A new sustainable piling technique is being trialled on a Crossrail job in west London with the hope that wider use will ensure work keeps to its challenging schedule.
Project: Paddington New Yard
Main contractor: Costain
Piling subcontractor: Bachy
Start date: June 2014
Completion date: December 2015
Tightly bound by a mainline railway, the Grand Union Canal and the elevated A40 Westway dual carriageway, Crossrail’s Paddington New Yard project is also constrained by its tight 18-month construction programme.
Located between Westbourne Park tube and Paddington National Rail stations, the site contains both the Royal Oak (tunnel) portal and Westbourne Park Crossrail worksites.
It is where Crossrail’s track leaves the overground western section of the route from Reading and dives underground into its central section tunnels (see box).
Before construction for the route commenced, the Paddington New Yard site was occupied by studios, a rail-fed concrete batching plant and an area for parking buses.
In 2009, the site was cleared for Crossrail to store tunnel section segments and allow tunnel arising to be transported away.
Supporting the new service
Contract C336 involves reinstating the batching plant and bus parking area (see boxes) and installing infrastructure to support the new Crossrail service.
A freight rail connection for the batching plant will also be reinstated, although this will not come online until Crossrail has been commissioned.
The studios have been permanently relocated nearby. Other infrastructure being reinstated includes a sewer, which is currently being over-pumped, a Network Rail safe access area and a wall adjacent to the new tracks, which will protect the bus deck structure should a train derail.
“Using smaller diameter piles reduces the number of concrete and muck-away lorries required to service the job, which is better for the environment and safer”
Simon Pledger, Crossrail
Considerable mechanical and electrical work is required to support the new railways, with landscaping and extra piles to support the canal embankment.
With work in the tunnels and stations continuing until early 2015, handover of the entire site to main contractor Costain will be in phases.
Costain and piling contractor Bachy have been preparing the ground and levels for piling to begin since April this year.
Sandwiched between the activities of fitting out Crossrail’s central tunnels and the route being tested and commissioned, contract C336 has a lot to fit in within just 18 months.
Predictably, the programme is being challenged by inclement weather – mostly heavy rain – as well as underground obstructions and archaeology.
Reinstating assets is a challenge in itself, as Crossrail has to meet the expectations of respective third parties: Lafarge Tarmac (the batching plant owner), Tower Transit (the bus operator), Network Rail and DB Schenker (freight rail operator).
Unconventional piling boosts safety and sustainability
When Construction News visited the site at the end of May, piling subcontractor Bachy was installing 900 mm-diameter, 14.8 m-deep conventionally augured concrete piles for the ‘derailment’ wall. Site workers were also performing a load test for a threaded pile.
A pile with a threaded shaft has increased skin friction compared with an unthreaded shaft and, therefore, increased pile capacity, explains Crossrail project manager Simon Pledger.
“But we need to understand the ground conditions and ensure that the threaded piles are suitable here,” he says.
“If they perform as we want, we’ll use them to replace [many of the] conventional piles because the threaded piles are of a smaller diameter.”
Using smaller diameter piles means less spoil is produced and less concrete is required to build the pile. “This reduces the number of concrete and muck-away lorries required to service the job, which is better for the environment and safer,” he adds.
Crossrail construction facts
- Crossrail is Europe’s largest construction project – work started in May 2009 and there are currently more than 40 construction sites.
- The service will transform rail transport in London, increasing capacity by 10 per cent, supporting regeneration and cutting journey times across the city.
- The route will run over 100 km from Reading and Heathrow in the west through new tunnels under central London to Shenfield and Abbey Wood in the east.
- The first Crossrail services through central London will start in late 2018 – an estimated 200m annual passengers will use Crossrail.
- The total funding envelope available to deliver Crossrail is £14.8bn.
- Tunnelling is now more than 80 per cent complete.
- The western tunnelling machines Phyllis and Ada have completed their journeys, building 6.8 km of tunnel each between Royal Oak to Farringdon.
Crossrail is expected to have results to determine whether the technique is suitable on clay soil and whether expected pile capacities have been achieved later this month.
If rolled out across the site, installation of these piles will involve attaching a special tool to the conventional pile-drilling head to cut into a stable, dry and unlined bore.
Cuttings are collected in a separate bucket and the quality of the bore is checked using closed circuit television.
Revised pile design speeds installation
The revised pile design will enable quicker installation of foundations and allow follow-on works to commence sooner and for the project to keep to its tight timetable.
“The site is a former railway yard which housed engine design workshops from the time of Brunel. This is one of our challenges. So far, we’ve found engine turntables and rails and possibly an old well”
Simon Pledger, Crossrail
The pile design could recover time lost due to unrecorded buried structures being found and having to be removed.
“The site is a former railway yard which housed engine design workshops from the time of [Isambard Kingdom] Brunel. This is one of our challenges,” Mr Pledger admits. “So far, we’ve found engine turntables and rails and possibly an old well.”
Two of three brick cylindrical structures have been removed, but the third lies under a haul road on site so has to be programmed for removal when the access route is no longer needed.
The site is currently a hive of activity and, for Costain, keeping to programme requires careful planning to ensure machinery keeps within safe working limits set by the Highways Agency for the A40 Westway and Network Rail near its track.
“The main challenges are the constraints imposed by how much is happening on site when all the while you’re next to a live railway and road and on a project with just an 18-month programme,” Capita senior engineer Shelley Galvin explains.
“The time taken for all working methods and operations to be approved by relevant stakeholders also has to be factored into the programme,” she adds.
The contract is expected to be complete in December 2015, three years ahead of Crossrail’s service commencing.
Bus deck gets off the ground
“Our scope includes building an elevated deck for the buses,” Crossrail project manager Simon Pledger says. “Because of where the new railway needs to be, the deck cannot sit on the ground.”
Designed by consultant Capita, the concrete deck will be 180 m long and 40 m wide and will connect to Westbourne Park bus garage.
Buses will refuel, wash and park on the elevated deck, which spans over an embankment supporting the Grand Union Canal, Alfred Road, the freight rail track (used to deliver material to the new batching plant) as well as an access road.
The deck structure comprises an in-situ concrete beam and ribbed slab deck, with a precast concrete perimeter parapet.
The majority of the deck is supported on a 12.5 m by 12.5 m grid of columns. Circular concrete columns which are typically 900 mm diameter and 6.5 m tall will support the deck.
Concrete batching plant
Its future owner’s designer will design the batching plant superstructure, but contract C336 requires Crossrail to install piles for a new below-ground structure called the Bottom Discharge Unit.
A steel hopper in the BDU will receive aggregate by rail and deliver it to the bottom of an inclined conveyor, which transfers the aggregate to the concrete batcher.
The BDU has a maximum excavation depth of 7 m and is L-shaped in plan with a length of 34 m and a width of 9 m, increasing to 16 m under the discharge location.
It is currently being constructed using a hard/firm secant pile perimeter retaining wall.