Final breakthrough on the last of three cable tunnels for UK Power Networks in London is achieved by contractor J.Murphy & Sons.
The 3km drive under east London, designed by AECOM and Parsons Brinckerhoff, is the culmination of almost two years of underground construction to create a 2.85m i.d. tunnel that has been threaded through the London Underground Tube network and which also runs under the River Thames.
The £27 million tunnel, lying 23m–30m below the surface, links substations from south-east of the Thames across to east London. It will house cables with a 400MW capacity, enough to power 130,000 homes.
Breakthrough into reception chamber in Tower Hamlets, east London, was achieved well within 50mm tolerance following launch of the 3.4m diameter contractor-owned Lovat TBM in Southwark, south-east London, in September 2014. An average advance rate of 23m/day has been achieved, at external water pressures of up to 3 bar.
Constant monitoring of ground conditions ahead of the face was required for successful completion of a drive that traversed a number of different geologies including heavy London Blue Clay, the Lambeth group of silty clay, fine silts and gravels, the Thanet Sands formation, and at the deepest vertical alignments, chalk-bearing flints.
J. Murphy, which in June 2014 completed another drive under the Thames for Crossrail in joint venture with Hochtief, will now spend two weeks dismantling the TBM on to 70m of back-up sledges, guiding all the equipment through a custom-sized junction chamber in Tower Hamlets before lifting the machine out of a shaft using a 350-tonne mobile crane.
The tunnel connects substations north and south of the River Thames to meet London’s demand for electricity. It is the final stage of a three-part tunnelling project along a 5.7km route between substations in south-east London and the City. The first stage, completed in 2011 by the same contractor, was a £1.1million 30m long interconnecting tunnel and junction chamber. The second stage was a £14million 2.7km-long tunnel between east London and the City, completed by Morgan Sindall in March 2013.
Darren Ramsay of Murphy said: “Each milestone in this last drive was successfully negotiated, from the challenges of tunnelling underneath the River Thames, the Jubilee Line and train tracks running into London Bridge. Final breakthrough in Tower Hamlets has been achieved with minimal disruption to Londoners and the environment. Despite difficult ground conditions there has been no deviation from the original alignment.”
|Table 1. Project factsheet|
|The contractor-owned Lovat TBM is 7.5m long, 3.4m diameter|
|37,000m of 132,000V cables will be installed|
|More than 6,000 loco journeys within the tunnel|
|290,000 man-hours completed without any lost injury time|
|77,000 tonne of excavated material transported from site|
|3,000 concrete rings line the tunnel from south-east London to east London|
|Each ring consists of six segments, 18,000 segments in total were used|
Nirmal Kotecha, Director of Capital Programmes and Procurement for owner UK Power Networks, said: “London’s power needs are increasing all the time and we are constantly investing in new infrastructure to ensure we maintain reliable electricity supplies. This essential project will increase the resilience of electricity supplies for residents and businesses for years to come. By working deep underground with the Murphy specialist team, we were able to avoid digging up roads along the entire route, helping keep the impact of this major project to a minimum.”
The tunnel will now be lined with hundreds of cable support brackets to carry 132,000 volt electricity cables, connecting substations between south-east London, the City and east London. Tunnel fit out will start this month (July) and last seven months, with the new 132,000-volt electricity cables due to go live in summer 2017.