Early testing has led to a project team installing a medley of soil nails on an environmentally sensitive tunnelling project in the south of England. Alexandra Wynne reports from the heath lands near Hindhead.
The Hindhead Tunnel is being dug along the route of the A3 road to complete the dual carriageway link between London and Portsmouth (GE March 2008). Importantly, the buried 1.8km portion of the new 6.5km road is passing beneath the Devil’s Punch Bowl as it is a Site of Special Scientific Interest.
Foundation company and sub-contractor for the job, Systems Geotechnique, had early involvement with the project to soil nail and stabilise the slopes leading to the portals. Southern estimating manager Paul Thurlow says this helped it come up with a design so detailed that the team knows which nail goes into which hole.
This stemmed from eight weeks of soil nail trials performed on site by the contractor in March last year. The trials aimed to test soil nail performance as well as coming up with savings on both project time and costs. These were carried out at the Hazel Grove area near the south portal. Slopes here represent typical strata (within the Upper Hythe Beds) across much of the site and are where the majority of soil nails for the project will be installed in the final stage of this work, planned to be completed next year.
The team divided the 2m high trial slope into six areas – ranging from 70˚ to 34˚ from the horizontal, – which it monitored for movement. One of the critical requirements involved maintaining the slope face while drilling the nails at 20˚ from the horizontal to ensure final bearing plates would sit flush to the face.
But the team encountered a problem. While using an auger with air flush to drill the nail bores during the trials, the face suffered severe spalling. So in addition to ensuring the final bearing plates would sit flush against it, the face needed to be protected. To do this, the team designed thin square, steel-plate drilling guides that could be attached to the rig. An aperture in the centre allows the auger stem to drill through, while the steel plate simultaneously protects the surface area surrounding the borehole.
Typically, soil nails are designed with a sacrificial length above the level of the grout. It is sacrificial because the grout in downward inclined bores only partially fills the hole near the aperture as its surface settles horizontally. But this would not give enough protection to the nail against the site’s aggressive ground conditions and the threat of exposure to air and water, which is greatest at the nail head.
On this project, the design team specified full grout cover along the nail to the face to provide a protective environment for the nails – a decision made more vital by the need for the 120-year design life needed for some of them.
Despite this plan, Mott MacDonald geotechnical site representative Antony Drake says it is more difficult than it sounds to fill a hole with grout to the top when it inclines downwards. The result is that some of the grout will be absorbed into the borehole while some overflows at the face leaving a space within the hole, above where the grout settles.
Throughout the trials, the team looked at a variety of options in an attempt to fill the hole up to the face. Following the success of the temporary anti-spalling drilling guide, it seemed fitting that the most successful method emerged as a second temporary plate, which sits over the borehole.
This time, the steel plate has two apertures – one a low grout feed hole and a second higher breather hole. This enables rig operators to grout through the lower plate hole and along the entire length of the nail. The plate is kept against the face for as long as it is needed for the grout to go hard.
There are two different types of nail at the north and south tunnel portals as well as along slopes that will flank the new road. Most will comprise steel bars, but in areas where nails are temporary, these will be glass-reinforced plastic. Drake says this is because they are easier to cut through at the face of the tunnel, where they are installed temporarily before tunnelling starts.
As well as the different materials, the nails vary from 16mm to 32mm in diameter and reach depths ranging from 3m to 10m. Thurlow says this set off another innovation on the project. To keep track of what nail type, length, diameter and bearing plate size is needed for each of the boreholes, the team came up with a tagging system – effectively a label next to each borehole that lists all of this information in accordance with the design.
Although acknowledging that this appears to be a very detailed approach, Drake says it was an exercise in value engineering and ensured a reduction in wastage regarding possible incorrect nail lengths and diameters.
Systems Geotechnique has finished installing 1125 nails or the north portal and 776 on a nearby, less inclined slope called Blackhanger, which flanks the road leading to the north portal. Tunnelling is now being done from the north to the south so the contractor is continuing to install the 600 nails needed for the south portal.
The last portion of the £3.2M contract is scheduled to take place in 2009. At this stage, site workers will install 9000 nails at the Hazel Grove area to complete the slopes along the roads leading to the south portal.