In 1858, the great stench forced parliament to commission one of the engineering marvels of the Victorian age: Joseph Bazalgette’s sewerage system. Between 1858 and 1875, Bazalgette built 82 miles (131km) of enclosed brick main sewers, six miles of huge intercepting sewers – London’s Embankment is simply the construction that envelops them – and no fewer than 1,100 miles (1,800km) of street sewers. Added to this were four major pumping stations driven by the most powerful steam engines in the world. He thus transformed the Thames from a lifeless, open sewer to a river that fish could survive in.
Bazalgette’s Marvel
Bazalgette over-engineered on a grand scale. As well as 310 million bricks (which are still in good condition) he used the newly-invented and durable Portland Cement, which is one reason his works have lasted. London’s population was then two million. He made his tunnels twice as large as needed to cope with the output of four million people. “We are only going to do this once” he said, “and there is always the unforeseen.”
There is indeed. London’s population is now nearly nine million, and Bazalgette’s system is not coping. It overflows on average once a week. Around 39 million tonnes of stormwater mixed with raw sewage flow though his overflow pipes into the river each year.
“Bazalgette’s system takes rainwater as well as sewerage, and when two millimetres of rain falls on London, it overflows and discharges into the river. That is not at all good, but it is preferable to the alternative of it backing up via lavatories into Londoners’ homes.” This is according to Richard Lewis, programme manager of the Tideway project – currently Europe’s biggest water infrastructure undertaking.
A New Marvel
Nicknamed the ‘super sewer’, the 16- mile (25.5km) tunnel is being built to broadly follow the path of the Thames, from Acton in west London downstream to Abbey Mills. It will intercept 34 of Bazalgette’s existing overflow pipes, provide a storage capacity of 1.6 million m3 and reduce untreated discharges into the Thames to around four events a year.
The main 7.2m-diameter tunnel will lie 30m below ground at Acton, falling to 70m at Abbey Mills, transferring the waste eastwards by gravity. At Abbey Mills it will connect with the Lee Tunnel, which was built by Thames Water in 2014, and thence to Becton sewerage treatment works, the biggest sewerage treatment works in Europe. A connecting tunnel of 4.8m diameter will extend four kilometres from Greenwich to link up with the main tunnel at Chambers Wharf, which is just downstream of Tower Bridge; a second connecting tunnel, the Frogmore tunnel, has recently finished its drive to link existing sewer outflows to the main tunnel at Wandsworth.
The project is divided into three sections. The western section, stretching 7km downriver from Acton, is being carried out by a joint venture of BAM Nuttall, Morgan Sindall and Balfour Beatty Group. The central section, which covers 13km through the heart of London as far downstream as Tower Bridge is by a JV of Ferrovial Agroman UK and Laing O’Rourke; and the 5.5km eastern section, which will take the work to the Lee Tunnel and thence to Beckton is by a Costain, Vinci, and Bachy Soletanche JV. Amey is responsible for providing process control, communication equipment and software systems for operation, maintenance and reporting across the Tideway tunnel system.
The project’s tunnel construction cost of £1.1bn is being funded directly by Thames Water. Average annual bills for Thames Water customers for 2019/20 will include £19 per household for the tunnel; this will eventually rise to no more than £25 a year before inflation.
Six TBMS will be used on the project. “Four of them are for the main tunnel” says Lewis. “Three of those are EPBMs and one is a slurry machine. The geology from Acton down the river to Hammersmith is London clay, to a depth of 30 or 40m. From there, the central section is Lambeth Group sand and gravel; and from Tower Bridge downstream it is chalk. That is where the slurry TBM will operate.”
“There are four entry portals, all of them down shafts. One is at Carnwath Road by Wandsworth Bridge. One TBM is going from there westwards, in other words uphill, to Acton Storm Tanks – an existing Thames Water facility comprising pumping station and six storm tanks. A second launch site is by Battersea Power station. That has sent TBMs off in both directions: one went westwards to Carnwath Road, which it reached in November. It hasn’t penetrated the shaft; it has been stopped and left in place just before that. The other went eastwards, downriver. It has just reached the Millennium Bridge, level with St Paul’s Cathedral, and is due to arrive at Chambers Wharf towards the back end of the year.
“Those two are EPBM machines from NFM. The Greenwich connecting tunnel will use a 4.8m-diameter TBM from Herrenknecht. On Wednesday 8 July 2020, the slurry machine for the chalk sections is arriving by jacking barge from Holland. The barge will dock at Chambers Wharf; it will be put on self-propelled modular transporters (SPMTs) and driven off the barge; that is the one that will tunnel towards Abbey Mills.”
The new system collects from Bazalgette’s original overflow points where rain and effluent flow into the river (combined sewage overflows (CSOs)). Vertical shafts have been dug down to the level of the new tunnel. Deflectors within the shafts make the stream flow in a vortex, like water down a plug-hole.
“The idea is to remove energy from the flow, and therefore reduce scouring and prevent it from eroding the connecting tunnels at the bottom of the shaft” says Lewis. “We have used all methods of tunnelling on the project” he says. “TBMs for the main tunnels, sprayed concrete for the connecting tunnels, and pipejacking for smaller links.”
COVID-19 Working
Construction began in 2016 and was on schedule and in full flow when Covid-19 struck. “When it hit we had two TBMs running that could not be stopped in mid-tunnel. The Action one had tunnelled as far as Hammersmith Bridge. We got them up to places where it was safe to pause, then stopped for three or four weeks while we worked out how to continue safely.
“We studied the problem with our workforce. We divided the project into three regimes: red, where there was no way round and we could not proceed at all; amber, where we could proceed with social distancing and safety measures, and green where we could continue unaffected and as normal. We began working again with a single shift, to see how the new procedures worked; and now, at the beginning of July, we are back to three shifts.
“In the first place, we modified how people get to work: instead of public transport we made car parking places for them. Above ground, we limit the number of people; the welfare stations and canteens are like school examination halls now, with tapes and yellow footprints on the floor to guide social distancing. Supervisors have added checking social distancing to all their other tasks. Underground, segment erection has to be done with people at close quarters but the TBM driver is normally at a distance from others anyway. And they wear full personal protective gear down there, with half-masks and so on. We made a conscious decision not to source these from any supplies that could otherwise go to the National Health Service.
“One effect it has all had is that shift changeovers take longer. Handovers and briefings are above ground. That slows things a little. But we are back to three shifts a day now. There was a stage when our workforce was two thousand strong. Nineteen hundred of them are back at work now, so nearly everyone who wants to be back is back.
“The main impact is on what you could call work-flow dynamics. Aboveground work that was scheduled to happen concurrently with tunnelling has had to be postponed because it would cause too many people to be on-site at once. So that will be carried out later. We are working on timings for that now.”
Maximising the River’s Benefits
The river dominates every aspect of the project. “The river gives us huge flexibility,” says Mr Lewis. “We are in a hugely crowded and congested capital city, but it lets us take muck out, and bring TBMs and lining segments in by barge, and each barge takes 40 or 50 trucks off the roads. We have mainly used existing fleets, but it has certainly called for some new barges, in particular in the fleet of jack-up barges.”
The segments that the barges bring in are produced by Morgan Sindall at the Ridham Dock site in Kent, for the western section, and by Spanish company Pacador on the Isle of Grain for the central tunnel. While for the eastern end of the tunnel, Tarmac and Max Bögl are producing at a manufacturing facility at Tallington near Stamford, Lincolnshire.
“Three million tonnes altogether will be shipped by river” says Lewis. “We have moved one million tonnes from Battersea so far. The spoil is taken downstream, where some goes for land recovery at Ingrebourne Valley capping a fly-ash tip, and the rest is used to create a new reclaimed-land wetland habitat on the Rainham marshes – the same project that took the spoils from Crossrail.
“And, perhaps unexpectedly given the riverine nature of the project, we had to build a lot of land first.” The dozen or so vertical shafts that take Bazalgette’s overflows down to the main tunnel are up to 60m deep (see diagram of the one at Victoria Embankment above). Most are on the river banks; their tops will be concreted over to give semi-circular public riverside spaces. “To build them we needed to build coffer dams. Then we pumped the water out and started digging down.” The hydrofraise machine that excavated them – essentially rotating cylindrical cutterheads lowered from a crane – was supplied by Bachy Soletanche and powered by renewably-generated electricity, which saves emissions and also reduces noise levels, by about 50%. The system was shortlisted for an Institute of Environmental Management and Assessment Sustainability Impact Award.
Two medium-size connecting tunnels join on to the main one. Tunnelling has yet to commence on the Greenwich connecting tunnel. The 1.1km Frogmore connecting tunnel was excavated from a shaft at Dormay Street, very close to its midpoint; the TBM first tunnelled 500m south to a shaft in King George’s Park where it was hoisted out, taken back to Dormay Street and set to work again tunnelling in the opposite direction to Carnwath Road on the river. Breakthrough there happened in July 2020.
Ciaran McQuaid was project manager for the drive. “On the Frogmore tunnel we emerged halfway up the Carnwath Road shaft, perfectly on target” he explains. “Navigation was by ‘Z system’: a station on the TBM takes snapshots of a target on the shield to guide the machine. The Frogmore tunnel was an easy one for navigation. It is a hockey-stick shape with just one curve in it. In contrast, the relatively straight Greenwich tunnel could well be less of a challenge.
“The Frogmore followed roads and then took the course of the River Wandle and followed that to where it enters the Thames. The main concern was settlement of the river walls. We had surveying teams out monitoring that; we had daily meetings with them in case we came close to trigger levels but the drives went well and there were no real settlement issues.
TBMs old and new
“The bigger TBMs on the main sewer muck out by screw conveyor to a conveyor belt in the tunnel, which takes them to a shaft, then by vertical conveyor to the surface. But the connecting tunnels are narrower, of 2.85m internal diameter, with no room for a conveyor belt. So we removed the muck by screw onto a short conveyor which took it through the TBM to a mini-electric railway which we laid along the tunnel. The railway also brought the lining segments up the tunnel to us. We used six segments per ring, the rings being one metre wide.”
This excavation was one of the exceptions to the river transport solution. “The river by the Dormay Street shaft is too shallow to allow barges in, so segments came in and muck went out by truck; a requirement was that they had to have special low cabs for visibility, so that the driver can see cyclists and the like more easily.”
The TBM itself (‘Charlotte’) is a veteran, and McQuaid is well acquainted with the machine. It was manufactured in 1994 by the Canadian company Lovat and was used on the Thames Water Ring Main, which Thames Water built between 1988 and 1993. For a machine that will be turning 26 in November she has done a marvellous job.
“Our best shift achieved eight rings – that is, eight metres – in a single shift,” added McQuaid. “Our best in 24 hours was 12 metres.” Now that it has broken through into the shaft the machine will be advanced onto a semi-circular platform then separated into various sections and lifted out by gantry crane. The secondary lining, when eventually fitted to the Greenwich overflow tunnel, will be cast in situ, on shutters made by Kern. When that has been completed, attention will focus on the Frogmore tunnel.
“On the large-diameter tunnel productivity has been good,” says Lewis. “We have been averaging around 115m a week in the clay; we are expecting around 80m a week in chalk.” The western and central sections are well on the way to completion. Work is about to start on the final, eastern section. During the two days it took to write this article, the TBM for it was delivered from Holland and safely unloaded at the Chambers Wharf site. It will take a month or two to prepare her for work, and she is expected to take 15 to 18 months to complete her drive to Abbey Mills. It is anticipated that groundwater pressures will be up to 7 bar.
The completion date for the whole project, pre-Covid, was scheduled for 2022. Handover is planned for 2024. The time in between is for testing. “We need several big storms to see how it handles large rainfall events, and when they come is in the lap of the Gods” says Lewis. After that, the Thames will certainly flow more sweetly than it has done since Bazalgette did his pionering work. No doubt he would consider the Thames Tideway project to be a worthy improvement.
