Kier Utility, working for Costain on behalf of Highways England, required a new 1,500 mm diameter concrete sleeve to be installed under the A30, near Truro, Cornwall, UK, as part of the A30 Chiverton to Carland Cross road improvement scheme. The concrete sleeve was to allow 2 x 710 mm diameter plastic pipes to pass through, also as sleeves, with ultimately 2 x 500 mm water mains then being housed within the sleeves. The 1,500 mm diameter tunnel was 75 m long and installed as a pipe jack using a full faced TBM.
Active Tunnelling Ltd (ATL) owns a large fleet of shoring equipment including sheets and frames, some of which was used to construct the drive and reception pits on this scheme.
The drive pit was 6 m x 4 m in plan x 7 m deep, with reception pit being 7.5 m x 3.5 m in plan x 5 m deep. ATL installed these cofferdams with the aid of its own 36 t zero swing excavator, which was also used to lift the TBM in/out of cofferdam during the tunnelling works.
ATL’s own TBM with rock head was used to dig through the Phyllite ground, which it excavated through with ease, nearly doubling forecast production and saving two weeks on the tunnel operation alone.
ATL proposed to cast timber rails, set in concrete to gradient within the 1,500 mm diameter tunnel, to allow the twin 710 mm plastic pipes to be slid along into position. ATL also proposed to install steel restraining bars (fabricated by ATL) within the tunnel to prevent the 710 mm plastic pipes from floating when the tunnel void was pumped full of grout later. Both details worked very well and the pipes were installed successfully with no issues.
The client was extremely pleased with all the work carried out on this site with the contract programme being reduced by a total of three weeks.
