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Large box-culverts enable watercourse diversion


Large box-culverts enable watercourse diversion

Some of the largest precast concrete box-culvert sections ever manufactured by FP McCann have been installed on a watercourse diversion project for one of the UK’s newest towns. Wixams in Bedfordshire is a 4,500-home development, which also includes schools and commercial and retail properties.

The town is situated on 750 acres of brownfield land and it was necessary for the developer, L&Q Estates, to temporarily divert a section of Harrowden Brook to prepare the existing trench for installation of the box culvert. The brook runs beneath the site’s Northern Distributor Road, an access road to two new warehouses which will help deliver jobs to complement the housing at Wixams.

Significant vehicle loadings on the access road during onsite construction meant that precast concrete box-culvert sections were considered the only option to channel the watercourse. Smith Construction Group, which is based in Milton Keynes, is undertaking the civil engineering and groundworks at Wixams and worked closely with FP McCann's Derbyshire-based box-culvert design team.

L&Q Estates’ project engineers specified a 65m-long precast concrete box-culvert with 4.8m internal width and 2.7m internal height. The 35 individual sections each weighing around 22 tonnes were manufactured at FP McCann’s production facility in Byley, Cheshire.

A diversion was created 25m from the original course of the brook and a new 100m long, 4.5m deep open-cut trench created to carry the flow to allow the construction of the culvert. The existing watercourse had to be correctly sized and lined with sand screed in preparation for installation of the box culvert units.

Each unit was delivered to site on a single articulated lorry and offloaded and installed using the largest crane available from the hire company. A schedule was put in place to manage delivery of the culvert units, with 60 to 90 minute intervals between shipments, starting at 8am. Three units were delivered on the first day and seven-a-day thereafter.

The first precast box-culvert unit was carefully lifted into the trench, which was repeated for each subsequent section. A culvert puller was used to winch each section tight to the next throughout the run.

The precast units were externally waterproofed with bitumen across the base and sides during the manufacturing process. An onsite contractor sealed all joints with bitumen and waterproofed over the top of the culvert. A 3mm rubberised board was laid over the culvert to protect the waterproofing.

With the culvert in place, headwalls were built using reinforced concrete and gabion baskets to improve the flow of water into and out of the conduit and to control erosion and scour. The brook was then reverted back to the original channel and the trench around the box culvert filled in. A landscaper was brought in to finish the job.

John Banks, commercial director, Smith Construction, said: “The box-culvert team at FP McCann has successfully helped design an offsite precast concrete solution to successfully manage the natural watercourse diversion at this location.

“The scale of this project and the size of the precast box-culvert sections meant we had to take extra care at the planning stage. From start to finish, the culvert run was completed in just eight days, which was as good as were hoping.

“Given how smoothly delivery and installation went, we might aim for eight or nine units a day in a future project. The work at Wixams demonstrates the flexibility that concrete box-culvert sections can offer.”

FP McCann’s box-culverts manager Alex Kirkham added: “The precast culvert sections for the infrastructure works at Wixams are amongst the largest we have ever manufactured at Byley and demonstrate our ability to offer a design solution to fit the most exacting of briefs.”

Gregg Wilkinson, group managing director at Warwick-based L&Q Estates, said: “As master planners of this site, it was vital we employed contractors we knew we could trust and would carry out this detailed work on time and to a high standard. This has been achieved through a united approach to the work and we are delighted with the final outcome.”