Project outline

Murphy is currently undertaking a Design, Build and Operate scheme at Stillorgan Reservoir on behalf of Irish Water. The existing facility provides drinking water to over 200,000 customers in the Dun Laoghaire-Rathdown and Dublin City region.

The existing site is a modular storage facility for treated drinking water, built in two stages between 1862 and 1885. Covering an area of 16 hectares, it comprises three open reservoir cells: the Gray, Upper and Lower Reservoirs. These represent the penultimate open storage system in Europe. The facility is fed from water treatment plants at Vartry, County Wicklow and Ballymore Eustace, County Kildare, via Saggart Reservoir. Constructed over 150 years ago, there are various issues with the structure and operating system, in addition to the challenges posed by open storage.

The aim of the current project to remove the open storage that is currently in operation and replace it with a covered reservoir of 160ML storage capacity. This will ensure a long-term, secure and sustainable water supply to the region, meeting current and future regulatory requirements.

Key challenges

Sensitive receptors are a key challenge on the Stillorgan Reservoir project. The majority of local residents are located close to construction works on the northern embankment of the Gray Reservoir in Stillorgan Heath. To address this, normal working hours have been reduced and the monitoring of noise, dust, vibration and odour has been increased.

As the Gray Reservoir has not been fully drained since its creation in 1885, there are a number of challenges involved in the drawdown operation. Six piezometers are being utilised in the upper embankment to monitor any movement and ensure stability. Siphoning will remove the need to use the existing valves and operating system, which have been defunct for over 30 years.

Project delivery/ innovations

The current project aims to deliver the 160ML reservoir, using the most sustainable and innovative construction techniques possible, while remaining at all times economical.

A significant innovation will be the removal of concrete baffle walls within the cells of the newly constructed storage units and replacing them with geotextile curtains. These will provide the same operational value but at lower cost and with less construction time.

Pipework design for the works has been optimised in order to ensure there will be no wastage or needless runs of pipe. Pre-cast concrete has also been explored for different aspects of the construction process and this too will have a large saving on time.

Key facts

• Drawdown of Gray Reservoir
• Internal and external pipelines
• Provision of CCTV
• New entrance at St. Raphaela’s Road
• New bridge within the site, spanning the Carysfort-Maretimo Stream
• Design and construction of a covered reservoir totalling 160 ML of treated drinking water storage, consisting of three cells with baffle walls to prevent short circuiting of flows. This will represent 1.5 days storage by 2031 demand.
• Design and construction of an ‘On-Site-Electrolytic-Chlorination’ (OSEC) plant for secondary disinfection at the site
• Pipeline modifications and improvements, including a manifold building (c.2,900m² area) for rationalisation of incoming pipework
• Drainage and drainage attenuation
• Site services and landscaping
• Internal road network
• Commissioning of new storage and disinfection dosing, followed by decommissioning of upper and lower reservoirs
• Operation of new reservoir and OSEC plant for a period of 3 months
• Materials will include:
  • 2,500 tonnes of steel
  • 24,000m³ of concrete
• At its peak the project will have 100 operatives at any one time
• Plant will include 3+ crawler cranes working at any one time
• Concrete base slab pours will consist of 450m³ per pour for 18 pours