Work on Transport for London (TfL)’s Silvertown Tunnel in east London has hit another milestone, with the project’s tunnel boring machine (TBM) reaching Greenwich. The new 1.4km road link, which includes 1.1km long twin bored tunnels, will connect Silvertown on the north side of the Thames with the Greenwich peninsula on the south side. The sections of tunnel not bored by the TBM will be formed by cut and cover tunnels and retaining walls as the access ramp reaches ground level. The TBM, named Jill, is driving the scheme’s first 1.1km long bore. Since its launch in August, the TBM has travelled around 900m and now just has a short way to go before it reaches the rotation chamber. Once there, main contractor Riverlinx – a joint venture formed by Ferrovial, Bam Nuttall and SK ecoplant – will work to rotate the TBM and send it back on the return journey under the river to the Silvertown site. Not only is Riverlinx using the UK’s largest diameter TBM at 11.87m, it will be turning the 1,200t machine around in the shaft at Greenwich with just 30mm clearance before it tunnels back to Silvertown. In addition, the contractor will use ground freezing techniques to stabilise the ground before mechanically excavating the cross passages. This is a method rarely used in the UK as a planned approach and one more commonly associated with emergency work.
The Latest Milestone Comes After a Conveyor Fire on 27 September.
halted tunnelling work for 10 days, when the TBM had barely bored 40m. An investigation by a forensic specialist into the incident has now resulted in an industry wide safety notice being issued by Riverlinx as there are concerns that the trigger for the fire is also a risk on other tunnelling projects. Analysis of the fire and CCTV showed that hot works earlier in the day, which finished at 5.30pm, caused smouldering within the hopper chute lining causing it to ignite at 10.20pm despite it being a wet day. The TBM and conveyor were not operating between the completion of the hot works and the fire breaking out. The resultant fire caused damage to the high angle conveyor used to remove spoil from the tunnel but Riverlinx project manager Ivor Thomas said that the organisation’s evacuation policy worked exactly as planned and no one was hurt during the incident. Prior to the fire, getting work on site underway had not been straightforward, with local stakeholders expressing concern that traffic from the operational tunnel will worsen air quality rather than improve it. However, the focus for Riverlinx is now on the construction programme. It has to meet TfL’s 2025 target opening date and, under the terms of the contract, TfL will only start to pay Riverlinx for its work when the tunnel is operational. “The programme is brutal, we have a big machine and constrained sites,” Thomas told NCE when asked about the main challenges. “Rotating the TBM is also complex and some of the cross passages are located in some tricky ground.”
On its journey Below the Thames, the TBM Passes Through the London Clay.
the water bearing Harwich Formation and the Lambeth Group, which can also contain water. This created the need for ground freezing during cross passage construction. The alignment just avoids passing into the Chalk. Riverlinx TBM manager Alessio Menghini told NCE he is expecting water pressures of up to 3.2bar during the drive and Riverlinx chose to use an earth pressure balance TBM for the work because of the ground conditions. Nonetheless, the contractor did adjust the vertical alignment of the bore to give slightly more cover as the TBM passes under the Thames. The original design had 6.5m of cover but that has now been changed to 8m. Looking ahead, breakthrough into the rotation shaft at North Greenwich is expected at the end of January and it is anticipated that it will take four months to turn the machine around ready for re-launch to drive the second bore. Riverlinx currently expects the second tunnel drive to be completed by the end of 2023 and TfL is currently slating 2025 as the opening date for the new road tunnel.