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GCL-sealed detention basins enable town to continue growing

Location of work/project:



United Kingdom – Horley – West Vale


Horley in the United Kingdom, has drawn continual development interest due to its convenient connections to major metropolitan areas and transportation options. To support the next phase of Horley’s development, a proposal for a major new development known as West Vale was accepted.

The West Vale site would support 1,500+ homes, a new primary school, and a neighbourhood centre with shops, pubs/restaurants, and other community facilities.

​Phase I of the project included roughly 600 residential properties, but the proximity to the River Mole, presented flood planning concerns. The Borough Council requested drainage works that could store and gradually release flood water at the same rate as water had discharged in the area prior to development. WSP Parsons Brinckerhoff, serving as the consulting engineering firm, proposed a system of four detention basins to provide attenuation with water quality treatment and a flow control device in each basin to manage discharge into the river.

​A geosynthetic clay liner system was specified to provide the essential seal in each basin. The ponds, which were excavated to an average depth of around 6m, were proposed with 20 – 25% inclines. NAUE carried out the slope stability analysis to ensure the lining system was compatible with the site’s engineering needs. Factoring in the underlying geology (Alluvium, River Gravels, and London Clay) and the planned usage of cement stabilised fill materials, Bentofix® X2 NSP 4900 was chosen.

This type of Bentofix® GCL is characterized by a uniform layer of natural sodium bentonite powder encapsulated between two strong and durable polypropylene geotextile layers – one layer woven and the other nonwoven. Importantly, a polyethylene coating is bonded to the woven layer. For Horley, it meant that prior to hydration an immediate low permeability barrier would be created and counteract any incompatibility between the sodium bentonite and the cementitious backfill. Though the GCL solution was significantly thinner than a conventional compacted clay layer would be, the GCL provided equivalent or greater sealing performance. Bentofix® also provided high internal shear strength and creep resistance that secured the GCL on the slope in ways that a compacted clay system could not match.

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