http://www.bbc.co.uk/news/science-environment-26242258

On Saturday, 15 February 2014, at 7:30 in the morning, a large sink-hole (about 35ft (9m) wide and 20ft (6m) deep) opened up under a house in Oatridge Gardens, Adeyfield, Hemel Hampstead:

20 families were immediately evacuated, so quickly that they had to leave their cars in the driveway. 12 of those families were evacuated directly because of the danger from the sinkhole, and others because their gas supply had to be shut-off.

The following Friday foamed concrete was injected into the sinkhole:

The concrete made that first sink-hole safe, but they then had to check the rest of the estate. 60 test-holes were drilled, and at least one further cavity was discovered - it was also filled with foamed concrete on March 6.

The big question, of course, is ‘why?’ Why did the sink-hole open up in the first place? Well, the first thing is that it was actually a “crown-hole”, and not a “sink-hole”...

This Adeyfield estate was built by the Jarvis Group in 2008 and, crucially, it was built on the site of a former brickworks. That site included clay pits and chalk mines (both necessary to to make bricks and tiles), although all such activity ceased in the early 1900s, a century before the land was re-developed. When the pits & the mines were exhausted they would have been filled in by waste sand. According to Dr Clive Edmonds, an expert on subsistence who is advising the developers, the key to the appearance of these crown-holes was the very wet weather in the weeks & months before it appeared, as the water could have moved the clay, chalk or sand & therefore have allowed the land to slump.

So, what is the difference between a “crown-hole” and a “crown-hole”? Well, the crown-hole is caused by human activity, whilst a sink-hole is due to natural causes (although I think that that may be a touch academic for you if it is *your* house that is affected).

Finally, if--like me--you enjoy watching gob-smacking moments, watch the video in the link at the top of this page. Right at the end is 20 seconds of the moment in Louisiana, USA when a sinkhole opened up & swallowed entire a stand of a dozen fully grown trees. The area was covered with floods at the time, so the sinkhole itself cannot be seen. Instead, twelve 100-foot-high trees vanish from sight, sucked down into the void. Truly, an awesome sight.

PS
Try not to worry over the idea of being sucked down into a sink-hole. Remember:-

In Hemel Hempstead, holes hardly hever happen.

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Alex Kemp