Evaluating Carbon Emissions during Slurry Shield Tunneling for Sustainable Management Utilizing a Hybrid Life-Cycle Assessment Approach

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The topic of this study is meaningful. But, the innovation of this manuscript is not clear. The following questions need to be replied.

(1) The introduction and background content of this manuscript is lengthy. The author should explain the key issues of this manuscript clearly. And there is no need to discuss more weakly related content.

(2) How are P-LCA and I-O-LCA combined in this manuscript? The left arrow and right arrow pointing towards the middle part (as shown in the flowchart in Figure 2) confused me. I couldn't see how the two methods are connected.

(3) In Figure 3, if there are two figures, each figure should have a subheading and be marked with a and b. In addition, the legend colors in Figure 3 are difficult to distinguish and cannot effectively display the results.

(4) The names of figures 8, 9, 10, and 11 are not standardized. A description of the specific content of the diagram should be provided.

(5) This manuscript combines two methods and forms an improved method. Then, a calculation was conducted through a case study. But I haven't seen the innovation of the method in this manuscript.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The construction sector is one of the principal contributors to carbon dioxide emission (CDE) and has a vital role to play in responding to the climate change emergency. This research aims to contribute to the calculation of emissions during tunnel construction, helping decision-makers identify and apply energy reduction measures. A process-based hybrid life-cycle assessment (LCA) model depending on processed-based LCA and input-output LCA has been developed for increased accuracy and testified with a case of a real tunnel with the hybrid model. emissions during the tunneling stage are negatively correlated with the efficiency of tunnel construction. The processed-based hybrid LCA model may not only help to accurately evaluate the amount of carbon emissions but also could facilitate more informed decisions. This work is suitable for the Journal of Sustainability. However, this manuscript needs a minor revision before being published in this journal.

1.      The H-LCA method, combined top-down IO-LCA and bottom-up P-LCA, has been developed relatively maturely since its proposal in the 1970s and has been widely used in the carbon emission analysis of highway engineering. Is there any innovative improvement in the proposed method compared to previous research, or has the application object been expanded?

2.      H-LCA model can reduce system truncation errors while retaining process specificity. However, there is a homogenization hypothesis for the upstream sector, which does not take into account the difference in emissions from different production sectors of the same material, is there sufficient benefit to breaking system integrity?

3.      The results of the life cycle assessment depend on the quality of inventory analysis, while this article lacks data quality inspection and comparison, uncertainty analysis, and sensitivity analysis for data sources. At the same time, the carbon accounting results studied in this article are described vaguely compared with similar research results, and there is no intuitive data comparison. What is the absolute difference and percentage of carbon emissions per functional unit? Is the material contribution ratio of 93.88% in this article too high, and there is a significant difference from the 65% in the reference (Miliutenko et al)? whether it is inappropriate to assume that the ratio of the two is equivalent in the article?

4. The carbon emission factors of China’s power grid have been updated to varying degrees from 2017 to 2019. Is the timeliness of taking the carbon emission factors of 2016 from the power grid in the article relatively backward? In the IPCC national greenhouse gas inventory, there are upper and lower limits of error for emission factors, and a single value in the text will cause significant uncertainty.

Comments on the Quality of English Language

Minor editing of English language required

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 3 Report

Comments and Suggestions for Authors

The paper refines the LCA modeling of tunnel construction by combining process based LCA and input-output LCA.

Lines 42-48 are missing citations.

Line 58 - "All" is a bold declaration.  "Many" might be a better word to use.

Line 58 and 59 sentence does not  make sense.

Line 63 - "achieve the discharges" seems like a strange choice of words.  Did you mean "reduce emissions"?

Line 70 - "environmental impact" - Is this an endpoint or a midpoint metric?  Did you mean GHG, climate change, or something else?  Total environmental impact sounds like an endpoint but still would need more clarification.

Paragraph lines 62-76 - It would be helpful to put tunnel construction into perspective.  What % of CO2 emissions in the construction sector come from tunnel construction?

Line 87 - "for the first time"  Are you saying that Miliutenko's first time using LCA was this 2012 paper?  Or that this paper was the first time a tunnel LCA was performed?  

Lines 90-93 - Are you saying that concrete and asphalt were the largest contributor to CED and GWP for the construction phase, but for the lifecycle, operational phase was greatest for energy consumption and GHG?  

Throughout this section, there is a lack of discipline for the terms GWP, CO2 emissions and GHG emissions.  

Line 102 - pots -> spots

Line 180-181 - A word seems to be missing in the sentence.

Line 185 - "not for other GHG like CO"  Do you mean methane or nitrous oxide?  

Lines 202-204 are redundant with lines 194 and 195 in the same paragraph.

Figure 1 in the top dashed line box says "Matiral"

Line 222 - LCA is a methodology, not a tool.

Line 224 - LCA could identify the energy reduction measures, but not necessarily.  It could be any impact category.

Line 249 - The term "industry" doesn't make sense in this sentence.  

Line 270 and 306 - date -> data

Line 306 - "will" is a strong verb.  Do you mean "may" or "can"?

Line 310 - Per the methodology, I-O LCA models don't take social, technological and behavioral factors into account.  That's not the purpose of I-O LCA.

Line 313 - tool -> methodology

Line 369 - Do you mean "of electrical energy for equipment i"?

Line 379 - Do you mean "of fuel for vehicle i ...amount of fuel for vehicle i"?

Line 384 - These idioms are difficult to translate.  Consider avoiding such terms.

Lines 399 and 400 - Do you mean "of fuel for equipment i"?

Line 417 - Functional unit is not clear. "new one ring equivalent to 25 rings above" is a strange FU and one that is not fully defined in terms of temporal, spatial, etc.  Perhaps you mean a FU such as "a 25 ring tunnel in Shenzhen China in 2023."

Line 418 - Do you mean Tables A.2-A.5?

Line 419 - Consumption and consumed in the same sentence might be improved by the term "utilized" on line 419.

Line 439 - This chart doesn't have a Figure label.  It also doesn't easily show that materials are 94% given the comparable height of the bars in the other 3 quadrants.  Consider if there might be another chart to better show this data.

Figure 3 - 41 rings?  The FU was 25 rings.  Why 41 rings now?  This chart isn't clear.  Check Figure numbers for remainder of paper since Figure numbers don't match the text.

Line 480 - While 5% is appropriate to capture, it's not appropriate to infer that prior researchers omitted something very significant or weren't transparent about the exclusion of auxiliary materials.  

Line 495 - A solution that threatens stability and durability is not a solution.  Scenarios need to be viable.  

Line 571 - Not sure that you can claim precision.  I-O LCA uses industry averages which can have significant error bars around the results.  

Line 591 - date -> data

Line 607 - 5% doesn't seem urgent.  

Line 448 - The chart doesn't show this, or at least it's not obvious.  

 

Comments on the Quality of English Language

The use of idioms can make the paper difficult for readers to understand.  Examples include "no mean feat", "more than a Band-Aid"

Consider having the paper reviewed for English language.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Round 2

Reviewer 3 Report

Comments and Suggestions for Authors

This paper uses a hybrid PLCA and IOLCA approach to perform a case study on CO2 eq emissions from tunneling operations in China. 

The paper is significantly improved.

At line 266, "five sources" are claimed, but only 4 are noted.  "Upstream indirect fuel" is omitted here.  Either add the 5th or correct line 266 to say "four sources".

On this same point, section 3.1 only discusses 4 of the 5 sources, omitting 3.1.5 Upstream Indirect Fuel.  Perhaps there's nothing the authors want to discuss, but I note the omission.

Section 4.1 does not state a Functional Unit.  An LCA paper should not omit a FU, goal, scope, boundary condition and so forth.  

Comments on the Quality of English Language

There are still sentences that are worded strangely, or not properly constructed.  The editor should make careful note to review for English language issues.

Author Response

Please see the attachment

Author Response File: Author Response.pdf