An underground wastewater treatment works, equivalent to the volume of 15 Olympic-size swimming pools, is to be built by Kvaerner Construction in Weymouth, Dorset.
The 20M target cost contract, which was won with a package which met special environmental conditions, is to be carried out in conjunction with process contractor OTV Birwelco and Wessex Water and will be complete by the end of 2000.
The 8,000m2 of playing fields above the new works at Wyke Regis will be fully restored for use by the local community. Hidden beneath the grass will be a 12m deep concrete cavern measuring 108m long and 72m across - roughly the size of a football pitch. It will house four fully-automated sequencing batch reactors capable of treating the effluent from a population of 130,000.
The new plant is being constructed alongside an existing headworks and long sea outfall to comply with the requirements of the Urban Waste Water Treatment Directive. Design and construction methods to reduce disturbance to the local environment and minimise earthmoving have been developed at the site, situated close to a housing estate and just a few hundred metres from Chesil Bank and the Fleet Lagoon, a noted tourist and wildlife area
Piling work has started to construct the perimeter walls and internal plunge piles before excavation of the interior. Excavation will be carried out in a controlled manner so as to load the structure evenly and to avoid local ground disturbances.
The foundation work involves constructing a watertight perimeter wall of interlocking large diameter secant piles with an inner concrete lining. To prevent the perimeter box shifting or distorting during excavation, a 'frame' of concrete beams and slabs will be constructed around the top of the interior.
Some 54 load bearing piles will be needed to support the concrete roof. These consist of steel columns plunged deep into wet concrete using Cementation's patented Cemloc system, specially developed for top down construction.
Beneath the base of the huge box, more than 100 tension piles resist flotation pressures and anchor the concrete base firmly. After the shell has been constructed and the four treatment tanks, each with a capacity of around 6.6M litres, are constructed, the box will be covered by concrete roof beams, and an insitu slab before disappearing under a final layer of topsoil and grass seed.
Brian McColm, who is managing the 30 month contract for Kvaerner, said: 'We have successfully applied this innovative approach before and, unlike other methods, excavations and engineering works are confined within the box itself, reducing noise and dust problems and hence reducing as far as possible our impact on the surrounding environment.
'Before piling operations get under way, we are conducting extensive analysis to confirm that ground and structural behaviour will
be exactly as planned.'