A hole that opened up next to a block of flats in St Albans yesterday is likely to have been caused by historic Chalk mining.
Source: Hertfordshire Fire and Rescue Service
The collapse resulted in a 6m wide hole at the surface that extends partly under the building and initial investigations suggest that the depth is up to 3m.
A spokesperson from Hertfordshire County Council said: “Hertfordshire Fire and Rescue were called to reports of sinkhole at Cedar Court, off Cedarwood Drive in St Albans at 6.35am yesterday and immediately attended the scene to ensure the area was safe.
“As a precaution the adjacent block of flats has been evacuated. A structural engineer has attended the scene. 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.”
Ground conditions at the site comprise Quaternary Lowestoft Formation – Diamicton (glacial deposits) over Chalk Group strata (Lewes Nodular & Seaford Chalk Formations undifferentiated).
According to Chalk subsidence specialist Clive Edmonds, who is a senior consultant at Peter Brett Associates, the size of the hole means that statistically there is a greater chance that it is a crown hole rather than a sinkhole and historic maps support this theory.
“The 1878-80 1:2,500 scale OS map shows that the collapse location occurs at the former location of Chalkdell Wood,” he said.
“There is no chalk at the surface and no pits/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. This was done commonly during the late 1700s and through the 1800s in order to lighten the soil texture, improve its drainage, make it easier to plough and increase crop yield.”