The Ripon area of North Yorkshire has unique geological and hydrogeological factors which can result in catastrophic subsidence occurring due to dissolution of thick gypsum below the region.
The solid geology below the site at Springfield Close was identified as the Edlington Formation (30m and 40m in thick). The Edlington Formation is dominated by reddish brown marls with interbedded mudstone and gypsum, passing down to massively bedded gypsum / anhydrite at the base.
As part of a reconnaissance survey a large historic subsidence hollow was witnessed on the eastern part of the site.This feature was also recorded on OS maps dated c.1856.A British Geological Survey map (extract below) also recorded the presence of historic subsidence hollows close to the site.
Gypsum Dissolution
The site was identified to lie within an area potentially subject to localised subsidence hazards, associated with both existing cavities as well as on-going dissolution of gypsum deposits in areas affected by groundwater moving towards the Ure Valley.
For these areas as a minimum, a geotechnical Phase 1: Desk Top Study Report and site appraisal are required, followed up by an appropriate programme of intrusive ground investigation designed to provide information needed for detailed foundation design.
The Phase 1: Desk Top Study identified that the site lies within an area where there is an approximate 40% – 50% relative possibility of gypsum dissolution occurring in the future.
Whilst the probability of gypsum dissolution related subsidence occurring at any location is very low, the results can be significant, and the consequences could be catastrophic if an occupied building were to be affected.
