Articles & Case Studies

Unique Hybrid Application Is The Solution For Residential Development

Posted: Friday 2nd February 2007

Four million new homes are planned to be built or redeveloped by 2021. The current issues regarding climate change, will have a significant impact on the methods used by building contractors and developers as they strive to accommodate the varying constraints and requirements of each individual project. Sustainable Urban Drainage Systems (SUDS) provide long-term solutions which reduce the impact of flooding and water contamination within the local environment.

Planning Policy Statement 3: Housing (PPS3) encourages the promotion of land efficiency in the development of new homes advising that 30 dwellings per hectare should be used as a national minimum to guide policy development and decision making. It is with this guidance in mind that local and regional planners alongside developers and architects are having to be more imaginative in their design and layout of developments. With space at a premium, underground sustainable drainage solutions are increasingly important to maximise the use of the land without compromising the quality of the local environment. Couple this with PPS25 and the soon to be introduced Water Framework Directive and it becomes apparent that it is no longer acceptable to ignore sustainable urban drainage solutions that significantly reduce the environmental impact of stormwater by controlling and storing at source.

Essex-based contractor, Knight Build, faced such problems during the redevelopment of an old private school into luxury apartments. The major issue faced on-site was the undulation of the ground which resulted in the below-ground SUDS application needing to be built on a slope. This translated to varying burial depths across a large area which had a significant impact on the type of drainage application that could be installed.

Following in depth consultation and technical input from Polypipe Civils, a bespoke hybrid solution was designed which integrated the Polystorm and Polystorm Lite stormwater management cells. Polystorm and Polystorm Lite both have exceptional drainage capabilities, with each cell offering a 95% void ratio in comparison to a typical rubble-filled soakaway which offers a void ratio of 30%. Both Polystorm and Polystorm Lite are often installed as standalone solutions on many projects; however this was the first instance where they have been used in conjunction.

Polystorm can be laid at a maximum burial depth of 3.7 metres, meaning that a modular cell structure could be constructed using Polystorm as the base layer, with Polystorm Lite cells placed nearer to the surface of the application. Using over 14,000 cells connected together, Polypipe Civils were able to design a structure that offered Knight Build the most viable and adaptable solution. Installed in July, the modular attenuation tank is capable of storing over 2.5 million litres of water run-off from the roof, car park and roadway at the development.

To facilitate the transportation of drainage water, Polypipe Civils also supplied over 1800 metres of structured wall Ridgidrain pipe. Whilst Polystorm, Polystorm Lite and Ridgidrain can all be used effectively as separate solutions, it is the design and ability of the products to be integrated together that allows for the greatest degree of flexibility.

As a below-ground application, the attenuation solution provided by Polypipe Civils offers little infringement on the footprint of the development, with the area on the surface of the application maintained as landscaped gardens. This satisfies the aesthetic objectives of the project, with all products buried out of sight of the residents.

Adapting to ever-changing regulations and guidelines can often place increased pressure on all parties involved in a project. Manufacturers however can use these legislative drivers to develop sustainable solutions, resulting in a proactive, rather than reactive approach to various issues faced on-site. The SUDS application installed at Bushey offered Knight Build the most practical solution to the issue of drainage and further minimised the need for heavy plant and increased labour during installation. Connecting the Polystorm and Polystorm Lite cells together quickly and efficiently enabled the modular attenuation tank to be constructed in less than six weeks, a major achievement considering the volume of cells involved.

Once installed and connected to the drainage area, the bespoke stormwater cell structure not only controls the release of excess rainwater back into the existing watercourse, but also reduces the levels of contaminants within the water to minimise the effects further downstream. Utilising a silt trap within the application set-up prevents the migration of dirt and other floatables which may hinder the performance of the cells, ensuring further cleaning of the water before it is discharged from the attenuation structure through Polypipe Civils flow control chamber, Stormcheck. Polypipe Civils advocates the need for treating stormwater to improve water quality and encourages the use of a silt trap or alternative method of filtration as part of a sustainable solution, especially with the introduction of the Water Framework Directive in the coming years.

The Knight Build project is a clear example of the flexibility of SUDS when tackling challenging site constraints. The technical consultancy and input offered by Polypipe Civils has resulted in a solution that meets both project and legislative objectives to provide long term and maintainable drainage onsite.

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