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Crossrail: Under the capital

With all of the 42km of tunnels for London’s Crossrail project now complete, NCE picks out some of the key facts and innovations in the programme.

Crossrail once again proved its ability to create a political photo opportunity last month as prime minister David Cameron and London mayor Boris Johnson went 40m below ground to mark the official completion of tunnelling for the £14.8bn project.

The epic programme to build 42km of tunnels below the capital began almost exactly three years ago when tunnel boring machine (TBM) Phyllis was launched from the Royal Oak portal in west London to excavate the first of the 6.2m diameter twin-bore tunnels in the 6.8km section to Farringdon station in central London.

Crossrail tunnels completion

In the headlines: Prime Minister David Cameron and London mayor Boris Johnson (front left) at the final TBM breakthrough last month

It ended when the cutter head for the final TBM, Victoria, broke through into the same station at 5.30am on 23 May having bored though its way from Limmo Peninsula in East London.

In total, eight Herrenknecht TBMs were used on the project. Six of these were earth pressure balance tunnelling machines weighing 980t and measuring 148m in length.

They were employed to dig through the London clays, sands and gravels that prevail beneath the capital.

Crossrail

Positioning: 3.4t lining segment

Two 110m mixed-shield slurry machines (Mary and Sophia) tunnelled through the wet chalk and flint beneath the River Thames for the section between North Woolwich and the Plumstead portal in south east London.

In total, the tunnels are lined with over 220,000 concrete segments weighing 3.4t each. In each section, seven segments and a key stone slot together to form a completed tunnel ring.

The average tunnelling rate for the entire project was 38m per day, although TBM Ellie broke the record for the fastest day of tunnelling by a single machine having progressed 72m on 16 April 2014 between Pudding Mill Lane and Stepney Green.

Crossrail tunnels completion

Lowering: TBM Elizabeth

Improvements to TBM technology - including real-time settlement control at the cutting faces - gave Crossrail the confidence to set maximum face loss limits as low as 0.5% to ensure the impact on the surface would be acceptable. Before moving to Thames Tideway, Crossrail programme director Andy Mitchell said this gave stakeholders like London Underground the confidence to allow tunnelling activity around its live lines.

“Feats like taking TBM Ada within 1m of the Northern line, in the middle of the day, during normal Tube operations, just wouldn’t have been allowed before,” he says. 

For parts of the project where tunnelling had to pass through predictable clays, gravels and sands, compensation grouting works were required.

TBM Ellie broke the record for the fastest day of tunnelling by a single machine having progressed 72m on 16 April 2014

For instance, at Farringdon station, five 5m diameter grout shafts were excavated between 15m and 25m below ground level and cementitious grout was pumped into the ground via tubes à manchette drilled through the sides of these shafts.

Another innovation proposed by the Bam Nuttall/Ferrovial Agroman/Kier (BFK) joint venture ensured that it won the contract for the western running tunnels between Royal Oak and Farringdon.

The JV proposed forming tunnels between these stations with TBMs before enlarging them to form the station boxes with sprayed concrete.

All of the TBMs are now in the process of being dismantled and returned to the manufacturer with parts to be recycled for future use.

However, the front “cans” of Victoria and Elizabeth will be left in the tunnel at Farringdon for Crossrail trains to pass through them when services start in 2018. Similarly the cutterhead and front “can” for Phyllis and Ada will remain 30m below ground in Farringdon.

  • Watch a drone’s eye view of Crossrail’s completed tunnels here

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