INTERNATIONAL | TBM TUNNELS - ROCK CLASSES FOR GROUND SUPPORT
ROCK CLASSES FOR GROUND SUPPORT OF TBM TUNNELS
A spotlight on the ITAtech report that provides guidance on rock classification for ground support on TBM-driven tunnels
Formally, the report is ITA’s Report No30, published
mid-2023, and comes from ITAtech Activity Group Excavation. Its official title is ‘ITAtech Guidelines for the use of Rock Classification Systems for Ground Support on TBM Tunnels’.
TBM TUNNELING CHALLENGE While rock classification systems have been developed over many decades, helping tunnelers to move ground support choices beyond isolated case-by-case decision- making about what to do, in what way and by how much, a challenge remained, for TBM tunneling. The report says that: ‘When these systems are applied
to rock support in mechanized tunneling it becomes a problem that the empirical data is obtained almost exclusively from the drill and blast tunneling method.” Challenges arising are the rock support is not then
optimized for TBM tunneling, and doesn’t account for the rock not being blast damaged but “relatively smooth”. To address the challenges, the report puts forward
a modified rock support selection methodology. The proposed alternative system, adapted to bored tunnels, draws upon published literature of both of the following established rock classification systems - Q (Barton, Lien, & Lunde, 1974) and RMR (Bieniawski, 1989), respectively. The report notes that, additionally, the adaptions are “aligned with the authors’ experience.” The report says that the modified system “is suitable
INTRODUCTION Being both well written and structured, yet perhaps shorter than anticipated - at only 16-pages in length, cover to cover - the ITA’s guidance on rock classification for ground support on open-type TBM bored tunnels comes sufficiently well illustrated, with graphics, tables and references. Given such brevity it usefully skips on a Contents page and gets straight down to practical briefing in five chapters and wraps with a short conclusion. Although A4, and likely to be a digital reference copy
rather than paper, given it would be quite thin, the report gives the sense of easily being a well-thumbed reference.
42 | Spring 2025
for use in design, cost estimating, and other rock support considerations” in relation to open-type TBM tunneling, focused on the areas immediately behind the cutterhead (L1 area) and the remainder of the machine (L2). But the proposed rock support schemes are not a guide for the needs of final lining, it adds - for such is likely to be determined prior to excavation. The TBM back-up area and farther back still are
classed as L3 areas, and as such are not covered by the proposed modified rock support scheme, which is focused on the needs of the L1 and L2 areas, respectively - L1 requiring more than L2. The report notes improvements by TBM manufacturers
and industry for installing temporary rock support. That being so, it highlights that temporary ground support
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