Subsidence caused by gypsum dissolution at Ripon, North Yorkshire

In the afternoon of Wednesday 23 April 1997, a large subsidence crater opened up in front of a house on Ure Bank Terace, on the northern outskirts of Ripon in North Yorkshire. Overnight its sides collapsed inwards, so the hole had doubled in size by the next morning. The subsidence crater was then 10m in diameter, and 5.5m deep to a choke of debris overlain by water 1m deep. Its sudden appearance was the cause of considerable concern to the occupants of the adjacent house, and the event was widely reported in the national press and media. A subsidence hollow as mapped at this site by the 1856 Ordnance Survey and documented by Cooper (1986). More subsidence had occured at the Ure Bank site in previous years, but this latest colpase had rather more impact. Creeping movement of the soil towards the new hole meant the adjacent house was destined for demolition. The event was the latest of a seris of ground collapses that have occured, at an average rate of about one per year, in and around the city of Ripon. While they are little more than an inconvenience in farmland, they have the potential to casue serious damage when they occur in built up areas.Ground cavities associated with subsidence features in an active gypsum karst may expand by dissolution at rates that are significant on an engineering time scale, as in the case of subsidence formation at Ripon, North Yorkshire. Gypsum dissolution may even be enhanced where groundwater flow is concentrated around the margin of an engineered plug, thereby propagating and spreading subsidence. Complete prevention of collapse in gypsum may only be possible by sealing from contact with groundwater, and this is probably impossible at Ripon, where water circulates into and from the buried valley of the Ure. Furthermore, any interruption of the natural groundwater flow may aggravate dissolution in the adjacent ground.