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Rainwater run-off for new dual carriageway section of major A-road

Precast concrete drainage products have enabled Highways England’s main project contractor Costain to safely deliver a new £192 million dual carriageway section of the A556 trunk road between the Cheshire towns of Knutsford in the south and Bowdon in the north. The dual carriageway, which opened in March, enables this strategic route to bypass the villages of Tabley, Mere and Bucklow Hill. Over 50,000 vehicles a day use the 7km stretch of road, which is an important link between the M6 motorway at junction 19 and the M56 motorway at junction 7.

Costain’s design partner for the scheme was Capita. Along with the task of designing the new road, Capita was also responsible for developing a drainage design solution with the capacity to deal effectively with rainwater run-off from the new road.

The drainage design uses precast concrete surface water channels at the edge of the carriageways to convey rainwater run-off to a series of V-gratings and then into catchpits. “Average catchpit spacing is approximately 80m but actual spacing is dependent on the gradient and width of a particular section of the road,” explains Anthony Bennett, an Associate at Capita’s Real Estate and Infrastructure arm.

The decision to use catchpits was the result of a value engineering exercise, which demonstrated that a mixed drainage solution was the most cost effective option. “The drainage philosophy was that a mixed drainage design would be an effective solution rather than having separate reception gullies and a separate a carrier system,” Bennett says.

Over 750 precast concrete catchpits have been used on this scheme in both 1200mm and 1800mm diameter. Manufactured by British Precast Drainage Association member FP McCann the precast concrete units incorporate a pre-cored monolithic base fitted with watertight, flexible connector seals to allow concrete drainage pipework to be connected quickly and easily to the unit.

The catchpits help improve the quality of runoff introduced to the environment. A catchpit is essentially an empty chamber with an inlet pipe, grating and an outlet pipe set above the floor of the chamber. This arrangement allows sediment carried by the run-off to settle out and collect in the base of the unit. The sediment is periodically removed using a pump. “A minimum depth of 350mm in the catchpit chamber allows desiltation to occur in each chamber rather than having a final silt catcher or trap at the end of each catchment,” explains Bennett.

There are four catchments areas on the new road, three are new and one is a combination of catchments remaining from the previous single carriageway road that has now been bypassed: Catchment A covers the first 1000m of the road; catchment B the next 2500m; and Catchment C, which includes catchment D from the previous road, covers the next 4500m and includes two large ponds.

Catchments A and B discharge into wetland ponds, both of which incorporate reed beds, before flowing into Tabley Brook, a local water course. Pond C is more of a traditional attenuation pond, which then discharges into balancing pond D and on into the River Bollin./p>

The ponds help deal with contaminants washed off the road surface in the run-off, such as engine oil, tyre fragments, brake dust and fuel as well as liquids spilled from collisions and breakdowns. The ponds allow pollutants to slowly filter from the water to help prevent contaminated water ending up in local watercourses. The ponds also store the run-off to allow it to be released safely, at a slower controlled rate, into the local river network to reduce the risk of flooding.

The scheme also includes over 100 Easi-Base precast concrete manholes. Catchpits are used where run-off discharges directly from the surface water channel into the chamber. However, where the run-off cannot discharge directly manholes are used, many incorporating flow control units, penstocks and pollution control devices. “If the run-off does not discharge directly into the chamber, the design was changed from catchpit to manhole,” explains Bennett.

Easi-base precast concrete manhole units in 1200mm, 1500mm and 1800mm diameter have been used. The units come complete with integral benching, channels and pipe connectors, with watertight seals, cast in place.

The use of precast concrete catchpits and manhole bases embraces off-site principles and helped Costain reduce the need for on-site wet trades including ready-mixed concrete and mortar, which helps reduce the number of vehicle movements and personnel on site improving safety. These off-site manufactured solutions are quicker, easier and safer to install and, because the system is manufactured under factory conditions and is quality assured, the units’ quality and finish are better than could have been achieved under site conditions. In addition, users can be sure that the precast units comply with all necessary technical standards.

The use of precast concrete drainage solutions helped Costain to ensure that the bypass opened in March 2017, on time and on budget.

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 for the A556 bypass”.

With the new dual carriageway now open, work will focus on converting the old A556 to the B5569 – with a new single carriageway road for local communities alongside a segregated green corridor for pedestrians, cyclists and horse riders.