First UK use of a reinforced soil retaining wall is helping one road scheme in Essex meet the client’s architectural and cost demands. Claire Symes reports
With 8,000 vehicles an hour trying to negotiate the “magic roundabout” at Sadlers Farm in Essex, it is no wonder that the junction between the busy A13 and A130 was a congestion hotspot.
But work is underway to change that with £63M being spent on a new link to bypass the roundabout and add capacity to clear the way for further development in the area.
Slotting the new road and building new bridges into the land available without closing the junction meant that main contractor Birse had to use some innovative ground engineering solutions.
These have also helped meet the architectural ambitions of the scheme’s client, Essex County Council.
“The main aim of the scheme is to remove a large volume of traffic from the A13/A130 roundabout with a new 1km long strategic link, remove the mini-roundabouts from the existing roundabout and add traffic signals,” explains Birse site agent Rudy Pilley. “The work also involves widening some sections of the A13 and building four new bridges.”
Although the upgrade only measures 4.5km from end to end, the work has involved first UK use of a soil reinforced wall system - MacRes - soil nailing and contiguous piled walls and piling for the bridge abutments.
The soil nailing and piling work on the scheme is being undertaken by Balfour Beatty Ground Engineering to a design by Mouchel.
But Birse turned to Maccaferri when it came to the soil reinforcement.
“This is the first time the MacRes system has been used in the UK, although it was developed in Italy a few years ago and has been used on a number of European projects”
“Soil reinforcement is needed to form the wing walls and abutments on three of the bridges,” says Pilley. “The challenge was to carry out the work to meet the architectural demands of the client. The designers looked at a number of options and selected soil reinforcement due to the lower costs and faster construction times.”
The architectural demands meant that the soil reinforcement needed to be carried out with a textured facing panel and is also being extended up higher than conventionally used to screen the bridge beams.
“There is more interaction needed between the structures and the earthworks than normal,” says Pilley. “We selected Maccaferri for the work because of the company’s ability to design, supply and install the soil reinforcement.”
Maccaferri’s work centres on the three semi-integral bridges - Church Road Bridge, Sadlers Farm Bridge and Sadlers Hall Bridge - and an extension of the subway at Rushbottom.
The fourth bridge - London Road Bridge - is a fully integral bridge which spans the strategic link.
The bank seats for the bridges are sited on 600mm diameter rotary bored piles installed into the London Clay by BBGE to a depth of 25m below ground level.
The piles have been installed with sleeves to isolate them from the surcharge effect created by the soil reinforcement.
“We first became involved in the project in May 2011,” says Maccaferri technical manager David Crowther.
“This is the first time the MacRes system has been used in the UK, although it was developed in Italy a few years ago and has been used on a number of European projects.”
Maccaferri site manager Ross Kennedy says that although the technique is established in Europe, this is the most challenging application of the technology so far because of the interaction with the bridge structure.
The challenges started with achieving the required architectural finish on the precast panels.
“ECC wanted the panels to have a blockwork style finish to match the planned blockwork facing that will cover the contiguous piles,” says Crowther. “The standard panel measures 1.5m by 1.5m and is cast in a factory in Belfast using a steel mould with a polymer compound to enable the pattern to be cast into it. The only mould we could find for the pattern was 30mm deep but our moulds are 20mm deep so the moulds had to be adjusted to cope.”
The panels are cast with galvanised steel loops in the back of the panel and a neoprene backing is also fixed to minimise water flowing through the panel.
On site, the other challenge has been aligning the panels so that the blockwork pattern lines up.
One of the main differences between the MacRes system and other reinforced soil panel systems is that it uses polymer reinforced straps - known as Paraweb - rather than steel to fix it into the slope.
“Rather than increasing or reducing the spacing to change the strength as you would do with steel, we can use different strength polymer straps to increase the capacity,” says Crowther.
“We are currently trying to get a British Board of Agrément certificate for the new version of the system,”
At Sadlers Farm the strap strength varies from 27kN to 45kN.
Crowther says the company is currently working on a system to replace the steel connection at the back of the panel with a polymer reinforced strap.
“We are currently trying to get a British Board of Agrément certificate for the new version of the system,” he says.
Before Maccaferri started work at each site Birse excavated the area to foundation level and installed a 175mm thick, 250mm wide concrete levelling pad along the line of the panel wall.
“Once the pad foundation was in place, we surveyed each site to ensure all the alignments were correct,” explains Kennedy.
“The first row of panels was placed in position and propped with acro props, which remain in place until the soil is placed behind the panels. The panels are also clamped together. Work on installing the straps then started.”
The straps are attached to the panel at 750mm centres with four attaching points on each one.
The strap is installed as a continuous loop attaching to the panel and into the bank with lengths varying from 3.5m to 7.2m and fixed into position with a pin to remove any slack.
The straps are supplied in 100m lengths and linked together with a buckling system.
A 300mm deep drainage layer is placed directly behind the panels before the backfilling gets underway.
The embankment is being formed in 150mm layers of granular 6I/6J material imported from another site in Essex and compacted using 12 passes of a Bomag 120 roller.
Immediately behind the panels, the granular fill is compacted using a hand-held wacker plate to prevent damage to the panels.
After the first layer is in position, the next layer of panels are placed using a specially designed beam attached to an excavator boom and loops at the top of the panel.
Work is currently focused on the Church Road Bridge north abutment.
All the other abutments are largely completed but cannot be finished until Birse completes the bank seat construction.
Maccaferri will also have to return to the site to complete the wall at the Church Road site as the new bridge is being built alongside the existing one.
“A sheet pile wall and soil nails have been installed to protect the existing bridge from the work on the new structure,” says Kennedy.
“We are currently working 24/7 on this structure to enable Birse to lift the beams into position during a four-day possession in late December. Once the new bridge is completed, the old one will be demolished and the sheet pile wall will be removed to clear the area for the soil reinforcement to be completed.”
Maccaferri will finish the top of the walls with a 740mm deep precast coping panel that will be secured using cast insitu fixing.
Pilley is pleased with the work and is confident not only that Maccaferri’s element of the scheme will be completed on time but that the overall project will meet its May 2012 completion deadline.