Wavin engineers a calm-headed stormwater solution

Wavin, the UK’s leading manufacturer of plumbing, rainwater and drainage systems is providing a stormwater management solution for a tricky two-phase car park construction project at a head office and manufacturing facility in Wiltshire. The project required Wavin’s expert design engineers to demonstrate some lateral thinking in order to value engineer the original system design and provide a more cost effective and feasible solution.

Planning

Phase 1 of the project, being delivered by main contractor MJ Church, is the excavation and conversion of a lawned area adjacent to the head office building into a new 50 m2 car park, required to support the facility’s future expansion. In order to comply with the Flood and Water Management Act 2010, surface water run-off had to be managed to reduce pressure on mains drainage, and the best practice solution identified was a SuDS (Sustainable Drainage System) approach.

The second phase comprises the redevelopment of some of the existing car park in order to provide additional yard storage and picking space for the increased activity planned on site. The SuDS solution chosen was a modular attenuation tank to cover both phases of the project, which required Wavin and the contractors to provide a solution that would enable both phases to be constructed separately but be fully integrated once connected.

To provide efficient management of water, Wavin’s design engineers initially specified a complete solution that comprised its AquaCell Stormwater Management System, Osma UltraRib pipework and connections, plus Wavin Range 600 Inspection Chambers, and finally a Wavin Mosbaek Vortex Valve.

 

Site challenges

Before work could start on site a number of challenges had to be addressed, not least the fact that there is no existing drainage in the area. A variety of underground criss-crossing high voltage electrical cabling necessitated careful positioning of the installation. Wavin provided a ground scan to give detailed information on the existing services which helped the contractors locate them by digging trial holes.

Following excavation, additional high voltage cables were discovered, which meant a slight redesign of the car park, and some repositioning of the tank installation were required.

Wavin’s Production Manager Paul Collins worked closely with the contractor throughout this process to overcome various technical issues.

Split tank design

Another challenge was that the original engineer’s design specification, as well as not being system-specific, also assumed a single attenuation tank would be provided for both phases of the project. Wavin in conjunction with the contractors identified that this would not be an appropriate solution due to risks posed by the phased construction. The tank’s geomembrane would have had to be exposed in an open trench until the second phase had been completed, meaning there was a risk of damage, as well as contamination from construction silt and other debris from the site. As a result Wavin and the contractors took the decision to design a split tank, with a 20m and 8m section for phases 1 and 2 respectively. Constructed separately, these tanks were then connected using Wavin’s UltraRib pipe.

The first task was to adapt the tank design in order to accommodate the Wavin AquaCell units. “The modular AquaCell units are 1 m long so we had to adapt the design to create a more efficiently sized attenuation tank,” commented Stuart Henly, Design Engineer at Wavin. “We achieved this by simply extending the excavation from 27.5m length to 28m.” A trench was then created to incorporate the 0.8 m depth of the tank, over which a 0.75 mm thick LLDPE Geomembrane was wrapped and doubled welded.

The BBA approved AquaCell system is available in a range of four units of differing strengths, AquaCell Eco, Core, Prime and Plus which can be mixed and matched to create the optimum installation for a range of applications including landscaped or trafficked for either shallow or deep applications.

Designed to deliver optimum performance under the heaviest loads, AquaCell Core was chosen for the project. To ‘future-proof’ the car park for possible future expansion and due
to the shallow cover depths, a 150mm protective reinforced concrete slab was installed over the tank for the second phase of the project. The concrete depth of the slab was specified by Wavin’s designers in accordance with CIRIA C680. The addition of the slab following Wavin’s structural calculations meant that the installation would be able to support the loads from delivery vehicles in addition to cars.

A Vortex Valve from Wavin will be retrofitted to the Flow Control Manhole during phase 2 of the project. Manufactured in 316L corrosion-resistant stainless steel the valve formed a crucial part of Wavin’s stormwater solution, enabling precise control of discharge flow rates from drainage, attenuation and infiltration systems. The custom-built Vortex Valves control flow using hydraulic effect, meaning they need no moving parts or power, and are self-activating. As a result of their efficiency, the vortex valves have much larger openings in comparison to an orifice plate, greatly reducing the risk of blockages.

To complete the project, Wavin UltraRib, in 225mm and 300mm diameter is being installed along with a total of six Wavin Range 600 Inspection Chambers.

Osma UltraRib is a range of structured wall pipes, suitable for both foul water and rainwater transportation. The products’ external ribs give the pipes exceptional radial strength and a stiffness classification of SN8. The comprehensive range is available in diameters from 150-300 mm, and meets all relevant regulation and compliance standards.

Phase 1 will see two 600mm plastic Wavin Range 600 Inspection Chambers replacing the initial specification of 1.2m concrete rings. The preformed Wavin Range 600 chambers offer health and safety benefits as their installation does not require heavy craning equipment. In phase 2, three further Range 600 chambers will be installed to the discharge side of the attenuation tank. A 1.8 m concrete ringed chamber will also be created to house the Vortex flow control valve.

Another novel design solution developed by Wavin for the project was for the vents required for each section of the tank. Instead of the normal air vent design which would require a gully grating, Wavin managed to arrive at a simpler alternative: “We have designed the system so that the vents come off the tanks and pass through the wall of a nearby inspection chamber which have vented covers,” commented Stuart Henly. “This provides a neater result and no additional concrete forming required on site.”

Stephen Daniels of MJ Church says that the phasing of the works was a key success factor to meet both site safety needs and the need for the car park to remain in use. “The phasing allowed a clear segregation of the car park while still allowing use of a section of the car park for staff. This approach meant it was a far easier task to provide an easily managed safety environment.”

Stuart Henly concludes: “The project is a great example of the expertise of Wavin’s Technical Services team. We have demonstrated how by value engineering a system design we can provide an efficient solution that is fit for purpose. It also shows the team’s quick thinking to overcome site obstacles such electrical cabling and provide an effective solution.”