The first TBM to be used on National Grid‘s $56M, 20km long North London Cable tunnel is due to go to face in mid-July this year. The 3m i.d. tunnel from Elstree to St. John’s Wood is one of the longest of this diameter to be constructed.
Contractor, J Murphy & Sons was finally officially awarded the contract last month and will build the tunnel in three drives using three Lovat TBMs. Each machine will be capable of working in EPB mode although the contractor told T&TI that it does not anticipate this being necessary. The contract also included the construction by underpinning of six new shafts ranging in diameter from 7.5m – 10.5m.
Murphy will tunnel concurrently from a shaft site at Elstree and in both directions from a shaft in Cricklewood. Drive 1, to begin in July, runs 5.6km from Elstree to Canons Corner; drive 2 runs 8.2km from Cricklwood to Canons Corner and is scheduled to begin in mid-August. Finally drive 3, due to start in mid-October, runs 6km from Cricklewood to St John’s Wood.
The Lovat TBM on drive 1 has a 3.5m diameter and will erect a trapezoidal lining to leave the 3m i.d. Drives 2 and 3 will be constructed by two 3.4m diameter Lovats. The lining will combine wedgeblock and bolted wedgeblock to leave a 3m i.d.
The tunnel, scheduled for completion by March 2003, will pass at depths of 25m through varied ground including clay, sands and sandy clay. The consultant for the client is Brown and Root whilst Murphy has retained the services of Maunsell.
National Grid project manager, Jim Street said, "This is National Grid’s largest tunnelling project to date. Tunnelling means we can keep the effects of construction on residents, businesses and road users to a minimum."
The project will reinforce the area’s electricity transmission system and meet future demand from the capital’s residential and business communities.
