Ground riddled with mine working, wildlife and earth dams all presented challenges to contractors working on the Heads of the Valley dualling project in South Wales.
In rural south Wales a transformation is taking place. A 50km section of the single Heads of the Valley road between Hirwaun and Abergavenny is being duelled as part of a programme of work to regenerate this severely deprived area. The Hirwaun to Abergavenny project is split into six sections. Section three, between Brynmawr and Tredegar junction is having one of the biggest impacts.
With help from the European Regional Development Fund, which has put forward £80M of the £140M project, work on the 7.8km section connecting Brynmawr in the east to Tredegar Junction in the west is almost complete.
The Heads of the Valley road runs through a region once known for its coal mining and industrial steelworks.
But one by one these industries have been shut down, and with the closure of the last steelworks in Ebbw Vale in 2002, the region has been in dire need of regeneration as its local population is far bigger than the local economy can sustain.
Despite being relatively short, section three of the road, which lies just south of the Brecon Beacons National Park, has had its fair share of engineering challenges. These are presented by old underground mine workings, numerous valleys, eight new bridges, four junctions, rock blasting and reservoir earth dams. The team has used innovate techniques to ensure that the section is delivered on time and on budget.
Contractor Carillion started on site in January 2013. It is carrying out 2.8km of improvements to the existing road and building 5km of new carriageway on old industrial land. A new cycleway will accompany the road along almost its entire length.
Geologically, the conditions presented a tricky challenge to the team. The road had to be built across ground riddled with old mining shafts and tunnels. Traditionally the approach would have involved drilling holes into the ground and pumping it with grout, says Carillion project director Mike Cummine. However this came with the risk that an area may have been missed and the subsequent danger of the road collapsing. Instead the team decided to take a risk-based approach based on the likelihood and consequence of a mine working migrating to the surface.
“In reality, it’s quite an unlikely event,” says Cummine. “By taking a risk-based approach, you end up targeting the expensive solutions to the structures that definitely can’t afford to deal with subsidence.”
With the risk-based approach, the team decided to put a geogrid capable of spanning around 2m under the carriageway. If a working migrated to the surface, he says it will form a low risk dent in the road. This would warn that something had happened and a repair can be made before a hole opens up. This approach meant that a cost-efficient, localised solution could be implemented without the need to incur the much larger cost of grouting large swathes of ground.
“Under our structures we’ve mapped out some of the [mining] roadways and grouted them up and others we’ve filled with gravel where we want water to flow,” says Cummine.
The team faced a big challenge to find and piece together all of the available historic information and confirm its findings by carrying out ground investigations, probing and drilling holes into the areas where the workings were thought to be.
Time also played a factor in the design of the road. When it was originally designed, the cost of disposal of excavated material was far less than at present costs. This presented the client with one of its big risks. Originally the design called for 750,000m3 of cut material to be disposed of, the team then managed to redesign areas to bring this figure down to a far more manageable 50,000m3 at the start of construction. However, at the end of the project this figure had been reduced to zero, through the use of the cut material elsewhere in the project. One of the major areas where the material was incorporated was at Carno, between Tredegar and Brynmawr.
Around half way along the route, the road had to cross a small valley in which an access road led to Carno reservoir. Originally the design called for the valley to be crossed with a steel bridge. But pylons and overhead power lines running parallel to the planned route would have presented a challenge to crane operators trying to manoeuvre the structure into place.
With the disposal costs for excavated material rising throughout the project, it was decided that the steel bridge structure should be replaced by a 28m-high built-up embankment with a tunnel created through it for the access road underneath.
“The Carnot pass used to be a bridge and now it’s an embankment and that has accounted for a quarter of a million [cubic metres],” says Cummine.
Client Welsh Government
Environmental Consultant TSP
“Also from a health and safety point of view we would have had to do a lot of craneage nearby, or move the cables, and moving the cables in that location would have been a very big diversion.”
Space around the embankment was tight as the toe of the earth dam of the reservoir encroached into the site. On this side, a 28m high vertical wall was created using precast concrete panels which had around eight to 12, 30m long, seat belt-width straps embedded in the embankment behind.
The friction of the soil compacted on these straps holds the panels in place forming the wall of the embankment.
Constructing the 18m-wide, 7m-high and 120m-long tunnel through the embankment was no mean feat either. Precast concrete arch sections were lifted into place in the valley. As fill was added around the structure to build up the embankment, the engineers had to carefully monitor its movement to ensure failure didn’t occur.
“Because they [the precast panels] deflect, the bottom of the arch actually came in about 50mm which is quite a lot for a structure,” says Cummine. “You do a balanced fill on each side - the bottom goes in and then as the fill goes in on the top, it deflects back out again.
“It was quite complicated because the depth of fill was a different depth from the front to the back, so there was quite a strict regime of monitoring and cloud scanning inside the arch as we filled, so we could keep an eye on what it was doing. It kept a team of engineers from Arup occupied for quite a while.”
“12% of our labour spend had to be on new entrant trainees, so on people who haven’t worked in the industry, people who were unemployed or had just come out of school or college.
Mike Cummine, Carillion
All along the length of the road small details such as these were put into place. Otter tunnels under the roads were created and along some sections, special fencing was erected around the road to stop them climbing over onto the road.
Lapwing habitats which were disturbed during construction have been relocated, constructed and are now being monitored as part of the ongoing contract of maintenance between Carillion and the client.
Throughout the project Carillion ensured that it engaged the local community at every step. This paid off at the public consultation where only three complaints were put forward.
“Being contractors and contractor-orientated we’re not about telling people we’re not going to interfere with their lives,” says Cummine. “It’s more about being honest with them about the scale of works we’re doing, what that physically means – how we’re going to control noise and dust and that sort of stuff.
“It’s too easy to say we won’t interfere and that’s not true.”
The company also gave a commitment to employ a significant number of people from the local community. In an area where unemployment is high, this had a real impact for the people who lived there.
“12% of our labour spend had to be on new entrant trainees, so on people who haven’t worked in the industry, people who were unemployed or had just come out of school or college. For us, that was in the region of about 60 employment opportunities for people
“Twelve per cent of our labour spend had to be on new entrant trainees, so on people who haven’t worked in the industry, people who were unemployed or had just come out of school or college,” says Cummine.
“For us, that was in the region of about 60 employment opportunities for people who have no industry knowledge, which is quite high for a scheme this size.”
The Tredegar to Brynmawr section was due to open last month. The whole length of road improvements is due to be completed by 2020.