Media attention following the fatal landslide at the Beaminster tunnel has put the local council under pressure to find a solution to the slope stability problem. Claire Symes reports.
Landslides during the wet summer have been a widespread problem for the UK, but there is one that stands out from the rest due to its initial impact and also the longer term implications at the site. While national media focus into the late discovery of two deaths from July’s landslide at the Beaminster tunnel in Dorset has now waned, the problem is still very real for the local community.
While local people are faced with the long term closure of the tunnel, which links the A3066 between Bridport and Crewkerne through Horn Hill, Dorset County Council (DCC) has been working to find a solution to the problem. After rapid mobilisation of framework consultant Parsons Brinckerhoff and ground investigation contractor Environmental Scientifics Group to site, DCC is working to have a soil nailing solution ready for the site in early 2013.
“The cost of the soil nailed solution is expected to be in the region of £2.1M, but it is hard to estimate the total cost of the work at the moment,” says DCC principal engineer Matt Jones. “My task is to just get it done. Our approach may not be the most effective one but we are trying to focus on the time issues as the road closure is impacting on local businesses.”
As well as the cost of the investigations and predicted construction costs, DCC has also had to carry out improvements on local roads that residents have been using as unofficial diversions to get around the closed route. The tunnel itself used to carry 5,000 vehicles a day and the official diversion can add up to 20km onto journeys and is increasing traffic levels along diversions routes, with some areas reporting double the normal vehicle numbers.
“We have made a commitment to grit unofficial routes to the same level as the main routes to try and keep traffic moving in the area,” says Jones.
Although the problem has now evolved into a major contract, the landslide did not initially seem to be a significant problem. Jones explains that when news came in about the landslip at the tunnel, the council took the decision with the local police and fire brigade to close the road.
“There was lots of damage around West Dorset during the heavy rain on 7 July, so at the time this was just one of many incidents we were dealing with,” says Jones.
When he visited the site on the Monday after the landslip, inspections of the slopes above the tunnel revealed that there was a large tension crack above the northern portal. This suggested that a larger landslip was possible, so the decision was made to keep the road closed until a solution could be found.
At this stage DCC called in its term framework consultant Parsons Brinckerhoff and has been working with the firm’s head of geotechnical engineering Ian King on the solution ever since.
Nonetheless, the full scale of the problem at Beaminster was not revealed until 10 days later when Somerset Police linked a missing couple to the area, and investigations of the landslide debris uncovered a crushed car and the bodies of the missing couple. Jones says he was shocked when the news about the fatalities broke, and says the size of the debris piles at the portals did not look large enough to be covering a car.
It was clear that reopening the tunnel was not a short term option.
“Whatever the solution was, it was clear that we needed to start discussions to ensure gaining listed building consent, as the tunnel portals are Grade II listed and planning consent did not further delay the work,” says Jones. “By starting the discussions in the early stages we were able to get some advice on what might be feasible and what wasn’t.”
Some of the early options that DCC looked at, due to demand from local residents, included demolition of the tunnel and replacement with a cutting.
“Our planning department said early on that the listed status of the structure, which was opened in 1832 and is believed to be the oldest road tunnel still in use in the UK, meant that demolition was not a viable option,” explains Jones. “The cost and time involved, not to mention the impact on the landscape, with taking the cutting approach meant that this was not favoured.”
A permanent diversion was not favoured either, so Jones’ team needed to find a solution that could allow the tunnel to be re-opened. The options were soil nailing to stabilise the slope or construction of a concrete hood structure to protect the portal approach and catch any further landslip debris.
Work on site started in late August, with Environmental Scientifics Group (ESG) undertaking a ground investigation involving sinking five boreholes to 25m below ground level to obtain samples for testing.
The ground investigation also looked at the foundations of the wing walls and portal to assess the stability and potential to support a temporary hood that could allow the tunnel to be reopened while the main work was carried out.
Unfortunately the foundations were not sufficient to allow this and ESG made a second visit to the site in late October to carry out further boreholes in the road in front of the portals to gain information that could be used to design a piled solution to strengthening the walls.
The ground investigation revealed that the tunnel passed through, and sat beneath upper greensand.
“We had expected to encounter the underlying gault clay within the boreholes,” says Jones. “The angle of natural slopes in the upper greensand in the area are 1:3 and the results of the ground investigation showed that the whole slopes above both portals were at risk of major failure.”
“By starting the discussions in the early stages we were able to get some advice on what might be feasible and what wasn’t”
Matt Jones, Dorset County Council
Before receiving the news about the scale of the instability, DCC had favoured the hood option, but once the scale of the problem was discovered, there were concerns about the size of hood that would be needed to contain a major failure. “There was also the question of long term maintenance too,” says Jones.
The options were presented to the council in early November and the soil nailing option was selected as the way forward.
According to Jones, the estimated £2.1M scheme is not the cheapest option but will give the best result in terms of restoring the site to its original appearance and reducing the risk of further failures.
Currently the cost of the work is being met from DCC’s own budget, but the council is trying to gain extra government funding for the work due to the scales involved.
Work is still underway on the final design for the soil nailing work and the listed building consents, but Jones believes the project can be delivered to allow the road to be reopened ahead of the 2013 summer holiday season.