London Tunnels breakthrough

Costain/Skanska/Bachy JV, the contractor on the US$120M Contract 240, on the UK's Channel Tunnel Rail Link (CTRL), is celebrating the completion of the first two tunnel drives of the mega-project's 19km long London Tunnels section.

The first of the 4.7km long twin tubes, that connect Stratford and Manor Park, broke through into the Barrington Road Shaft on Wednesday 3rd December, with the second bore following a week later.

Boring began in September 2002, from the Stratford Box heading east under the Great Eastern Line and London Underground's Central Line Tunnels. The JV used two 8.1m diameter Wirth TBMs to bore the 7.15m i.d segmentally lined tunnels through dense water bearing Thanet Sands.

Alan Dyke, managing director of the client, Union Railways (North) Ltd, said of the first breakthrough, "the completion of this drive represents a major success for the CTRL project, made possible by the dedication and skill of the JV and the reliability of the Wirth TBM."

Both C240 drives came in well ahead of programme, the downline drive taking just 43 weeks in total to complete. The upline tunnel was delayed by the incident at Lavender Street where a garden subsided (T&TI, March 2003, p6), but once going again completed the remainder of the drive (4.2km) in 29 weeks.

Extensive investigations were carried out into the Lavender Street incident and into methods of identifying subsurface voids, but the way the subsidence occurred was never finally isolated. Old uncharted wells, which had deteriorated, is a suggestion supported by some anecdotal evidence. There are a number of deep wells in the area.

Tunnelling restarted when the UK's Health & Safety Executive was satisfied that further possible hazards had been addressed and that the necessary processes of route risk assessment and strict definition of driving parameters had been re-checked.

Apart from a couple of incidents, the performance of the TBMs in terms of settlement control has been described as 'exceptional' with volume losses kept to between 0.5 and 1% of the tunnel face area.

CTRL Section 2 'London Tunnels' involves the construction of two 19km long high-speed rail tunnels between St Pancras and Dagenham (the 2.5km long C320 'Thames Tunnels' broke through last year). Aside from C240's two Wirths, two TBMs are being driven west from Dagenham to Manor Park, and a further two TBMs are boring west from Stratford to the north of St Pancras Station. All the London Tunnel drives are currently ahead of programme and have achieved good outputs (Table 1).

The US$8.3bn CTRL project is scheduled to open to Eurostar Train services in 2007 (for details see the T&TI CTRL Section 2 supplement, distributed in September 2003).