On 1st October, 2015, a giant sinkhole opened up in Fontmell Close, St Albans forcing families to evacuate their homes and cutting off utilities to over 50 properties.
• October 2015, a 66ft wide, 30ft deep crater opened in a residential street in St Albans, 5 days after a small hole was first spotted.
• 6 families were evacuated immediately with more than 50 properties left without gas water or electricity while temporary solutions were being devised.
• It was determined that the hole was from heavy rainfall and inundation causing a previously unknown chalk mine void to collapse.
• The hole required 48 lorry-loads of foamed concrete to fill, with full site investigation and remediation costing in the region of £650,000.
• The last families were allowed to return to their homes 14 months later.
A majority of Hertfordshire rests upon chalk with some clays and silts of the Thames Group to the south. The county has a notable mining legacy from the Medieval times of small-scale mines in rural locales, predominantly of chalk but with minor clay and sand extraction. Unfortunately many of the subsurface workings are concentrated within some of the region’s urban centres.
The hole that appeared in St Albans was the result of a collapse of a previously unknown sub-surface chalk mine void after a period of heavy rainfall. Further investigations determined this void to not be part of a larger network of sub-surface workings allowing residents to return, however, it highlights the risk of uncharted mines in urban areas. Successful remediation was completed allowing all families to return home safely with the hole and utilities repaired, however, remediation works were time consuming and costly. Similar chalk mine collapses have occurred nearby to St Albans: In Hemel Hempstead, chalk mines beneath the Highbarns area were discovered after a patio collapsed as well as subsequent other holes appearing which requiring 8 years of remediation works to repair. In High Wycombe, Buckinghamshire, a 30ft deep sinkhole from an old chalk mine ‘swallowed’ a car after it opened in the driveway of a sub-urban house.