Dublin's Liffey water supply may be better known as the key ingredient of the city's most famed beer. But the city is drinking more and more of it in its natural state, inspiring a visibly spectacular addition to its civil engineering infrastructure.
Beside the airport and M50 motorway to the north of the city, an elegant new water tower is taking shape. Shape indeed, for this is no ordinary functional vessel, but a curving, elegant, vase-like structure, its lines owing as gently as the waters it will contain. The team behind the e52M (£32M) North Fringe Water Supply project are proud of its most visible emblem.
'The tower gives us a good excuse to show off our work, ' jokes Dublin City Council project engineer Adrian Conway. 'One of the problems of being a water engineer is that often almost everything is underground and people can't see what you are doing.' Indeed, this is probably Ireland's most public construction site, being beside the country's busiest road. A recent concrete pour, with three 58m concrete pump booms dangling like synchronised gymnasts, slowed rush-hour trafc as curious motorists craned to see the action.
The water tower's vase-like shape and formwork, splaying radially outward and upwards like branches of a tree, has given an almost organic quality to its construction. The tower appears to grow rather than being built, its elegant lines easing it into the environment.
And once it is finished, the engineers will not be hiding away their achievements. 'The structure will have three coats of special paint to enhance its look. It can't obviously be seen in the dark and the design is for it to be continuously lit. We're in discussions with a company which lights structures so we may have something more imaginative with changing colours, ' says Metin Amet, project engineer for the joint venture design team of Hyder and Dublin-based consultant PH McCarthy.
The 39m high tower at Sillogue is just one aspect of the project, which includes a new 38km water artery running east to west along the northern fringes of the city (see box, p32). The new main is being built in three contracts - two complete and the last leg to start next year - and will link into the existing high pressure mains network serving the north of the city from the reservoirs in the south, easing pressure and supply difculties.
Water will be fed into the new facility at Sillogue from Ballycoolin reservoir in the west of the city, which has a treated supply from the Liffey at Leixlip.
Inadequate mains capacity and Ballycoolin reservoir's relatively low top level has meant poor pressure for some customers in higher-lying areas across the north of the city. The tower will boost these water pressures.
'The tower gives you a lot of water at peak times when you need it. It is like a family using water from the tank in the house - it lls up between times of heaviest demand in the morning and evening. The tower is better than having pumps running at peak times requiring huge power at certain times of the day, ' says Amet.
The tower's form was designed with appearance in mind and its geometry is deceiving: standing 39m tall, its diameter at the top is 38m.
'It's almost as wide at the top as it is high. Being near the airport, there were restrictions on how high we could go. It's a double parabolic shape that looks pleasant to the eye. We had intended to go for a domeshaped roof that would be easier to design and construct, but the Irish Aviation Authority limited the height horizon, ' says Amet.
'We were able to get our water height, but had to have a at roof.' The structure of 5,000m 3 of concrete (reinforced with 580t of steel supplied by Midland Steel) is founded directly on Dublin's typical stiff boulder clay on a 25mx25m concrete base slab, without a need for piles, given the good ground conditions.
It consists of a shaft with access staircase and ladders supporting a bowl that holds 5Ml of water. The shaft's external diameter ranges from 16.8m at the base to its most slender at 8m, widening upward to a diameter of 38m at the top.
Wall thicknesses vary too, from 800mm to 1.7m.
An innovative formwork system from Austrian rm Rund-Stahl-Bau meant that the elegant varying shape was, itself, constructed with elegant simplicity. 'The formwork was done speedily. Everything arrived on site pre-prepared and assembled. The shutters inside and outside the structure are independent and there are no tie holes, ' says senior resident engineer Martin Hession. 'It's really a freestanding shutter.' Temporary eelwork supported the shutters forming the inside of the tower.
Externally, the concrete itself provides the support: for the rst pour the external shutters were risen off the ground while for each subsequent pour, temporary xings in the concrete that had been previously cast support the shutters for the next pour. No tie bars meant no tie holes to be back grouted.
Construction joints were conventional along with the C28/35A concrete for waterretaining structures.
An alternative to the proposed insitu concrete roof suggested by contractor John Cradock - using precast beams spanning radially from the centre of the tower topped with in situ concrete - eliminated the need for formwork and steelwork, which would have posed 'extreme difculties' in construction, says Hession.
Water from the storage reservoir being built alongside will be pumped from a station serviced by three variable speed pumps working at a maximum of 700l/s. M&E installation is by Earth Tech Ireland.
The 92m long, 66m wide, 6m deep storage reservoir will be buried. It consists of two cells each of 15Ml capacity, with provision to build a third if required. The reinforced-concrete structure contains high-density polyethylene curtain walls inside to ensure an s-ow and suf cient circulation to avoid water stagnation. But internal columns are precast - not just for ease of construction.
A high-quality finish was sought, but not for appearances.
'The finish inside the waterretaining part of the tower and storage reservoir was important.
We don't want any indentations that would allow bacteria to collect. In the cells we were going to use a controlled permeability formwork liner that would allow bubbles to dissipate so you don't get blowholes. But we found that the steel shutter alone was just as good, ' says Conway.
Dublin's water supply system
Most of Dublin's water is supplied by the River Liffey, rising in the Wicklow mountains to the south and flowing in a ring along the southern and western flanks of County Dublin before flowing east through the centre of the city. Victorian reservoirs at Bohernabreena in County Dublin (two dams harvesting a tributary of the River Dodder and feeding Ballyboden treatment works) and Roundwood in County Wicklow (from the River Vartry and feeding Vartry treatment works) were added to by a huge 166Mm 3 reservoir at Ballymore Eustace, built in the 1930s and 1940s near the upper reaches of the Liffey. This was no mean feat as it is second only in size in the British Isles to Kielder Water at 199Mm 3. This network supplies Dublin's southside and centre, as well as parts of the north of the city via highpressure arterial mains. A separate supply for the north side also comes from the River Liffey at Leixlip - from the Norse for 'Salmon Leap' - where water from a treatment facility is pumped to a reservoir at Ballycoolin nearby in the west of Dublin. But rapid urban expansion, as well as the growth of in adjoining counties of Meath and Kildare, has pushed existing infrastructure to the limit.