Saving Money on Surface Water Attenuation with Marshalls Priora Permeable Paving

When Trafford Housing Trust proposed a development of mixed ownership apartments in Stretford, Manchester, one of the major planning requirements was that the new car park should not increase the ood risk on the adjacent high street. To satisfy this requirement, the original design specified 70m3 of underground plastic crates to store attenuated rainwater beneath a macadam access road. The cost for the design and installation of the crates was quoted at £35,000.

Although this was a technically e cient solution, Marshalls paving engineers saw an opportunity to reduce cost, time and carbon over the timeline of the job.

The surface of the car park was changed to Priora permeable paving to provide catchment across the site. To satisfy structural requirements, the depth of the permeable sub-base was calculated at 325mm – but to provide equivalent water attenuation to the crate system this was increased to 500mm. By simply adding 175mm to the sub-base, we created sufficient storage capability to remove the crates completely.

While making this change, Marshalls also switched the macadam access road to Keyblok and the surrounding paving to Saxon.

The total cost for all Marshalls paving materials, including Priora sub-base aggregate and membranes, came to just over £30,000. This was nearly 15% less than the cost of the design and installation of the storage crates alone, not including the additional savings from eliminating:
• Drainage channels
• Connecting pipework
• Macadam surfacing
• Auxillary flag paving

In addition to saving significant amounts of money, this solution also required less digging over less time and created a safer working environment.