Articles & Case Studies

Cost effective concrete drainage is Costain’s choice

Posted: Thursday 22nd June 2017

The need to deliver a cost effective drainage installation quickly, safely and with minimal environmental impact was behind Costain’s decision to use a precast concrete drainage design in the construction of a new dual carriageway section of the A556 trunk road between the Cheshire towns of Knutsford in the south and Bowdon in the north for Highways England.

The £192 million scheme has over 750 precast concrete catchpits and over 100 precast concrete manhole bases. These quality-assured off-site manufactured solutions were used because they were quicker and easier to install. More importantly, embracing off-site construction enabled Costain to reduce the need for wet trades on-site, such as ready-mixed concrete and mortar, which helps reduce the numbers of vehicle movements and personnel on site, which helped improve safety.

The catchpits are used to improve the quality of runoff from the new road before it enters the local watercourses. A catchpit is essentially an empty chamber with an inlet pipe, grating and an outlet pipe set above the floor of the chamber. A minimum depth of 350mm in the catchpit chamber allows desiltation to occur. This arrangement ensures sediment carried by the run-off settles out and collects in the base of the unit so that it can be removed periodically using a pump.

In addition, this innovative new bypass also includes over 100 precast concrete manholes in its construction. Catchpits are used where run-off discharges directly from the surface water channel into the chamber but where the run-off cannot discharge directly manholes are used, many incorporating flow control units, penstocks and pollution control devices.

Costain used precast concrete manhole units in 1200mm, 1500mm and 1800mm diameter. The units came complete with integral benching, channels and pipe connectors and with watertight seals cast in place. When compared to traditional manhole construction methods, the use of precast concrete manhole construction helps speed installation, reduces costs and it minimises the need for personnel to work in confined spaces to form the base in-situ. It is also better for the environment because the precast manhole base system uses less concrete than traditional in-situ construction and there is less waste and less excavated material to be disposed of to landfill.

Using precast concrete drainage solutions helped Costain meet its construction programme so that the bypass opened in March 2017, on time and on budget. But don’t take the CPSA’s word for it; commenting on the installation Simon Ellison, Costain’s sector director, said: “These off-site manufactured precast concrete drainage chamber systems have helped us achieve our build programme and site safety targets. They are simple to install and the integrity of the finished units together with the cast-in openings and seals are integral to our long life, low maintenance drainage design solution for the A556 bypass”.

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August 2021

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