Sinkholes: checking records before development starts
19 September 2014
Checking records is essential in identifying areas prone to sinkholes before development starts, as Clive Edmonds explains
An unprecedented number of sinkholes appeared in the UK last winter as parts of the country experienced record rainfall. Peter Brett Associates LLP (PBA) collects data on sinkholes and in February alone, its databases show that there were at least 13 such events. To put this into context, in a typical year, there might be an average of 1 or 2 collapses per month. Usually, the sizes of holes are, at most, a couple of metres in both width and depth. But during this period, large-scale holes up to 10m wide and deep were appearing (see Figure 1).
Figure 1: Sinkhole activity v rainfall 2012-14
Damage to buildings and alarm among local residents is worrying from a building control perspective, where the aim is to ensure that development is safe and that geohazards are correctly addressed by developers.
So why the sudden flurry of sinkholes? Although they open for various reasons, the most common cause is a sudden influx of water. Not just in the summer of 2013 and the wet winter of 2014, over the long-term since 2000, there has been a notable increase in rainfall. This has to go somewhere, so it soaks into the ground increasing the loading on soil arches above voids, softening the soil and eroding fines. Where the collapsing voids are associated with solution features, the hole that appears at the surface is called a 'sinkhole'; if the void is associated with a man-made feature (e.g. mine working) then it is called a 'crown hole'.
What are the implications of sinkholes and crown holes for property? When they suddenly appear in urban areas, they can cause extensive structural damage and can sometimes result in properties being demolished, in particular when repair costs exceed insured value.
Looking forward, it is clear that developers and home buyers need to take a more proactive approach to identifying the risk. Checks can be carried out at the home-buying survey stage, but sometimes this data is conflicting and diffi- cult to understand. It is important to recognise the geohazard risk in advance of development since it can be expensive to mitigate using ground treatment, special foundations and drainage control.
Some areas of the country are more susceptible than others to sinkholes. This is particularly the case in the South where sands, gravels and clays overlie chalk. Elsewhere, land underlain by limestone, gypsum and salt can also be prone to sinkhole development in certain circumstances. The national PBA natural cavities database, which has more than 32,000 records, shows that chalk hills of Kent, Essex, Berkshire, Buckinghamshire and Hertfordshire make them particular hotspots. Limestone is also vulnerable to sinkholes in areas such as South Wales, as are places such as Ripon, underlain by gypsum.
PBA is currently working on many of the reported crown holes. Most are associated with old brick and tile works where chalk was mined beneath the surface clay pits. A hole that opened up in Hemel Hempstead, Hertfordshire, in February affected 50 properties causing many families to be evacuated and took almost 200m3 of foamed concrete to fill – equivalent to almost 20 truckloads. Another collapse at Gillingham, Kent, the same month took a similar quantity. A car was swallowed up at Walter’s Ash, Buckinghamshire, and another left teetering on the edge in Upper Basildon, Berkshire, areas known to be riddled with old brick works with chalk mines below.
Building control should expect developers not only to check for sinkhole potential in natural cavity-prone areas but also those areas with old mine workings. PBA's database contains around 16,000 records revealing the legacy of all types of mining – from tin to iron ore, slate and chalk. Records can pre-date historical OS maps so developers need to be wary and seek specialist help. Standard desk studies may not highlight the risk.
Dr Clive Edmonds is a Partner at Peter Brett Associates