ITA GUIDES | MECHANISED TUNNELLING
SEGMENTAL LINING: INVESTIGATING AND ASSESSING DURING OPERATIONAL LIFE
The tunnelling sector is dominated by attention to the design and construction phases of underground infrastructure, and on the latter by no small measure - not least as that is where the bulk of the budget is spent by necessity, on building the civil engineering assets as they are realised in site-specific challenges. Often there is the argued case for more attention to be usefully spent, along with budget, on the upfront planning and decision- making. Even so, at the other end of the spectrum of an asset life there is the least attention, especially from a construction- dominated community. Yet it is by far the longer of all the phases and how it performs, and what it costs - economically and financially - depends so much on what has been decided and done before, at big and small scales. It is not unknown but it is certainly uncommon to find a report looking at assets over the operational lives of tunnels. ITA has
produced such guidance, which will provide information, data and insight into this latter stage of an underground asset’s life and, in so doing, not only help with decisions required in operations and maintenance works but also feed-back , or at least raise awareness of problems - and opportunities - to those making the early stage planning, design and indeed construction stage decisions. The ITA report No. 46 is ‘Damage Categories and Inspection Techniques for Single-Shell Segmental Lined Tunnels during their
Operational Phase’. Over its 35 pages it is dominated by textual narrative to begin with, and for some way, then moving for the remainder - and majority - of the guide to a well-balanced combination of illustrations and tables. WG 6’s Activity Group 2 has been around for almost five years now, having been established in 2021 to focus on preventative maintenance and refurbishment works for tunnels. This operational stage study, therefore, fits under WG 6’s remit to consider the full scope of maintenance and repair of underground assets. It was the prior work of another WG - WG 2 - that led to the focus that would come on operational stage condition and
performance of tunnel lining. Its study, then, was focused on damage in bored tunnelling works, seeking to minimise such problems during the construction phase - such as due to production, transport of construction activities underground. A key recommendation was that all such damages, and associated repairs, should be in a compiled database - an ‘As-Built’
register, if you like - for the handover of the underground asset. The anticipated benefits included enabling follow up of the lining database during the subsequent operational life of the bored tunnel. The latest report, from WG 6 and focused on operational life performance, notes that the behaviour of the segmental lining of bored tunnels is subject to change with time. “Therefore,” it says, “the main focus of this damage catalogue” is consequently the operational phase, and also to be able to distinguish between the ‘stable’ damage at the start, caused at the construction phase, and what is viewed as ‘developing’ damage, that is either “initiated or evolving during the service life” of such a tunnel. For with the latter, damage can be progressive and may “ultimately lead to failure.” How? The damage mechanism may originate from construction defects or “is often not foreseen in the design,” says the guide,
“or is due to changing circumstances in the surrounding of the tunnel.” As such, inspection and monitoring are clearly important in mitigating further damage - all of which should “be unnecessary to say,” but emphasis upon such a message may not be misplaced. The discussion on the topic is covered in only three chapters: Bored Tunnelling and Induced Damages; Techniques of Inspection and Monitoring; and, Damage Categories. The last chapter is the biggest. The report’s opening chapter discusses bored tunnelling methods and the ways that damage to lining (structural, water, chemical, accidents, usage) - and also emphasises “the importance of distinguishing” in which phase it happens (design, manufacture, transport, construction, service, improvement/renewal). Under the discussion of design as potentially being the origin of problems, the guide comments that errors made in this phase “can have severe consequences,” ranging from limited to partial or total collapse pf a section of tunnel. Chapter 2 goes into many inspection and monitoring techniques. It notes that technology changes quickly, leading to field techniques changing on resolution, data acquisition, and data storage and processing. On data acquisition, technologies discussed, albeit briefly, include laser scanning, photogrammetry, thermal imagery, ground penetrating radar, and variations of these along with a host of other survey sensors for more a multi-dimensional mapping approach. The final chapter, on Damage Categories, looks at the different types of damage and their potential consequences for tunnel - not
only for the structure but its users. It addresses the categorisation in six sections: Segment Misalignment; Ovalisation; Concrete Cracks; Leakage; Spalling and Concrete Chipping; and, Complete Cracking across the Full Cross-section. The guide off rounds off with References - including both a bibliography and a listing of the tunnel inspection regulations and guidelines of various nations.
With so many ways to pursue such outcomes, the
guide invests heavily in discussion of possibilities in its last section, Chapter 6. Even so, it cautions to take what is offered not itself as prescriptive but as an impetus to investigating such questions and developing appropriate strategies, project by project, to balance technology and procurement. All that said, Chapter 6 is excellent in presentation of highly structured and thus accessible
information, drawing as it does from multiple international tunnel projects.
OPEN MODE: THRUST FORCE IN WEAK ROCK As the shortest of the three reports under review, having a focus on thrust force for weak rock conditions is a narrow intent, and is even more narrow in considering only open mode shield tunnel driving in such ground. An important niche focus
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