Pavement construction for Manchester Airport's second runway starts this week - with the concrete surfacing being laid twice as fast as originally planned. The last-minute arrival of a second £750,000 paver to speed the operation is just the latest example of accelerated construction in a reprogrammed project desperate to save time.
Wet weather delayed the crucial 2.4Mm3 earthworks programme by up to seven months. The vast muckshifting fleet should have finished its task last October but the last of the scrapers is only now leaving site.
As the contractual completion date which was scheduled for next January began to look more like the following autumn, joint venture contractor Amec-Tarmac began totting up its punitive late completion penalty of £250,000 a week. But then came the compromise.
Cut the delay to four months and promise us a completed runway by next May, said client Manchester Airport. In return we will waive the £4M worth of late completion liquidated damages.
A new agreement was signed last month and the JV's acceleration plans are already in full swing. Extensive use of lime has helped hydrate the wet clay needed to infill the 30m deep River Bollin valley so that the runway can be routed over the top. Observational engineering, optimising the best position for drainage layers, has speeded settlement of the infill and many of the follow-on runway commissioning tasks, such as lighting and services installation, are being brought forward to overlap with the main construction.
'Operations originally planned as sequential will now run in
parallel with current work and we are confident of meeting the revised completion date,' asserts joint venture project director Alan Roberts.
Overcoming Manchester's notoriously wet climate has proved the toughest of a string of challenges faced by the £108M high-profile design and build runway contract.
A five-month long and very public battle with invading protesters at the start of the contract was immediately followed by a state of the art environmental operation to remove everything else in the way of the new 3km runway (see box).
The region's wet autumn, winter and spring was good news for those responsible for planting trees and other vegetation - and for filling new ponds - but not for earthworks subcontractor VHE. A roughly balanced cut and fill operation centred on infilling the 800m wide Bollin valley with 2Mm3 of clay.
A 270m length of river was culverted through a semi-circular 18m high concrete tunnel, cast insitu in 14 sections. Around and above it, the clay was placed in 300mm layers with a thin aggregate drainage blanket installed every 2.5m to aid settlement.
After a full muckshifting season last year, October marked not the programmed completion, but only the 60% stage. Limited earthworks over the winter marginally eased the backlog.
Lime spread over the clay and an observational engineering exercise to determine the best thickness and positioning of drainage layers has significantly increased settlement rates in the new 30m high embankment.
The fill, heavily instrumented and overlying an alluvium river bed, is predicted to settle by 800mm. And the contractor must remove 90 per cent of it before handing over the completed runway.
By observing settlement rates as the soil was positioned, the contractor could fine-tune clay and drainage layer thicknesses
to ensure maximum consolidation.
'We would normally need at least a year to remove this amount of settlement but here we have met our target in half that time and two months faster than programmed,' claims project manager Chris Jackson.
This time saving helped mitigate the overall delay, as did reprogramming the £6M earthworks so that runway pavement and drainage areas could be released early. The 400mm thick concrete pavement, and its 150mm lean mix base, are both being slipformed, with expansion joints saw-cut later at 3m intervals.
Two site batchers keep the project's total concreting requirement of 2000m3/ day an in-house operation. Nearby, a vast derelict aircraft hangar, once home to Lancaster bombers and - according to rumours - Churchill's private plane, is now piled high with aggregate for both concrete and drainage.
A £2M purpose-built 2km rail siding channels two trains a day, and their cargo of 3200t of aggregate, direct to the hangar. Though the contractor had no other transport option, as aggregate trains replacing at least 186,000 lorry journeys were part of the environmental brief, construction manager Sam Robinson reckons rail deliveries are a far less stressful solution.
'It would have been cheaper by road but, with a lorry arriving on average every minute, that solution would have been a logistical headache,' he says. 'This way it just happens, and the aggregate is always available.'
Also 'just happening' is the continual arrival and departure of aircraft on the live runway, only metres from the site. This provides Robinson and his team with some unusual restraints.
Crane movements must be logged daily with the airport control tower as their jibs show up on radar. Ditches cannot be excavated in lengths greater than 50m to avoid the remote scenario of a plane skidding into one, and spoil heaps need covering to avoid attracting birds.