In order to comment on the updated ITA WG 12 Report (published October 2020), l invited a number of known industry SCL practitioners to share their views. The ITA Working Group (WG) reports are not codes and ITA is not mandated to insist on compliance with them.
The WG and AG reports are for guidance around best practice, and intended for those readers who do not know what they do not know. But they are more likely intended for those who know that they do not know and seek further guidance and knowledge. Immediately the ITA WG reports are published, they are by definition out of date, particularly if the report has taken much time to prepare (sometimes this can take many years).
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Capturing essential knowledge for guidelines is a rare skill. The WG animateur has to balance the input of everyone on the WG and consequently most guidelines have too much content which can confuse even the experienced reader; at worse, it can misdirect novice readers who do not know what they do not know.
The document is intended to inform, i.e. “…to give infrastructure owners and their advisors the confidence to incorporate PSCL into the underground space design…”. This aspiration can be undermined if the supporting detail is not clear, disputable or incorrect.
And thereby lies the challenge for ITA when publishing WG reports. The supporting detail must be beyond reproach and clear for those less familiar with the application of PSCL. Experts and day-to-day practitioners of PSCLs will usually seek guidance from specific references, such as those recommended in the report (e.g. Alun Thomas’s excellent second edition of his book `Sprayed Concrete Lined Tunnels`)
My colleagues and l found the WG12 report can be challenged in a number of areas and we trust that future updates and edits will address those parts we have commented on. However, our general reaction is summed up in the following quote by one of our colleagues: “In general, the document is well written and it has a balanced blend of the critical issues associated to [sic] permanent shotcrete linings. [We] find [it] positive that it indicates pragmatism in acknowledging that in many instances, like for instance the construction of the invert, the best solution might be simply casting the concrete. This aspect is important to avoid this document feeling like a selling pitch for shotcrete.”
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By GlobalDataI have used unattributed quotations from the many informed responses that l received in order to illustrate the range of observations that we have shared.
Main Categories of Observations
I chose six categories to reflect the diversity of the responses and selected those quotes/comments (unattributed) that capture the common themes and observations. Each correspondent commented on most of the chapters in the WG report and there was particular emphasis on Chapters 5 and 6. Some of my colleagues may feel that l have not included all of their individual responses. The proposal is to share the sum of all the responses with ITA WG 12.
Differentiating the principles of PSCL for use in hard rock and soft ground
It was generally observed that in Sections 5 and 6 the report needed to be clear when differentiating between different design principles and approaches.
“In my opinion it (the report) also gets rather caught between principles of soft ground SCL tunnelling and hard rock tunnelling (two very different approaches).”
“Definition of hard rock – too simple as you need to take account of the joints, discontinuities etc to characterise the rock, just > 60MPa is not sufficient; and why have these when there are perfectly good definitions of rock already?”
Cl 5.2: “Also need to consider potential rock wedges formed by joint orientation in the rock.”
Cl 6.2: “The heading is misleading, following items do not correctly differentiate between soft ground and rock tunnels, whereas rock tunnels are restricted to ‘hard’ rock, which refers to tunnels behaving stable or friable”.
Interpretation of standard testing methods for use in design
This is a common area of debate when selecting design parameters for PSCL.
“My main concern is the confusion that exists between strategies to characterise materials by testing and then how to use this information for design”
“…in particular there are oversimplifications regarding…beam testing… both three-point tests (with a reference to a moulded beam test sample rather than a sprayed sample) and four-point loading tests that are not aligned with the design approaches noted, further complicated by the text in cl 14.2 referring to EN 14487-1, which refers to the four-point test rather than the three-point test referenced in Table 7…(in my experience I can adapt the approach and standards to suit and to make something work, but recent experience has demonstrated inflexibility by some of our more theoretical colleagues).”
“It should also be pointed out that strength tests on panels will not give the same results as the lining. They are often not sprayed using the same settings, and will not experience the same temperature history, which is crucial to strength development. I have been in tunnels where panels are placed near to a compressor (nice and warm) and where panels are near the ventilation (cool) and also where different parts of the lining are experiencing very different temperatures, sometimes as low as 10-12°C due to water ingress or as high as 35°C (during the first two hours). For every 10°C, the rate of strength gain doubles, so temperature is the most important factor.”
The structural behaviour of composite linings including short/long-term considerations and the influence of waterproof membranes
Not surprisingly, Sections 5 and 6 were by far the most commonly observed sections of the report and there were challenges regarding the strength and behavioural contributions of composite linings, particularly when waterproof membranes are applied.
“I feel that the term ‘composite lining’ is mentioned throughout the document, but as it is actually not properly explained what the real meaning of the term is, it feels as if it means different things in different references. It would have helped to have an explanation, and also to make clear, that in most of the instances for projects, the designs have not yet adopted the principle of composite action and clients seem to be still reluctant due to the difficulties of demonstrating that this action can clearly be demonstrated in the long term.”
“5.2: Sprayed concrete vs cast concrete Support
The heading is misleading because in the text PSCL is described also as immediate support, which does not apply to cast concrete. Also, it is not correct to define loads onto the PSCL, because it is in interaction with the surrounding rock or soil and during excavation, stresses are redistributed, resulting in displacements which cause stresses in the PSCL.
Therefore, self-weight loads on PSCL do not exist. Note again, that not all PSCLs contain accelerators. In fact, some of the high-quality permanent linings do not use accelerators, but special fast-setting cements and/or Microsilica.”
“5.2: Paragraph column 2, paragraph 2: “Were the sprayed lining…”, need to also add that even if not part of the permanent lining, the degraded SCL properties need to be taken into account as, per the next paragraph, the degraded lining will still have some structural capacity over the design life of the structure, but any degradation needs to be taken into account.”
“5.2: Column 3, need to add the design needs to take account of the short term and the build-up of loads over the long term.”
“Section 5.2 seems to be confused about whether it is discussing a permanent primary lining or a sprayed secondary lining. It talks about early-age loading in the middle paragraphs, but in the last paragraph comes to the conclusion that because it is only loaded once it has hardened, standard design approaches can be used.”
“Section 5.2: In the latter part of this section, the explanation for the structural behaviours re: early loading and strength build up is very confusing as there is a mix of design approaches.”
“Section 5.2: Differences between PSCL v CI concrete do not acknowledge comparative influences re: hydration temperatures or during heat distress generated during fires in the tunnel.”
“Also geometry (of the excavated cross-section) needs to take account of in-situ stresses. You can optimise the shape to take account of horizontal versus vertical stresses.”
“Section 6.2.4 final paragraph:…there should be some recognition that an in-situ lining may be the best solution – lowest cost and fastest, as the material costs of in-situ are less and if you have a standard shutter shape or one designed to accommodate changes then this can be cheaper and more efficient.”
“Section 6.2.4 also says that if the secondary lining is sprayed, then composite action between a primary and secondary lining can be assumed, and the secondary lining can be thinner than a cast-in-place (CIP) secondary lining. I don’t see why it matters if the secondary lining is CIP or sprayed, what matters is that there is either no membrane, or a bonded membrane between the two linings, rather than a sheet that is not bonded.”
“6.2.6: what about single-pass linings with no membrane in appropriate ground conditions?”
Reliance on Nordic and central European References and case studies
This was self-evident when judging the case histories. I invited SCL practitioners from North America to join the dialogue. However, l received no responses up till the time this article went to press. It occurs to me that this might have been a similar experience with the WG?
“It does quite a good job of reconciling different practices in different parts of Europe, particularly single pass ‘NMT’ in hard rock in Nordic countries vs soft-ground ‘SCL’ in the UK vs alpine hard rock/NATM. However, (within the body of the report) there seems to be little about practice in North America or elsewhere, apart from in the project case studies at the end.”
“…there does not seem to be that many references to experiences in North America. It would have been useful therefore that they had sought some contribution from our colleagues in North America. For instance, it is my understanding that for the construction of the cavern next to Grand Central Station in the LIRR East Side Access, they used concrete sprayed on a sheet waterproofing membrane with quite successful results. (We are sceptical this side of the Atlantic but their experiences should also be recounted).”
Dust
The problem of dust is becoming increasingly an important health and safety topic as was demonstrated at a recent online BTS webinar lecture given by Australian colleagues in 2020.
“The section on dust is a little isolated and at-odds in comparison to coverage of other, just as relevant, SCL risks.”
“9.1.4 mentions silica dust is now a carcinogen in the UK.”
“I am wondering also what the compatibility of the statements in relation to dust in Appendix 2 is with respect to the latest recommendations in the (UK) Health and Safety Standards recently published…(defining) the requirements for dust associated with sprayed concrete…”
“Section 11.3: No mention of airflow helmets (which filter the air the operative is breathing during spraying, as a last line of defence in case the ventilation and mix design have not reduced dust levels to a safe value), or of real-time dust monitoring systems. Also, it should be pointed out that regulating layers produce a lot more dust than standard sprayed concrete, and these are used quite often for PSCLs prior to spraying a waterproof membrane or as a finishing layer.”
Other
I could have added a lot more information under this particular heading, but publishing constraints have meant that l have had to be selective.
“It is difficult to differentiate which aspects (of the report) are mandatory to be regarded and which are recommendations.”
“It shall (also) be borne in mind, that the application of PSCL depends on issues such as available labour ability and access to high-cost equipment, as well as materials, which obviously limits its application.”
“Objective: [on] page three, the report claims that lower capital and operational costs and carbon footprints can result from the adoption of PSCL, compared to in-situ linings. This has not been demonstrated in either the report or in the case studies. It would be helpful to provide evidence as owners would be interested in this.”
“I think the safety section was a little light and there is no mention of pressurised systems and plant/human interaction for example, although reference to BS 6164 should cover most issues. I can understand concentrating on SCL falls.”
“…A fall [of SCL] in an exclusion zone is not a near miss. It should be recorded though as [it] could indicate a problem.”
“12.1.3: I feel that all operators should go through a VR simulation and pass to a certain level and be recertified biannually?”
Concluding Remarks
I received over 50 pages of collective insight and comments from my colleagues and l had the task of ensuring that this review reflected the diversity of their comments. I propose to feedback the entirety of the responses to the ITA WG 12 for its consideration and for a possible update to be arranged as soon as is practically possible .
My concluding view is that given the number of clarifications, strength of comments and suggestions received from my correspondents, the WG 12 report would require an update for the reasons given in my introduction.
The global demand for underground and tunnelling infrastructure is on the increase and our industry is attracting new and diverse talent and the interest of adjacent industries. PSCLs are a major feature of underground works. Any WG report that aims to inform best practice in the name of the ITA must ensure that emerging users can be confident of receiving the benefit of current, clear and correct guidance and recommendations. We believe that our review should be seen as a contribution to that aim.
The Working Group is to be congratulated on its efforts to publish the recent guidelines, particularly given the current global constraints. WG12 is acknowledged within ITA to be one of the proactive Working Groups. Copies of the report are available from the ITA website: https://about.ita-aites.org/ wg-committees/working-groups
