THE TUNNEL boring machine (TBM) breakthrough for a cable tunnel for the London Olympic Park has been delayed by severe water ingress into a 10m diameter, 26m deep shaft that will carry power cables into an electricity sub-station in Hackney.
Contractor Murphy had dug down to 16m when it hit a high pressure aquifer. As a result, a secant piled wall around the shaft must be built for the final 10m to prevent water ooding the tunnel.
The emergence of the water ingress into the shaft - part of a scheme to underground cables - came after tunnelling work was suspended for ve weeks in February and March after the discovery of chemicals naphthalene and benzene following a breach in the overlaying ground (GE April).
The Olympic Delivery Authority (ODA) said extra work would not delay the final project as mechanical and electrical contractors could start their work on the rest of the tunnel.
'The water ingress and substances encountered in overlying grounds are issues that are known to be encountered during this kind of operation, and didn't have a signicant impact on the progress of work, ' an ODA spokesman told GE.
He added that TBMs on two of the four tunnels had broken through.
The other two would break through by early summer in time for the tunnels to be lined later in the year.
ODA chief executive David Higgins said the tunnels would be operational by summer 2008.
Higgins was speaking at the launch of 10 key milestones - to be completed by the end of the Beijing Olympics in summer 2008 - which include the cable tunnels.
'These milestones are tough targets, but when we welcome the Olympic Flag to London in 2008 I am con dent we will hit every one, ' he said, adding that the ODA had entered into a period called Demolish, Dig, Design.
The four tunnels, each 3km long and up to 30m deep, will allow the removal of 52 pylons on the Olympic Park site.
Contractor Murphy said the operation had been challenging due to the variable ground conditions, which meant it had to use additives to stabilise the ground.
'The geology has been extremely variable. It quickly moves from hard clays to very, very soft silts and muds, and we have been working under three bar pressure for most of the drive, ' a spokesman said.