Client: Tesco Stores Ltd
Sub-contractor: White Young Green (Bristol Office)
Type of works: design of permeable pavement drainage and the extension of an existing Tesco store car park.
The project was an extension to an existing Tesco store car park, completed in 2007. The proposed car park area was to the east of the store and joined the existing car park at a gradient of approximately 1:50. The car park was to serve the store with typical loadings of cars and vans. The car park was to be built on a rock subformation.
The proposed Formpave solution was a tanked system using an impermeable membrane. The impermeable membrane was wrapped around the sub-base to form a storage tank within the sub-base for the storm water runoff.
The pavement was the typical Formpave Aquaflow construction depth of 480mm, with the Inbitex geotextile below the laying course to assist the bioremediation process and filter out heavy metals.
The sub-base was a clean crushed stone with the lower sub-base, 250mm deep, being a
10 to 63mm grade and the upper sub-base, 100mm deep, being a 5 to 20mm grade. The laying course, 50mm deep, was a 2 to 6mm grade. Aquaflow 80mm rectangular block was laid with a 3mm grit for brushing on the surface. It was essential that crushed stone and not crushed gravels were used to ensure a good interlock of the aggregate material.
Due to the slope on the site and the discharge restriction imposed by the EA, the standard depth of 480mm was not adequate for the entire site. Therefore Formpave designed the system so that the lower edge of the car park had additional stone sub-base material to store the water in. To achieve this, the subformation was slackened to a gradient of 1:125, whilst keeping the surface blocks at 1:50. This achieved the additional storage without upsetting the final look of the car park surface.
The primary reason behind the slackening of the gradient was to achieve the storage without cutting too far into the rock since this would have been too expensive and may have stalled the contract.
The overriding factors that led the client to choose the Formpave Aquaflow system for this scheme was the rock formation. The use of traditional piped systems with storage tanks and petrol interceptors would have been economically prohibitive to construct due to the excavations in the rock. There was also a time issue as excavation in rock can be exceptionally lengthy and subject to unforeseen hard spots and ultimately costs and time.
By using the Formpave Aquaflow System, the client and engineers achieved an affordable attenuation system meeting the EA discharge restrictions on both water quality and quantity within their programmed timescale.