GROUND SUPPORT, LINING | INSIGHT


and derailment, and that paper says case studies and the use of non-linear material behaviour and staged loading conditions provide more realistic assessments of lining performance. The paper adds that while they do not govern


lining design, such loads “can still present significant challenges” and it proposes an approach that can avoid “overly conservative solutions” such as increasing segment thickness or replacing steel fibre reinforced concrete (SFRC) with full RC segments. The results confirm that a “well-considered SFRC design can still satisfy” requirements – when supported by sound engineering judgement and validated analytical approaches.


Sprayed concrete Operators trained in sprayed concrete application “are the cornerstone” of getting a lining of that type bult well. As Chittenden says in a paper: “Their skill determines not only the structural integrity of the lining but also the pace of excavation, the volume of material waste, and the level of site safety.” Such wastage can be more than 50% and both training and continuous monitoring have roles to play in reducing such inefficiency. “The competence of the operator ultimately


determines performance.” Wastage and inefficiency also inform


sustainability performance. In a paper, de Rivaz discusses reductions in CO2 equivalent (CO2e), having the Sustainability Index and Mechanical Index, and how designers and the construction supply chain need to work together – incentivised by the project client – to optimise design, have less steel and Portland Cement used, and no microplastics. Such design optimisation can enable SFRC to work for permanent lining. Looking at layered SCL (first and secondary), in


their paper Falaknaz and Thomas focus on the performance of sprayed waterproofing membrane at the boundary. Such a focus on how the membrane behaves and evolves within the lining system “is crucial to ensuring the structural integrity and long- term performance of tunnels constructed in rock.” The paper considers parameters of the interface


and water pressure on load distribution, and the how construction sequence affects load sharing in the layers. Chemistry in SCL also features in the paper by


Xu et al, which considers early strength gain using low clinker cements to reduce carbon footprint and improve sustainability performance. The focus comes from negative effects on early strength gain from the cement replacement, which otherwise gives benefit on carbon, sulphate resistance and durability. Given the intended use is SCL, slow early strength gain is a particular problem. Research discussed in the paper shows that LCS-


series admixtures in mixes, such as with CEM II/A 52.5 N-SR cement, can boost performance of low


NUMERICAL MODELLING: NEW GUIDANCE


While numerical modelling is common on tunnel projects and other engineering disciplines there are relatively more unknowns or uncertainties presented by limited information on underground settings, which has led the French tunnelling association (AFTES) to develop guidelines and recommendations on using numerical simulation and this was the subject of a paper by Pavel and Bouzeran. The aim of the guidance from AFTES Working Group 30.2 (WG30.2) is to


establish a common technical framework that draws upon best practices, improves design quality and enhances safety – “and ensures consistency across the tunnelling and underground construction industry.” In undertaking the work, a number of factors were looked at including: the fundamental questions faced by engineers on each project (such as how appropriate is modelling, what level of detail to go to, and which numerical modelling approach is most suitable); and critical steps in performing reliable numerical simulations. The paper outlines the main factors influencing numerical modelling – type


(continuous or discontinuous, geometry, boundary conditions, initial stress state and “the need for a calculation sequence consistent with the construction process.” There are also soil and rock properties (including groundwater and its management), tunnel supports and linings, and the soil-structure interface. Looking at how models are utilised throughout project development, the


paper says a gradual study approach is “essential” to choose location and design of structures and to control their costs and completion times. That said, it notes that numerical modelling varies in complexity depending on the stage of studies and issues involved. The guidelines and recommendations consider objectives by stage, following the classification of tasks in French standards for geotechnical tasks. The paper presents insights and outcomes from development of the guidelines and recommendations. It does, though, highlight that key areas of emerging technologies for data, computation and digitalisation – such as BIM, virtual reality, AI and machine learning – are not addressed by the guidelines and recommendations. It is stressed that “it will be essential” for the guidelines and recommendations to be dynamic and updated regularly.


clinker SCL. The key is the LCS-series admixtures (patented) influencing the timing and rate of ettringite formation to happen in steps. The papers says the admixtures work for both normal wet sprayed concrete and dry-wet mix systems. Research on replacement of traditional cement


in sprayed concrete was also the focus of a paper by Jameel et al, investigating how use of such supplementary cementitious materials present problems for mix rheology, sprayability, setting time and some variation on mechanical properties – all of which can impact safety, tunnelling progress and then durability of the lining. The research looked at familiar alternatives (Fly


Ash, Silica Fumes and Ground Granulated Blast Furnace Slag (GGBS)), which increasingly have supply issues, and newer types (Volcanic Pozzolan and Calcined Clay). The mix design studies considered three factors: availability; mechanical properties; and replacement percentage. While the research noted many viable options from the alternatives there were noted advantages with Volcanic Pozzolan.


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