Northern Line Extension Tbms Lowered Below Ground

The cutting heads and shields of Helen and Amy, the two 6 metre diameter tunnel boring machines (TBMs) that will excavate and line the major part of the new running tunnels, have been lifted from the surface and lowered down into the crossover box, from where the tunnelling to Kennington starts.

Helen was lifted and lowered on Sunday 12 February, and Amy on Tuesday 14 February.

Derek Whelan, FLO TBM Project Manager, said, “Successfully lifting what both together was 600 tonnes of machinery is undoubtedly a major feat. This operation has been nine months in planning and seeing it come to fruition was very rewarding! It has been a great team effort and I am proud of the hard work and dedication of everyone involved.”

Lifting a 295 tonne, 6m diameter TBM is a highly technical process with many challenges. Prior to the lift, enabling works had to be completed safely and on time. Crane pad foundation construction, crossover box construction, TBM assembly, SCL adit construction and TBM slide rail installation were just some of the works that have been carried out in preparation for the lift. 

The lift was carried out using a 750 tonne Liebherr LR1750 crawler crane. 35 lorry loads delivered the enormous crane. The first delivery arrived on Monday 6 February, upon which assembly began. A test lift took place prior to each of the TBM lifts.

The TBMs were each lowered into the ground perpendicular to the direction they will be tunnelling in as a result of the constraints placed by the props running along the crossover box. Following this, each TBM was rotated 90 degrees to line up with the launch rails and then jacked into the launch tunnels. 

After the first lift, the crane was relocated and reloaded to facilitate the second lift. It was a challenge to manoeuvre the 750 tonne crane due to space constraints. Following this, reloading the ballast, or steel weights placed at the rear of the crane to provide stability, in such a tight time scale was a testament to the expertise of our team members involved. Each lift took around three hours to complete. However, it comprised many hours of planning and preparation, with the involvement of a team of more than 20 people.

It all came down to planning, precision in the execution and the right weather conditions. Jonathan Cooper, London Underground Project Manager said, “The team have demonstrated an outstanding commitment to safety whilst working incredibly hard to ensure the date for the contract lift was met. This gives us the best chance of launching the TBMs on time. Well done all!”

David Darcy, FLO Project Director, said: "Everyone involved in enabling this milestone to be achieved can be very proud. It is a very significant step in the overall engineering and construction of the project. 

Now the TBM shields are down, with each one being positioned in its prepared launch chamber, there is a further intensive period over the coming weeks to finally assemble the machines and to connect the back-up gantries and their equipment to each TBM and to test and commission each TBM to enable the start of digging. It is planned that Helen starts digging the southbound running tunnel in late March, and Amy the northbound one month later. Between them, Helen and Amy will excavate over 286,000 tonnes of ground and install 3,288 precast concrete tunnel lining segments. 

It is not just the TBMs that are necessary.  They are part of a system, fitting the physical constraints of the Battersea site, that will see ground excavated, conveyed out of the tunnels to the cross-over box, up to the surface using a high angled conveyor, conveyed at high level across the Battersea Power Station development site that is under construction by others, directly discharged into barges and transported away down the River Thames to landfill near Tilbury. Going into the tunnels, and erected simultaneously with the ground coming out, will be the tunnel lining segments. This whole system has to be ready for the TBM start.

We look forward to this exciting phase."