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One hit wonder

IRELAND - Cork drainage: The streets of Ireland's second city are undergoing dramatic surgery to provide it with a complete new drainage system. Diarmaid Fleming reports from Cork.

Corkonians are fiercely proud of the River Lee where their heroes 'sported and played', as the song goes. But even a brief swim in its gentle reaches would probably guarantee a spell in one of the hospitals of Ireland's second city.

Cork's drainage scheme is a simple affair, unchanged from the foundation of the city centuries ago, with untreated sewage going straight into the Lee. Today, that means nearly 60M litres of raw sewage and polluted water join the river which runs through the heart of the city. The situation has long been regarded as unacceptable, and the mammoth drainage scheme now under way dates from the consultant, local firm EG Petit's detailed report as far back as the 1960s.

The city centre sits on an island in a fork of the Lee, with an almost Venetian network of waterways. Designing and constructing a modern drainage scheme for the city represented a massive engineering challenge.

While £16M was spent from 1977 to 1997 constructing new sewers to take the flow from the numerous outfalls from the city, it was not until the 1990s that the money became available for a complete new sewer network, along with state-of-the-art treatment works. The £160M cost is 80% funded by the EU, to help Cork conform with its standards, with the rest coming from the Irish government.

Once completed, flow from the city centre will enter a 3m diameter inverted siphon crossing, taking it 140m under the river to connect with the drainage system on the southern side of the river. A 3m diameter 2.65km sewer will then run 2km to a pumping station.

Twin 1.5m diameter rising mains from this connect to a header chamber, which takes flow via a 1.2km long rising main from the city's existing pumphouse at Rochestown, which is to be modified.

From the header chamber, twin pressure pipes pass under Lough Mahon to a £45M treatment plant at Little Island to the east, where a diffuser will discharge treated effluent to the sea.

'The city centre works are a massive job, ' says Cork Corporation project engineer Denis Duggan. 'There are hundreds of little outlets going into the river, so we decided that every street needed a new foul sewer. This meant that every property needed a new connection. But the streets would be riddled with roadworks and enormous disruption.'

To minimise the chaos, the work was divided into three separate contracts before it began in 1999. A total of 17km of sewer pipes is being laid by local firm Sorenson, a Morrison-Leneghan joint venture, and the Corporation's direct labour force - to minimise claims for delays in an area of rich archaeological significance where finds have included the remains of a 12th century house.

The dramatic surgery on the city's streets is painful for residents, but there is much to gain.

In near Mayor Giuliani speak, then city manager Jack Higgins determined that there would be a 'one hit on the streets' approach. It represents a zero tolerance of sorts, meaning no more roadworks for the long term.

'We are using the opportunity to do everything: water, gas - all utilities. We are asking companies what's wanted. They pay us for the ducting and we put it in.

While there's enormous disruption, at least we can say to people that when we've gone, we've gone, ' says Duggan.

In the city, sewer work has involved GRP relining of old culverts or replacing them with new pipes and grouting up. Several near collapses have had to be repaired.

Downstream, joint venture contractor Ascon Nuttall is constructing huge 30m deep insitu concrete sunk caissons to form two shafts north and south of the river, linked together by the inverted siphon. Driving the 2.65km, 3m diameter main trunk sewer took the joint venture 10 months with a TBM in gravel to depths of 8.5m.

Ascon is also nearing completion of the new pumphouse - one of Ireland's largest - as well as a number of minor branch mains.

The twin rising mains, along the path of an old railway line, are being completed by contractor McGinty O'Shea.

Dutch contractor Van Oord ACZ won the £21M contract to construct the header chamber and 4.6km of twin 1.2m diameter pressure pipes to the treatment plant. PJ Hegarty undertook the header chamber and 1.1km of pipework on land as a subcontract, with Van Oord laying the 3.6km of underwater pipe under Lough Mahon, and in the process attracting national media attention.

The GRP pipes were towed in rafts of 450 sections from Norway, and jointed with concrete collars cast and fitted at a dockyard in Cork, before being towed and sunk into position into a dredged trench. The laying work was completed ahead of schedule in December.

Work on the treatment plant has already begun, with the construction of huge new berms and 450,000m the 15ha plant on its 33ha site.

The joint venture design, build, operate contract was won by Consort JV, comprising Ondeo Degremont and Vinci Construction of France, EPS Pumping and Treatment Systems and PJ Hegarty of Ireland, along with the UK's Northumbrian Lyonnaise International.

Consultants for the scheme are EG Pettit and Mott MacDonald. The plant will have a population equivalent operating capacity of 423,200 persons, with a total design flow of almost 400,000m 3a day - six times the dry weather flow. Treated effluent will flow by diffuser to tidal reaches of the sea, while sludge will be converted to dry solid granular material, suitable for agricultural fertiliser and soil conditioner.

Construction is due to end in late 2003. After commissioning in mid-2004, the new facility will provide water of bathing standard, as well as a priceless new environmental amenity - and even the chance to swim by the Banks of the Lovely Lee.

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