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Contractors World  - UK & Ireland
2013 Vol 3 No 1
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Crossrail tunnelling in southeast London gets underway - page 2 of 4 >>>

Two machines, Sophia and Mary will excavate over half a million tonnes of material

 

Two machines, Sophia and Mary will excavate over half a million tonnes of material and be under the control of 20 person ‘tunnel gangs’ working in shifts - 12 on the TBM itself and eight working between the rear of the machine to the tunnel entrance.

Sophia is different to the tunnelling machines being used elsewhere on Crossrail. Known as a ‘slurry’ machine, she is specially equipped to deal with the chalk, flint and wet ground conditions that she will encounter in southeast London.

As the machine advances forwards, precast concrete segments are put in place to form concrete rings to line the tunnels.   Sophia has a rotating cutter head at the front, and a series of trailers behind housing all the mechanical and electrical equipment.

As the machine advances forwards, precast concrete segments are put in place to form concrete rings to line the tunnels.

The segments are positioned with millimetre precision and held in place by hydraulic rams. Once the segments form a complete ring, the structure is extremely strong and stable. In the final stage of the process, adjacent rings are bolted together.

Different geological conditions

Unlike the tunnelling machines being used north of the Thames, Sophia needs to be able to deal with the chalk and flint that is found south of the River Thames. She has a sealed, pressurised, air-locked chamber behind the cutter head where the excavated soil is mixed with bentonite (a mixture of clay and water) to form a liquid. This slurry liquid is removed from the tunnel through a system of slurry tubes to the bespoke treatment plant located at the Plumstead site.

A different type of TBM, Sophia is designed to cope with the more demanding conditions while tunnelling under the Thames. The second tunnel will be driven by similar machines, to be called ‘Mary’ due to start work in a few months time.   As part of the tunnelling process, the excavated soils will be pumped out as liquid slurry to a special treatment plant at Plumstead.

A different type of TBM, Sophia is designed to cope with the more demanding conditions while tunnelling under the Thames. The second tunnel will be driven by similar machines, to be called ‘Mary’ due to start work in a few months time.

The slurry will be separated into sand, gravel, water and chalk. The chalk will come out in ‘cakes’ or slabs of filtered chalk particles.

Stephen Hammond, Crossrail Minister said: “The launch of Sophia, Crossrail’s fifth tunnelling machine, demonstrates the great progress the project is making, stimulating the economy, generating thousands of jobs during construction and delivering huge transport improvements to people living in southeast London.”

Andrew Wolstenholme, Crossrail’s Chief Executive said: “The start of tunnelling in southeast London marks another milestone for Crossrail, a project that will transform public transport in and around the capital. The benefits for southeast London are huge - for the first time people living between Abbey Wood and the Royal Docks will be able to travel right through the centre of the capital without having to change trains, bringing Canary Wharf, the West End and Heathrow closer than ever before.”

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