Work on London’s super sewer is set to reach a new milestone in October with launch of its first tunnel boring machine. Claire Smith takes a look at the geotechnical challenges of reaching this key point.
Battersea is a hub of ground engineering work at the moment with work on the Northern Line Extension, Battersea power station redevelopment and work on Tideway’s super sewer all underway in one small area. But while the results the other two project may be clear to see, the only indication of the geotechnical challenges currently being tackled for Tideway at Kirtling Street may be no more than a conventional man hole cover.
Nonetheless, what will become London’s largest manhole has called for some complex ground engineering solutions and there are still a number of challenges ahead as work moves to the tunnelling phase. Work so far has seen a 30m diameter, 60m deep shaft excavated, two 37m long adits for launch of the tunnel boring machines (TBM) dug out and the TBM cutter heads moved into position.
Work by the Ferrovial Agroman and Laing O’Rourke (FLO) joint venture on Tideway’s Central contract started with the diaphragm wall for the shaft two years ago and the TBM drives will take another two years to complete.
Subcontractor Geocisa undertook the diaphragm wall work and even getting that started was not simple.
“The shaft is 31m in diameter and the diaphragm wall extends to 86m into the Chalk but has been excavated to 60m for the tunnel construction,” says Tideway project geotechnical engineer Tim Newman. “The work was not straightforward as the shaft passes through the full spectrum of the London Basin geological horizons.”
The Environment Agency also had concerns about the techniques due to the shaft being located just 3m from the river wall.
“The close location of the shaft to the river raised fears that the lateral stresses from the construction could compromise the river wall so a sheet piled coffer dam which extended 12m into the London Clay was constructed to eliminate the issue,” explains Newman.
“This solution then created potential for lumps of made ground to fall into the diaphragm wall panel during construction so an inner and outer ring of CFA piles was constructed using 343 450mm diameter piles with the guidewall cast on top as a capping beam.”
Newman describes the shaft as one of the most challenging on Tideway in terms of location.
The ground conditions continued to cause challenges once Geocisa started constructing the diaphragm wall too.
The shaft was formed by 42 panels – 21 primary and 21 secondary – that were 2.8m wide, 1.2m thick and extended to 86m below ground level.
“Geocisa started work with a hyrdomill but the London Clay clogged the wheels and took time to remove so work through the clay switched to a grab,” says Newman.
“The grab was only suitable to 50m due to the verticality needed but it was actually the geology that dictated that work had to switch back to the hydromill at 40m.”
At 40m the geology changed to the Lambeth Group which are topped by the Upper Shelley Beds at Kirtling Street and these beds were too hard for the grab. The geology then continued into sandstone and the Upnor Formation before reaching the Thanet Sands and Chalk.
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The crown of the 10m diameter, 37m long adits that are being used to launch the TBM are located at 40m below ground level so pass through the bottom of the London Clay and upper parts of the Lambeth Group.
By the time adit excavation started in March, it was on the critical path and the TBMs were being ready to make their way from Herrenknecht’s factory in Germany to arrive at the site by boat on the Thames.
The excavation faced both ground water and geological issues.
“The face of the adits also passes through water-bearing sands with hydrostatic heads of up to 2.5bar,” says Newman.
“WJ Groundwater undertook a rigorous programme of dewatering for both the sand and Chalk to mitigate the effects of basal heave.
“WJ also undertook internal dewatering to dry out the sands ahead of the mechanical adit excavation.
“These areas were still damp when they were excavated which presented some challenges for the shotcreting as it could not create an effective seal. These issues were overcome through use of accelerants in the shotcrete mix.”
The Coquina – layer of shells - at the top of the Upper Shelley Beds had an upper and lower band also presented challenges.
“We knew it existed from the ground investigation but the thickness was too small to recover a core for testing so we only had indicative strengths to work on, which suggested that it was up to 50MPa,” says Newman.
As a result of the poor core recovery, Tideway is now specifying sonic drilling to ensure cores are recovered to enable more accurate laboratory testing.
Tideway did not have borehole data for the actual shaft location as it was moved 100m westwards late on but FLO did commission Structural Soils to undertake three boreholes once on site which gave the more detailed information.
In addition to the hard material in the Upper Shelley Beds, there was also a 0.5m thick sandstone layer that had to be excavated.
Cores from the sandstone were recovered but Newman says that results were under representative as they suggested the strength was around 25MPa but in reality it was probably closer to 100MPa.
“Hydraulic breakers were used to get through these hard bands during the adit excavation but the sandstone could be anything up to 2m thick once the TBM drives get underway,” he says.
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Adit excavation was completed in May and the first of the TBMs was brought to site on 21 June, which gave the project team time to ready the shaft and adits for the lowering of the TBM cutter heads with casting of the base slab, installation of slide rails and construction of the concrete end wall at the cutting face.
The ground conditions encountered during the adit work is also a concern for the TBM launches and a head intervention is planned for a few metres after the launch in order to inspect the teeth.
Tunnelling will start on the western bore first with Millicent planned for launch in October. The eastern bore, which will be driven by Ursula is expected to start a few months later.
“It is a challenge to launch both from the same shaft,” says Newman.
The central tunnel is 12.5km with the western bore covering 5km to Carnwath Road in Fulham and the eastern bore covering the 7.5km to Chambers Wharf in Bermondsey.
“The drives are expected to take two years,” says Newman.
“The western drive is fairly straightforward geotechnically as the tunnel will exist the Lambeth Group after about 500m and pass into the London Clay. This has a few fault zones but nothing of concern.
“It is the eastern drive that will be more challenging.”
The drive is within the Lambeth Group for almost the whole 5km drive until about 500m from the Chambers Wharf shaft.
“That last 500m is very variable and the TBM team will have to constantly change face pressures to accommodate the changes,” says Newman.
Between London Bridge and Tower Bridge there is a significant fault with a large graben and up to 15m of vertical throw and then just to the east of Tower Bridge the geology changes to Chalk.
Millicent is an earth pressure balance machine which was selected for the bulk of the drive but is not ideal for the Chalk ground conditions.
“The flint band spacing is around 0.6m so there could be up to 12 flint bands in the face at any one time,” explains Newman.
Newman says that testing on the flint puts the strength at up to 900MPa, which is a contrast to the 2MPa to 5MPa of the Chalk.
“We can’t do any face inspections in this area either due to the faulting but we have an intervention planned ahead of reaching this area,” he says.
“The one plus side of the faulting is that it means there is not much Thanet Sand to contend with – maybe just 1m or 2m in the inverts for a short distance.”
In addition to the ground condition challenges, ground water and ground gas are also creating concern.
Groundwater with pressures of up to 3bar are expected but Newman is less worried about that than the ground gas issues.
“The ground gas concern comes from deoxygenated air within the Upnor Formation of the Lambeth Group is not a concern for the tunnelling itself as the ground will be saturated,” he says. “It is the exposure in shafts and spoil transport does create potential for hypoxic conditions to form. FLO has installed special ventilation and has a programme of gas monitoring to maintain safety.”
Newman says that the Blackfriars shaft is within the Upnor Formation and will be of particular concern.
However, the project is some way from reaching these locations and, while there are considerable challenges ahead, the excitement surrounding the first TBM launch on the Tideway project is tantamount. The site may be destined to be commemorated with a simple man hole cover but it has an important role to play between now and when Tideway is completed in 2023.