Gypsum-related sinkhole remediation Magdalen Road, Ripon, Yorkshire UK

Unfortunately the recent sinkhole in Magdalen Road, Ripon (10 November 2016) was not the first to affect Ripon. The impact of the dissolution of the buried gypsum is notorious (Cooper 1998) in this area. It results from the generation of drop-out sinkholes with a potential to cause significant damage to buildings and infrastructure. The geology beneath Ripon comprises Permian marls, dolomitised limestones and gypsum, capped by approximately 10 m of glacial deposits. Dissolution of the soluble gypsum at depth results in buried voids that propagate to the surface as sinkholes. The detail of how the sinkholes develop is not fully understood (size and position of the dissolutional openings that develop in the gypsum). This is because the cavities are inaccessible and water filled. Once a sinkhole develops the ground adjacent to it is unstable; access to the sinkhole is not possible, and any intrusive investigation (such as boreholes) have the potential to further destabilise the ground. Consequently, the sinkholes are usually remediated to make them safe as soon as possible. Historically, microgravity geophysical techniques have been demonstrated to be the preferred technique for detecting buried cavities and disturbed ground associated with the dissolution. Here we propose to use a range of geophysical techniques to monitor the remediation (infilling of the cavity) and record changes to the condition of the ground throughout the depth of the sinkhole in order to learn more about its geometry and depth and hence the likely formational processes. In parallel we propose to use the borehole records held by the National Geoscience Data Centre to generate a 3d geological of the Ripon area and attribute this with the geometry of the sinkhole and the associated zone of disturbance in order to further the understanding of the geological context of the sinkhole, i.e. in relation to the distribution of the host sediments and the groundwater table.
As well as contributing to the process understanding, this approach will (i) allow the comparison of a range of geophysical techniques (micro-gravity, electrical resistivity and seismic) to assess their viability for void detection and remediation monitoring, which will help inform guidance for any subsequent events and (ii) it will feed back to inform investigation requirements for proposals for any future developments.

This proposal offers the potential to build on the impact of NERC's growing body of peer reviewed sinkhole related research by monitoring the effectiveness of the remedial measures based on geophysical surveying and monitoring using a range of techniques in order to further process understanding with respect to the dissolution occurring at depth. Additionally, the research will provide information on the benefits of the geophysical techniques in providing a measure of the success of the proposed remediation methodology. By working in collaboration with Harrogate Borough Council there will be an opportunity for these findings to contribute to further development of the existing guidance (e.g. Cooper and Calow, 1998) as well as preparing peer review literature for dissemination to the engineering community. This is particularly timely in that the Draft Local Plan is currently out for consultation, something that the BGS would normally contribute to. Clear guidance on how to monitor remediation has clear benefits for any future sinkhole occurrences.