BTS HARDING PRIZE COMPETITION 2025 | MECHANISED TUNNELLING


Left:


Attention to ring assembly design


The concept for TBM transition was for it to break


through, mine through a temporary invert block in ‘free air’, and then enter the ground on the opposite side of the shaft. With shaft internal diameter of only 9m, the confined


space constraints made TBM maintenance especially challenging, requiring close coordination between the TBM crew and shaft team. The effective length of the shaft was significantly


shorter than the 300m-long TBM, meaning it wasn’t straightforward to simply stop the machine in the centre of the shaft, complete maintenance, and then continue. Instead, the machine advanced in stages, with multiple tasks happening between each ring build. After mining, rings were built in the shaft, followed by


miners strapping them from the inside while temporary works were installed to support the rings. Once the TBM broke through, cutterhead tools were replaced before mining resumed. This continuous cycle meant that, at any given time, critical operations were taking place both inside the shaft and within the tunnel. This paper analyses the methodologies employed by


the engineering team to conduct maintenance activities and safely transition the TBM through the Kidbrooke shaft. It covers technical considerations, challenges, and solutions, offering insights for future tunnelling projects.


SHAFT ENTRY OF THE TBM HMJV worked with OTB Engineering on the temporary works design for the shaft transition, which included the TBM parameters. At Kidbrooke, sinking the shaft presented significant


challenges due to water ingress in the Harwich Formation. To manage this, dewatering wells were installed around the shaft to relieve ground water pressures. The presence of the Harwich Formation at the base was also mitigated through a reduction in shaft depth. Even after the TBM had broken through at the Eltham site, water ingress at Kidbrooke’s shaft’s New Cross portal was observed. To address this, the wells had to be reactivated, and the portal was subsequently back grouted.


The Harwich Formation was found only in the


tunnel invert, and with the tunnel horizon positioned above the groundwater table, a ‘free air’ breakthrough was possible, eliminating the need for further ground improvement or complex reception & launch assemblies. To assess potential ground movement, OTB Engineering carried out face loss calculations and developed settlement contours. Instrumentation and monitoring were established along the tunnel route and at the shaft, with ongoing measurements to track settlement. At 80m before the breakthrough at Kidbrooke


intermediate shaft, the TBM operated at an EPB pressure of 2.3 bar in the tunnel crown. As it approached the shaft, this was gradually reduced to zero to enable the ‘free air’ breakthrough, a transition influenced by the geological conditions, particularly the stability provided by the London Clay at the crown and the localised dewatering. To maintain stability, the five rings installed before the


shaft, as well as the five rings following the shaft, were reinforced. Within the shaft, fibre-reinforced rings were used, following a specific ring pattern that incorporated two removable segments at the crown to facilitate the replacement of the screw conveyor. To ensure the seamless assembly of the rings within


the shaft, I prepared a structured guide for the miner responsible for cutting gaskets and installing dowels. I also suggested removing the gaskets on the side of the shaft ring, which made both removal and later reinstallation easier. This approach minimized damage to the segments during handling and eliminated the need for a full bridge beam across the shaft. Instead, we only had to use the beam for the length of two rings, which was both cost-effective and supported by the design. The list I prepared clearly outlined where gaskets


needed to be cut—specifically on removable segments and on the sides in contact with those to be removed—helping to prevent confusion during construction and ensuring an efficient ring installation process.


July 2025 | 11


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