Recent underground investigations of abandoned chalk mine workings beneath Norwich City, Norfolk

Abstract

The city of Norwich is underlain at shallow depth by abandoned mineworkings made for the extraction of chalk and flints. Early workings took the form of surface pits and quarries, with later working of underground mines. The latter, often several hundred years old, are liable to collapse, and may reach the ground surface. Most of the mines are now inaccessible, although the recent investigation of two such mines has included the use of direct mining techniques to gain access to the old workings. Stability at Harford mine has been found to be adequate, due to adequate rock cover, moderate age and a rural setting. Collapses at Earlham Road mine have been found to be well developed, due to inadequate rock cover, great age, poor rock quality and disturbance due to the urban location. The principal collapse mechanism is identified as localised void migration, following roof collapse, with a possible secondary contribution from pillar over-stressing.