The Ground Moves Beneath Our Feet
For any homeowner, there is no worse nightmare than their new property collapsing, having been unaware of subsidence potential or underground ground cavities in the area. If you're looking to purchase a house, it's vital to understand the impact ground movement can have and the best ways to avoid common pitfalls.
Natural ground subsidence is one of the most common forms of ground movement. It occurs as a result of many natural geological hazards and refers to the upward, lateral or downward movement of the ground. Such movements are typically small, but under exceptional circumstances they can be larger. Significant natural ground instability has the potential to cause subsidence damage to some weaker buildings and structures. 1
The most common way for sinkholes to occur is when a soluble rock, like chalk, is dissolved when exposed to acidic rainfall or groundwater. A more dramatic collapse can happen when the overlying material is more cohesive like clay, which can suddenly ‘drop out’ after a period of dissolution below.
Natural dissolution is only one way sinkholes are formed and human impact can also play a part. As experienced in Fontwell, West Sussex in 1985, burst water mains or leaking drainage pipes can increase subsurface water movement assisting and speeding up dissolution, sometimes leading to the sudden appearance of sinkholes.
One of the most devastating occurrences of subsidence on record happened in Ripon, Yorkshire. Large parts of Ripon are underlain by gypsum that has dissolved to form an elaborate cave system. Occasionally, further dissolution of the ground can expose the caves, forming a sinkhole at the surface. In February 2014 one such sinkhole opened up on the east side of Ripon, causing devastating damage to a house there. The residents had returned home one evening to find the back of their house collapsing, and a huge visible crack which appeared to be widening up to 40cm. After rescuing the dog and shutting off the utilities, surrounding neighbours were quickly evacuated 2. No casualties were caused by this incident, however the damage sustained to the building and on-going ground movement at the site meant that the house had to ultimately be demolished. 3
Increased rainfall exacerbates subsidence and the formation of sinkholes. Over the last few years ground movement has become much more prevalent, and as the weather is predicted to become more extreme it is likely that we will be seeing more incidences of these hazards.
In October 2015 a 10m deep sinkhole appeared in a residential street in St Albans and several families were forced to evacuate in the night. Over 50 homes in the surrounding area were also left without gas, electricity and water. 4 The sinkhole was filled with 48 lorry loads of concrete in order to stabilise the site while the root of the problem was investigated. Surveys were carried out which found a void under the close, related to ‘irregular style of old chalk mining’. 5 Some of the families in the immediate area of the sinkhole have still been unable to return to their homes over a year on 6 and the investigation and repair has cost councils over £600,000 7.
After an overnight storm in May 2016, Greenwich residents woke up to see a car partially swallowed by a sinkhole which had opened up in Woodland Terrace. 8 This sinkhole was the most recent of many ground movement events which have occurred in the borough in the last few years. Several large holes have recently appeared and it is believed to be due to the presence of underground chalk caverns and disused mines in the area. 9 In a region where property values are so high, regular ground movement events could negatively impact the housing market and a local estate agent stated that the house prices of properties near to new sinkholes could decrease. 9
Householders weren’t the only ones affected by sinkholes in 2016. Commuters were caused severe delays in July 2016 when a four metre deep sinkhole opened up under a railway line near Forest Hill during rush hour. 10 Trains going in and out of London Bridge Railway Station had to be diverted or cancelled while Network Rail worked to secure the hole using 50 tonnes of ballast. 11 Unfortunately this didn’t solve the problem and the following month another hole opened up in a nearby location causing further disruption to passengers. It has been proposed that the second hole may have been related to the material used to fill the previous hole settling. 12
Modern building regulations have been put into place to mitigate the risk of subsidence affecting new build properties. When planning an extension, it is important to follow the Government guidelines as increasing the property footprint and pressure can cause an increase in the amount of ground movement.
As plants can take a lot of moisture from the ground, the amount of vegetation close to the property can affect subsidence. Each type of tree extracts a different amount of water from the ground - ash and oak trees take up a lot of water whereas evergreens use a smaller amount. Before removing existing trees, it’s a good idea to seek an expert opinion and consult the local authority as it could result in the ground moisture content increasing and the subsequent swelling of the soil. 13
Groundsure reports provide a first step to investigating these kinds of hazards, using data from the British Geological Survey. The data comes from geological maps of England and a rating is obtained from the evaluation on six natural hazard data sets, including the shrink-swell potential of clays and the compressibility of loose sediments. In addition, most Groundsure reports contain data including records of mining and natural cavities.
One of the earliest signs of subsidence is a diagonal crack suddenly appearing in the plaster or brickwork. 14 If you are concerned that a property you are looking to buy may be affected by subsidence we recommend the following:
- That buyers look at the property themselves in more detail for any signs of existing damage.
- That they contact the local Building Control Office at the Local Authority who may possess additional local information pertaining to subsidence in the area, and may be able to impart historical subsidence knowledge and suggest protective/reductive measures.
- Seek professional advice from a professional property surveyor from the Royal Institute of Chartered Surveyors.
- Groundsure, 2009. Groundsure GeoInsight User Guide [online] Available at: http://www.emapsite.com/downloads/product_guides/GroundSureGeoInsightUserGuide.pdf [Accessed 15th March 2015]
- BBC News, 2014, Ripon sinkhole forces three homes to be evacuated [online] Available at: http://www.bbc.co.uk/news/uk-england-york-north-yorkshire-26236667 [Accessed 23rd Feb 2015]
- ITV News, 2014, House demolished after Ripon sinkhole damage [online] Available at: http://www.itv.com/news/calendar/story/2014-02-17/sinkhole-appears-in-ri... [Accessed 23rd Feb 2015]
- BBC News, 2015, Sinkhole opens up on street in St Albans [online] Available at: http://www.bbc.co.uk/news/uk-england-beds-bucks-herts-34410423 [Accessed 7th October 2016]
- BBC News, 2016, St Albans sinkhole: further collapse ‘unlikely’ [online] Available at: http://www.bbc.co.uk/news/uk-england-beds-bucks-herts-36845750 [Accessed 7th October 2016]
- The Herts Advertiser, 2016, St Albans sinkhole first anniversary: Evacuated couple become media stars [online] Available at: http://www.hertsad.co.uk/news/st_albans_sinkhole_first_anniversary_evacuated_couple_become_media_stars_1_4716954 [Accessed 7th October 2016]
- BBC News, 2016, St Albans sinkhole: Council costs top £600,000 [online] Available at: http://www.bbc.co.uk/news/uk-england-beds-bucks-herts-35864807 [Accessed 7th October 2016]
- The Guardian, 2016, London sinkhole partially swallows car [online] Available at: https://www.theguardian.com/uk-news/2016/may/12/london-sinkhole-partially-swallows-car [Accessed 7th October 2016]
- The Telegraph News, 2016, Greenwich sinkhole may have ‘serious impact’ on house prices, say experts [online] Available at: http://www.telegraph.co.uk/news/2016/05/12/car-falls-down-sinkhole-in-greenwich/ [Accessed 7th October 2016]
- Guardian, 2016, South London sinkhole causes rail misery in evening rush hour [online] Available at: https://www.theguardian.com/world/2016/jul/18/south-london-sinkhole-network-rail [Accessed 7th October 2016]
- Evening Standard, 2016, Fresh mayhem as second sinkhole opens up in Forest Hill [online] Available at: http://www.standard.co.uk/news/travelnews/fresh-mayhem-as-second-sinkhole-opens-up-in-forest-hill-a3311521.html [Accessed 7th October 2016]
- Get Surrey, 2016, Forest Hill sinkhole: everything you need to know about disruption affecting Southern Rail and Thameslink services [online] Available at: http://www.getsurrey.co.uk/news/surrey-news/forest-hill-sinkhole-everything-you-11703271 [Accessed 7th October 2016]
- Gocompare.com, 2015, Subsidence and home insurance [online] Available at: http://www.gocompare.com/home-insurance/subsidence-and-home-insurance/ [Accessed 24th March 2015]
- EAR Sheppard, 2011, Subsidence, heave and landslip [online] Available at: http://www.earsheppard.co.uk/subsidence-heave-landslip.php [Accessed 24th March 2015]