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Engineering Newcastle's triple-decker

2017 01 18 a19 a1058 aerial progress (03) 039

From the top of the A19/A1058 Coast Road roundabout near Newcastle, streams of gridlock can be seen stretching as far as the eye can see. But not for long.

Today, congestion in this area of the North East is severe during weekday rush hours but also bad at weekends when motorists stream to the retail park close to the junction.

Daily around 30,000 vehicles travel from east to west along the A1058 Coast Road, while 20,000 use the A19, which runs from north to south. Accident rates around the intersection have trended higher than the national average. Local authorities have been crying foul for years, as the constant traffic jams cripple the Tyneside economy.

But that is soon to change as the intersection is receiving a major improvement.

At present A19 traffic travels around the roundabout, while the A1058 carries vehicles above it on bridges which cross the roadway on each side and on an embankment across the central section.

The A19 will be lowered and put in a cutting, which will run beneath the roundabout. This will require new slip roads to allow access between the roundabout to the lowered A19.

In addition, a new single span structure will carry the A1058 over the cutting in the centre of the roundabout. Two more single span structures will carry the roundabout across the A19 cutting.

A19 orange cropped

A19 orange cropped

This project is not a “first” by any means – another three level interchange is currently being built in Glasgow. But the scheme provides an interesting construction and logistical challenge nonetheless.

Three level solution

Lead designer WSP Parsons Brinckerhoff tried out plenty of options in early 2012, with the preferred ones all involving a three-level solution.

“There were three options discussed to try and achieve the three-level objective: a tunnel option; a flyover where the A19 went over the coast road, and a cut excavation option which we went with in July 2012,” says WSP project manager Ian Smith.

A number of options for crossing the embankment were discussed, before the most efficient design was arrived at.

 “There were efficiencies made here at the start of the design phase. Starting estimates of about £137M fell by about 40%, due to various factors,” says Smith. “The most notable being that, when we started the design process, we anticipated the provision of a freestanding multi-span bridge, which meant the demolition of existing bridges. But what we’ve done is we’re putting in smaller 25m to 30m single span bridges.”

2017 01 18 a19 a1058 aerial progress (03) cropped 063

2017 01 18 a19 a1058 aerial progress (03) cropped 063

The site is very cramped, limiting working areas

In the final design, the two bridges taking the roundabout across the lowered A19 are formed from 19 precast Y-beams. The bridge carrying the A1058 across the cutting will be formed from 22 precast Y-beams. Decks will be cast insitu.

A Sisk Lagan joint venture is the contractor for the £75M project. It is hoped the project will ignite economic growth in the North East, and it is visibly billed as part of the government’s Northern Powerhouse initiative.

“As a local myself, I recognise Newcastle and the North East are the very extreme of the Northern Powerhouse,” says Sisk Lagan project manager Stephen Marshall.

“There’s quite a large investment in that M62 corridor near Manchester and there’s always that thought that the investment in infrastructure won’t extend this far. But Highways England appears quite committed to the A19 and the A1, so it seems the North East is going to benefit.”

This is a major improvement and it’s not envisaged it will be modified

Ian Smith, WSP

The lowered A19 will draw 50% of existing traffic away from the roundabout according to WSP Brinckerhoff. It is expected that the extra capacity created will last for the next few decades at least.

“There have been many minor improvements to the junction over the past decade, but this is a major improvement and it’s not envisaged it will be modified, certainly not until the mid-2030s,” says Smith.

First on site has been geotechnical specialist Bauer which installed 120 of 583 piles. These will act as retaining walls for the lowered A19. The largest piles, up to 1500mm in diameter and incorporating two layers of steel cages, will also act as structural supports for the roundabout bridges and A1058 bridge across the cutting.

“In the centre, the piles have a dual function, as a retaining wall for the dive-under, and at the same time they act as bearing piles for the cross-bridges of the roundabout, the middle level,” says Bauer project manager Gustav Jahnert.

Pile testing

One pile per structure is being tested using the O Cell method, which involves using a hydraulic cell to test the pile’s strength. The sacrificial loading device is embedded within the base of the pile. Then as the load is applied to the O-cell it expands to test how far the pile can be loosened.

“You concrete the pile as normal and after the pile has hardened… this hydraulic cell is activated and expands and tries to push the pile out of the ground,” says Jahnert.

“At first a conventional pile load test was specified, but we recommended to the client to use the O Cell method, due to the constrained nature of the site. We’re working in the middle of a live roundabout, so we’ve only got a little island where we can work,” says Jahnert.

“We’ve tested one, and the pile moved less than 1mm. So basically it didn’t move at all.”

Video pile inspection

In another high-tech move, one engineer on site came up with the bright idea of strapping a torch and a video camera to some plant for an inspection of the pile shafts.

“And it’s looking nice and clean,” says Jahnert.

The ground comprises a few metres of made ground, then some glacial till and finally sandstone 11m below. There were also occasional boulders, the biggest of which was “about the size of a bathtub,” says Jahnert. The boulders forced some changes to piling methods, but there was no alteration to the timetable.

These bored cast insitu contiguous pile walls, up to 30m deep, which vary from 600mm diameter to 1500mm diameter, form the retaining wall within which the lowered A19  can be excavated.

1km cutting

In total the cutting is about 1km long. Depending on where you take as datum, the depth of the roadway to the roundabout above will be between 5m and 7m, rising to 15m in the central section.

A capping beam and canopy structure and bearings are to be installed prior to the three bridges’ installation. Each bridge is formed of precast Y-beams, with an integral deck on top.

In total, about 80,000m³ of material is to be removed.

Logistically it is very challenging, the working spaces are tight

Gustav Jahnert, Bauer

Currently on site, sheet pile walls are going up to prepare for the sliproads work, and excavation has started for the north and south roundabout bridges.

Keeping traffic flowing at all times throughout the project will be a tough logistics challenge.

“Logistically it is very challenging. The working spaces are tight. We are using in our planning, building information modelling to look at opportunities and interfaces with our plant, our people and make sure we maximise our space on the site,” says Marshall.

Thankfully bringing materials to site is not too difficult, with the local batching plant close enough to be visible. But Bauer’s Jahnert says there were still concerns on possible delays to site. “One big question when we started was: are our lorries going to be affected by the [motorists] traffic on the A19? We spoke to the drivers… and they’re now probably taking every possible side road in the area.”

60% local workers

A maximum of 200 workers is expected to be on site at peak, compared to about 150 at present. Contractor Sisk Lagan claims about 60% are local workers and the aim is to keep the economic benefits local.

Highways England has been pushing a collaborative working environment, which Bauer’s Jahnert says has provided genuine benefits in the early piling phase.

“This is reflected in a geotechnical design point of view with WSP Parsons Brinckerhoff as the designers having two full-time designers in the site office at all times, one structural and one geotechnical. So as soon as anything unforeseen comes up on site it’s very easy to go and speak to them.

“You get problems resolved quickly. Whereas on other jobs, they’re offsite, you might have to form a queue, and a lot of time is lost to bureaucracy.”

Work began in June 2016. All three levels of the intersection are expected to open to traffic in March 2019.

 

 

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