Beds, Herts & Bucks

St Albans sinkhole: Concrete lorries start pouring

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Media captionThe first lorries arrive to pour quick-drying cement into the sinkhole, as Ben Ando explains

About 77,000 litres of quick-drying concrete have been pumped into a giant sinkhole in an attempt to repair a street in St Albans.

The 66ft (20m) diameter hole is spread across driveways and front gardens in Fontmell Close and is 33ft (10m) deep.

Experts believe the sinkhole, which has left 50 homes without power, was caused by the collapse of a unrecorded mine.

Hertfordshire County Council said a further 100,000 litres of concrete would be poured in on Saturday.

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Media captionChalk in the soil dissolved, causing the hole to open, say geologists

The concrete will be left to set on Sunday.

On Monday morning a further 100,000 litres of foam concrete, brought in by lorries, will be poured in and later in the week it is hoped the utilities services can be restored.

A private security firm has been appointed to keep a watch on homes in Fontmell Close and Bridle Close from Friday evening onwards, and a temporary emergency access road was created for residents to get their cars out via a playing field behind the site.

Richard Thake, cabinet member for community safety, said: "At this time we cannot say exactly how long this will take, but it is likely to take several days.

"All agencies are working together to get residents back to their homes as soon as it is safe to do so.

"Our priority is to ensure the safety of residents in the area and minimise disruption and to resolve the situation as soon as possible."

Five homes were evacuated in the early hours of Thursday after residents heard a crash and the hole appeared.

Image copyright South Beds News Agency
Image caption Council teams were called to the cul-de-sac collapse at 01:30 BST on Thursday

A small hole in the footpath was cordoned off last week, but Hertfordshire County Council said there was "no reason to suspect that the hole would collapse".

One resident told the BBC his wife had previously seen a postman fall in the smaller hole and he had looked "quite badly injured".

Residents have been told it could take weeks to repair the road.

What causes a sinkhole?

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Media captionSinkholes are often caused by an abnormal amount of water seeping into the ground, but they are difficult to predict

The British Geological Survey says there are several different types of sinkhole, sometimes known as "dolines".

  • Some happen as a result of surface dissolution of the soluble rock, by rainfall or acidic groundwater.
  • They can occur where a thin covering of loose material such as sand, clay or soil covers soluble rocks beneath.
  • In other cases, the gradual collapse of a cave passage at depth can cause a sinkhole.
  • Some are caused by the erosion of weak unconsolidated material by flowing water.
  • If a giant hole is caused by man-made factors such as a collapsed mine then it is technically called a crown hole, rather than a sinkhole which is formed naturally.

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