As ever, time is money in the civil engineering industry and anything which can deliver high quality results in double-quick time is worth investigating.
Hence in the specialised business of storm drainage and flood attenuation – currently a rare growth area in the industry – engineers and contractors are catching on to the considerable advantages of using pre-cast concrete components.
Where previously they would not have thought twice about specifying in-situ concrete, utility contractors now realise the benefits of being able to crane-in precast structures such as pumping chambers, control chambers, manholes and CSOs which are delivered to site complete and ready to install.
Kijlstra, which has pioneered the use of precast in the Dutch water industry, has recently spearheaded a campaign to encourage its wider use in the UK – with considerable success.
In Milton Street, Airdrie, North Lanarkshire, main contractor Byzak has just completed the most recent installation of a Kijlstra precast CSO as part of its £15 million contract with Scottish Water to upgrade the sewage infrastructure and flood attenuation system at the Airdrie Business Centre and surrounding houses.
Here, Byzak has installed a 60 m long, 1.5 m diameter storage sewer to collect excessive surcharges from the combined sewer during periods of heavy rainfall.
Upstream of this storage sewer is a 3.9 m long, 2.7 m wide Kijlstra CSO. During normal flows, the CSO feeds into the existing 450 mm diameter sewer which discharges into the South Burn, a culverted watercourse running underneath Milton Street.
Traditionally, this CSO would have been built with in-situ concrete, a labour-intensive and time-consuming activity for the contractor. But the model supplied by Kijlstra comprised only three main components – the main chamber, the roof slab and the weir wall - which were simply craned into position and assembled on site.
“We chose precast here for a number of reasons, and cost was not the main one” says Byzak’s site agent on the project, Kris Cameron. The main reason was installation time, which of course goes hand-in-hand with cost. “In terms of up-front costs, precast and in-situ were like-for-like” says Mr Cameron. “But the precast option is so much quicker”.
The Kijlstra CSO, supplied by SCP Environmental, was delivered to site on 20th April and installed in less than four hours. “Of course we had to prepare the site beforehand, which involved sheet piling the area, digging the excavation and preparing the base, but the actual installation was very quick. The main chamber was lifting in and the other components slotted into position and grouted in place” says Mr Cameron.
Once in place, it only remained for Byzak to make the connections to the gravity sewer and the storage sewer before installing the 6mm static peak screen supplied by Hydrok. “Building the CSO with in-situ concrete would have taken about four weeks instead of the five working days with the precast CSO” says Mr Cameron.
With time reductions of this magnitude, life becomes a lot easier for the contractor on a project like this. And there are other benefits as well. Because precast units are built in factories, rather than put together by the site team under temporary working conditions, quality is generally much higher than is obtainable with in-situ concrete.
And by minimising several lengthy and complicated site operations such as formwork erection, labour costs are reduced and site safety enhanced.
This latest project follows hard on the heels of another scheme for Scottish Water with main contractor Clancy Docwra. This project, in a residential area of Saughton, Edinburgh, was also designed to alleviate storm surcharges which were flooding nearby houses.
Here, the ability to install and complete the CSO quickly was of the utmost importance since the excavation was in the middle of a residential street and on a major bus route. Another benefit which emerged here was that the excavation required to install a precast CSO was significantly smaller than that needed to erect formwork for an in-situ CSO.
Back at Milton Street, Kris Cameron is convinced that precast is the way to go. “We have a couple more CSOs for this project and we’ll definitely be using precast” he says. “It takes out a lot of site work, it’s reliable and it’s quick. It’s got to be the way forward”.