Contractors have overcome challenging logistics and difficult ground conditions to build an 100m long embankment underpass on the Heads of the Valleys road South Wales.
The A465 Heads of the Valleys Road in south Wales is a vital part of the Trans-European Network link between Ireland and the Midlands. However, in places it is just single carriageway, often congested, as lorries struggle up the steep valley sides. As a result, work to widen a 40km stretch between Abergavenny to the east and Hirwaun, west of Merthyr Tydfil, has been ongoing since the 1990s in an effort to improve journey times and drive regeneration in the area.
Construction of the latest phase of the project, Section 3 Brynmawr to Tredegar, began at the end of 2012 and is being built by Carillion in partnership with design consultant Arup and environmental consultant TACP on behalf of the Welsh Assembly Government.
Much of the project involves widening the road from single to dual carriageway but at the village of Rassau, on the northern edge of Ebbw Vale, a completely new stretch of road is being built.
Here, the route runs between the northern edge of the Rassau housing estate and the Carno Reservoir, crossing the deep narrow valley of the River Ebbw on an embankment, in the shadow of the 27m high Lower Carno Dam. The project includes construction of an underpass through the embankment for a Welsh Water access road, a footpath and the dam spillway, which will be supported on piles installed by Cementation Skanska.
“The underpass will be an 18m wide, 9m high and 100m long concrete arch structure, sitting on two rows of 75 rotary bored piles either side of the River Ebbw,” explains Cementation Skanska senior project manager Jonathan Manning. Piling was due to finish as GE went to press after just over two months on site.
The first challenge for Cementation was getting plant and materials to the small working area. Access was restricted, with everything having to come down a steep haul road from the nearby Rassau Industrial Estate. The haul road was built by Carillion after it had cleared trees and vegetation, including Japanese Knotweed, and ensured the local wildlife – birds, bats and lizards – was protected from construction work.
Once on site, piling had to be carefully coordinated, with the two Bauer BG28 rigs and the two BM700 support cranes having to work around one another. The logistics of the job presented a major challenge as Carillion’s excavators and dumper trucks had to move around site, while concrete and steel cage deliveries were arriving all the time.
“Each rig worked on one row of piles, either side of the river,” Manning explains. “This gave us enough space for the teams to work around each other and allowed us to coordinate concrete and steel deliveries, ensuring our target of two piles per rig, per day was achievable.”
With very little storage space on site, materials were typically brought to site “just in time” to minimise rig downtime. Concrete was supplied by local firm Hope Construction Materials, with piling cages supplied by Express Reinforcements coming from Neath and Newcastle.
Logistics aside, another challenge for Cementation was the ground conditions. Site geology comprised granular glacial deposits (mainly cobbles and boulders) overlying mudstone and siltstone. Beneath this was a 2m thick layer of strong to very strong quartzitic sandstone (with a design strength of 50MPa), with weak to moderately strong mudstone and siltstone (with a design strength of 8MPa) below. Groundwater was encountered at the base of the glacial deposits.
However, towards the north-west corner of the site, ground investigations had indicated that the sandstone layer had been scoured out and replaced with the weaker mudstone and siltstone.
The geology did not make for easy drilling, Manning explains: “We used a combination of inverted and progressive augers and coring barrels to get through the glacial deposits and the rock, which was very hard in places.”
The variable ground meant Arup had to estimate a range of pile lengths, specifying rock socket depths and reinforcement, depending on the material encountered during piling. Two foundation options were used. Where the piles were founded in sandstone, 1m deep rock sockets were cored and in the weaker mudstone and siltstone, 3m deep sockets were formed. On the western row, pile lengths ranged from 6m to 21m where the scour feature was at its deepest and between 8.5m and 11m on the eastern row.
All the piles were 1,200mm diameter, with spacing on the western row varying between 1,550mm down to 1,400mm across the scour feature and between 2,960mm and 1,400mm on the eastern row. Piles were installed on a “pile one, miss three” basis.
All the piles were reinforced for their entire length, with three cage types used: Type A, comprising 20, B40 and 16, B25 bars; Type B, comprising 20 B40 bars and Type C, comprising 20, B25 bars. “Type A reinforcement was in effect a double cage, used in the most onerous conditions, where the piles were longest and ground weakest,” Manning explains.
“Cages were supplied in standard lengths which included an allowance for the uncertainty of the pile lengths and we cut them to length on site. Where the piles were deepest on the western row, we had to couple two type As together. On the shorter piles we could splice cages together if the additional allowance was not sufficient”.
Allowable combinations were an A and B cage, a B and a C cage or two C cages. All the piles were completed with C50 grade concrete and buried in the 1.3m thick piling platform. The piles will be connected by a capping beam, built by Carillion.
“The successful completion of the piling is not only down to careful planning, design and execution,” Manning adds. “The project has been extremely challenging but our excellent working relationship with Carillion and Arup gave us a degree of flexibility that ensured the project was carried out with minimal problems.”
The Welsh Government identified the need to improve the A465 Heads of the Valleys Road as part of a regional traffic study in 1990, with the aim of improving journey times and to drive regeneration in the area.
The scheme now supports the Welsh Assembly Government’s Turning Heads Regeneration Strategy, which provides training and employment opportunities to stimulate economic growth.
In 1994, alternatives for improving the 40km of road between Abergavenny to the east and Hirwaun, 11km west of Merthyr Tydfil, went out to public consultation, and in 1995 the preferred route was chosen.
The project involves dualling the A465 along most of the route (with additional climbing lanes), apart from in at Hirwaun and Rassau where two new stretches will be built.
Construction is split into six sections:
- Section 1: Abergavenny to Gilwern (opened in 2008)
- Section 2: Gilwern to Brynmawr (planned to start 2014)
- Section 3: Brynmawr to Tredegar (under construction)
- Section 4: Tredegar to Dowlais Top (opened in 2004)
- Section 5: Dowlais to A470 Junction (planned to start after 2014)
- Section 6: A470 Junction to Hirwaun (planned to start after 2014)
Section 3 extends from Brynmawr to Tredegar. The 7.8km long route includes four junctions, eight bridges, three underpasses and six retaining walls, with extensive tree planting and noise mitigation measures.
The project involves 900,000m3 of earthworks, including 350,000m3 rock and 95% of construction waste diverted from landfill.