Tunnel boring machines

To construct the 42km of tunnel required for Crossrail the tunnel boring machines will undertake ten individual tunnel drives to construct the 6.2m diameter rail tunnels.

Each TBM weighs approximately 1000 tonnes and will be up to 140m in length with an external diameter of 7.1m. This allows for an inside tunnel diameter of 6.2m once the concrete tunnel segments are in place.

At the front of the TBM is a full face cutter head which rotates at 1 to 3 rpm. As the TBM advances forward the cutter head excavates the ground.

The first of eight TBMs is currently being tested at the Herrenknecht factory in Germany and will shortly be dismantled and transported via road and ship to London before being re-assembled at Westbourne Park

The first two TBMs will be delivered to Royal Oak Portal, just west of Paddington; two TBMs will be delivered to Limmo Peninsula in London’s Docklands and two will be delivered to Stepney Green. One further TBM will be delivered to the Plumstead Portal site for the Thames Tunnel but will have two cutter heads - the main section will be attached to a new cutter head to complete the tunnel.

The eight TBMs will undertake a total of ten individual tunnel drives to construct the 6.2m diameter Crossrail tunnels.

There will be two different types of TBM to reflect the differing ground conditions along the Crossrail route. Six will be Earth Pressure Balance Machines, which will be used for the main running tunnels between Royal Oak and Pudding Mill Lane.

These will pass through ground which is predominantly London clay, sand and gravels. The Thames Tunnel, which is predominantly constructed through chalk, will use Slurry TBMs.

The eight tunnel boring machines will be used as follows:

• Royal Oak to Farringdon (Drive X) – 2 x earth pressure balanced TBMs
• Limmo to Farringdon (Drive Y) – 2 x earth pressure balanced TBMs
• Pudding Mill Lane to Stepney Green (Drive Z) – 2 x earth pressure balanced TBMs
• Limmo to Victoria Dock Portal (Drive G) – 1 x earth pressure balanced TBMs re-used from the Stepney Green to Pudding Mill Lane drive
• Plumstead to North Woolwich (Drive H) – 2 x slurry TBM

To help reduce the likelihood of settlement while the tunnels are constructed, the TBMs have to run nearly 24 hours a day, 7 days a week, 365 days a year. There will be scheduled breaks to allow maintenance on the TBMs to take place.

Each TBM will be operated by a ‘tunnel gang’ comprising of around twenty people - twelve people on the TBM itself and eight people working from the rear of the machine to above ground.

Herrenknecht AG has been selected by the tunnelling contractors to manufacture all of the TBMs that will be required for construction of the Crossrail tunnels.  

The first two Crossrail TBMs are currently being manufactured. Factory assembly of these TBMs begins in October and will complete during November. The tunnel boring machines will then be factory tested before being dis-assembled in readiness for shipping to London. The first TBM components will begin arriving in the UK at Tilbury Docks in December with the second TBM arriving later in January. The TBM components will then be transported to Westbourne Park for re-assembly.

TBM in numbers (Earth Pressure Balanced Machine):

• Total length of TBM: approx. 140m
• Total weight: approx. 1,000 tonnes
• Speed: approx 100m per week

Shield

• Shield diameter: 7,080mm
• Operating pressure: max. 3 bar
• Cutterhead diameter: 7,100mm
• Power main drive: 1,920kW
• Main drive max. torque: approx. 9,800kNm
• Cutterhead rotation speed: 0 - 3.19rpm
• Nominal thrust force: approx. 58,000kN
• Screw conveyor diameter: 900mm
• Power screw conveyor: 400kW

Backup

• Number of gantries: bridge + 10 gantries
• 2 belt scales
• 1 Refuge chamber
• 1 Personnel container

Gallery - images of the TBM cutterhead being installed, 31 January 2012

Removing material from the tunnels

Excavated material will be removed from the tunnel face at a controlled rate and deposited onto a series of belt conveyors (pipes for a slurry machine) which remove the excavated material through the TBM and out of the tunnel. The TBM advances forward by using a series of hydraulic rams at the back of the cutter shield which push forward from the last precast concrete ring erected.

Precast concrete segments will be designed to be built in rings behind the TBMs.

Once the cutter shield has advanced forward by a defined distance the next precast concrete ring can be erected.

These concrete segments will be made in a factory and transported to the tunnelling worksite. Arrangements for the production of the concrete segments will be confirmed later this year as the successful bidders for C300, C305 and C310 are currently identifying the best locations for these facilities for their respective tunnel drives.

Sprayed Concrete Lining will be used to build the larger platform tunnels at stations and the shorter passenger circulation tunnels at stations. The sprayed concrete lining is applied in layers to the excavated ground to form a robust lining. This method of lining is slower to construct but is more suited to larger and more irregular shaped tunnels and tunnel connections. The tunnel lining will have a design life of at least 120 years.

Materials, including concrete tunnel segments, will be brought to the TBM by a temporary railway constructed in the tunnel.

Tunnel construction schedule: