Taking the Crossrail route from below ground to the surface presents some challenges. GE reports on the work at Royal Victoria Dock Portal
There will be many piling projects on smaller sites than that of Crossrail’s C340 Royal Victoria Dock Portal contract in East London but there will be very few with such challenging logistics.
The work to build the massive secant piled box structure at Royal Victoria is certainly the largest project currently being undertaken in the UK by Bachy Soletanche.
At this point, a stone’s throw from London’s Excel Centre, trains from London on the south east leg of Crossrail between Whitechapel and Abbey Wood will emerge from tunnel about 10m below ground. By the time the new railway opens in 2018, the piled box will form
the first half of a 320m long transitional cut and cover tunnel and U-trough structure between the tunnel portal and Custom House 400m to the east, the only new surface level station to be built on the Crossrail route.
Either one or two Crossrail boring machines will break into the box towards the end of next year after driving from the Limmo Peninsula roughly 1km to the west. Client Crossrail Ltd has yet to decide whether a single machine will bore both of the twin tunnels from Limmo in turn instead of two TBMs working in echelon.
The decision makes little difference to Crossrail’s C340 contract. Its principal contracting team of Vinci Construction and Taylor Woodrow aims to have the 175m long secant piled box ready to receive either one or both TBMs before the end of January next year.
The idea is to hand the 60m of box closest to the bored tunnel to the adjacent tunnel boring contractor first. Vinci and Taylor Woodrow will then continue construction of the remainder of the transition structure, its floor slab, walls and roof (which extends to 105m from the portal), before returning to finish the first 60m, complete with roof slab, once the last TBM has been lifted out.
Bachy Soletanche, as Taylor Woodrow’s piling subcontractor, is constructing the secant piled walls of the receptor box, as well as temporary and permanent piling for the U-trough section of the transition structure.
The total pile count is 730, including 348 continuous flight auger (CFA) tension piles. These are 450mm in diameter and installed to 19m deep on a grid pattern to resist the buoyancy of the U-trough’s base slab as it rises to ground level beyond the extent of the secant walls. These are impressive structures, with 1,200mm diameter females and 1,180mm males.
“The logistics are the biggest challenge – we have smaller sites, but none with so many big rigs and cranes”
Alistair Briffett, Bachy Soletanche
Bachy has five big piling rigs on the project – four rotary and one CFA – as well as two heavy duty telescopic crawler cranes and ancillary equipment; and all of this is on a site just 18m wide, hemmed in by a road on one side and the tracks of the Docklands Light Railway (DLR) on the other.
“The logistics are the biggest challenge. We have smaller sites but none with so many big rigs and cranes in use. On such a narrow site we have to be careful not to get the rigs working on top of each other,” says Bachy Soletanche project manager Alistair Briffett.
Bachy is helped somewhat by the length of the site. It has allowed enough room for storage of reinforcement cages and spoil before it is transported from the site, but the phasing of the work has been made all the more tricky by the demands of the project’s ground monitoring procedures.
These are being carried out by Soldata, which has an array of monitoring systems on site including inclinometers installed to check for lateral movement (see box). Vinci has obligations to Crossrail’s neighbouring stakeholders including Newham Borough Council, Thames Water and the DLR to check the integrity of the secant piled box once excavation begins in earnest.
Only when enough baseline data is established can sections of the site be cleared for piling. “Areas we could work in have tended not to follow in a linear sequence and the monitoring requirements have held us up at times, but we’ve remained on programme,” Briffett says.
Key to it has been keeping well ahead with construction of the quicker CFA primary piles, Briffett adds. This has maintained a clear path for flexibility of programming the movements of the four rotary rigs installing the reinforced secondary piles.
The secant side walls of the receptor box are a hard/firm design, with C32/40 males and C16/20 females. The former are installed to a maximum depth of 19.8m, the latter to a minimum of 16m.
“Ideally we would have kept the two operations closer together to prevent the concrete of the females from hardening as much before following up with the males, but we had to take a risk on that because of the necessity of always having plenty of areas of work ready for the rotary rigs,” says Briffett.
Complicating matters further is the high water table on site – 1.8m below ground level due to the close proximity of the Victoria Dock and River Thames. Good London Clay occurs at a depth of 13m and above it are layers of silts and gravels and 6m of made ground.
“It has been a challenge to keep the piles dry,” Briffett says.
“The water pressure is high enough to force water down the annulus between the pile casing and the surrounding clay. We’ve been placing concrete by tremmie pipe to get round that problem.”
The piles of the two end walls of the receptor box differ significantly from those of its long sides.
The ends are obviously a lot shorter than its sides – the box narrows from 16m wide where the TBMs will break through, down to 12m wide at its eastern end. The end piles are of the same principal dimensions and depths, but the eastern wall is temporary and has a soft/soft design, to be demolished once Taylor Woodrow has excavated for construction of the eastern half of the transition structure.
At the western end, the TBMs will break through a “soft eye” secant wall of piles containing glass fibre reinforcement.
Bachy was progressing with the very last phase of primary pile construction when GE visited the C340 contract early in July. Despite the challenges of the site, progress has been maintained at five primary piles and three secondaries per day since piling work started in April this year, according to Briffett. As GE went to press, Bachy was closing in on completion of its work just ahead of its 25 July target.
The piling contractor has also driven two rows of sheet piles into the London Clay over a distance of 100m beyond the secant walls to make ready for the excavation of Taylor Woodrow’s U-trough half of the transition structure.
Taylor Woodrow is due to start excavating the receptor box and U-trough and installing props for both this month.
Soldata’s monitoring work has involved installing three shallow datums and four extensometers to monitor Thames Water’s Victoria Dock sewer.
There are also four manual inclinometers and one automatic inclinometer to monitor ground movements between the site and Docklands Light Railway tracks, plus six piezometers to monitor the water level around the secant pile walls. All were installed in boreholes drilled by Soil Engineering.
Soldata is also providing full time data management on the project. Automatic alerts are set up within the system to notify the construction team when trigger levels are breached. All data is also uploaded to the Crossrail monitoring database. Manual surveys are being carried out on precise levelling points and prisms monitoring surrounding roads, footpaths, third party properties and electricity pylons.
Noise and vibration monitoring is being carried out to satisfy Section 61 conditions including assistance in the reporting of environmental monitoring data to the local authority.