A large historic clay pit sits beneath the surface of last week’s mammoth sinkhole, as seen in images prepared for this paper.
Landmark Information Group, a land, property and environmental data specialist, has composed images which line up historic Ordnance Survey data with the current layout of Fontmell Close in the Bernards Heath area.
The historic use of land, such as quarrying, coupled with an area’s geology - in this case ranging from clay with flints to chalky material - can increase the risk of sinkholes occurring.
Chalk extends through most of Hertfordshire, and it has been extensively quarried and mined in the county since Roman times to produce agricultural lime, clay for use in brick making, and cement.
Landmark’s images show not only the location of a former claypit, used for the local historic brick-making industry, but also manmade mining cavities and a lime kiln nearby, all of which could have an impact on the lay of the land.
Samantha Ashton, a data consultant at the firm, said: “Sinkholes are very dramatic and shocking when they occur. But when I saw photos of the one in St Albans, I thought the sinkhole was only part of the problem at the site.”
She explained that, put simply, the sinkhole occurred as, “something under the ground has dissolved. The top then sinks, because the underlying support has dissolved, for example by water, and a cavity is formed so the weight on top cannot be supported.”
Samantha advised residents living near the sinkhole to watch for subsidence; there might be cracks slowly widening in walls or a conservatory which is separating from a house.
During the home-buying process, reports should be sought on ground stability and whether a property is on contaminated land - possibly a former landfill.
There have been other sinkholes reported recently in Herts, including in nearby Hemel Hempstead, and just last month the Herts Advertiser carried a story about fears of a sinkhole forming in Jersey Farm after the appearance of deep cracks and holes in a road.
Despite this, the county council has downplayed the occurrence of such cavities turning into large sinkholes as ‘rare’.
But Haydon Bailey, chairman of Hertfordshire Geological Society, said: “Sinkholes are not uncommon. They tend to be a consequence in a number of areas where chalk is close to the surface.”
Haydon, who lives in St Albans, explained that the ground collapsed when something disturbed the underlying layers, most likely water coming into contact with the chalk and dissolving it.
He spoke too of the impact of the historic use of the Bernards Heath area, saying that while the road would have “provided a nice cap” over the previous clay pit, “eventually the ground has given way”.
Haydon urged the county council to carry out a thorough investigation of Fontmell Close and Bridle Close, “to see if there are any more potential cavities developing underground. That should be the first thing they do, to put people’s minds at ease - or residents can have their own surveys carried out on their own properties”.
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