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South coast cover-up

Wastewater treatment Secondary works

Constructing a new wastewater treatment plant in a 16m deep excavation next to an existing facility posed some unusual problems for the contractor. Diarmaid Fleming reports from Dover.

The challenge facing contractor Gleeson at the Broomfield Bank Treatment Works site is to eliminate any movement or vibration during the construction of a new secondary treatment plant which could damage sensitive equipment on an existing completed primary plant directly alongside. Too much movement could even put it out of action.

As part of the £130M Dover & Folkestone wastewater treatment scheme, the £11M design and construct project for Southern Water will complete the treatment works needed to comply with 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.

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

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

'The level of detail involved made this one of the most difficult parts of the project, ' Hall adds.

Hydraulic design of the primary plant by process engineer OTVB had made provision for the possibility of secondary treatment later, as now required.

However, the civil and structural works for secondary treatment could not be similarly shoehorned in, leaving the complex task of constructing a new plant alongside the existing one.

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

This means a significant excavation. As drawn up by civils designer Balfour Maunsell, the design for the treatment plant requires a hole in the chalk of 90m by 25m on plan, sloping to follow the contour of the hill at the top from a depth of around 16m at its deepest to around 12m downhill. Skanska installed contiguous bored piles up to 1,200mm in diameter to support the sides of the excavation.

Seven reinforced concrete struts, 800mm square, span between the capping beams on the piles and a spine beam running along the centre. Capping beams at the higher end are 1,600 by 1,500mm wide and 1,750mm by 1,850mm at the other. The 1,700mm square spine beam is in turn supported on a row of 305x305UC columns running from the base slab below. These are protected from horizontal forces, as the loss of a column could collapse the strut, leading to deformation of the walls and possible progressive collapse.

The columns will later be encased in concrete.

Excavation had to be carefully controlled to avoid movement, monitored on a series of gauges inside the existing plant. 'The building moves around 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, ' says Hall.

Excavation was carried out to a detailed design to ensure that the sides of the excavation remained balanced. On the south side furthest away from the existing primary treatment plant box, plate jacks bearing against the capping beam have been installed to load the concrete struts up to 300t. The 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 existing structure.

On a bright sunny day, the dry chalk crunches underfoot, giving a false picture of the difficulty encountered during excavation. 'The wet weather in the winter caused plenty of difficulty, with excavated material looking almost like sludge during the worst of it, ' says Gleeson project manager John Marshall.

At the excavation bottom, a combination of props and ties ensures enough restraint to prevent movement. The existing building has floors at two different levels adjacent to the new excavation. Where the existing building is shallower, at around the mid-depth level of the contiguous piles at this side of the excavation, a series of stressed ground anchors incline under the existing structure, tying in the contiguous piles to prevent deflection.

However, where the existing 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, ' says Marshall.

A new cill beam cast alongside the floor of the existing 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 anchorages. The arrangement of the anchorages further influenced the construction, with concreting of the floor slab having to be done in bays.

'Because of the restriction in space, it is not feasible to powerfloat the finish, which is done by hand instead, ' says Marshall, as he observes a busy gang carefully tamping a finish almost as smooth as the head of a newly poured pint of Guinness. Final restraint to the walls will be provided by the floors when cast. Concrete is generally C35 with 50% replacement ground granulated blast furnace slag.

Standard precast concrete highway inverted T-beams spanning between the capping and spine beams will form the roof, with topping concrete designed for light loading such as grass cutting. A layer of around 400mm of topsoil will then be planted with grass to blend in with the existing hillside.

'A considerable amount of detailed work has gone into the landscaping, with trees already planted to ensure that once the treatment works are operational, they will be invisible to the public, buried under the hill, ' says Hall. State of the art odour treatment equipment will be installed to mask smells from the plant, which has a capacity of 1,005 litres per second and is designed for a population of 140,000. Design of the biological aerated flooded filter (baff) facility is by Brightwater Engineering with Balfour Maunsell and Gleeson's in-house design team.

The plant is due to become operational in October next year, and the project is ahead of schedule after reprogramming to make up for difficulties posed by the weather. The deadline for completion is strict, with Environment Agency penalties due to fall on Southern Water if nonsecondary treated effluent continues to flow through the outfall past the October deadline, reflected by undisclosed penalties for delay in the construction contract.

On the spot

Name: John Marshall Age: 37 Job: Project manager, MJ Gleeson Group Best thing about this project:

Building an unusual and challenging wastewater treatment structure, a lot more than just circular and square tanks on a hill.

. . . and the worst: The weather when it was bad.

Best job ever: Finishing a hotel in Jersey and enjoying a few beers afterwards by the sea.

. . . and the worst: Working on a series of railway overbridges on night possessions in the middle of nowhere far from home.

Best thing about engineering:

The degree of self-reliance and autonomy, and the craic when a job's going well . . . and the worst: When things are going against you when a job's not going well.

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