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Don't move, we've got you covered

CONTRACTS: Careful construction was needed to form the massive excavation for a new wastewater treatment works next to existing plant, in a environmentally protected area. Diarmaid Fleming reports.

PROTECTING SENSITIVE equipment in a water treatment plant near Dover was a prime consideration during construction of a secondary plant alongside.

The project was further complicated by the fact that the site lies in a designated area of outstanding natural beauty.

Part of the £130M Dover & Folkestone wastewater treatment scheme for Southern Water, the £11M design and construct project at Broomfield Bank Treatment Works will meet the requirements of the European Bathing Water and Urban Wastewater directives.

Work on the primary plant was completed in 1999, but to comply with the latest directive, a secondary plant was needed.

Design of the primary plant had provided for the possibility of secondary treatment later. However, civil and structural works for secondary treatment could not be similarly shoehorned in, leaving the complex task of building a new plant directly beside the existing one.

During construction, contractor MJ Gleeson had to eliminate any movement or vibration that could damage equipment on the primary plant box.

Wastewater from Folkestone and Dover is pumped through 18km of new sewers and tunnels to the treatment plant in the Farthingloe Valley, 3km west of Dover.Treated effluent flows through a pipeline for discharge 2.7km offshore.

'The site is in a designated Area of Outstanding Natural Beauty, so detailed planning was needed before we could proceed, ' says Southern Water project manager Jeff Hall.

Various schemes were examined and lengthy consultations were needed with local authorities, the Environment Agency, conservation groups and organisations and individuals affected by the scheme.

Environmental considerations were paramount in the design. An above-ground solution would have destroyed the natural beauty of the lush green valley and surrounding hills. The much more expensive and technically demanding option of building underground and into the hill was chosen instead. The roof of the plant will be surfaced over and landscaped.

Designed by Balfour Maunsell, the treatment plant requires a 90m by 25m excavation in the chalk, sloping to follow the contour of the hill at the top from a depth of about 16m at its deepest to about 12m downhill. Cementation Foundations Skanska installed up to 1,200mm diameter contiguous bored piles to support the sides of the excavation.

Seven 800mm square reinforced concrete struts span the pile capping beams and a 1,700mm square spine beam running along the centre, supported on 305mm square steel columns.Capping beams at the higher end are 1,600mm by 1,500mm wide and 1,750mm by 1,850mm at the other.

Excavation had to be carefully controlled to avoid movement of the box alongside, monitored on a series of gauges inside the primary plant.

'The building moves about 0.25mm from thermal expansion and contraction a day, so we installed the gauges two months beforehand to give us background readings before construction began, 'Hall says.

Excavation was carried out to a detailed design to ensure that the sides remained balanced. On the south side, furthest from the first box, plate jacks bearing against the capping beam were installed to load the concrete struts up to 300t. Excavation to a stepped profile was carried out in five lifts, with jacks incrementally loaded to replicate the passive resistance provided by the ground at each lift stage, ensuring no movement in the primary structure.

At the excavation base, a combination of props and ties ensures enough restraint to prevent movement.The existing building has floors at two different levels next to the new excavation.Where the building is shallower, at around the mid-depth level of the contiguous piles at this side of the excavation, a series of ground anchors has been used to tie in the pile wall beneath the building. However, where the building is deeper, this arrangement is not possible.

'The only way to fit anchors would be to fix them lower down the pile, but this would have caused excessive deflection which we could not allow to prevent movement. So we had to look at another way of doing it, ' Marshall says.

A new sill beam cast alongside the floor of the primary works, but separated with polythene sheeting, supports a series of temporary tubular struts at 4m centres which bear at their other end against temporary anchors. The arrangement of the anchors further influenced construction, with concreting of the floor slab having to be done in stages. Final restraint to the walls will be provided by the floors when cast.

The plant, which has a capacity of 1,005 litres per second and is designed for a population of 140,000, is due to become operational in October next year. The project is ahead of schedule after reprogramming to make up for difficulties posed by wet weather during excavation of the chalk.

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