the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 03 Aug 2026
Understanding Spatial and Temporal Evolution of Progressive Excavation Damage Zone in Granitic Rocks – Development of a Novel Mine-by and Laboratory Experimental Framework at the BedrettoLab
Abstract. The brittle damage around tunnels in the form of spalling is common in massive crystalline rocks subjected to high in-situ stresses. Understanding the short-term development and long-term evolution of the Excavation Damage Zone (EDZ) is particularly important for deep geological repositories (DGRs) for heat-generating nuclear waste, as the EDZ may provide preferential pathways for radionuclide migration. This paper presents an overview of the PRECODE mine-by experiment at BedrettoLab, a newly developed underground research laboratory (URL) in southern Switzerland designed to investigate brittle fracturing and EDZ evolution in crystalline rocks under repository-relevant conditions. The PRECODE tunnel was excavated in Rotondo Granite using both controlled drill-and-blast (D&B) and line-drilling and rock-breaking (LDRB) methods to evaluate the influence of excavation technique on EDZ characteristics. The experiment combines multidisciplinary in-situ monitoring including acoustic emission, distributed acoustic and strain sensing, hydraulic monitoring, electrical resistivity tomography, and terrestrial laser scanning with laboratory-based fracture mechanics and time-dependent testing under varying environmental conditions. The main objectives are to characterize short-term EDZ formation, investigate long-term crack growth and stress corrosion processes, quantify permeability evolution, and assess the interaction between excavation-induced damage and nearby fault zones. Preliminary results indicate clear differences in AE activity and EDZ development between excavation methods, while observations from the mature EDZ of the more than 40-year-old Bedretto Tunnel suggest ongoing time-dependent brittle fracturing. The coupled in-situ and laboratory datasets provide a unique basis for improving predictive models and long-term safety assessments for nuclear waste repositories in crystalline rocks.
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Status: open (until 14 Sep 2026)
- RC1: 'Comment on egusphere-2026-3474', Anonymous Referee #1, 07 Aug 2026 reply
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- 1
This is an interesting paper, which combines a review of research related to excavation damage zone characterization and prediction with a presentation of new preliminary results for an ambitious and valuable field experiment. The paper is strong overall, notwithstanding a few technical concerns to be addressed and a need to more clearly articulate the contribution. The writing is generally good, but there are many areas where some minor editing would improve the readability and presentation quality of the work.
Specific Major Comments:
- The literature review in the Introduction is appropriate, but the contribution of the paper is not clearly presented in the context of the review. The paragraph from Lines 110-114 is where this would normally happen, but the current version of this paragraph only includes two sentences that are not well-connected, with the second summarizing the study without any reference to the specific contributions.
- Section 4.5: By definition, if imaging is being done using laser scanning (which does not penetrate into the rock), could one not at best be characterizing the HDZ based on visible overbreak? More generally, this raises a question of whether or not the "EDZ growth" that is attributed to time-dependent damage process could instead be attributed to differences in characterization approaches (or, for example, time-dependent transitioning of parts of the EDZ to HDZ without actual EDZ growth). Perhaps these alternative possibilities are not likely for reasons known by the authors, but it should be addressed explicitly either way. I also believe that currently the explicit/direct comparison between the newer results and the older tunnel is made in the abstract (Lines 39-40), but not fully addressed in the discussion within the paper.
- The simplified presentation of AE rate over time in Section 6.1 may not be sufficient to understand the trends. I think it is necessary to add a visualization of the accumulation of AE activity relative to the total excavated volume, which is a better representation of changes in the energy balance within the system. In this framing, I actually think that the AE rates may be quite similar between the two excavation methods. Statements like "... indicating a greater degree of excavation-induced damage associated with drill-and-blast excavation" in the conclusions therefore may be misleading (since the total damage per unit-volume excavated, or equivalently per unit length of advance, may be similar).
- I would argue that there is a great deal of uncertainty associated with the resistivity results and that there is a risk of overinterpretation. For example, consider the area around (0.8,-0.2), which is a prime example of the statement "higher resistivities are observed in the soft excavation region compared to the D&B part". Here, the resistivity varies between the two figure panels from ~2.300 Ohm.m (left) to ~3.700 Ohm.m. For a quantity that naturally varies over orders of magnitude, such differences are not necessarily particularly large, and well within the range of variability that might be expected to be induced based on propagation of various uncertainties through the inversion process. At the very least, this possibility should be acknowledged (or explicitly refuted). Relatedly, since the rock is so impermeable, why is it a reasonable assumption that one year would be sufficient time for water to enter small (sometimes isolated) cracks within the EDZ?
Specific Minor Comments:
- The "MS type" is listed as a "Review article", but this article is not really framed as a review, in my opinion.
- Lines 70-72: If you are going to mention "A variety of studies", you should cite more than one here, and ideally independent studies (i.e. not just later papers that also cite back to Diederichs, 2007)
- Lines 142-143: This statement is not universally true. It is well-established in the literature that for porous rocks and under confined conditions, CD and volumetric strain reversal are not coincident. A more accurate statement might be something like "Under unconfined compression conditions, low porosity crystalline rocks typically reach the CD threshold at the point where volumetric strain reversal occurs.
- Lines 154-156: A bit strange to only see one reference on relevant studies using DIC when five studies on DSS were provided in the immediately prior sentence.
- Table 1: Not something to be addressed in this paper, but it is curious that the dynamic Young's Modulus is lower than the static Young's modulus.
- Line 602: Wouldn't this represent intact rock properties rather than fracture or rock mass?
- Line 737: I believe you may mean "point spacing" rather than "pixel spacing"
- Lines 740-741: In this comparison, how do you differentiate EDZ (or really, HDZ) development from local structural overbreak?
- Line 757-761: the distinction between EDZ and CDZ (and the way it is determined) here is getting a bit blurry.
- Lines 806-808: why is the number of iterations only known "roughly"? Should this not be a fixed and known number?
- Lines 853-854: what is meant by "Most AE events"? This is vague. If you're using this metric to delineate the EDZ (at ~1.5 m), you should present a more repeatable criterion (why not 1.4 m or 1.6 m?).
- Lines 847-860: much of this is interpretive and new (i.e. not previously presented in a discussion section). The conclusions should summarize rather than introduce new interpretations.
Technical Corrections (Minor Writing Suggestions):
- Please double check the journal's guidelines, but you need to (or wish to) use commas rather than periods to separate thousands in large numbers
- Lines 22 and 44: I suggest removing the initial "The" (starting "Brittle damage...")
- Line 47: "may or may not to be violent" (typo - erroneous inclusion of "to")
- Line 52: I believe "disposals" should be "disposal"
- Line 58: I suggest "In-situ observations ... *have* revealed ..."
- Line 73: Should the first sentence of this paragraph actually perhaps be the last sentence of the prior paragraph?
- Line 85: Is the comma after "demonstrated" here appropriate/necessary?
- Lines 90-94: I found this sentence a bit awkward/tortuous, and suggest re-wording.
- Line 106: "... in rock, which impacts the long-term strength."
- Lines 134 & 142: "During a compression test..."
- Line 149: Suggest removing "More recently,"
- Line 214: "are" should be "were"; similarly, "is" should be "was" in the next sentence. The conditions documented by Collins (1997) are not static, and therefore should be referred to in past tense.
- Line 215: Start this sentence "Brittle failures ..." (remove "the").
- Line 229: "... for the TBM section the EDZ depth was limited to a few centimeters"
- Line 230: "... prior to and after excavation ..."
- Lines 231-232: "... limited to the EDZ and no enhanced permeability was observed beyond this zone."
- Line 239: suggested to remove "a" before "heat transfer"
- Line 260: "... and shape of the EDZ"
- Line 260: "Using a Hoek-Brown constitutive ..."
- Line 291: "Using an FDEM approach ..."
- Lines 306-312: I know you need to touch on this here, but this is longer than it needs to be and feels repetitive, repeating some earlier text almost verbatim.
- Line 356: "Even a massive rock mass ..."
- Line 359: "... around underground openings strongly interact with ..."
- Line 360: The repetitive use of "and" reads awkwardly here
- Lines 362-363: "... tips of microcracks ..."
- Line 431: "... of the field stress tensor"
- Line 433: Here "in situ" is not hyphenated, while it is elsewhere. I prefer "in-situ", but I suppose the most important thing is that it is consistent throughout the document either way.
- Line 462: "shows" should be "show"
- Lines 465-456: can specific citations be added?
- Line 522: "... from the Bedretto Tunnel, which has an azimuth of ..."
- Line 539: there appears to be a run-on sentence here that could be solved by adding a semi-colon between "sensors" and "after"
- Line 540: maybe "slower" rather than "more time-demanding"?
- Line 567: suggest replacing "post" with "after" for better parallelism with "during"
- Line 623: I believe "system" should be "systems"
- Lines 653-654: "The objective of the geophysical monitoring is to collect time lapse data to track EDZ evolution and allow for comparison of the shape and extent of the EDZ between areas with different excavation methods."
- Line 657: repeated with what frequency?
- Line 666: remove "up"
- Line 670: suggest replacing "however," with "but"
- Line 671: "... and each electrode is connected to takeout cables ..."
- Lines 735-736: "... a number of laser scanning and outbreak mapping activities were conducted ..."
- Line 736: "... characteristics of the EDZ ..."
- Line 739: "... the cloud-to-cloud distance algorithm ..."
- Line 747: suggest removing "the" before "normal"
- Line 748: suggest a comma before "with"
- Line 751: should "location" be "locations"?
- Line 774: "... we expect that saturation had taken place ..."
- Line 785: "Regarding the latter, ..."
- Lines 806-808: this sentence switches from past tense ("was run") to present tense ("is reached")