Effects of Entrance Shape and Blast Pocket on Internal Overpressure Mitigation for Protective Tunnels Exposed to External Detonation on the Ground

Round 1

Reviewer 1 Report

Summary

The authors present a numerical study of a highly specific tunnel complex design and its overpressure response resulting from a single externally placed explosive charge setup. Two variations in the structural geometry are considered, namely the slope of the tunnel entrance and the depth of blast pockets within the rigidly modelled structure. A mesh sensitivity is presented at length to build confidence in the model results and provides a good example as a design study approach. An interesting result is observed for the optimal blast pocket geometry but is unfortunately not further investigated or discussed with reference to the literature. The tunnel entrance geometry evaluation is flawed as it utilises a poorly chosen reference case which negates this portion of the analysis.

1. Introduction

As the model presented is limited to a rigid structure, the paper would benefit from some discussion about deformable vs rigid tunnel analyses.

The introduction briefly touches on mitigation techniques in tunnels, however there are many analogous scenarios when considering enclosed civil structures. The introduction would benefit from including some mention of these as they both benefit readers concerned about tunnel design and similarly expand the results of tunnel analyses to these fields. Whilst this is a broad suggestion, a recent review paper https://doi.org/10.1177/20414196221118595 may help the authors identify some such papers/fields for inclusion.

The methodology employed in the study relied upon convergence studies to validate the model, which is understandable given the challenge of experimental validation of such scenarios. A brief commentary of the approaches used in the literature to validate tunnel models would provide the reader with both context and confidence of the challenge/method.

2. Protective tunnel

Wall thicknesses are specified despite the model being rigid – clarify?

Figure 2 introduces the problem with the entrance shape analysis. As the figure shows it is not the angle of the entrance that results in the “capture” of a larger segment of the blast, but rather the proximity of the entrance to the blast.

The location of the top edge of the tunnel entrance should be fixed in space, representative of where the tunnel enters the surrounding land. Instead the authors fix the front bottom edge of the tunnel geometry and pivot the angle about this point. This implies that somehow for the α < 90 case the land around the sloped part of the tunnel is suddenly non-existent and for the α = 90 case that the tunnel entrance extends beyond the land.

As such the slope entrance analysis is merely proving the obvious, that a tunnel entrance that is closer to the blast captures more of the blast. This flaw transfers through into the rest of the slope angle analysis and discussion throughout the paper.

3. Numerical modelling

Line 136 – There is absolutely no justification for needing 4 references to define blast scaling.

Lines 139-145 – A large series of technical phrases are used without context to the scenario being modelled. Link the relevance of the features of the code to the problem at hand.

Line 156 - There is absolutely no justification for needing 4 references to define JWL.

Lines 171-173 – How can validating the blast pressures at the entrance of the tunnel address the differences that may occur from using a rigid structural model when any deformable effects would occur within the tunnel structure?

Line 174 – table label incorrect

4. Validation

An excessive number of gauge points are considered, and when plotted merely obfuscate one another whilst only peak pressures are considered for the convergence study.

For the validation study consider reporting the specific impulses of the positive and negative phases to evaluate effective loadings.

The readers may benefit from the authors using better descriptions than “tunnel entrance” and “main tunnel entrance” to differentiate the two different locations in the structure. Consider explicitly calling the outside facing tunnel the “exterior/external tunnel entrance”

Figs. 5 & 17 – far too many overlapping plots presented on the same axes.

Fig. 11 & Section 4.3 – please show the model boundaries for the blast loading – indicating where the blast is mapped to and where the flow out boundaries are for the fluid.

5. Blast analysis

Please clearly indicate the gauge point location better and include it in Fig. 15. Mention is made in the discussion of Section 4.4 that “a distance of 80m” is used. Whilst that section only considered gauges up to 30m from the main tunnel entrance.

Please also report the specific impulses – this provides a better measure of the effect of the diffraction effects and whether the reduction in overpressure is due to deflection or rarefaction by the geometries tested.

Fig. 17 – select less plots per graph so they are interpretable. It is just a mess at the current scale and number of subplots. Select contrasting signals and interpret them in the text.

The results show a trend of peak pressure reducing with decreasing pocket depth up until the 0.5 ratio, after which at a ratio of 0 (no pocket) the peak pressure is maximum attained. This is an abrupt change to the trend established by the analysis. This indicates an area of interest for investigation and further discussion which is completely not addressed by the paper.

Line 398 – Assume this is a copy paste error and this should be the title for the conclusion section

Given the previously mentioned issues with the tunnel entrance angle analysis the conclusions need updating once this has been fixed.

The bulk of the conclusion section comprises of an introduction and mention of future work, with the remainder relating to the validation study for this specific model. Besides the single pocket depth comment nothing else mentioned is conclusive.

Lines 424-429 – The authors list extensive set of parameters to vary to further the analysis, which together constitute an infinite parametric study. These are the limitations of the analysis, not a conclusion.

Author Response

Please see the attached responses to reviewer comments.

Author Response File: Author Response.docx

Reviewer 2 Report

This study presented a numerical investigation of protective tunnels to propose a tunnel model for mitigating maximum internal overpressures. A computational fluid dynamics model of the protective tunnel was developed to predict the maximum overpressures. The effects of the slope angle of the tunnel entrance and blast pockets were examined through a parametric study to reduce the maximum blast overpressure. The MS is well orgnized. The idea is easy to follow. The MS can be accepted after the following issues are addressed.

1) Please justify why the authors conduct this work.  I notice that the authors menthioned 'as such....'. It seems to be insufficient.

2) Table 1 refers to whose parameters.

3) Whether the Mesh sensitivity will be dependent on the physical scale of your model. Please comment. Thank you.

4) It is strongly recommended that the authors explain the rationale behind the results rather than the results alone, e.g., the 4th conclusion.

Author Response

Please see the attached responses to reviewer comments.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Thank you to the authors for addressing my comments in a concise and polite manner, especially when addressing topics considered out of scope of the original project. I am happy that all my concerns have been addressed.

The revised manuscript reads very well and only wish to add clarification to the original comments for the authors consideration. 

Fig. 2 consider aligning the lower edges of the tunnel front in the three subfigures (a)-(c) so the graphical position relative to the detonation matches that described by the text.

Lines 429-438/Section 6.3 - My comment on not listing the limitations as conclusions was two-fold, one it was not really a conclusion and secondly it becomes an infinite list of variables when the geometry of a structure is a variable. I appreciate that the authors removed the limitations section from the conclusions and for integrity still included it as a separate section. I am also happy if they'd rather exclude Section 6.3 altogether as it is unfair to the work presented in its declared scope to list all the real world variables that are out of the scope as "limitations".

My main comment on the excessive gauge points was related to the graphical representation of the overpressures where the individual curves densely overlap making comparisons between curves challenging. The suggestion to use impulses was to simplify these curves into a simpler plot for differentiating between curves. The authors modifications to the plots have improved this differentiation.