Tideway Is an Epic Civil Engineering Project. at £3.8 Billion, It Is the UK's
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CLEANING UP LONDON’S WASTE WEALTH CREATION CLEANING UP LONDON’S WASTE Tideway is an epic civil engineering project. At £3.8 billion, it is the UK’s largest ever water infrastructure undertaking. It includes 25 kilometres of large-diameter tunnel deep below the River Thames, with 23 huge shafts and a workforce of 4,000. To Tideway’s Chief Executive Officer, Andy Mitchell CBE FREng, it represents even more than that. At the halfway point of the project he spoke to Hugh Ferguson about his mission to reconnect Londoners with their great river. The new Thames Tideway of brick-lined sewers feeding than four million at a time when Tunnel will divert the millions of into 131 kilometres of large the actual population was only tonnes of raw sewage that flows interceptor sewers running two million. Even so, rainfall is into the Thames each year, and west to east, parallel to the river. variable and Bazalgette realised take it, by gravity, through a new They stretched from Abbey Mills that his system could not cope tunnel to Beckton sewage works pumping station (and then to with the greatest storms, so he in east London. Construction Beckton) on the north side of the included a series of 57 combined started in 2016 with all work due river, and to Crossness pumping sewer overflows (CSOs). During to be completed in 2024. station in the south. He built the extreme events, these overflows Tideway will supplement tunnels to last – 150 years later, allow rainwater and sewage to rather than replace Sir Joseph they are still in remarkably good overflow into the Thames, rather Bazalgette’s great network of condition, and can continue than cause a back-up that would sewers, completed in 1865 to to transport most of London’s flood streets with sewage. combat the ‘Great Stink’ from sewage for years to come. Three factors have combined the polluted Thames. Bazalgette Bazalgette also designed and the system can no longer estimated that the system, one the tunnels so that, unlike cope. Bazalgette’s ‘design of the greatest engineering some other cities’ sewerage population’ of four million has achievements of the Victorian systems, they carry both sewage increased to nine million and era, prevented around 12,000 and storm water run-off, thus much of London’s space has deaths a year from cholera and avoiding the need to dig up been transformed from open A cofferdam was constructed on Blackfriars north foreshore so that a dry workspace could exist below the level of the River Thames. A drop shaft has been designed to intercept both the LL1 sewer located within the listed Bazalgette river wall and the existing Fleet sewer outfall, which is located under Arch 1 of other waterborne diseases. London’s streets twice. He ground (which can absorb a the Blackfriars Bridge, to the new Tideway tunnel. The shaft is now excavated to a depth of 23 metres and when completed will be 53 metres deep (to invert Bazalgette’s system planned ahead, designing a great deal of rainwater) into level) with an internal diameter of 22 metres © Tideway comprised 2,100 kilometres system for a population of more buildings or ‘hard standing’, 14 INGENIA INGENIA ISSUE 82 MARCH 2020 15 CLEANING UP LONDON’S WASTE WEALTH CREATION BLACKFRIARS BRIDGE WORK Tideway’s most visible construction site is at Blackfriars, in the City of London overlooked by people crossing Blackfriars Bridge. It is also prompted a major redesign, undertaken to avoid the risk of damaging the Victorian gas mains. In the summer, this will result in the largest ever structure to be floated on the Thames in Central London. A combined sewer overflow from the River Fleet discharges under the north arch of Blackfriars Bridge. The plan was to build a twin-walled, sheet-piled cofferdam (a watertight enclosure that enables construction below the waterline). Within that, new concrete culverts would transfer the flow with water draining directly and long dry spell, when the system main feature of the network into a new shaft some 200 metres upstream and down the online shaft directly into the main Thames immediately into the sewers. fills with sewage, the ‘first flush’ is a huge 7.2-metre internal Tideway Tunnel. Thirdly, a consequence of into the Thames is concentrated diameter tunnel (large enough Analysis showed that the cofferdam and particularly its heavy concrete tie-ins to the river wall, climate change is more erratic sewage. to fit three double-decker could cause movement to the wall and in turn movement to the Victorian gas mains. Damage to the and extreme weather, so that buses) running 25 kilometres gas mains could impact the gas supply to the City, while relocation of the gas main would delay the the water engineer’s benchmark from Acton in the west, project and, worse, require closure of the Embankment – a major road artery – for several months. of a ‘hundred-year storm’ may WHAT TIDEWAY DOES beneath the Thames through Instead, the engineers came up with an ingenious solution. Reducing the twin-wall cofferdam to now happen three times in Tideway’s job will be to capture central London, and then the upstream end, where the shaft is being sunk, would reduce wall movements enough to avoid five years. all of this overflow and initially veering north to Abbey Mills damaging the gas main at its critical point further downstream. A large dry dock, drained of water, The overflows, initially store it in its new network in the east. Some £1.1 billion replaces the central section of the planned cofferdam. Inside the cofferdam, the section of concrete required about twice a year, of shafts and tunnels, which of preliminary works have culvert connecting to the Fleet CSO is being prefabricated. Meanwhile, a prepared bed is being are now used 60 times a year, together form a huge 1.6 million already been completed by created beneath the bridge arch back to the cofferdam using crushed rock capped with a concrete and rising, tipping a staggering cubic-metre underground Thames Water – including the mattress on which the culvert will sit. 40 million tonnes of sewage a reservoir, releasing it only as fast Lee Tunnel, the easternmost Around May, the dry dock will be flooded to float the culvert. The downstream end of the dock A combined sewer overflow releases rainwater and sewage during peak flows year into the Thames. Worst of as the pumps and sewage works (and deepest) section of tunnel, will open and the culvert winched some 100 metres before lowering into position on its bed. It may 60 times a year © Tideway all, when heavy rain follows a at the eastern end can cope. The from Abbey Mills to Beckton. be only a short float-out, but its massive size – 100 metres long, roughly seven metres square in The tunnel is deep, section and weighing 3,500 tonnes – breaks all records. principally to avoid London’s Then the culvert will be connected to the CSO (with a temporary gate to allow sewage outfall myriad existing underground to release downstream until Tideway is complete), and the dry dock will be re-sealed, locking in the Abbey Mills Pumping Station services: 30 metres in the west, upstream end of the new culvert. With the dock re-drained, the section of concrete culvert linking to 23 the shaft will be built, resting on piles sunk below the Thames. Lee Tunnel with gravity flow to 66 metres Beckton Sewage TOWER HAMLETS Treatment Works at Abbey Mills (and deeper Finally, the whole Blackfriars site will be backfilled and landscaped to create a new public open se Cut 24 still at Beckton). Tideway has space adjoining the river. The new space is Tideway’s largest and its proposed name is ‘Bazalgette CITY OF The Limehou LONDON NEWHAM Embankment’. 22 23 sites for constructing its CITY OF WESTMINSTER 15 21 huge shafts that connect the EALING 14 overflows on either side of the 16 1 HAMMERSMITH 17 AND FULHAM river to the main tunnel. Some are also used for lowering KENSINGTON 18 AND CHELSEA and lifting the massive tunnel Greenwich 2 13 GREENWICH 10 connection boring machines (TBMs). tunnel 12 11 19 Digging the Lee Tunnel RICHMOND 9 20 UPON LEWISHAM required considerable technical THAMES SOUTHWARK innovation, in part as a testbed 3 for the work on Tideway itself. 7 8 LAMBETH 4 These innovations included 5 Frogmore hydrofraises (large crane- WANDSWORTH connection tunnel 6 mounted drilling machines with twin counter-rotating cutters mounted within a steel frame) for sinking panels Map key under bentonite slurry to Main tunnel drive site Long connection tunnel drive site Connection tunnels West works site create the primary concrete Main tunnel reception site System modifications Lee Tunnel Central works sites linings of shafts and the use CSO site Main tunnel Drive direction East works site of slip forming to build the Short connection tunnel drive site inner concrete linings of the The 25-kilometre-long Tideway Tunnel runs from Acton Storm Tanks in the west, beneath the Thames, to Abbey Mills Pumping Station, where it will connect to shafts. Another innovation Blackfriars north foreshore. A 100-metre, 3,500-tonne interconnecting culvert will be the largest structure to pass the already completed Lee Tunnel to Beckton. Tideway’s 23 construction sites include Hammersmith Pumping Station (2), Blackfriars Bridge Foreshore (15) and was the use of fibre optic along the Thames later this year. It will be lifted into the central section above and then connected to the CSO drop King Edward Memorial Park Foreshore (21). Several additional connection tunnels bring sewage to the main tunnel.