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Taking cover

Sewage treatment plants are ideal for location underground in order to reduce surface land requirements, lessen odour and visual pollution problems and improve the purification processes affected by extreme cold or strong sunlight.

Water treatment facilities can also be located in underground plants where they offer less aesthetic impact and obtain operational advantages due to constant temperature conditions. The first completely enclosed underground wastewater treatment works in the UK is at Swansea where it forms the central component of a major scheme to improve the water quality in Swansea Bay.

The works provides full treatment to the flow from the local population of 165,000. To meet planning requirements, the 160,000m3 structure was buried 6m below ground level and landscaped over.

Economics dictated a small footprint area which necessitated a compact arrangement of concrete tanks and plant rooms which formed the foundation for the building above. When completed, the treatment works were the largest covered works in Europe.

The substructure - 200m long by 53m wide and 25m deep - employs both precast and insitu concrete components. The site investigation had revealed underlying peat at a depth of approximately 45m, so a rigid, watertight structure was required to resist buoyant uplift and minimise differential settlements.

Piling of the whole structure would have been extremely expensive. The concrete structure was therefore designed to balance the weight

of excavated material so as to minimise potential settlements. Concrete was the ideal construction material for the complex structural forms as it was able to provide the necessary durability, maintenance and strength/weight characteristics.

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