Preparation is underway for tunnelling work on the Tideway scheme, but the result should mean that there is little to see.
As you get closer to the Chambers Wharf site in Bermondsey on the Tideway mega-sewer project in London, you would be forgiven for thinking you’d taken a wrong turn as you pass through a quiet housing estate. Yet on the waterfront just beyond is one of the scheme’s key sites from which one of the tunnel boring machines (TBMs) will be launched and many millions of tonnes of spoil will be brought out.
It sounds like most people’s worst nightmare to have that on your doorstep – and it is not without its challenges – but the Costain, Vinci Construction Grands Projets and Bachy Soletanche joint venture (CVB) is working to minimise the impact. Solutions range from secondary glazing and community liaison right through to development of an electric hydrofraise diaphragm wall rig, a warehouse structure to cover the tunnel headworks and use of river transport to bring in materials and take away spoil.
The ground engineering work needed before CVB can start tunnelling is also wide-ranging with cofferdams, grouting and diaphragm wall construction for the access shaft.
Once the above ground work is completed and the shaft excavated, work will then start on a massive spray concrete lined underground cavern that will be used to assemble the TBM before it is launched on the 5.5km drive to Abbey Mills in East London. The cavern will also be used as a reception shaft for the TBM being used by Laing O’Rourke and Ferrovial Agroman to drive the central section of the Tideway scheme (see box), as well as the TBM used for the Greenwich connector tunnel.
CVB moved onto site in April last year and the first challenge was to create the land for the work to be delivered by reclaiming an 8,000m2 area within a twin wall cofferdam constructed from sheet piles.
“The cofferdam is formed with twin 20m long sheet pile walls that are connected by tie rods to effectively form a gravity structure,” says CVB geotechnical construction manager Martin Stanley.
In total, 90,000t of marine dredged material was placed to reclaim the area impounded by the cofferdam.
Tunnelling on the eastern section of Tideway will be wholly within the Chalk, but the shaft will have to pass through the River Terrace Gravels, alluvium, London Clay and Thanet Sands before reaching it.
Groundwater was clearly going to be a challenge, especially with hydrological connectivity to the tidal waters of the Thames, so a comprehensive grouting programme was undertaken in the gravel for the shaft work and at depth within the Chalk for the cavern construction.
“So far we’ve installed over 20km of grout holes to create a cut off in the gravels and isolate the Chalk,” says Stanley.
Work on the diaphragm wall for the 27.7m diameter shaft started this summer and, when GE visited site, work was underway to concrete the seventh out of 36 panels. Each panel is 1.5m thick, 2.5m wide and 73m deep.
“To meet the programme here, we needed to be able to carry out the D-wall work up until 10pm at night, which, with a conventional rig, would have been challenging because of the noise,” says Stanley.
The solution was the development of an electrically powered Hydrofraise diaphragm wall rig to meet the programme and verticality demands of the work but without impacting on local residents.
“An electricity supply was planned at the site for the TBM, so we looked at whether this could be brought in early and be used to power the Hydrofraise,” says Stanley.
“Operationally it is no different from a conventional rig but it is 3dB(A) quieter, which effectively halved the noise.”
Shaft construction is programmed to finish in mid-February but CVB is aiming to fast track the work to be completed in January. Nonetheless, tunnelling is not expected to start at the site until mid-2019 as first a warehouse type structure has to be built over the construction area before shaft and cavern excavation can get underway.
Along with the warehouse, river transport is a key part of CVB’s solution to minimise the impact of the work on local residents. Already 100,000t of demolition waste from the former warehouses that occupied the site have been taken out by river and steel reinforcement cages for the diaphragm wall are being brought in by boat. Barges will also be used to take away spoil from the shaft, cavern and tunnelling operation and bring in the precast concrete tunnel lining The TBM – an 8.8m diameter Herrenknecht slurry machine – which will start its journey from Chambers Wharf is currently being designed in Germany and it is hoped that it will also be brought to site by river.
With the warehouse structure over the tunnel works, materials and waste will be taken in and out by day but tunnelling operations will operate around the clock. With the doors to the warehouse structure shut, CVB hopes that the impact of the work on local residents will be minimal so that it can get to work on delivering its aim to cut 18 months off the programme for the eastern tunnels.
The Tideway lowdown
“The driver for the construction of the Thames Tideway Tunnel is to clean up the River Thames,” says Tideway construction manager Maurice Gallagher. “At the moment around 39M.m3 of sewer overflow is released into the river but the tunnel will minimise this to around 2M.m3 a year by transferring the bulk of it to Thames Water’s facility at Beckton.”
The £4.2bn Thames Tideway Tunnel is a 25km long gravity-fed inception, storage and transfer system, which will run from west to east across London at up to 66m below the River Thames.
The tunnel generally follows the path of the river from the start of the system in Fulham in west London to Limehouse in east London. From Limehouse, the tunnel cuts inland towards Abbey Mills Pumping Station where it will connect with the Lee Tunnel, which will transfer sewage to the Beckton Sewage Treatment Works.
Construction of the tunnel is split into three contracts. The western section is being constructed by a joint venture of Balfour Beatty, Bam Nuttall and Morgan Sindall. The central section – the longest tunnel drive of the three – is being built by Laing O’Rourke and Ferrovial Agroman. The western section is being delivered by the CVB joint venture, which is formed from Costain, Vinci Construction Grand Projets and Bachy Soletanche.