LONDON’S DEEPEST TUNNEL AND SHAFTS WEALTH CREATION LONDON’S DEEPEST The Lee Tunnel – the UK water industry’s largest project since its privatisation in 1989 – is the deepest tunnel ever built in London. Richard Sutherden, design manager for the project management team on this Thames Water project, describes the challenges and solutions TUNNEL AND SHAFTS for the venture that will become operational at the end of the year. London’s deepest tunnel is now nearing of 1858, the ‘Great Stink’ in the river was so water to discharge into the Thames completion in east London. When it is overwhelming that it affected the work of downstream of the capital, and later, into finished, the Lee Tunnel, with a total cost of the House of Commons. That same year, sewage works. The interceptor sewers north £635 million, will stop the pollution of the Parliament passed an enabling Act for the of the Thames converge on the Victorian nearby River Lee, which currently has to Metropolitan Water Board’s main drainage Gothic Abbey Mills pumping station, where accept sewage and storm water beyond the scheme, devised by Joseph Bazalgette, the the sewage is lifted to flow in huge brick capacity of the capital’s 19th century Board’s engineer. Bazalgette’s system pipes above ground along the Northern sewage system. The Lee Tunnel is the first of comprised 2,100km of sewers feeding into Outfall Sewer to Beckton, beside the two tunnels in Thames Water’s London 131km of large interceptor sewers running Thames. By Bazalgette’s own estimate, the Tideway Improvement Programme. The 6.9 west to east, parallel to the river. The system scheme saved around 12,000 deaths a year kilometre-long tunnel forms the eastern, carried a mixture of sewage and storm from cholera and other waterborne diseases. downstream end of the system. The Lee Tunnel will store and carry sewage and storm water from Abbey Mills pumping station in East London to Beckton Sewage Treatment Works. Constructing the tunnel posed unprecedented challenges in dealing with high groundwater pressures. Its deepest sections run over 75m below ground level, where the groundwater pressure is up to 8 bar. But the greatest innovations came in construction of the five shafts, the largest ever sunk in the capital. They involved the UK’s deepest diaphragm walls, and a world first in building the inner linings as huge stand-alone, concrete chimneys. LONDON’S SEWAGE SYSTEM In the 1850s, some 400,000 tonnes of The Lee Tunnel overflow shaft shortly after construction of the shaft secondary lining and before the launch of the TBM. The 9.9km-long Lee Tunnel runs from Joseph Bazalgette’s Abbey Mills pumping station in East London The excavation of this shaft was 79m deep and the diaphragm walls supporting this 20m internal diameter shaft (the sewage were flushed into the River Thames to Europe’s largest sewage works at Beckton, beside the River Thames. Later, it will connect to the larger Thames tunnel from West London © NCE smallest on the project) are 90m in depth. © Thames Water each day. During the unusually hot summer 32 INGENIA INGENIA ISSUE 63 JUNE 2015 33 LONDON’S DEEPEST TUNNEL AND SHAFTS WEALTH CREATION presented both a challenge and an process exploits the extraordinary thixotropic a ‘blow-out’ of groundwater before the base Bazalgette anticipated the growth of the first stage will upgrade all sewage treatment opportunity. The project was challenging properties of bentonite slurry: the slurry is was capped. The massive, heavily reinforced metropolis and designed his system to last, works, the second is the construction of the because of the depth, the groundwater semi-solid when static and can therefore base slabs, inverted domes, transferred the with the highest quality of construction and Lee tunnel and the third will be the Thames pressures, and the tight tolerances on support the sides of a trench, but returns to forces from the upward water pressure into considerable spare capacity. To cope with Tideway Tunnel. The Thames Tideway Tunnel verticality. These ruled out conventional its liquid state when agitated. Trenches are the walls. The largest of these was 4m thick, occasional extreme events, such as major will run 25 kilometres from Acton in West shaft construction techniques such as excavated under the slurry, with cages of with 4,000m³ of heavily reinforced concrete: storm surges, he included 57 combined London, directly below the Thames before caisson sinking, or piling and underpinning reinforcing steel lowered in, and then it is the largest shaft base ever in the UK, yet sewer overflows (CSOs), allowing the excess veering northeast to Abbey Mills. From with shotcrete (sprayed wet-mix concrete). It tremied (underwater) concrete is pumped to another record for the Lee Tunnel. to overspill back into the Thames to avoid Abbey Mills it will connect into the Lee was also an opportunity, because more the bottom, displacing the bentonite as it Construction took 48 hours, pouring up to flooding. Tunnel to take the sewage to Beckton shafts will be needed further upstream, most rose. The technology, borrowed from the 150m³ of concrete per hour. The system has coped remarkably well for Sewage Treatment Works. The tunnel is of them much closer to existing buildings: mining industry, was first introduced into a century and a half, but Bazalgette’s ‘extreme deep throughout, to avoid the myriad the Lee Tunnel shafts could be a test-bed for construction in the 1950s – see Deepest-ever events’ now happen about once a week. existing services under London, particularly developing safe and economical solutions. diaphragm walls. CREATING THE SHAFT’S Every year, some 39 million tonnes of sewage the cable tunnels under the Olympic Park. The first decision was to use traditional With the wall complete, the soil inside INNER LINING mixed with rainwater flow through the CSOs The sewage flows by gravity, so it is deepest diaphragm walling techniques to create the could be excavated, with internal relief wells The smooth-faced inner lining, with its many into the Thames. About 40% of this discharge at the eastern end, which is the Lee Tunnel. outer linings of the shafts. The construction to relieve the hydrostatic pressure and avoid cut-outs and details, still had to be added is from Abbey Mills CSO, from where it flows into the Thames via the River Lee. Thames Water’s proposed Thames BUILDING THE SHAFTS One of more than 50 combined sewer overflows DEEPEST-EVER DIAPHRAGM WALLS Tideway Improvement programme is Sinking the shafts – two at Abbey Mills and which, on an average of once a week, discharge untreated sewage and storm water into the River designed to minimise these overflows, as three at Beckton, with the largest at 38m Thames in London, a total of 40 million tonnes a well as improving the sewage works. The internal diameter with walls 98m deep – year © Thames Water The primary lining for the cylindrical shafts was formed as a series of vertical primary panels, linked together by secondary infill panels. The trenches for the operator of the plan position, inclination and twist of the cutter body. panels were excavated under a bentonite slurry using a hydrofraise (pictured right), a crane-mounted drilling machine with counter-rotating cutters within a Hydraulic rams and pressure plates on the cutter body can be jacked steel frame, and then filled with concrete. © Thames Water against the side of the panels to change the inclination of the cutter Made ground/alluvium heads and correct any errors in verticality. As an innovation on the 25m 25m River terrace deposits 25m 38m Diameter Diameter Diameter Diameter London clay deposits The Lee Tunnel shaft was to be the deepest ever sunk in the UK and Lee shafts, this was supplemented by lowering an ultrasonic echo 68m 68m Harwich formation 78.5m 86.5m presented several challenges. In particular, how could verticality be sensor into the trench, and configuring the output to give three- Depth Depth Depth Depth 1.5m 1.2m Lambeth group 1.5m 1.8m 20m maintained within 1:300 tolerance over the full depth of nearly dimensional imaging of the excavation. The data confirmed that the Diaphragm Diaphragm Diaphragm Diaphragm Diameter 100m, and how could the resulting structure be as near as possible maximum out-of-plumb at the bottom of the five shafts was just wall wall wall wall thickness thickness Thanet sand formation thickness thickness 74.5m 15 cm – well within the required tolerance for these 38-metre- Depth to a regular cylindrical shell? With the shafts working in compression 86m 83m 92m 98m to support the ground, verticality was essential to ensure sufficient diameter shafts. Diaphragm Diaphragm Chalk group Diaphragm Diaphragm 1.5m wall depth wall depth wall depth wall depth Diaphragm wall thickness overlap between panels to bring down the compressive stress in the The shaft walls were sunk as a series of panels, 7.2 m long and up 90m concrete to within design limits. to 1.8 m thick, with gaps between panels. Each panel took a week to Diaphragm wall depth The key to controlling these variables was in carefully monitoring build, with concrete poured continuously to avoid the formation of what was going on. A conventional crane-mounted grab dug the ‘cold joints’, where there could be cracking and water penetration. trench in the upper levels. At the lower levels, a special ‘hydrofraise’ The concrete mix was designed so that it did not reach its minimum took over: a crane-mounted drilling machine fitted with two cutter compressive strength until 56 days after pouring. This gave the team drums rotating in opposite directions to break up the soil within the enough time to construct the secondary panels through the primary bentonite.