Ireland has long used water towers to secure water distribution throughout the country. But until recently the water reservoir capacity of these water towers was in the range of 150 to 1500m3. Therefore, consulting firm P. H. McCarthy & Partners had to think big when the company was employed by Meath County Council to design a water tower for Kiltrough near Drogheda.

The Kiltrough water tower was designed for a volume of 5000 m3 and its water reservoir is supported on a cylindrical concrete shaft with a diameter of 8m and a height of 31.5m. The tank serves an area of 3200ha with a population of approximately 12,205. The area is a mixture of farmland, small villages and holiday resorts with a water demand of 4930 m3/day.

The project was a challenge not only for the design team but also for the contractor. The right people, infrastructure and equipment had to be provided to secure the construction of the water tower in the shortest possible time and to cope with the impressive dimensions of the structure.

With an overall height of 54m and a maximum bowl diameter of 36m the water tower is the largest ever built in Ireland. The dome shell covering the tank bowl is designed as a free span dome with 32m in diameter. Top water level in the tank is 11.8 m. The construction required some 3400 m3 of concrete and the placing of 570 tonnes of reinforcement.

Since projects this like are not the daily bread and butter of a general contractor it is common to subcontract special portions of the construction work (formwork, mild reinforcement and mechanical work) to specialist subcontractors. By co-ordinating these subcontractors and focusing on the concrete works project contractor Pierse could achieve optimum progress and results.

For the formwork system the main goal was to find a technically sound and cost-effective solution. RSB-Roundtech, an Austrian-based company specialising in custom-made formwork systems for axisymmetric concrete structures was subcontracted to supply 381 tonnes of formwork.

Essential experience

RSB-Roundtech has extensive experience in formwork for water towers. In the past 15 years RSB has been involved in the construction of most of the major Irish water tower projects: such as Tipperary (volume 800 m3), Mullingar (volume 850 m3) and Limerick (volume 900 m3).

RSB’s experience was very helpful when it came to the formwork design for the Kiltrough project. Shortly after contracts had been awarded, the RSB design team sat down with the consultant and the contractor to work out formwork design specifications. Since formwork is critical, early co-ordination is very important to ensure optimum and safe progress during the construction phase.

Ensuring water tightness

The most important goal in connection with a water retaining structure is the water tightness of the concrete in the bowl section of the water tower. Since each cold joint and any kind of through wall tie is a weak spot in this regard RSB was eager to reduce the number of cold joints to an absolute minimum and to avoid through wall ties in general.

Based on these requirements the bowl formwork has been designed so the entire bowl section including the ring beam on top could be constructed in just four lifts. In addition, the RSB system limits the maximum concrete drop to 0.5m. This allows visual control and optimum conditions for placement and compacting of the concrete – minimising the potential for honeycombs.

Another advantage of RSB is the fact that the inner and outer form of the bowl section are designed as independent, self-supporting systems eliminating the need for through wall ties entirely. Thus the potential source for leakage is eliminated.

No support structure required

By designing the inner and outer bowl formwork as two independent and self-supporting units the loads carried by the formwork during construction can be carried directly on the cured concrete structure of the main shaft.

This eliminates the need for a support scaffolding with provision for proper foundation and time-consuming erection and takedown procedures.

Quality requirements

The engineer’s quality requirements in respect of the concrete surface of the water tower structure were achieved by covering the outer formwork with 4mm BETOPLAN plywood, giving the concrete a very smooth and uniform appearance. To underline the slenderness of the structure, a pattern to the outer concrete surface was applied with 24 equally-spaced vertical feature ribs.

The construction of the Kiltrough water tower structure took approximately 15 months. About 6 weeks were lost due to bad weather conditions.

At the end of the project there is no doubt that the dimensions and the size of the water tower makes it a dominant landmark within Meath County.

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