BTS, LINING – SLIPFORMING | TECHNICAL


strength/stiffness and not cementitious chemical hardening. Testing was undertaken on unconfined and confined


(triaxial) samples using bespoke equipment (Figure 6) and revealed the STLM has a significant very early-age strength and stiffness. Testing at different confining pressures plus understanding of strength and stiffness stress dependencies helped to derive an advanced constitutive material model. This has proved useful for 2D and 3D numerical studies, process validation and understanding material behaviour in a variety of ground conditions. In the long term, such slipformed lining is expected to


be highly durable and capable of providing a high-quality structure in demanding environments, Henry said. Qualitatively, with a 28-day strength meeting the


requirement of at least C40/50 and with X-ray CT scanning demonstrating that long-term STLM samples have a high density (2,500 kg/m3 and higher), high aggregate ratio and low porosity, the lining is expected to be highly durable (Figure 7). Quantitative testing for various durability metrics has also proven this to be the case. On shrinkage, long-term comparative testing with other mixes has demonstrated the STLM to have very low shrinkage, with shrinkage strains below the tensile strain capacity of the material at all ages; this would yield an uncracked and hence watertight lining. For hydraulic tunnels, abrasion is a concern and


comparative testing, using a modified version of an ASTM C1138 underwater test, shows that the STLM performs comparably to other high-performance tunnel lining concrete mixes such as fibre reinforced structural concretes. Ongoing testing for sulphate attack (using a modified ASTM C1012 test method) has found that STLM samples exhibit minimal expansion and mass change when subjected to a highly concentrated sulphate solution, owing to the density of the material and beneficial chemistry of the LC3 binder. From water penetration testing, the calculated permeability is very low, Henry said. A few questions have come on what happens if there is a stoppage in the injection process and if a cold joint would result, Henry said. Testing for this was performed with a half-height sample, topped up by mechanically pressing more material after seven days. The findings did not show any visible joint or formation of a preferential, water path weakness, he said. Consequently, stoppages are not expected to cause issues using the system for tunnelling. Binder development was performed with cognisance of the necessary standards, such as EN 197-5 in Europe or Type IT Ternary Blended Cement (ASTM C595) in the US; and, the recent BSI Flex 350 in the UK and ASTM C1157 in the


US. Mix components are also compliant, Henry added. He handed back for Colin to discuss the next section of


the presentation.


Cost and Sustainability Having provided major clients with cost metrics on tunnel projects, Colin said they had standing in this area and used the same approach to consider cost and carbon for the new system versus conventional tunnelling methods. There are savings on both, coming from: reduced excavation diameter; no ring annulus to grout; no precast production/ logistics; an eco-efficient mix design; and a zero-waste factor. Fewer processes and increased production rates, along with fewer personnel required, would further reduce total cost and carbon. Overall, comparative savings of 50% on total cost and carbon have been estimated for the system, measured against two reference projects: a sewer tunnel (Thames Tideway, which also included secondary lining); and a rail tunnel (part of High Speed 2: twin 5km- long drives) as in Figure 8. Further sustainability benefits could result from some


(albeit marginal) reuse of excavated material, which would be a fit with circular economy principles, as would innovations such as getting recycled steel fibres from used car tyres (e.g. Dramix Loop).


Safety, Production and Logistics In comparison to conventional tunnelling, the new system eliminates several processes with benefits for safety underground: no precast rings to build; no bolts; no grouting; no secondary linings; no waterproof membranes; no sprayed concrete; and no rock bolts. The system lends itself to automation and remote operation. While there would be fewer underground workers overall, some would be needed in the tunnel to extend the services. A key driver in developing the system was to increase


the advance rate compared to conventional methods. The control system is driven by two primary parameters: face pressure; and advance rate. It can use feedback control to manage injectors individually to modify their speed, noting that material placement is much slower than in conventional concrete pumping. Verification of the quality of the lining that is formed,


before it appears out the end of the short shutter, will be vital, and the University of Warwick is looking at options to systematically measure its integrity, Colin said. It should also be noted that, with the system being somewhat analogous to pressure grouting, he added, voids outside of the shutter must be filled with STLM before the machine can move on.


Figure 6. Bespoke testing apparatus for undertaking triaxial testing of STLM specimens rapidly after forming.


Figure 7.: X-Ray CT scans - STLM sample.


Figure 8. Construction cost and embodied carbon - (left) cost comparison and (right) embodied carbon comparison.


October 2026 | 13


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