Work to bore Thames Water’s £635M Lee Tunnel is now about to get into full flow after nine months of tunnelling. Claire Symes reports.
Tunnelling projects are never simple schemes but the depth at which Thames Water’s new, 6.9km long Lee Tunnel sewer is being bored makes it more challenging than most. First the depth of its diaphragm walled shafts pushed the boundaries of geotechnics and now the launch of the tunnelling boring machine (TBM) is forcing
further bespoke solutions.
Main contractor MVB - a joint venture between Morgan Sindall, Vinci Construction Grands Projets and Bachy Soletanche - lowered the £16.9M Herrenknect Mixshield slurry TBM into position into a shaft at the eastern end of the scheme in mid-December last year. Nine months on - and 450m into the drive - the machine is on the verge of reaching the final stages of its launch.
The £635M Lee Tunnel is part of a planned £5bn investment to improve London’s sewage transfer network and treatment facilities. The 7m diameter Lee Tunnel will create a new link between Abbey Mills pumping station at its western end and Beckton sewage works in east London, adding 380,000m3 of storage capacity in the process.
This will prevent 16Mt of stormwater and sewage entering the River Lee each year during periods of heavy rainfall.
The gravity system has been designed to eventually take flow from the planned Thames Tunnel which will run from west to east London.
The Lee tunnel lies between 75m and 80m below ground and passes mostly through the Upper Chalk. Work started on site in 2010 with installation of four shafts. From west to east they are known as Abbey Mills shaft F, Beckton connection shaft, the Becton pumping shaft and the Becton overflow shaft (see diagram overleaf).
The TBM was launched from the Beckton overflow shaft, the smallest in terms of diameter at 20m, and 74.5m deep. The 78.5m deep, 25m diameter Beckton connection shaft will be the next to receive the TBM. Work there is complete.
The adjacent 86.5m deep, 28m diameter Beckton pumping shaft, just off the main tunnel drive is now excavated to its full depth and construction of the concrete base is about to start.
The final destination for the TBM is nearing completion too. The 25m diameter, 68m deep shaft at Abbey Mills only has another 5m to be excavated to reach its full depth, clearing the way for the lining to be constructed.
“Lowering the TBM was just the first stage of the launch process that is still underway,” says MVB project director François Pogu.
“Lowering the TBM was just the first stage of the launch process that is still underway”
François Pogu, MVB
The TBM - formed of the cutter head, the drive unit and the five gantries - is 110m long and weighs in at 1,646t in total, so getting the whole system into the 20m diameter shaft was never going be simple.
“TBMs usually use an umbilical cord to allow progressive launch with the cutter head and drive unit in position at the bottom of the shaft and the main body connected by the cord at ground level,” says Pogu. This so called umbilical cord is normally a tube feeding power cables, grout lines and water to the TBM.
“But here it was not possible due to the depth of the shaft. To overcome the problem, three levels of steel framework were constructed within the shaft to allow the necessary equipment to power and supply the initial launch with materials.”
This arrangement allowed the site team to drive the first 100m and place the first 67 of the 1.7m wide precast concrete tunnel lining rings. Even this was challenging and Pogu said that the umbilical cord used for the initial launch was the longest ever installed by Herrenknecht.
“We completed the first bore by February and then had to stop tunnelling for seven weeks to allow the “three storey building” in the shaft to be removed and the gantries lowered one by one into position,” says Pogu.
Even this was not a simple task. Each gantry weighs 120t and had to be built at a site 1km away from the overflow shaft and brought to site in sequence through Beckton sewage works by a multi-axled low loader.
The gantries were lowered into position with a 1,000t mobile crane and then the whole system went through a commissioning process to allow tunnelling to restart in May.
“In the first 100m of the tunnelling, the aim was 2.5 lining segments per day. The aim is to reach 14 rings per day when the TBM installation is complete”
François Pogu, MVB
TBM installation has still to reach the final stage, even after nine months. “Logistics are still being installed and up until now there has only been room for one locomotive to deliver rings to the construction face,” says Pogu.
The aim for this stage was to install seven rings per day. Earlier this month, the team had managed 33 rings in one week.
The tunnelling operation is currently operating two, 12 hour shifts from Monday to Thursday, one eight hour shift on a Friday and maintenance is carried out over the weekend.
“In the first 100m of the tunnelling the aim was 2.5 concrete lining ring segments per day. Each ring represents 1.7m of progress,” says Pogu. “The aim is to reach 14 rings per day when the TBM installation is fully complete.”
Pogu says the aim is 10 per day, but he believes 14 is achievable, and that the average number could actually be higher.
The next phase will take place when 257 rings - to reach 450m - have been placed. The site expected to reach this milestone soon after NCE went to press.
Three week halt
“When we get to 450m, work on tunnelling will stop for three weeks,” says Pogu. “This will allow us to empty the bottom of the shaft and install a station with track crossovers that will allow us to operate two trains.” These trains will then be used to carry precast tunnel lining segments to the tunnel face.
When work restarts in early October, Pogu expects the TBM to take another seven weeks to reach the connection shaft. But the TBM will not be allowed to see the light of day as the shaft will be backfilled with a weak grout mix to help maintain the pressures at the tunnelling face.
“We will use a 2N cement/sand mix to 5m above the top of the TBM to balance the pressure and we will add water on top if this is not enough,” says Pogu.
Regular checks are being carried out on the cutter head at other points in the construction. “The head was checked at 50 and 150m and will be checked again at 450m to check for wear caused by flint layers within the chalk.
So far the inspections have not shown anything out of the ordinary. “From excavating the shafts we know that the flint level varies to up to 25% but currently we’re only seeing 8% to 10% flint,” says Pogu.
The inspections were carried out by lowering the pressure down from 6bar to enable the head to be inspected easily. But Pogu’s team has plans in place in case it is not possible to lower the pressure sufficiently.
Fractures within the chalk and the high water table mean that pressures of up to 8bar are anticipated and the slurry is there not to support the rock face but to cope with the water pressures by providing equal and opposite pressure to water within fractures in the chalk. “We are trying to avoid the use of hyperbaric interventions but they cannot be ruled out because working at such a depth has an impact on the rings and grout, as well as the TBM itself,” explains Pogu.
Lowering the pressure
Part of Pogu’s strategy is to lower the pressure to check conditions ahead of the TBM. “The TBM has the on board pumping capacity to cope with water flows [from fractures in the chalk] of up to 220m3/hour. So far the two interventions that we have carried out to check for wear have only resulted in water flows of 30m3/hour,” says Pogu.
Nonetheless, the TBM has yet to pass through the most challenging ground conditions, which are expected in the highly fractured Greenwich Fault zone which also puts some water bearing sand beds within the alignment of the TBM.
“If water flows are higher than our 220m3/hour capacity, then we plan to move forward 10m to cover the fractures and hope that the grout will seal them. If this doesn’t work then we can drill ahead and radially around the TBM face to inject grout. While this won’t be effective in the chalk itself, it will help to seal the fractures.”
Work so far is on schedule and Pogu seems confident that there are plans to cope with the ground conditions, even when the drive reaches the complex Plaistow Graben later in the scheme. If his confidence proves well-founded, the tunnel will be operational in April 2015.