TECHNICAL | BTS, LINING – SLIPFORMING
Figure 9. Marsh cone rheology testing to assess pumpability of the cementitious slurry component of the STLM.
On logistics, while pumping or using bullet cars for
getting fresh STLM to the face on short tunnels would be feasible; for longer tunnels it is proposed that aggregates, sand and fibres are taken to the TBM on conveyors, and cementitious components, along with water and the necessary admixtures, are pumped as a slurry. Laboratory work on rheology suggests this is possible (Figure 9). Components would be mixed at the face before delivery to the injectors.
NEXT STEPS Colin said the Slipform Tunnelling system represents a paradigm shift in tunnelling technology. CECL registered the trademark for the system in 2020 and was granted the patent in 2024. The company and consortium partners are looking for client in the US or UK with a project (possibly to use a re-purposed 3m-diameter Robbins machine) to deliver a first-of-a-kind slipformed tunnel and bring the technology to market.
Q&A Following their presentation, Colin Eddie and Henry Pairaudeau took questions from the audience at the BTS meeting.
The Q&A has been edited for space.
Q: How do you turn the machine, given eccentric thrust? What of slipform curvature? How does the control system work, with fuzzy logic, and what of asymmetric hoop stresses? A - Colin M Eddie (CME): The system is designed to negotiate tighter radii curves than in conventional bored tunnelling, and this is helped by having a short shield and an articulated short shutter. Thrust eccentricity is a significant issue for segmentally lined tunnels but in Slipform Tunnelling the injector control logic will keep STLM placement going until the target pressure is reached. On a curve, more material is required on the outside of the bend and the control logic ensures this is achieved.
Q: What of ground heave risk when placing the lining material under high pressure? A - CME, Henry Pairaudeau (HP): Whilst the pressure inside the injector is high (around 30MPa) based on testing, we have found that the lateral pressure exerted on the ground is somewhat reduced, and numerical modelling shows it to be localised at the rear of the bulkhead. In someways, this can be likened to compensation grouting and could be beneficial in some instances, reducing out-of-balance loads in the long-term and achieving mechanical prestress of the lining.
Q: On logistics, mention was made of mixing the STLM in the TBM backup. What sort of volumes, and does this constrain the minimum tunnel diameter with method? A - CME: Logistics discussions are under way with consortium partners, considering pumping the STLM for short sections but transporting on longer tunnels to mix near the face. Specifics on volumes, rates and equipment constraints would be decided project by project.
Q: Is the system suitable for all soil/rock types, including
difficult ground conditions such as squeezing ground, high water pressure, permeable gravels and open joints? A - CME: Thrust needs due to varied ground conditions would be balanced through control of the injector pressure and the number operating, which provides flexibility in placing the lining.
Q: How does the system handle torque generated by the cutterhead? A - CME: At the rear of the shield, a bulkhead with a castellated profile ensures that the machine is ‘keyed’ into the newly formed lining. This provides roll resistance.
Q: How does the system work with tunnel openings and shaft connections, etc? A - CME: Such connections and openings would require bespoke engineered design solutions. The system has no provision to introduce rebar and could not accommodate the high tensile forces generated at openings without supplementary temporary and permanent works arrangements.
Q: As Slipform Tunnelling creates a single-pass, permanent lining, it would need to meet permanent works standards? How would you produce the test specimens for production and quality assurance? A - HP: The ‘benchtop’ test equipment allows for more rapid characterisation of STLM and could be used for pre-production and production quality assurance. For longer-term properties (e.g. flexural testing), there is the possibility of forming beam specimens but this may be challenging. In any case, our testing indicates that the properties of conventionally cast specimens (without injector pressure) represents a lower bound to results, which could be used conservatively for design values.
14 | October 2026
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