With tunnelling work is set to start in earnest on Tideway this month, GE takes a look at the preparations so far at the Kirtling Street site.
The cutterhead on the first tunnel boring machine (TBM) – known to the Tideway team and TBM B but the public as Millicent – started turning below ground in December at Kirtling Street in Battersea. The TBM cutterhead is now 100m from the shaft and has been sat 20m out under the river since 22 December when the drive was paused for reconfiguration during the umbilical launch process.
Work on the launch is now nearing completion and TBM is B is set to start to move forward in the next week or so and will be followed shortly by the launch of TBM C, which is better known as Ursula. TBM B will drive the 5km tunnel westwards to Carnwath Road, while TBM C will drive the 7.6km tunnel eastwards to Chambers Wharf.
TBM B has not been completely idle while work to configure the main conveyor has been underway.
The cutterhead on TBM B is currently turned through a couple of rotations each day to ensure that no adverse torques are developing while the machine is paused.
“The current face pressure is 1 bar and the worst case we’re expecting on this drive is 3.5 bar,” says Tideway site manager Paul Hallows. “To the east we are expecting to use higher pressures – up to 5 bar.”
Until the pause, TBM B was achieving advance rates of around 14.5m per day and is currently sat 20m out below the river.
“We need to get both TBMs several hundred metres into the tunnels before we will reach pure production rates,” says Hallows. “During the initial launch we were advancing 25mm/min but there is potential to double that.”
The cutterhead for this TBM C is in position and the gantry will be positioned over the next month with start of tunnelling expected towards the end of February.
“Ursula is just over three months behind Millicent in terms of launch,” says Hallows.
According to Hallows, ground conditions were one of the drivers for deciding which TBM to launch first, as well as the number of assets the TBM will cross.
Ground conditions westwards are expected to be more uniform compared to the eastward drive, which will also pass directly below residential properties and other utilities during its initial launch.
Work on the shaft at Kirtling Street, which will become London’s largest manhole at the end of the project, gave some insight into some of the ground condition challenges for the tunnel drive eastwards.
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.”
Newman describes the shaft as one of the most challenging on Tideway in terms of location.
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.
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 on their way from Herrenknecht’s factory in Germany. The excavation faced both ground water and geological issues but adit excavation was completed in May and the first of the TBMs was brought to site on 21 June.
“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.