The process of buying a property has a plethora of considerations resting on the decision to buy or not – Contaminated Land, flooding, radon, planning applications in the area, local energy infrastructure and ground stability should all be factored in.

Planning applications such as extensions can easily be scuppered by issues relating to ground stability, such as…

Ground stability issues can manifest in a number of ways – small cracks in walls can be indicative of minor ground stability issues, while more severe cases can involve holes opening in the ground.  Sinkholes are probably the best-known instability features, and they will often attract the attention of local, national and international press.

1

How do Sinkholes form?

Sinkholes are typically formed by geological processes or heavy rainfall events, but sometimes also human activity. Sinkholes are formed by subterranean voids which allow material to slump in, creating a surface-level sinkhole. The way in which this occurs is largely dependent on geological conditions – there is a wide variety of bedrock and superficial geologies which each have their own characteristics. The most susceptible geologies across the county are Gypsum in the north east of England, salt deposits around Cheshire, and carboniferous limestones in the Mendips, across Wales, the Peak District and the Yorkshire Dales. ²

Sinkholes are generally categorised either by the way the cavity is formed, or by how they manifest at surface level.  Solution sinkholes are formed by the dissolution of soluble rock. These are among the more common sinkhole types.  Suffosion sinkholes are sinkholes where sandier superficial material slumps into below- ground fissures, eventually revealing a hole at surface level. Collapse sinkholes describe a similar process to Suffosion, except the material falls more abruptly into a cavity.  The shock value of spontaneously formed, large sinkholes typically attracts dramatic headlines and it’s often collapse sinkholes behind those headlines. Finally, caprock sinkholes occur when a cavity forms below otherwise stable rock, disrupting that stability and causing material to collapse into the newly-formed cavity. ²

Whilst sinkholes can be created through natural processes, humans can also affect ground conditions which subsequently trigger sinkholes to form. For instance, building works modifying drainage or loads imposed on the ground. Changes to the level of the water table can impact underlying structures, ultimately leading to subterranean collapses. Raising the water table can bring it in contact with dissolvable geological material. Furthermore, the interception of clay filled voids during mining can lead to collapses as it allows the softer clay material to sink into the cavity created during the mining process which then propagates up to the surface. Mining can also affect the water table through of a site which occurs when water is removed, typically by pumping, to allow access to the desired deposits. This removes the support provided by the water and can subsequently cause them to collapse.²

In Old Woking Road, Woking there was recently a large sinkhole that was attributed to a broken sewer connection. As the road on which the sinkhole formed is a major thoroughfare for commuters, local press emphasised the frustration of local commuters.³ The road was subsequently closed, repaired and reopened on the 1^st February. However, 48 hours later the sinkhole re-emerged, this time substantially larger.⁴ The affected section of Old Woking Road was reopened to the public on the evening of the 10^th February – fortunately no one was hurt during the occurrence of the sinkhole.

Sinkholes are often popular subjects for sinkholes – Groundsure has covered them a few times in the past, such as this article detailing sinkholes in Ripon, Yorkshire in 2014, and a sinkhole in a residential street in St Albans.

International examples

Internationally, we have recently seen a sinkhole in Hoboken, New Jersey, USA, in which a burst water main washed surface soil material away to create a sinkhole at street level into which a car fell (luckily, no-one was in the car at the time).^5,6

How can ground stability issues be investigated?

Given the variety of ways ground stability issues can affect a property it is important to investigate them thoroughly before deciding to purchase a property, but the question of “How can I do this?” often arises. Property search reports can provide a screening for susceptibility including Groundsure’s suite of environmental searches which utilise data from the British Geological Survey (BGS), Peter Brett Associates (PBA), the Coal Authority and Groundsure’s own Historic Land Use Database (HLUD).

Further avenues to pursue include, but are not limited to;

• Contacting the Building Control Department at the Local Authority to ascertain if they hold any records of subsidence occurring in the area
• Commission a structural survey to be undertaken by a professional accredited by the Royal Institute of Chartered Surveyors (RICS) to ascertain if the property has been affected by subsidence to date
• In some cases a geotechnical investigation might need to be undertaken and where necessary appropriate remedial measures can be implemented to mitigate against the risks

For all new developments the developer is required under the National Planning Policy Framework (NPPF) and Building Regulations to ensure the property is constructed to the appropriate standards. With that in mind, when investigating ground stability issues with a new build property you may wish to contact the Planning and/or Building Control Departments at the Local Authority to ascertain the property has been built to the correct standards.

References

1. Mirror (2017) Enormous 50ft sinkhole appears at £400m UK shipping terminal just three months after it opened [online] Available at: http://www.mirror.co.uk/news/uk-news/enormous-50ft-sinkhole-appears-400m-9817510 [Accessed 4^th April 2017]
2. British Geological Survey (2017). Sinkholes (or dolines) [online] Available at: http://www.bgs.ac.uk/caves/sinkholes/home.html [Accessed 1^st March 2017]
3. Get Surry (2017) Woking sinkhole: Old Woking Road set to reopen after carriageway collapse forces week long closure [online] Available at: http://www.getsurrey.co.uk/news/surrey-news/woking-sinkhole-old-woking-road-12584854 [Accessed 1^st March 2017]
4. Get Surry (2017) Woking sinkhole appears BIGGER after 48 hours despite council assurances it was fixed [online] Available at: http://www.getsurrey.co.uk/news/surrey-news/woking-sinkhole-appears-bigger-after-12544254 [Accessed 1^st March 2017]
5. Daily News (2017) Large sinkhole forms at ste of water main break in Hoboken, engulfs SUV [online] Available at: http://www.nydailynews.com/news/national/large-sinkhole-forms-site-water-main-break-hoboken-article-1.2984875 [Accessed 1^st March 2017]
6. Nj.com (2017) Hoboken water main break repaired after sinkhole swallows SUV [online] Available at: http://www.nj.com/hudson/index.ssf/2017/02/hoboken_sinkhole_swallows_suv_after_water_line_bre.html [Accessed 1^st March 2017]