UK first on CTRL breakthrough

The 8.2m diameter Herrenknecht slurry TBM being used to bore Contract 320, the 'Thames Tunnels', under East London, has successfully broken through on the first of C320's two 2.5km long bores. The landmark event, on the 4th March, saw the first of eight major bores completed on Section 2 of the UK's US$8.2bn Channel Tunnel Rail Link. It also saw the first use of an innovative breakthrough technique in the UK.

The Hochtief/Murphy JV began tunnelling in July 2002 and was the first to start boring on the project.

The tunnel alignments pass through clay, gravel, then chalk containing flint. The slurry TBM, which included a jaw stone crusher for flints larger than 150mm, easily handled the mixed ground conditions. The cutterhead is now being dismantled and returned to Germany, while the backup is reversed through the tunnel to the second Herrenknecht cutterhead at the launch shaft in Swanscombe.

The second bore, which will include the construction of the three cross passages, is scheduled to begin before the end of this month.

The US$236M contract was originally tendered on the basis that the machine would bore 85m per week. However, by achieving an average of 100m per week, the contract is now approximately two weeks ahead of schedule.

Elsewhere on CTRL 2, while going to press, one Wirth EPBM on the 4.7km long twin bore Contract 240 was still waiting for permission to restart, following a collapse above the tunnel that swallowed two gardens in East London (see T&TI, March 2003, p6). "The TBM will not start for at least another couple of weeks. We still have to satisfy the Health & Safety Executive, as well as the residents," Alan Myers said, head of tunnelling for the client's consultant Rail Link Engineering.

The previously suspended second TBM working on the C240's other bore recently re-started after London Underground gave it permission to continue under it's Central Line, with a minimum clearance of 4m. "The second machine is going well – there were no problems under the Central Line," Myers said.