In November 2018 a ‘sinkhole’ appeared adjacent to a block of flats in a residential area of St Albans. The first reports of the collapse noted a 6 metre wide hole next to an exterior wall of an apartment building at Cedar Court. The location of the ‘sinkhole’ which was close to one of the walls led to the flats being evacuated as a precaution.
This is not the first time a ‘sinkhole’ has appeared in St Albans, with a ‘sinkhole’ in 2015 at Fontmell Close gaining global media attention. The collapse at Fontmell Close resulted in several homeowners being evacuated from their properties, some not returning for over 12 months and with an estimated council remediation bill of over £1,000,000. The ground collapse in 2015 at Fontmell Close was later confirmed to be linked to ancient brick clay quarrying and it’s associated underground chalk mining. The Bernards Heath area of St Albans was extensively exploited for clay and chalk over a period of several centuries, with the earliest recorded clay quarrying in the 15th Century.
Historical clay extraction and the associated chalk mining is a common cause of ground collapse or sinkholes that have appeared over the past decade (and before), with notable collapses in other areas such as Hemel Hempstead, High Wycombe, Plumstead, Greenwich and Dartford since 2012. Despite the ancient nature of the mining that is causing these sinkholes, the expert interrogation and analysis of available records can often provide an accurate insight into where and even when they might occur. As with the examples above and the sinkhole at Fontmell Close, the collapse on the 6th November at Cedar Court and particularly the historical, geological and topographical context of the vicinity shows startlingly similar characteristics.
Cedar Court, St Albans is located on the northern boundary of a former 1920s-30s brickworks and clay pit, north of Hill End Train Station With geological conditions similar to the nearby Fontmell Close, historical chalk extraction and their associated underground voids cannot be discounted. In fact, at Cedar Court, chalk appears even closer to the surface and with underground chalk extraction such a common but poorly recorded association with brickworks.
Ancient chalk mining is not always related to nearby clay pits and a huge number of smaller but sometimes prolific chalk mines are associated with ancient agricultural practises. There are many hundreds of them recorded across south and east England and they are often found above glacial soils and near to field boundaries or ‘dells’, ancient woodland designated for chalk extraction. The chalk, or the lime within the chalk, is spread across the surface to improve the productivity of the soil. These glacial soils are present beneath the Site and the site also lies adjacent to the historic ‘Chalkdell Wood’ (figure 2) suggesting that this area of St Albans may have been linked to the extraction of chalk for many centuries. Although mining is not often associated with towns like St Albans, numerous chalk mines are recorded across Hertfordshire and therefore a link to mining activity beneath Cedar Court.
Until extensive geotechnical investigation and remedial works take place, it is not possible to know for sure whether the cause of the collapse is linked to ancient agricultural chalk mining from Chalkdell Wood associated chalk mining from 1920s brickworks, or potentially another cause, such as the natural dissolution of the chalk beneath the site. However, the nature of the collapse and the impact it has already had on residents only serves to highlight the importance of understanding the ground beneath our feet.
A local surveyor’s knowledge is key when it comes to knowing about past industrial and agricultural uses within a particular area, which could impact on the structural stability of buildings constructed on such a site. Sinkholes occur gradually over time but the collapse of the ground is usually a sudden event and cannot often be predicted. However, if the surveyor knows that a particular area has suffered from ‘sinkholes’ in the past and during their inspection identifies slanting trees and fence posts or even large dips and depressions in the ground then these will serve as indicators that further investigations are necessary. Furthermore, if the surveyor finds any evidence of recent subsidence to a building – downwards movement of a foundation – or fresh cracks to walls and driveways then further investigations will also be recommended to confirm the cause and any necessary mitigation.
If you require any further advice in respect of ground instability problems please feel free to contact Hamblin Sullivan Associates for advice in this respect.