Image above: Three homes had to be evacuated after a 25ft-wide (7.5m) sinkhole opened in a street in North Yorkshire. (Courtesy of BBC News)
Across the country, sinkholes have been swallowing up earth at 4 to 5 times their usual rate but do you know why they occur or whether your client is at risk of them?
You might imagine that this uncharacteristic rise in the emergence of sinkholes has been caused by the recent inclement weather that has swept the UK but, as Dr Vanessa Banks asserts, this is only part of the problem. Speaking to BBC news, Dr Banks said “Sinkholes have not been caused by the weather; they’ve been triggered by the weather.”
To discover one of the key causes of sinkholes, one must delve underground where the underlying rock, such as salt or gypsum, often has naturally occurring cave structures. Gypsum is a solid rock but is surprisingly susceptible to water: a van-sized chunk of the rock would take just 18 months to be dissolved in a fast-flowing river. All that these cavities have to support the earth above them are mere bridges of rock or soil that are inherently unstable due to their geological composition.
The increase in rainfall has indirectly caused the spate of sinkholes to manifest, simply by increasing the amount of water that’s passing through the ground. The groundwater further disrupts these already unstable bridges and widens the cavity, creating the sinkhole into which land – and often, property – is pulled.
This phenomenon does occur, albeit more infrequently than present, from other triggers such as burst pipes or damaged drainage systems and also repeatedly transpire as a by-product of natural land subsidence so it’s important to note that sinkholes aren’t simply unique to rainy periods.
Nor is the phenomenon restricted geographically, as sinkholes have been showing up suddenly across the country at speed and it’s difficult to predict where they might appear next. Indeed, within the past week, three homes had to be evacuated in Ripon, North Yorkshire after a 25ft hole grew beneath one detached house leaving it on the brink of collapse. The unfortunate home owner had spent barely a week in the property after recently completing their purchase; had their solicitor practised sufficient due diligence and checked for environmental risks?
In Sittingbourne, Kent: an area underlain by chalk and limestone, a 15ft hollow was spotted on the central reservation of the M2 last week, causing severe delays to motorists. Sarah Fray, director of Engineering and Technical Services at the Institution of Structural Engineers, told the Independent that it was likely caused by the corrosion of the alkaline chalk and limestone rock by acidic rainwater. But this only serves to prove the unpredictability of sinkholes, as that particular process is thought to have taken millennia.
And it seems that even sites that were “thoroughly investigated” and had “piled foundations [that] were designed by a structural engineer, taking in to account the geo-technical reports” are vulnerable to the problem. This month, 48 properties in Oatridge Gardens, Hertfordshire had no option but to be evacuated after a 35ft sinkhole opened up in a quiet, suburban cul-de-sac: an estate that was developed as recently as 2008.
So how can you help your clients prepare? Certainly, being conscious of the proximity to underground cavities – either natural or artificial – is essential, as is knowledge of the geological make-up of the area and the risk of natural subsidence.
We offer environmental and other mining reports from Groundsure and Landmark which highlight a number of environmental hazards to a given property, including groundwater flooding, natural ground subsidence, former coal mining sites and other potential environmental threats.
However, relying on the top level findings can be fallacious, as a ‘pass’ grade could simply mean that there is no contaminated land nearby. It’s imperative to drill down within the report to ensure that you and your client are suitably informed; failing to look beyond the top level results could result in disaster.
To find out more, call us on 0844 249 9200.