While many Crossrail projects are moving away from the ground engineering phase, there is still a major piling contract to be delivered at Pudding Mill Lane.
At just 1km long and 250m at its widest point and without the use of a tunnel boring machine you’d be forgiven for thinking that the work being delivered on Crossrail’s C350 contract is a small task. In fact the work at Pudding Mill Lane is anything but and involves interfacing with both Network Rail and the Docklands Light Railway, as well as tunnelling work on the adjacent C305 contract and some major utilities.
Work on the second phase of piling by Bachy Soletanche for main contractor Morgan Sindall has just got underway and is making use of the UK’s largest piling rig to ensure the work goes to plan.
“This contract will take the Crossrail rail line from the portal up to join alongside the GNER railway line as it goes into Stratford,” explains Crossrail construction manager Sam Dring.
Piling work for the scheme started in 2011 but could not be delivered in one go as the Pudding Mill Lane DLR station lay right on the alignment so a new station has been built 40m to the south on an earth embankment and a six-span viaduct supported on piles installed in the first stage.
“The new station was opened in April and the old station has now been demolished to make way for this stage of piling but that means that we now have an island site with a live railway to the north and the DLR station to the south,” says Dring.
As well as coping with working close to the DLR and Network Rail, there are a number of other constraints on the site that have had an impact on how the project is being undertaken. The site is close to two rivers – the River Lea and the City Mill River – and the route also crosses the Northern Outflow Sewer and a 20in water main that was the only supply into the Olympic site to the north of the rail lines. Elsewhere the new alignment crosses a 4,000V power cable that is the main supply for London, an EDF cable tunnel and the North London Flood relief sewer.
If this wasn’t enough to contend with, Network Rail is also currently upgrading its overhead gantries which currently encroach on Bachy’s work area and the piling work needs to be planned around Network Rail’s work.
The first stage of work focused on building the foundations for the new station building and work involved extending a diaphragm wall built by Cementation Skanska under C248 at the western end of the site with a further 95m of diaphragm wall panels. The 18m to 20m deep wall will form the supports for the cut and cover tunnel which will take Crossrail trains from the bored tunnel portal.
Bachy also installed 800, 900mm and 750mm diameter continuous flight auger (CFA) piles to depths of up to 30.5m for the station structure.
The phase of work that is now underway centres on the ground engineering for the cut and cover tunnel and four span bridge to take Crossrail over other infrastructure and up to the same grade the main line railway.
The main challenge is construction of a 230m long cased secant piled (CSP) wall that will follow on from the end of the diaphragm wall and create a retaining wall between the emerging Crossrail line and the adjacent Network Rail lines. The wall is a hard hard design and formed from 280, 1,050mm diameter piles to depths of 20.4m and installed at 815mm centres to give a bite of 230mm into the female piles.
Delivering this work is the largest rig in the UK at the moment – a Bauer BG46 – which has been hired in for the work. It is one of only four in Europe and 15 worldwide.
“We chose the rig for this job because we were constructing a hard hard wall and we wanted the high torque the rig delivers to drive the casing,” says Bachy Soletanche project manager Alistair Briffett. “This is the first time we have driven CSP piles of this diameter to this depth. The piles are also cased to full depth, which adds to the demands of the work.
“Our Soilmec rigs can go deeper but we would need two rigs to deliver the work here and there is no room for that.”
The need to deliver 1 in 200 verticality also drove the decision to use a machine that can case to the full depth as it gave Bachy greater certainty that this tolerance could be achieved.
Even once the pile is bored the work is not simple as the cages, each weighing 3.7t, will go to full depth. “We have worked with London Concrete to develop a flow mix using plasticisers to prevent segregation rather than using a conventional slump mix,” explains Briffett.
Ground conditions are not straightforward either with up to 8m of made ground with possible obstructions overlying weak London Clay and gravels. The piles Bachy is installing will toe into the Thanet Sands at 20m below ground level.
“There is a calcrete layer in the top of the Thanet Sands, which also meant the torque of the BG46 was necessary,” adds Briffett. “The Thanet deposits also have the potential to lock up augers so we can’t sit on any position and phasing has to be carefully planned.”
To deliver the CFA element of the second phase, Bachy has Soilmec SF 120 and CM120 rigs on site, as well as a BG24 set up for coring work as needed. There are 95 CFA piles in the second phase with the 750mm diameter piles driven to 25m and the 1,050mm piles to 30.5m.
“We are carefully planning the work to try to avoid the need to swap augers,” says Briffett.
On average there are 16 piles being installed per pile cap for the new four-span viaduct, but some of the more complicated pile caps have 24 piles.
Work on site got underway in July and Briffett hopes to have completed the work by October this year but adds that this is dependent on the interface with other contractors. “Using the same project team as the first phase has helped smooth the start of work on site and we have regular meetings with Morgan Sindall to plan ahead,” he says.