Jubilee nearly sinks DLR

Jubilee Line Extension contractors had to carry out emergency nightwork last week as a tunnel breakthrough threatened to undermine the foundations of a railway viaduct.

A 200t tunnel boring machine - named Tracey - emerged on the north side of the River Thames last Wednesday two weeks ahead of schedule. But the operation almost turned to disaster as the high water table in the area - caused by January's heavy rainfall - destabilised the ground at the tunnel portal.

The portal - on contract 110 of the JLE - is just a few metres from the Docklands Light Railway viaduct and ground movement threatened to undermine the structure.

Following on so closely from the catastrophic tunnel collapse at Heathrow Express last year, this could have been another massive body blow to the industry's image.

Contractor McAlpine - Wayss & Freytag - Bachy battled overnight to sink a 20m length of sheet piling into the ground to protect the foundations of the viaduct.

A ceremony marking the tunnel breakthrough was hastily cancelled by the project team last Friday and rearranged for this week. The statement said that the machine would surface this week instead. In reality, the TBM actually appeared as expected last week but the event was cancelled while water was pumped away from the site and the emergency piles sunk.

A spokesman for the Jubilee project said: 'The event has been delayed by water problems.' He did not comment on the piling around the DLR.

The contractor set two TBMs going last August and called them Sharon and Tracey. Both made excellent progress and Sharon broke through with no difficulty on 20 December into a prepared 10m deep and 10m diameter portal pit adjacent to the Docklands Light Railway. The 1.2 km running tunnels have encountered sand and gravel lenses in the clay - despite this progress has hit 154m per week.

But as Tracey approached breakthrough in late January, engineers feared that the wet ground - caused by this month's downpours - could lead to the collapse of the DLR into the portal pit just metres away. This prompted the emergency sheet piling. The TBM then emerged into a 3m deep pool of mud filling the portal pit.