Summary of issue:
On the 9th November 2016 a naturally occurring sinkhole opened up on Magdalens Road near the River Ure in west Ripon. The collapse, reaching depths of 9m, measured 10m x 20m and extended into the back gardens of three households. The sewage system connecting the residencies collapsed and a safety parameter was set up resulting in the evacuation of 12 homes. On the 11th November British Geological Survey undertook investigation works concluding that the site remained at risk of further slippage. Monitoring continued, especially of the surface tension fractures which when recorded, had extended to twice the distance of the collapses 10m surface footprint. The BGS response team, after completing data collection, liaised with Harrogate Borough Council, North Yorkshire Fire and Rescue Service, and utility services in regards to ground stability. Currently, the team then returned to the BGS to analyse the field data. Following this, the Harrogate Borough Council officers met with representatives of North Yorkshire County Council highways department, Yorkshire Water and Northern Powergrid. As of the 14th of November all residents remained displaced, with utility companies working towards making plans for temporary solutions.
What is the cause of the issue?
Between 1834-2016 a total of 48 sinkholes affected Ripon and the surrounding area. Of these, 33% occurred in built up areas and three, occurring in 2014, 1980 and 1979, took place close to the site on Magdalens Road. Ripon is built upon the Permian Brotherson Formation (marls, limestone and gypsum), capped by glacial deposits. In the area groundwater flows from W-E into gypsum dominated successions due to the areas topography. Gypsum solubility allows for evolving Karst water filled cave environments to form on a human time scale with two joint sets (N-S & E-W) controlling orientation of these ever developing cave systems. The formation of such voids beneath the surface promotes instability issues and collapses despite, in this case, surface loading not being altered.
Can data relevant to the site be sourced, provided and used as evidence:
There are several national databases for karstic and natural cavities, recognised within the industry. This alongside groundwater, rainfall and other climate data allows for modelling of such sub-surface environments and direct evidence for such events.
What is the professional interpretation according to Terrafirma?
Although Ripon’s ground stability issues arise naturally through gypsum dissolution, the potential for further damage to property is a very real prospect. Furthermore, the likelihood of future events is high, with increasing wet weather in this century potentially leading to more subsidence issues than seen in previous years. Within this, such events hold the very real prospect of causing injury or death, something that will remain as long as understanding of the subsurface remains poor and the built environment resides on potential high risk zones without correct site investigation, risk analysis and potential remediation if required.
Overall, additional investigation and data analysis are essential to provide a clear picture of the subsurface and the possible effects increasing wet weather may have in the coming months and years to determine the potential threat subsidence has and will continue to have on this historic city.