Tunnelling on Section 2 of the Channel Tunnel Rail Link got under way last month. Ground Engineering looks at the progress of this key phase of the UK's first high-speed railway.
Milly was the first of eight tunnel boring machines to begin work on Section 2 of the Channel Tunnel Rail Link last month. The Herrenknecht machine is now boring beneath the River Thames to form one of the twin tunnels for the highspeed route between central London and the Channel Tunnel.
Four pairs of TBMs are being used for CTRL's London and Thames tunnels. All are essentially earth pressure balance machines with similar specifications, laid down by CTRL designer and project manager Rail Link Engineering and its contractors.
Each pair of machines is being built by a different manufacturer, tailored to the ground conditions of the part of the route they will be working on.
Contract 320 Thames Tunnel
The 2.5km long twin tunnels under the River Thames form most of the 3.6km long Contract 320 between Thurrock in Essex and Swanscombe in north Kent being built by main contractor Hochtief Murphy jv.
Milly, the German-built Herrenknecht mix shield TBM, arrived at the Swanscombe launch chamber in May and last month began boring northwards on the 8.16m diameter downline tunnel (for coast-bound trains).
The 95m long,1,100t TBM will bore about 85m per week through alluvium, Terrace Gravel and the Upper Chalk, which contains flint bands, at up to 40m below the river level.
Once it reaches the Thurrock portal in January or February next year, the TBM backup train will be returned to Swanscombe, where it will be fitted with a new cutter head and begin work on the upline tunnel in May 2003. A total of 33,000 concrete ring segments will be installed on each tunnel with 280,000m 3of spoil produced.
Contract 240 Stratford to Barrington Road
German TBM manufacturer Wirth used its hard rock tunnelling knowledge to give Contract 240's earth pressure balance TBMs an edge to deal with hard flint bands on the route.
The machines' Tungsten carbide picks and teeth can be swapped for hard rock roller disk cutters if needed.
Although most of the 4.58km route between Stratford and Barrington Road is in Thanet Sand, the 8.05m diameter tunnels dip into the chalk in places and its flint capping layer the Bullhead Beds.
The machines also have double articulation of the main shields. The front section of the 10.9m long three-part shield is actively articulated from the centre body by 29 hydraulic thrust cylinders.
Where hard material such as flint is met in only one part of the face, the shield can be steered sharply to compensate for the tendency for the whole machine to be thrown off line.
Passive articulation of the tail section is intended to reduce overbreak beyond the precast concrete lining, and help it follow through the ground with minimum resistance.
Probing ahead of the face can be done using a large drill on the axis of the machine and from an array of kit around the perimeter of the central body section.
These drills can push probes out through slanting, reinforced holes formed in the double skin of the shield.
Eight ports around the rim of the rotating cutter head itself will allow a polymer foam to be injected to the ground to fluidise excavated material and give some lubrication to the shield.
While one machine was assembled and tested in Wirth's factory in Erkelenz, the other had to be put together in sister company NFM's factory in Lyon, France because of the sheer size of the 950t shield and its extensive back-up equipment.
Main contractor Costain, Skanska and Bachy Soletanche jv expects delivery of the two machines in late September, with tunnelling due to start in late November.
Contract 220 Stratford to London West Portal
Two Kawasaki earth pressure balance tunnelling machines will be used to bore the twin 8.05m diameter, 7.6km long tunnels through Upper Chalk, Thanet Sand and London Clay between the Stratford Box in east London and King's Cross.
The Japanese machines were tested by joint venture contractor Nishimatsu/ Cementation Skanska at Haverton Hill near Teeside before being assembled on site at Stratford in late June.
Tunnelling is due to start late this month.
Contract 250 Ripple Lane to Barrington Road
Canadian manufacturer Lovat is supplying joint venture contractor Edmund Nuttall/Wayass & Freytag/Kier Construction with dual mode open face/earth pressure balance machines for Contract 250's 5.5km long tunnels between Ripple Lane and Barrington Road.
The TBMs will drive 2.2km in London Clay in open face mode before changing to closed earth pressure balance mode to bore through the sands and clays of the Lambeth Group and the Thanet Sand.
The upline machine arrived in July and is due to start tunnelling this month. The downline machine will arrive sometime in August or September, with tunnelling due to start in October.