed in order to precisely determine the type of statistical distribution. In underground projects, such as this, sufficient amount of data (of one random variable) is seldom available, so we made reasonable assumption that normal distribution is the most appropriate statistical distribution of random variables in question.
To support our assumption In description of methodology in Section 2.2.2 Probabilistic method, we quoted Phoon et al. [6] who stated that the selection of probability distributions are site and parameter specific, and that there is no universally “best” distribution for ground properties. As explained in the text, considering central limit theorem for sample variance, we can assume that as the sample size gets larger i.e. >30 the distribution of the sampling means approaches a normal distribution. Given the large number of samples (e.g. thousands in our case) a normal distribution of random variables was considered appropriate for this research study.
Comment 10
L613: Why is the overbreak measured on m2. Most of your context and discussion are based on wedges and volumes (in m3). This approach seems not consistent with the described problem.
The overbreak at each chainage was measured in m2 per meter length of the tunnel by geodetic survey, and this was the form of the input data we have to use in our analyses. The overbreak area was then converted to volumes using the procedure described in section 3.3 (subsections 3.3.2 and 3.3.3).
Comment 11
Figure 11: The logarithmic fit is highly arguably. Also, RMR has a minimum rating of 3 for RQD < 25% and 5 for spacing < 60mm. This gives a minimum possible value of RMR of 8 before applying the factor for discontinuity orientation. Can you graph the overbreak against this last factor?
We would like to thank reviewer for this insightful comment. It gave us some deeper understanding of the nature of overbreaks as it will be explained bellow.
We agree that the scatter of data is relatively large, but this is the result of the combination of natural variation of the rock mass and probably some subjectivity that is integrated in any in situ characterisation of rock mass.
Among the other distributions (e.g., linear, exponential that we also tried) the logarithmic function best fits the analysed data. Also, the same trend is obvious on figures 11a and 11b, which we find more important than the type of function used to describe the data. It was important that methodology based on both deterministic and probabilistic method could have been statistically evaluated to give us some realistic assessment of quantification of the overbreaks.
Regarding your second question: Yes, the minimum RMR value is 8 (if all the other parameters are assumed equal to zero), but these low values RMR=3 and 4, are a consequence of negative quoting for joint orientation. In summation ratings of parameters these values could be readily achieved.
The requested figure is provided in the attachment.
It can be seen in the figure that negative quotation of -5 for joint orientation, which had given the total value of RMR of 3 was also giving the largest measured overbreaks e.g of up to 25 m2 per meter length. This indicates that there was a particular combination of factors which were concentrated in one type of damaged structure of rock mass (e.g fault zones). When checking the higher values for the negative quotation (i.e. higher than -5 such us -10 and -12) we found that these conditions were not always met as the total RMR value was also highm so negative joint orientation did not necessarily result in high overbreaks.
[1] D. Zou, “Contour Blasting for Underground Excavation,” in Theory and Technology of Rock Excavation for Civil Engineering, Singapore: Springer Singapore, 2017, pp. 503–508.
[2] Z. Zhou, R. Cheng, X. Cai, J. Jia, and W. Wang, “Comparison of Presplit and Smooth Blasting Methods for Excavation of Rock Wells,” Shock Vib., vol. 2019, pp. 1–12, Apr. 2019, doi: 10.1155/2019/3743028.
[3] A. Pomasoncco-Najarro, C. Trujillo-Valerio, L. Arauzo-Gallardo, C. Raymundo, G. Quispe, and F. Dominguez, “Pre-split blasting design to reduce costs and improve safety in underground mining,” Energy Reports, vol. 8, pp. 1208–1225, Nov. 2022, doi: 10.1016/j.egyr.2022.07.109.
[4] S. P. Singh and P. Xavier, “Causes, impact and control of overbreak in underground excavations,” Tunn. Undergr. Sp. Technol., vol. 20, no. 1, pp. 63–71, 2005, doi: https://doi.org/10.1016/j.tust.2004.05.004.
[5] S. P. Singh, “Blast damage control in jointed rock mass,” Fragblast, vol. 9, no. 3, pp. 175–187, Sep. 2005, doi: 10.1080/13855140500293280.
[6] K. Phoon, F. Nadim, M. Uzielli, and S. Lacasse, “Soil variability analysis for geotechnical practice,” in Characterisation and Engineering Properties of Natural Soils, Taylor & Francis, 2006.
Author Response File: Author Response.docx
Reviewer 3 Report
A methodology for determining overbreaks in hard rock tunnel construction using drill and blast technique is presented in the manuscript. The quantification of overbreaks including the threshold value distinguishing technological from geological overbreaks is proposed. Finally, The application of the methodology, demonstrated on the example of pressure tunnel in hard rock, is presented. The manuscript puts forward the method of overexcavation quantification, and has practical application examples. However, there are also some small problems that need to be pointed out here.
1. In the abstract, the authors pointed out that “Technological overbreaks caused by inappropriate use of drill and blast excavation are not easily distinguished from inevitable overbreaks dictated by geological conditions (geological overbreaks) with which they interfere and overlap with.” If the rock mass is relatively broken, so the blasting effect is inevitably poor, and it is easy to cause overbreak. It is obviously caused by two reasons, not only blasting. Is it meaningful to distinguish?
2. In the abstract, The application of the methodology, demonstrated on the example of a 12 km long pressure tunnel in hard rock, is presented. However, in the “Introduction”, (see line 109), the Tunnel length becomes 8.1 km. It seems to be inconsistent.
3. In the manuscript, two methods are proposed, namely, deterministic and probabilistic methods., and pointed out that the methodology can be used in any jointed hard rock mass. However, the work of these two types of methods is considered to be a rather tedious task. Moreover, for many joints, only the surface can be seen, and the interior of surrounding rock masses cannot be observed, so the volume of wedge is difficult to be determined, so how do you think about it?
4. Continue to discuss the third question, How to verify the reliability of the methods you proposed?
Author Response
Ljubljana, 08.1.2023
Subject: Reply to the comments of Reviewer 3
Dear Editor,
We would like to thank the reviewer on his/her valuable comments, which helped us to clarify and better formulate our manuscript. The changes in the manuscript are marked red.
We accepted all the comments of Reviewer 3 and made due changes as appropriate, which were added to the text. In the continuation we give detailed itemised answer to reviewer´s comments, as follows.
Comment 1
A methodology for determining overbreaks in hard rock tunnel construction using drill and blast technique is presented in the manuscript. The quantification of overbreaks including the threshold value distinguishing technological from geological overbreaks is proposed. Finally, The application of the methodology, demonstrated on the example of pressure tunnel in hard rock, is presented. The manuscript puts forward the method of overexcavation quantification, and has practical application examples. However, there are also some small problems that need to be pointed out here.
We would like to thank reviewer for the summary that well represent the research work presented in the manuscript- We hope that the our response to his comments as well as changes in the document will correct small problems indicated by the reviewer.
Comment 2
- In the abstract, the authors pointed out that “Technological overbreaks caused by inappropriate use of drill and blast excavation are not easily distinguished from inevitable overbreaks dictated by geological conditions (geological overbreaks) with which they interfere and overlap with.” If the rock mass is relatively broken, so the blasting effect is inevitably poor, and it is easy to cause overbreak. It is obviously caused by two reasons, not only blasting. Is it meaningful to distinguish?
We believe that there is a reason to distinguish as overbreaks can cause severe problems in NATM tunnelling, as pointed out in the text. The consequences of large overbreaks are seen in terms of excess excavation and additional removal of excess material, larger and inappropriate use of shotcrete for the primary lining and excess use of cast in place concrete for secondary lining. If the overbreaks are uncontrolled additional structural measures are needed (eg. the need for the reinforcement of the secondary lining to compensate for the asymmetric loading of the self-weight of the cast in place concrete etc.).
It is relatively easy for blasting contractors to be relieved of the responsibility for poor blasting by ˝poor geology˝ which brings about a certain degree of inevitability of the occurrence of overbreaks. The paper presents a methodology, which can be objectively used to distinguish between ˝geological˝ e.g. justified overbreak and the overbreak caused by inappropriate blasting. This information is very valuable for the client, as he can react on time to mitigate the damage and/or accept additional costs if that is justified.
Comment 3
- In the abstract, The application of the methodology, demonstrated on the example of a 12 km long pressure tunnel in hard rock, is presented. However, in the “Introduction”, (see line 109), the Tunnel length becomes 8.1 km. It seems to be inconsistent.
We agree with the reviewer. We changed the sentence in the “Abstract” ,as follows:
The application of the methodology, demonstrated on the example of a 8.1 km long section of a 12 km long pressure tunnel in hard rock, is presented in the paper.
Comment 4
- In the manuscript, two methods are proposed, namely, deterministic and probabilistic methods., and pointed out that the methodology can be used in any jointed hard rock mass. However, the work of these two types of methods is considered to be a rather tedious task. Moreover, for many joints, only the surface can be seen, and the interior of surrounding rock masses cannot be observed, so the volume of wedge is difficult to be determined, so how do you think about it?
The aim of the deterministic and probabilistic methods in the methodology was to check the reliability of the input data that had been gathered during the construction of the tunnel. It was demonstrated on the key examples that the input data gave reliable indications on the type and form of wedge failure. This was a necessary pre-condition to extrapolate these findings and use statistical evaluation which was also a tedious task but very much less so.
The interior of the surrounding rock masses mass was seen in the process of the excavation of the tunnel as every 3m of the face of the excavation was mapped and evaluated. The shear strength of joints were tested in the laboratory so these data were determined using scientifically recognised methods. It was important for us that methodology based on both deterministic and probabilistic method gave acceptable results (based on the given input data) in terms of calculated and observed wedge failure so that overbreaks could have been statistically evaluated to give us some realistic assessment of quantification of volumes.
Comment 5
- Continue to discuss the third question, How to verify the reliability of the methods you proposed?
We would like to thank reviewer for emphasising this important issue.
Every scientific method is directly dependent on the quality of the processed data so equally is valid for the research work in our manuscript. The first part of the methodology was developed and carried out on the case example to check the reliability of the data. The good agreement between deterministic, probabilistic and observed wedge failures gave us credibility to carry out appropriate statistical analyses of quantification. In that sense we consider that the reliability of the method was confirmed on the case example we presented. We understand that this might not be the case for some other case examples in which the input data are poor and unreliable. But the proposed methodology would process those data, establish poor relationship between calculated and observed behaviour, and inevitable conclusion would be that statistical evaluation would not give any reliable results and would not be carried out.
To clarify this in the manuscript we added the following paragraph in ˝Summary and Conclusions˝:
One of the aims of the deterministic and probabilistic analyses developed as part of methodology was to check the reliability of the input data that had been gathered during the excavation of the tunnel. It was demonstrated on the key examples that the input data gave reliable indications on the type and form of wedge failure. This was a necessary pre-condition to extrapolate these findings and use statistical evaluation of the volume quantities for the whole tunnel. In that sense, the reliability of the method was confirmed for the presented case example. For the cases, in which the input data are poor and/or unreliable the proposed methodology would establish poor relationship between calculated and observed behaviour, and inevitable conclusion would be that statistical evaluation would not give any reliable results and would not be carried out.
Author Response File: Author Response.docx
Round 2
Reviewer 2 Report
Good changes. Paper must be proof read by a native English speaker
Author Response
Ljubljana, 15.1.2023
Subject: Reply to the comments of Reviewer 2
Dear Editor,
We would like to thank the reviewer on his/her valuable comments, which helped us to clarify and better formulate our manuscript.
We carefully checked the second round of comments of Reviewer 2 who only required the edition of English. This was carried out by MDPI English edition services.
Comment 1
We would like to thank Reviewer 2 for his encouraging comment. The paper has been proof read and corrected by a native English speaker.
Author Response File: Author Response.docx
