Works for the 1.9km, A3 road tunnel near Hindhead in Hampshire are set use a sprayed concrete lining (SCL) despite fears that strict new Health and Safety Executive rules would make it too costly.
The Highways Agency told GE it had reached agreement with the HSE to relax rules governing exposure to nitrogen dioxide and dust to enable the SCL method to be used on the £371M tunnel.
The diesel powered machinery for SCL generates nitrogen dioxide and the excavation would create dust from the sandstone. Highways Agency project manager Paul Arnold told GE that using a tunnel boring machine had been considered but it was found to be too costly.
‘The new laws from the HSE are quite onerous and this is one of the st big tunnelling projects to be done since they were brought in, ’ said Arnold. ‘As a result we looked at a TBM but even at a length of 1.9km it wouldn’t be cost effective, ’ he added.
Instead, the HA’s team, led by contractor Balfour Beatty, agreed a compromise regime with the HSE under which more electrically powered excavation equipment can be used with more conveyor belts for the spoil and more ventilation to limit exposure to dust.
The team is preparing to start work after roads minister Stephen Ladyman gave the green light to the project last month. There will be two tunnels, one in each direction, freeing up traf c congestion in and around Hindhead which is a major bottleneck on the A3.
The original alignment of the 7.3m diameter tunnel was curved to avoid the Devils Punch Bowl because it was feared that the ground underneath was weak. But the investigation, which included boreholes up to 80m deep, allayed fears.
‘At one stage we were steering clear of the Devils Punch Bowl itself, ’ said Arnold. ‘We’ve since made minor changes to the alignment as we’ve become more con dent about the ground. There are sandstone layers and clay and occasional water tables but it’s not going to give us too many headaches.’
The tunnels will be excavated in three small sections before a primary sprayed concrete shell is applied to support the ground. A waterproof membrane will then be installed followed by a cast concrete lining to complete the structure. The tunnels will be constructed between 20m and 60m deep and will be connected by cross passages every 100m.
Enabling works will start in 2007 and the tunnelling works are due to start in March 2008 and will take around 10 months to complete.
Around 300,000m 3 of spoil will be removed. The tunnels are expected to open for trafc in 2011.
When complete, it will be the longest non-estuarial crossing tunnel in England, although the proposed A303 tunnel at Stonhenge in Wiltshire would be longer if it is completed.