TBM Transition through Kidbrooke Shaft

ABSTRACT

This report explores the transition of the tunnel boring machine (TBM) through the Kidbrooke Shaft as part of the London Power Tunnels Phase 2 Project – Package 2 Tunnels & Shafts. It highlights key engineering challenges, maintenance interventions, and design modifications implemented to ensure a seamless TBM passage. The paper also discusses lessons learned and innovative solutions that contributed to the success of these works.

INTRODUCTION

National Grid’s London Power Tunnels Phase 2 Project aims to enhance the electrical grid’s reliability by constructing 32km of underground tunnels to house high-voltage cables. The scheme is split into several packages, of which Hochtief-Murphy Joint Venture (HMJV) were contracted to deliver two: Package 2 Tunnels and Shafts; and, Package 5 Headhouses and Mechanical & Electrical (M&E).

The project included five tunnel drives undertaken from three drive sites: Kings Avenue, New Cross, and Hurst through the full range of the South London geology, with the New Cross site being unique as it serviced two drives extending west and east simultaneously. The New Cross east drive covered 11km and accommodated significant engineering challenges.

As part of the wider permanent operational requirements of the scheme, an intermediate shaft was constructed on the New Cross east drive, at Kidbrooke, approximately 7km from New Cross.

To ensure the TBM operated efficiently over this 11kmlong drive, the intermediate shaft at Kidbrooke enabled crucial maintenance activities and helped to ensure smooth tunnelling progress. Maintenance tasks at the shaft included cutterhead tool replacement, tailskin brush replacement, and screw replacement following the TBM’s breakthrough at Kidbrooke.

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.

TBM SEQUENCING INSIDE THE SHAFT

When the TBM cut through the concrete portal and the cutterhead was exposed in the shaft, the following activities took place:

• A mini digger was lifted into the Kidbrooke shaft to remove broken concrete via a boat skip and the excavation chamber of the TBM was completely emptied of broken concrete;
• The screw conveyor was inspected, revealing considerable damage to the wear plates due to mining through various ground conditions. Consequently, a decision was made to replace the screw conveyor; and,
• The cutterhead tools, particularly the over-cutters, were found to be worn and required replacement.

TEMPORARY WORKS FOR TRANSITION

A concrete plinth was built before the breakthrough at Kidbrooke.

After the cutterhead breakthrough, grouting operations proceeded as usual for the first two rings. However, during the excavation of the third ring, with the tailskin no longer within the ground, grout began to seep into the shaft, necessitating an immediate halt to the grouting process. I ensured effective communication between the miners operating in the TBM and the personnel in the shaft, which was crucial to promptly cease grouting activities.

Supporting Rings inside the Shaft

To support the rings that were no longer within the tailskin, a combination of bullflex and lytag support systems was utilised to bear the load of the TBM.

1. Upon full exposure of a ring in the shaft, a specifically designed bullflex sock was positioned beneath the ring at the inbye and outbye circle joints;
2. A grout mixer pan and pump were used to batch and inject OPC cement into the bullflex until air and water seepage confirmed full saturation and hardening;
3. The gaps between the bullflex seals were shuttered at the concrete plinth;
4. Using a piccolo pump, lytag was injected into the gap until refusal; and,
5. This process was repeated for each ring that extended beyond the tailskin within the shaft.

Knee Support and Ring Anti-Roll Brackets

In addition to invert support, the tunnel rings required knee support and anti-roll brackets. As the cutterhead rotates in one direction, the shield tends to rotate in the opposite direction. This effect is particularly pronounced for rings in free air, necessitating adequate support measures.

At Kidbrooke intermediate shaft, the following supports measures were put in place:

1. An 1800mm super slim soldier was vertically bolted to the concrete plinth on each side of the ring;
2. Two 720mm super slim soldiers were installed diagonally, bracing the vertical soldier on either side of the ring;
3. A timber block was wedged between the ring and the vertical soldier, and an anti-roll bracket was positioned beneath the timber block, securely placed against the extrados of the ring and the concrete plinth; and,
4. This procedure was systematically applied to each ring constructed within the shaft.

SCREW CONVEYOR REMOVAL

To facilitate the removal of the screw conveyor, the two crown segments had to be removed. The following sequence was followed:

• The operatives working in the tunnel were briefed at New Cross, and the lifting team was briefed at Kidbrooke. Effective radio communication between these teams was crucial;
• The lifting beam, designed by OTB Engineering to match the tunnel curvature, was lowered into the shaft, and rested on the segments of the ring that needed to be removed. Anchors were bolted into the beam to secure it to the segments, with resin injection used to reinforce the fixing;
• The radial spear bolts were removed from the crown and shoulder segments;
• Removing the first two segments proved challenging as they did not dislodge easily. A miner inside the TBM used a hydraulic jack to push the segments from within, while a crane pulled from above;
• The segments, along with the lifting beam, were extracted from the shaft and marked for reinstallation; and,
• These steps were repeated to remove the segments from the remaining five rings in the shaft.

Once the crown segments were removed, the screw removal process commenced:

• The screw was fully retracted, and all mechanical and electrical components were disconnected;
• A winching point was installed at the entry portal, and a pneumatic chain hoist was connected to the screw auger;
• The screw, composed of two sections, was split into halves to facilitate removal;
• Inbye section was secured to prevent it falling within the casing once the outbye half was removed;
• The outbye half was lifted out of the shaft using the crane at the top of the shaft at Kidbrooke; and,
• The inbye half was subsequently removed in the same manner.

SCREW REPLACEMENT

• A refurbished screw was prepared and ready for installation at Kidbrooke;
• The screw was split into two halves and installed in the same sequence as the removal process. The angle of the auger was adjusted to facilitate insertion into the casing;
• The connection point of the split screw was positioned outside the casing, and the two halves were welded together; and,
• The joined augers were slid into the casing, and the main motor was lowered into the shaft and secured to the screw.

REINSTALLING RINGS AND BRIDGE BEAM INSTALLATION

Weekly meetings were held with OTB Engineering to align with the design brief. We discussed our requirements, and they provided options to ensure we built the right solution. One option involved installing bridge beams across the entire shaft due to concerns about reinstalling segments, but as they were placed effortlessly, a more efficient approach was chosen.

Instead, we left out two rings to install ventilation from Kidbrooke to Eltham, doubling as a critical emergency exit. Given the tunnel length, this was a key safety measure. Dräger stations and a staircase were also installed to provide a safe escape route in case of an incident.

• After screw replacement, the removed segments were reinstalled. A bridge beam was installed to enable the thrust cylinders to apply load to the tunnel lining;
• The segments stored on the surface at Kidbrooke were reinstalled into the crown of the rings. The segments were strapped, and radial spear bolts were installed. This process was repeated for three additional rings;
• An opening of two rings was required to facilitate the installation of the ventilation booster fan, which would enhance ventilation from Kidbrooke to Eltham;
• Four lifting beams were lowered into the gap, and end base plates were bolted to the extrados of the last full ring. The gap between the beam and segment was filled with grout;
• A lifting beam (without segments) was lowered and placed over the centre of the bridge beam, then bolted to the axis segments of the exposed ring in the shaft; and,
• Once the lifting beams were installed, the TBM was ready to resume advancement.

TAILSKIN BRUSH REPLACEMENT

The tailskin brushes were originally scheduled for replacement in the Kidbrooke shaft; however, due to re-sequencing of works at New Cross, an enforced downtime created an opportunity to complete the replacement earlier. The team adapted their plans and carried out the brush replacement while still in the ground.

This replacement was essential due to the wear sustained by the brushes throughout the tunnel excavation. Failure of the brushes could result in water and grout ingress, as well as a loss of pressure. This approach minimised risk for the remaining 4km of tunnelling to Eltham, where non-cohesive deposits and groundwater inflows were anticipated. Two of the three rows of brushes were replaced, ensuring optimal sealing. The previous two rings were secured by longitudinal strapping between each ring and radial strapping between each segment.

To access the brushes, it was necessary to dismantle a ring after mining. The last constructed ring followed a U11 sequencing pattern, so the subsequent sequence required an even number and selecting U12 positioned the key segment at the top of the ring. This placement was optimal for its removal, as the key segment needed to be extracted first. The ring designated for dismantling was constructed without dowels to facilitate the removal of segments after assembly, while bolts were installed to secure the ring in place.

The thrust cylinders were extended to 1600 mm, following a safe stop procedure. After allowing an hour for the grout to settle, an additional 100 mm extension was applied to separate the tailskin brushes from the grouted gap seal. At this stage, the key segment was carefully removed using the erector, which was specifically designed to extract segments to aid with tailskin brush replacement and placed onto the segment feeder. The thrust cylinders were then incrementally extended farther until the second row of brushes was exposed.

Once the brushes were exposed, the remaining segments—except for the invert—were removed using the erector and placed on the segment feeder in their original order for easy reinstallation. The invert segment was then lifted onto the tailskin after pulling back the feeder. Before replacing the brushes, accumulated dry grout and grease were jet washed. The old brushes were then removed and replaced with new ones, ensuring no gaps that could allow water ingress.

To ensure thorough lubrication of the tailskin brushes, WR90 grease was applied using an injection lance, ensuring the grease penetrated all areas within the brush assembly. In some instances, manual application with a brush was necessary to guarantee complete coverage of hard-to-reach spots. The entire greasing process required three 8-hour shifts to complete, ensuring optimal lubrication and functionality of the brushes.

After the brushes were installed and greased, the ring was reassembled following the sequence below:

• The bottom segment was installed back using the erector;
• The segment feeder was pushed forward;
• The remaining segments were installed as per standard procedure;
• WR89 (tailskin grease) was injected; and,
• The erector was repositioned to its normal operational state.

Since this task involved hot works in the TBM, I ensured all safety measures were in place, including obtaining permits. I coordinated with the tunnelling gang and engineering team to manage site operations and liaised with the miners to ensure correct procedures were followed. I also made sure the erector components were covered to prevent grease contamination.

CHALLENGES ENCOUNTERED
TBM Breakthrough Alignment and Portal Transition Challenges

After the TBM breakthrough, the machine only had the plinth to mine through, meaning there was no pressure on the top thrust cylinders. As a result, the TBM began to lift, and within two rings, I realised that we were not going to align with the portal. To counteract this issue, the team implemented corrective measures by exerting maximum pressure on the top thrust cylinders while applying minimal pressure on the bottom cylinders. This strategic adjustment aimed to gradually bring the TBM down to the required level for proper alignment with the exit portal.

Throughout the mining process within the shaft, meticulous monitoring was conducted after the completion of each ring build to assess the TBM’s positioning.

As part of this effort, I was stationed at the face of the TBM, relaying instructions from the engineering manager to the TBM operator. Every 100mm to 200mm of advancement, I coordinated back and forth, ensuring real-time adjustments to the thrust and articulation cylinders. This close communication played a pivotal role in achieving continuous alignment corrections, ultimately enabling the TBM to successfully pass through the portal.

Despite the successful breakthrough, an unforeseen complication arose. The hinge plates, which were installed on the portal to facilitate a smooth transition, had sheared off at the top right corner due to the TBM entering at a higher-than-expected position. While positioned at the TBM face, I observed that these dislodged plates were adhering to the cutterhead and being dragged along with the advancing machine.

To prevent damage to the excavation system, the TBM’s advance speed was reduced to 5mm per minute while the cutterhead rotations were increased. This adjustment allowed the plates to dislodge safely, preventing them from being drawn further into the excavation system. A total of five plates were recovered from the excavation chamber. This precautionary measure was critical, as any of these plates entering the screw conveyor could have caused significant damage to the screw.

One of the key sustainability initiatives trialled at the Kidbrooke intermediate shaft site was the use of Earth Friendly Concrete (EFC). The trial aimed to assess its pumpability and placement characteristics before its planned permanent use at the Hurst shaft site. At Kidbrooke, EFC was used in the construction of the plinth over which the TBM transitioned. The insights gained from this trial directly contributed to the record-breaking EFC pour for the base slab at the Hurst shaft, marking a major milestone in the use of low-carbon materials in tunnelling.

Resolving these challenges highlighted the importance of proactive monitoring, clear communication, and adaptive strategies in TBM operations. My involvement in relaying instructions, ensuring precise TBM adjustments, and identifying hazards during breakthrough reinforced the value of hands-on site presence in overcoming tunnelling challenges.

CONCLUSION

The London Power Tunnels Phase 2 – Package 2 came with its fair share of challenges, especially during the TBM transition through the Kidbrooke intermediate shaft. Key maintenance tasks, like cutterhead tool replacement, tailskin brush replacement, and screw conveyor replacement, were critical in keeping the TBM moving forward.

Getting the TBM through the shaft highlighted the need for quick decision-making and adaptability on site. Good communication between the New Cross and Kidbrooke shaft sites was key to dealing with unexpected wear on components and keeping alignment on track during breakthrough. Using reinforced rings, temporary works, and support systems helped maintain tunnel stability and allowed the drive to continue smoothly.

After the maintenance at Kidbrooke, TBM availability for the remaining 4km drive to Eltham was the highest on the project, significantly exceeding the performance measure agreed within the TBM contract. The TBM completed this section without significant stoppages, demonstrating the effectiveness of the cutterhead and screw maintenance.

The last 2km were completed in just under two months, achieving the highest productivity rates of the entire project.

The overall tunnelling on London Power Tunnels Phase 2 was completed three weeks ahead of the original Clause 31 programme, with Package 2 finishing ahead of the contract date. The efficient TBM transition at Kidbrooke played a crucial role in mitigating risks on the longest tunnel drive of the project, ensuring uninterrupted progress toward Eltham.

I fully immersed myself in the TBM transition at the Kidbrooke intermediate shaft, choosing to be in the tunnel while much of the attention was on the west drive’s final breakthrough. This was an opportunity to develop my understanding of the TBM. To ensure a smooth ring build, I created a gasket-cutting plan for the miner and stayed late into the back shift after starting on the day shift, ensuring the work was done properly rather than relying on a handover.

When the TBM was set to begin exiting the intermediate shaft, and started breaking into the concrete wall, I was on the cutting wheel with a level, checking how close the top of the cutterhead was to the portal. Before this, my work as a shift engineer had been routine—steady mining and ring building. But real learning comes from challenges, not routine. I knew this was my chance to grow, so I stayed longer on shifts, and some miners stayed back with me.

One shift, we were meant to finish at 3pm, but we needed to build a ring in the shaft for temporary works. Two miners stayed with me until 5pm to get it done. When the hinge plate issue arose during shift handover, I caught it before it could damage the already worn screw.

This experience pushed me in ways routine work never could, giving me a deeper understanding of TBM operations and the confidence that I had made a real impact in delivering this critical phase of work.

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REFERENCES

• Kidbrooke: National Grid ET (no date). At: https://www.nationalgrid.com/electricitytransmission/network-and-infrastructure/london-power-tunnels/in-your-area/kidbrooke
• Hakimian, R. (2024) ‘National Grid switches on first 2.5km circuit of £1bn London Power Tunnels 2,’ New Civil Engineer. At: https://www.newcivilengineer.com/latest/national-gridswitches- on-first-2-5km-circuit-of-1bn-london-power-tunnels-2-06-08-2024
• Hochtief-Murphy Joint Venture Ltd (2024). ‘Method Statement for TBM Transition Through Kidbrooke Shaft.’ Internal document, London Power Tunnels 2, Hochtief Murphy JV, London, March 2024. (Note: The referenced method statement is an internal document and is not publicly available.)
• Kundan, R., Daniele, S. and Martin, S. (2023). ‘Challenges of delivering 32.5km of major tunnelling infrastructure in South London (UK), and the influences of an innovative “Enterprise” project delivery model.’ Hochtief-Murphy JV, London