TECHNICAL | BTS, LINING – SLIPFORMING
repeating the cycle as managed by the control system. “We all know tailskin grouting
systems are the bane of contractors’ lives” – as they keep blocking and causing delays, Colin said. One TBM manufacturer informally said wet concrete and TBMs don’t go together – in which he is in agreement and, as such, ensured there would be an integral cleaning system for the injectors. “We’ve got to keep the system clean.” Each of the injectors is separately
controlled, under three laws, he said: 1) push the material until a maximum pressure is reached; 2)
Figure 3. (Top) Detail of the injector showing the STLM delivery arrangement (grey) and the flushing and cleaning circuit (blue).
when fully extended then return; and 3) if going into cleaning mode then push farther by an extra 100mm. The control system manages the injectors simultaneously. “The rate of advance of the machine is governed by just one parameter – the speed of the forward-stroke of the injectors,” he said. The system has been designed for shield advance of more than 50mm/min but for the Synchronous system it depends how quickly the excavation proceeds. Colin recalled the sentiment of Siegmund Babendererde – former Chief Engineer of Hochtief and an early pioneer of ECL system application in Germany, who is most associated with extruded linings – who said: “Revival of the concept would have to start from scratch and be based upon developments in modern concrete technology.” In developing the new system, Colin said his own focus
has been on the machinery while his colleague, Henry, looked at materials in collaboration with the University of Warwick. He handed over to Henry for the next section of the presentation.
Figure 5. STLM specimens - (right) flow table testing immediately following mixing and (below) following compaction in the forming apparatus (approximately one hour after mixing).
LINING MATERIAL For the lining material design, the effort went back to first principles, and asks what does the material fundamentally need to do at each stage of the process? Henry said it needs to be compatible with the mechanised system; and also use components that are sustainable, low cost and easily available to ensure commercial viability. A five-year research programme at Warwick developed
a novel, one-pass lining material – the Slipform Tunnel Lining Material (STLM) – which is a composite cementitious material. It is formulated with components familiar in modern, high-performance concretes – aggregates, cementitious binder, water, admixtures, steel fibres and polypropylene (PP) fibres. It is workable when fresh and
Figure 4. Development testing arrangements at the University of Warwick’s heavy structures laboratory. The setup simulates injection of STLM at high pressures.
formulated to – when under high mechanical pressure from the injector – quickly gain the required form and density, and ‘very early-age’ strength and stiffness. Prior to injection, the STLM needs to be flowable for
distribution and achieve high filling efficiency. It has low binder content and a low water/binder ratio; admixtures enable the mix to achieve suitable rheology. During injection, the material is subjected to high
pressures. In the laboratory, bespoke apparatus was developed to simulate injection and test the material. This was performed at full scale using a 1000kN servo- hydraulic actuator and a modular reaction frame, and fabricated split-cylinder equipment (simulating the injector) to form 150mm-diameter, 300mm-high cylindrical samples of the pressed material (Figure 4). More recently, a smaller ‘benchtop forming’ test rig has
been developed to test smaller samples, as it can fit in a standard concrete compression test equipment, Henry said. It can be handled by one person and so enables more samples to be tested in a period. Ahead, it could be used to support quality assurance, he added. Several features enable the material to rapidly transform from its flowable fresh state to a competent structural material under injector pressure (Figure 5): ● The aggregate grading is designed to enable crushing during injection, generating a dense aggregate matrix with a high degree of particle interlocking;
● The cementitious binder is an LC3 type (a ternary blend of Portland Cement, fine limestone and calcined clay). The fineness of the additions gives a dense lining with low porosity. The pressed material is highly durable without relying on Fly Ash or Ground Granulated Blast- Furnace Slag (GGBS) as cement replacement materials, due to their availability issues but also questionable sustainability credentials.
● The head of the injector allows drainage and some excess mix water is pressed out under high pressure, which adds further to lining density and durability.
Laboratory testing sought to characterise the STLM
strength and stiffness, immediately after forming. This ‘very early-age’ testing of STLM was undertaken within two hours of mixing, Henry said. It was well before the initial set time when hydration begins, based on isothermal calorimetry tests. Hence, the tests measure a mechanically-derived
12 | October 2026
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45
