We were instructed to undertake BRE365 compliant soakaway infiltration testing for a large housing development in Highworth, Swindon, on behalf of a large national housing developer (Persimmon). We previously undertook a full geotechnical and geo-environmental investigation and assessment for the site.
Following receipt of a Planning Committee Report, further soakaway feasibility assessment was required in order to support recommendations for the use of sustainable drainage systems. Three additional trial pits were excavated across the site.
The site is recorded to be underlain by bedrock comprising the Jurassic Corallian Group bedrock. The south western area was underlain by the Stanford Formation (formerly the Coral Rag of the Upper Corallian) with the remainder of the site (north and centre) underlain by the Hazelbury Bryan Formation (formerly the Lower Corallian). The Corallian strata is recorded to be underlain by the Oxford Clay, but this was not encountered in the investigation.
During the previous phase of works, spalling/collapse of the exploratory hole sides was observed in all positions with the addition of water. In order to carry out additional testing and to maintain the integrity of the soakaways, the test pits were filled with aggregate and a central monitoring installation constructed.
During the testing, sufficient infiltration was achieved to allow an infiltration rate to be calculated and this enabled finalisation of the sustainable drainage solution design. Overarching cautionary recommendations were also made for due to the potential for the creation of solution features within the underlying limestone bedrock.