I'm sure that you remember the St Albans Sinkhole that swallowed up the road outside the houses (1 October 2015). Well, here it is, finally filled with foamed concrete from 48 lorries:-
Council engineers have declared that all the houses local to this sinkhole have been built within the boundary of a clay pit that was excavated in the 19th Century, and then backfilled with waste from the area. The precise reason for the sinkhole, however, is almost certainly former chalk excavation in the bedrock beneath the clay.
A contiguous layer of clay is impervious to water. A void left after excavations could, however, be scoured by water & leave unstable ground under an apparently stable surface.
The question that the council has applied itself to was "is this the only void?".
Geotechnology Ltd., working under instruction from the council, made precise measurements of gravity under the road. Those measurements have suggested that the sinkhole probably has a brother even bigger than itself which, crucially, extends under the houses:-
This new anomaly is estimated to be a void of about 20m depth (the original sinkhole was 10m deep). The council will repair & pay for all damages below the road, but anything under the houses is the responsibility of the land-owners (who are also the householders) - they will need to speak to their insurers.
--------- Alex Kemp