Extensive design changes made as a result of technical challenges at two of Tideway’s sites are to blame for a cost increase of £280M on London’s super sewer, the company’s chief operating officer told New Civil Engineer.
The challenges at both Blackfriars in central London and the King Edward Memorial site in east London were announced in November last year, with the cost hike confirmed last week.
At the site in Blackfriars, Tideway chief operating officer Mark Sneesby said that the original plan was to build the shaft by constructing a cofferdam in the river, backfill it to ground level to create a working platform from which a diaphragm walled shaft could be constructed.
However, when detailed design of the diaphragm wall took place, contractors Ferrovial and Laing O’Rourke found the action of backfilling and movement caused by the diaphragm wall construction would cause two Victorian gas mains, embedded 5m away in the river wall, to rupture.
As diverting the gas mains would have caused the adjacent main road to be closed for six months, the joint venture (Flo) had to come up with an alternative design for building the shaft.
“We had to take some big decisions,” said Sneesby. “We’ve built up some really good relationships with all of our stakeholders and to then say we have to close that road for six months was just unacceptable. We had to find an alternative solution.”
The alternative design for the shaft is now a secant pile wall solution down to 25m with a sprayed concrete lining (SCL) solution beneath to 52m. Sneesby said this means that pilling can be carried out from riverbed level and the cofferdam no longer needs to be backfilled.
“We can start piling from lower down and so with the combination of the jet grouting, piling and sprayed concrete lining, we’re anticipating much less ground movement than with diaphragm walling and filling,” said Sneesby.
However, the use of the secant pile wall and SCL for the shaft created the need for dewatering and jet grouting in two rings to stabilise the ground before it was sprayed. It is believed that this is the deepest jet grouting ever to take place in London at 55m below the river bed level.
Prior to carrying out the procedure at Blackfriars, a test for the jet grouting was carried out the Kirtling Street shaft and then excavated around it to verify the technique.
However, the design change has had repercussions for the project’s programme.
The delay caused by the redesign means that the shaft may not be built by the time the tunnel boring machine (TBM) reaches the Blackfriars site. This means the planned inspection and maintenance of the TBM will not be as simple as originally planned.
To try to mitigate this, a jet grouted canopy above where the TBM will enter the shaft has been created. With stable ground above the TBM, the cutting heads will then be able to be accessed from within the TBM to be maintained without compressed air, before completing the next stage of its drive.
At the King Edward Memorial site, similar to Blackfriars, the cofferdam was to be backfilled. But as the backfilling was taking place, Sneesby said the sheet pile walls deflected more than was expected.
The cause was found to be thicker, soft alluvium layers than the design allowed for which meant that the ground had to be strengthen for both the temporary and permanent cases.
To strengthen the temporary works, 150, 25m deep CFA piles were installed along the bottom perimeter of the cofferdam with a beam and slab at the top to support the sheet piles.
However, to strengthen the ground where the permanent works will be, the 100m by 50m area of the cofferdam has had to be strengthened using deep soil mixing. This will involve installing 1,400 1.2m diameter, 8m to 12m long “columns” in the ground to strengthen it.
“The augers go into the ground and as they come up the ground is injected and mixed with grout,” he said. “This reconditions the soil and makes it stronger.
“You’re effectively covering the whole are with columns which are cement bound to give extra stability and strength of the soil.”
Once the ground strengthening is complete, the shaft will then be constructed using a diaphragm wall technique.
Again, the shaft now will not be ready before the TBM arrives due to the additional work being carried out. However, as there is no maintenance on the machine planned at the site, the alignment of the tunnel is being changed to skirt round the shaft with a small connection tunnel between the two.
Sneesby estimated that the redesign for both sites will have cost the project over £100M each but said the additional cost is still within the baseline amount of £4bn set aside for the project.
On top of these additional costs, Sneesby said more has been spent because of Tideway’s decision to transport more materials by river than by road, as originally planned.
“The decision to move lorries from the road to the river was a really positive one, and one we’re very proud of,” he said. “We’ve taken around 360,000 HGVs off the road and moved it onto the river.
“But by doing this I think we’re leaving a lasting legacy as it’s all about the economy of the river and upskilling the people who work there.”
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