A large ‘sinkhole’ that appeared in St Albans may have been caused by historic chalk mining, experts believe.

People living in the Cedar Court flats off Cedarwood Drive were evacuated on Tuesday morning and the site was cordoned off.

Residents were told that they would not be able to return for at least a week.

Two experts have said that the size of the pit points to it being a crown hole rather than a sinkhole, as originally thought.

A crown hole is a cavity caused by mining activity that is suddenly exposed in the ground.

 

The pit may be a crown hole

Dr James Ford, who has a PhD in chalk mining, wrote: “The type of collapse and the historical maps all point to a classic chalk mine collapse - either a 'crownhole' event or a failure of shaft capping.”

And chalk subsidence specialist Clive Edmonds told industry publication Ground Engineering: “The 1878-80 1:2,500 scale OS map shows that the collapse location occurs at the former location of Chalkdell Wood.

“There is no chalk at the surface and no pits or quarries shown so I suspect that it might refer to shafts excavated down into the chalk to extract the chalk for burning at the surface in clamp kilns to produce lime for spreading across the glacial clay soils present.”

The Herts Fire and Rescue team was sent to Cedar Court in just after 6am on Tuesday after reports of a sinkhole appearing near the block of flats.

Read more: Flats evacuated after sinkhole appears

A spokesperson from the county council, who said they do not want to speculate about the cause, stated on Wednesday: “Our priority is to ensure the safety of residents in the area.

“Utility services have also been on site to ensure gas, electricity and water supplies are isolated. We are working closely with St Albans City and District Council's resilience team at the scene to ensure the safety wellbeing of residents in the area.”

The council put a 20 metre wide hole that appeared in Fontwell Close near Batchwood Sports Centre in 2015 down to 19th century chalk excavation.

A report read: “The work found that the cause of the collapse was due to an irregular bellpit style of old chalk mine working at depth below the highway.”

 

The sinkhole in Fontmell Close in 2015 was caused by historic chalk mining