Software analysis has proved to be a vital tool in designing the slope stabilisation for the Beaminster Tunnel in what is believed to be the first soil nailed scheme in the Upper Greensand.
With the A3066 through the Beaminster Tunnel closed for over a year following a double fatality landslide at the site, road users were putting pressure on Dorset County Council to reopen the route. However, the stakes for the solution were high with the design calling for what is believed to be the first soil nailing undertaken in the Upper Greensand.
Add access issues, on-going slope instability and a very wet spring into the equation and the challenge facing main contractor Raymond Brown and ground engineering contractor Can Geotechnical in delivering the scheme increased. According to consultant Parsons Brinckerhoff, it was use of geotechnical software to validate the design and check it as the scheme evolved that was invaluable to providing confidence in this pioneering technique.
As well as in-depth analysis, teamwork between the client, consultant and contractors involved was vital and as a result the project is now in the final stages of completion and the tunnel has reopened to traffic ahead of the main summer season.
“The nails are being used to pin the mesh at the surface but the main aim is to control the deep-seated slope failures that were identified during the investigation phase of the work,” says Parsons Brinckerhoff geotechnical engineer Vicky Weare. “The design of the soil nails is pretty typical but the scale and shape of the bowl has been a challenge for the design and for the layout of the work too.
“The geology also presented some concerns and we believe this is the first time soil nailing has been undertaken in the Upper Greensand.”
DCC principal engineer Matt Jones says the geology presented challenges during both the design and the construction phases. “The difference between how the material behaves on a wet or dry day is incredible,” he says.
During the work no evidence of the deep-seated failures predicted by Parson Brinckerhoff’s analysis have been found, but the shallow instability that caused the initial mudslide on 7 July 2012 has been an on-going problem. “The shallow failures that occurred after the design work started proved that the road closure was necessary,” says Jones. “We have organised visits for local people and media to demonstrate the scale of the problem.”
Can started on site in April and has installed 860 soil nails – 540 on the south side and the rest above the northern portal.
Local pressure meant that DCC wanted the work fast-tracked so the team from Can has been working 12-hour shifts with 11 days on site, followed by three days off.
Can has had to work very closely with Raymond Brown to coordinate the work as during the Can team’s three-day rest period, Raymond Brown was working on the site to ensure it had access to the next section.
The programme was dictated by access to the land and work started on the northern side before the rigs moved onto the southern slopes on 15 May and then work was carried out in parallel.
Once the land access issues had been resolved, DCC faced a final hurdle that could have halted the whole project no matter what the outcry from the local population – badgers. “There was a sett on the east slope above the southern portal,” says Jones. “We used infrared cameras to prove that it wasn’t in use and Natural England quickly gave us permission to close the sett and progress with the rest of the work.”
Trees above the tunnel have been reduced in height and Jones says that vegetation management will be an ongoing issue at the site. “The area above the tunnel is a designated ancient woodland so we can’t just clear the trees,” he says.
Access was one of the main issues and Can overcame the challenge of installing soil nails on such a steep and potentially unstable slope by mounting drifters onto two excavators, including one long reach machine. A roped access A-frame rig was also used where the excavators couldn’t reach.
Much of the early work on site for Raymond Brown focused on constructing additional access ramps so that the excavators could get to the site above the portal.
“It was bad enough trying to walk on the slope at that stage, let alone trying to get a machine up there,” says Jones.
Can Geotechnical project manager Hazel Tant adds: “The nails themselves vary in length from 8m to 14m but all are inclined at 10 degrees, grouted along the whole length and installed at 1.5m centres around the bowl above the tunnel portal where the slopes reach up to 45 degrees.”
The 32mm diameter hollow Dwyidag bars are grouted into 100mm diameter holes using conventional 40N/mm2at 28 days strength grout.
“The load on each nail is dependent on the effective stress so each nail is different, so as the surface slips occurred we had to revise the design,” says Parson Brinckerhoff’s Weare. “We used the software package Slope W and designed the nails to BS8000 N6: part 2 to design the initial solution and checked any changes using Slope W as the work progressed.”
The soil nailing solution was tested at various locations across the site at the start of the project and in most areas the design proved to be viable but in one area close to the southern portal the nails failed to reach the test load. Acceptance testing called for the nails to cope with loads of 110kN. Sacrificial testing showed that in some areas loads of up to 300kN were achieved but in the wet area near the southern portal, the nails failed at 60kN.
The solution involved installation of 11m long inclined drains to reduce the ground water with a 2m high gabion basket retaining structure close to the road and a reduced slope angle above. “It has been difficult to install the drains as the ground was so soft,” says Tant.
The surface of the slopes above the portals has been covered by erosion protection matting that has been hydroseeded. The soil nail face plates are being covered by Geobrugg Tecco mesh, which is an active system that will retain any shallow instability and prevent the debris from reaching the road.
The work on site included restoration of the listed tunnel portals but they will not be put in the position of having to act as retaining structures in future as a reinforced concrete wall is being built behind and is attached to a 3.5m concrete slab that spans the barrel of the tunnel. “The concrete structure will help waterproof the final few metres of the tunnel,” says Jones. “The first few metres of the tunnel at either end is stone lined rather than brick built and we have always had an issue with water washing out the pointing. The concrete structure will reduce the maintenance related to this.
“We are not expecting any further slips but there is a potential for a build-up of fines and the concrete wall behind the masonry headwall will retain this material.”
“The nails are being used to pin the mesh at the surface but the main aim is to control the deep-seated slope failures that were identified during the investigation phase of the work.”
Vicky Weare, Parsons Brinckerhoff
Cost of the work has been estimated at £2.6M by Jones but he adds that some of that cost was for maintaining the diversion routes.
DCC has used the opportunity to install a new drainage channel through the tunnel while the stabilisation work has been underway. “We have invested a further £60,000 at the site but it would have been much more expensive, not to mention challenging locally, to have closed the tunnel again in a year or two to carry out the work,” says Jones. He adds that a major inspection of the tunnel was carried out before the full re-opening to ensure disruption to traffic in the near future is minimised.
With the soil nails in place and the tunnel fully reopened at the end of July it looks like efforts put into analysis of the design have paid off – a relief for the design team and local people alike.