INSIGHT | GROUND SUPPORT, LINING
SPOTLIGHT ON SUPPORT
Ground Support and Lining Design were together as one of the many technical themes
discussed at the recent World Tunnel Congress in Canada. T&TI outlines a selection of the key interesting papers, including outlines of two new guidance documents.
At the latest ITA-AITES World Tunnel Congress (WTC 2026), which was held in Montreal, Canada, the information-packed event had so many technical papers and presentations that Ground Support and Lining Design was but a sub-theme within a section – Engineering Design and Underground Structures – but even so it has 42 papers in the excellent published proceedings. Those papers, in the sub-theme alone, cover 343 pages. Below, T&TI shares some highlights from groupings of
topics within the conference papers on Ground Support and Lining Design. A key paper discussed forthcoming guidance on tunnel face stability to be published by ITA-AITES and another outlined the guidance on use of numerical modelling on tunnel projects by the French tunnelling association (AFTES) – see box panels.
TUNNEL FACE STABILITY: NEW GUIDANCE
ITA AITES’ Working Group 2 (WG2) is producing international recommendations on assessing tunnel face stability, as briefed in a paper by Berthoz. The impetus for WG2’s work is the lack of “synthesis” or “sufficient guidance in practical application” on problems that can result from tunnel face instability, such as serious injury or worse, infrastructure damage, recovery cost, project delays, etc. Tunnel face stability is not subject to an international standard; it is not included in
Eurocode 7, for example, and there have not been design recommendations from ITA- AITES, the paper notes. The nearest to addressing the need are in Germany, from the tunnelling association (DAUB), in 2016 but is focused on TBMs, and in Hong Kong, again looking at bored tunnelling. The new recommendations from WG2 are to provide guidance to identify geotechnical hazards for tunnel face stability in different situations; a review of calculation methods to avoid collapse and blow-out; and how to practically use the methods. However, what is not being addressed are choice of calculation parameters, “in
particular the shear strength”; assessment of induced displacements; and impact on nearby works. On practical construction guidelines, the paper notes that conventional tunnelling
gives regular and systematic opportunities to perform face mapping but data from drilling (exploratory, drainage, placing bolts or dowels) adds to the information available. The value of reverse head extensometers – which “allow measurement of extrusion of the face and, by extension, the anticipation of a face collapse” – was noted but so also was its uncommon use due to cost as well as slowing up tunnel progress. The paper adds that closed mode pressurised TBM boring does not presently give
accurate information about the nature of the ground or face stability, but back-analysis of surface displacements can help in evaluating these data against prior assumptions. Effective training and the presence on site of sufficiently experienced teams were underlined as vitally important.
SCOPE The papers under Ground Support and Lining Design covered a number of areas – face stability, loading, pre- excavation and excavation support, sprayed concrete lining (SCL), mix design, analysis, crown support, design approaches, primary and secondary/permanent lining, artificial ground freezing (AGF), settlement, rock mass classifications, fire risk, and applications in the context of a variety of tunnel projects. Here, within limited space, a selection of a few interesting papers in some of these topics are discussed.
Loading on linings How stress distribution in precast concrete segmental lining is affected by thrust eccentricity from a TBM jacking forward was the focus of a paper by Abedi et al. They note that the governing load case for lining design is often the thrust forces from the jacking pads against the lining, which give rise to compressive and tensile stresses in different zones, and also tensile spalling stresses – which does not have an equivalent analytical method of evaluation as for other, bursting, tensile stresses. The paper discusses use of non-linear finite
element analysis (NFEA) for approaching the spalling stress evaluation problem, where the studies looked at combinations of: three precast segment types (key, reverse key, counter key); two mixes; two sources of radial thrust pad eccentricity (intrinsic; construction); and two stress distributions (rectangular; trapezoidal/triangular). Across all options the critical criterion was crack width, for serviceability performance against water infiltration and this durability. While the governing condition was mostly
increased thrust pad eccentricity towards the extrados, the governing segment type varied depending on assumed stress distribution – all of which, the paper says, highlights the complexity of design and analysis of concrete segments under TBM thrust forces. In a paper by Chau et al, the design needs
for accidental load cases on segmental lining is discussed from the perspective of the use of a rational design philosophy and analytical tools. The load scenarios include fire, blast, flood, collision
20 | October 2026
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