A range of geotechnical techniques have come into play as contractors improve links from an expanding Northamptonshire town to the roads network.
If you were asked to design a project to give young civil engineers the opportunity to experience as many aspects of the job as possible, you probably could not imagine anything as perfect as Stanton Cross in Northamptonshire, where Galliford Try is building the infrastructure for a new housing development.
Like a cartoon island in a children’s story book, the site has absolutely everything in a relatively small space: river training works, bridges over a river and a railway, ground improvement and stabilisation, piling, culverts and road construction.
“There aren’t many jobs for young graduates with this number of disciplines,” says Galliford Try project director Adam McAllister. “It’s got everything: earthworks, structures, culverts. It’s near enough the perfect civils job for a young graduate.”
The site is on farmland east of Wellingborough, which is being developed in a scheme promoted by Bovis Homes to create a massive mixed use extension to the town.
It will eventually include 3,650 new homes, shops, leisure and commercial space plus community facilities, a 58ha country park, three schools and a doctor’s surgery.
The £900M development is the largest growth the town has undergone since the 1960s, and is set to take around 10 years to complete. Key to the first homes being built is the construction of two new roads linking the site to the existing road network and the rest of the town.
At the moment, the Midland Mainline railway acts as the eastern boundary to the town, and Stanton Cross is on the other side of the tracks, so the 9km of new roads are essential for opening the site up.
The first new road, known as Route 4, ties into the town’s road network outside the railway station, runs north along the west side of the tracks, then heads east to join the Irthlingborough Road. It crosses the railway line and the River Ise, and much of it is on the floodplain of the River Nene.
The second new road, Route 9, runs north to south between the railway line and the River Ise until it meets the Finedon Road.
Galliford Try has a £24M design and construct contract for the new roads and bridges, and arrived on site in October 2015 to start the enabling works. At that stage the scheme’s main designer WSP Parsons Brinckerhoff was still taking the detailed design through the planning process, but the contractor was able to start work on essential flood alleviation work, as well as building a new car park at the station.
Route 4 crosses the flood plain on an embankment up to 6m high. Most of the land to the north of this embankment will become part of the development’s country park, but flood alleviation measures are still needed to ensure the scheme does not increase the likelihood or severity of floods in the area.
Galliford Try has excavated a series of ponds that will store water in the event of heavy rain, with the excavated material being stockpiled for the embankment construction. And the main construction works include diverting the River Ise to slow it down before it passes under the new Route 4 road bridge.
When the contract began in earnest in June 2016, the river diversion works and the rail bridge were the two main items on the critical path, as well as getting all the third party consents.
The rail bridge team had a fixed point to aim for: a 54 hour possession of the railway in February 2017 during which the beams had to be installed.
The rail bridge was the subject of an early value engineering exercise. It was originally designed to have a deck structure that spanned the railway perpendicular to the tracks, with the road built on a curve on top of the deck. “The early bridge structure had a huge footprint, so we made it curved,” explains McAllister.
This decision made the bridge design and construction more complex, as the steel deck beams are curved – with a 112m radius – and canted. But it greatly reduced the amount of material in the structure.
Foundations for the rail bridge abutments consist of a total of 102, 23m long continuous flight auger (CFA) piles installed into the underlying ironstone and grey marl. They are topped with insitu concrete pile caps and base, all constructed adjacent to the operational railway.
“We did the majority of that work in a high street environment behind a fence,” explains McAllister.
“We had to put track monitoring in throughout the piling, but we did all the piling without possessions up to the point of the main wall shuttering going in.”
The front face shutters for the 8m high abutment walls went in under possession, but the walls were poured with the live railway operating alongside. “The key was to get in and out to hit the delivery date for the beams,” says McAllister.
The bridge’s composite deck consists of six 41m long steel I-beams topped with in situ concrete. The beams – weighing up to 40t each – were fabricated by Nottingham-based Briton Fabricators and delivered to site along the Finedon road to sit alongside the east side of the railway line. Here they were fixed into pairs and the external beams were fitted with RMD Kwikform’s Paraslim parapet formwork system before being lifted into position using a 1100t crane supplied by Baldwins.
A 100t crane was then used to fix the permanent formwork in the form of EMJ Permadec panels between the beams.
Galliford Try handed the railway line back to Network Rail just 23 hours into the 54-hour possession, and has since poured the deck concrete, taking the total volume of concrete in the rail bridge to 1,520m3. The 26m long bridge over the river Ise is a more conventional structure, with piled foundations, insitu concrete base and abutment walls, and precast concrete deck beams although a planning requirement means the bridge will have arched facia.
Again, it is built on CFA piles. Piling subcontractor Van Elle installed around 70 piles to a depth of 18m. Van Elle is also installing all the sheet piles for the river diversion works and sheet piled walls around the base of the river bridge. This bridge is effectively being built off line, while the river is put through a temporary diversion.
In addition to the two bridges, construction of Route 4 also includes installing two culverts to take water from two existing swales from the new flood ponds to the River Nene. One is a double culvert and the other a triple culvert, and both are constructed from 2.7m square precast sections placed on a concrete blinding bed, together with concrete wing walls and aprons.
While Route 9 has a balance of cut and fill, Route 4 includes a substantial embankment to take it over the flood plain and to line up with the higher ground at the eastern side of the site. This is being constructed using material excavated from the flood alleviation ponds, which has been stockpiled, and will be lime stabilised before being placed in the embankment.
Galliford Try has undertaken some ground improvement beneath the embankment structure in the form of stone columns and controlled modulus columns, installed by Vibro Menard. There is also a stone fill load transfer platform beneath the embankment where it runs through the flood plain.
“The embankment doesn’t need pre-loading for settlement, but we have put in settlement markers where there are no stone columns,” says McAllister. “The ground improvements and stabilisation are something we developed to minimise the settlement periods.”
With work going on in so many areas at once, the project is reliant on an effective haul road infrastructure, he adds: “The job is severed in two places – one by the railway and one by the river, so the infrastructure of haul roads is vital. We have three main access points, and there is a logistics plan in place that determines the point of delivery, which haul road to use etc. We’ve built a substantial temporary road infrastructure to build the permanent works.”
Galliford Try has just started work on Route 9, and is continuing with the work on Route 4 and the river diversion. It is scheduled to complete the work early next year.