McAlpine jv hits trouble on Jubilee Line drive

The Sir Robert McAlpine-led joint venture on Jubilee Line Extension Contract 110 (Canary Wharf-Canning Town-North Greenwich) hit problems last Friday (10 February) when an unexpected mud hole halted tunnelling work near the Blackwall Tunnel.

The contractors were able to keep disruption to the crucial arterial road that serves the tunnel to a minimum but three one-minute traffic stoppages were necessary in order to take ground readings.

The jv, which includes Wayss & Freytag and Bachy, was removing a sheet-piled cofferdam left behind during work on the tunnel in the 1960s. Working to a method-statement agreed with the client, Jubilee Line Extension, the jv had to sink a segmental shaft and drive a heading out from the shaft to the piles to cut them off from within the heading.

A grout mix was used to stabilise the ground with approval from Greenwich borough and the DoT.

But last Friday a mud hole was encountered 8.5m into the heading, which created an invert that filled with water and gravel. Three men working in the tunnel evacuated immediately.

After immediate consultation between the client and contractor, the heading was backfilled with an approved foam concrete from a Pioneer plant across the road. Monitoring carried out has shown there was no deviation around the location.

JubileeLine Extension said there had been no risk to life, limb or structure and there will be no 'serious delay' to the project.

Four options for completing the pile removal exercise are now being considered.