Report on a responsive visit to a sinkhole at Magdalen Road, Ripon on 11 November 2016.

Aerial image of the sinkhole using an unmanned aerial vehicle (UAV)

On 10 November 2016 the British Geological Survey received a call from the emergency planning team of Harrogate Borough Council. BGS has responded by sending a multi-skilled responsive team comprising three people.

Sinkhole description

On 10 November the sinkhole was reported to be in the order of 10 m in diameter and of unknown depth. By the time of the visit, cracking associated with the sinkhole had extended to almost twice this footprint (Figure 1). The BGS responsive team estimated the depth to be 5 m. There was no visual evidence of damage to services (Figure 1). The BGS responsive team have obtained aerial images of the sinkhole using an unmanned aerial vehicle (UAV) for close-range aerial photography.

Geographical and geological setting

The geology of Ripon area

Ripon is a historic city and market town in North Yorkshire. The sinkhole was situated at NGR 431607 471684, on the eastern side of Ripon close to the River Ure.

The geology comprises Marls, Limestone and Gypsum of the Permian Brotherton Formation, capped by glacial deposits (Figure 2). Karst in gypsum (hydrated calcium sulphate) can form water-filled cave systems. The rapid solubility rate of the gypsum means that the karst is evolving on a human time scale (years-decades).

Groundwater flows from the high ground west of Ripon, through the rock into the gypsum. While passing through the gypsum water dissolves it. It then emerges as artesian sulphate-rich springs in the low ground along the River Ure. The dissolution features and caves are controlled by the joints in the rock and form the resultant maze of caves.

The joints in the rock beneath Ripon trend approximately north–south and east–west defining the orientation of the caves. The water flow under Ripon city runs approximately from west to east.

Frequency of sinkhole collapse

Close to this sinkhole, previous collapses have occurred in 2014, 1979 and 1980. The wider area of Ripon periodically encounters sinkholes; in the 1980s and 1990s, one was appearing every two to three years (Cooper, 1998).

Triggering mechanisms for these sinkholes

The triggering mechanisms for these sinkholes to form can be one, or a combination of mechanisms, the most common mechanisms include:

  • enlargement of the caves due to subsurface dissolution
  • infiltration of water from the surface washing down fine materials from the covering deposits
  • fluctuations in the groundwater levels; rising groundwater wets the covering materials and falling groundwater leaves the cover saturated and without hydraulic support.

Anthropogenic impacts

Karst environments are sensitive to anthropogenic impacts, e.g. leaking drainage pipes, burst water mains, irrigation or changes in groundwater level.

Mitigation

The BGS response team have collected data and information on the event and conditions at the site. They have liaised with Harrogate Borough Council, North Yorkshire Fire and Rescue Service, and utility services with respect to ground stability. They are returning to BGS to further process and analyse the field data.

References

Cooper, A H.  1998.  Subsidence hazards caused by the dissolution of Permian gypsum in England: geology, investigation and remediation. In: Maund, J G and Eddleston, M. (eds.) Geohazards in Engineering Geology. Engineering Geology Special Publication 15. The Geological Society of London, 265–275.

Contact the Sinkhole Response Team

British Geological Survey
Keyworth
Nottingham
NG12 5GG
E-mail: Sinkhole team