The Hemel Hempstead sinkhole may have been caused by underground chalk mines, former clay pits and building homes on sand, the UK’s leading expert on subsidence suggests.
Engineering geologist Clive Edmonds has recommended that 200 cubic metres of foamed concrete are pumped into the Oatridge Gardens cavity tomorrow. About 20 lorries will be required to transport the material to fill the 20-foot deep and 35-foot wide hole, and it will take all day to do.
Dr Edmonds has studied sinkholes for more than 20 years – and also helped repair the one that appeared in Highbarns, Nash Mills, in 2007.
He said: “As soon as I went to Highbarns, I said: ‘This is most likely going to be a mining problem.’ I thought the same thing in Oatridge Gardens.”
The Nash Mills chalk mines – a sinkhole threat to 430 homes when they were discovered – have now all been filled in.
They are thought to have been dug in the 19th century to produce chalk for papermaking at John Dickinson.
The Oatridge Gardens chalk mines were used in the late 19th century as part of the production process of a brickworks that was then based on the site.
Former clay pits used by the facility are thought to have been filled in with sand – on which the homes were later built.
Mines of that period were generally 10 to 12 metres under the Earth’s surface. Their tunnels were between three and five feet high with a chalk roof of about two metres.
Bricks and tiles were made at the Oatridge Gardens site and each one would have been made of about 75 per cent clay and 25 per cent chalk.
It is not known how far the chalk mines and clay pits extended across the site – which now houses 48 flats and houses on it.
Chalk mining is thought to have taken place in south east England for thousands of years.
Dr Edmonds said: “In principle, there’s not a great deal of problem with building on such land – providing you know what you are building above.”
Water can dissolve rocks such as chalk, limestone and gypsum, making existing underground cavities bigger.
It can make soil heavier, putting greater weight on the roofs of tunnels below.
The UK has received about 200 per cent more rainfall in January and February than it would do in an average year, leading to 10 reported sinkholes.
Dr Edmonds said the last ‘bumper’ year for sinkholes was 2001 – when seven or eight were reported across the entire 12-month period.
In a more average year, there would only be about six in the UK.
Dr Edmonds said subsistence issues can persist as the land dries out after a downpour, which can lead to sinkholes later.
He said: “I can easily see further sinkholes appearing yet.”
After the Oatridge Gardens cavity has been filled with concrete tomorrow, geo-technical probing will begin on the site – which will take up to a week.
A steel cone will be drilled into the ground to test its resistance and strength. If a chalk mine is discovered, a camera will be sent down to get a better look at it, and then it will be filled in with a concrete-based grout material.
> Keep checking this website for updates about the progress of the repair operation.