RECENT UNDERGROUND INVESTIGATIONS OF ABANDONED CHALK MINE WORKINGS BENEATH NORWICH CITY, NORFOLK

The city of Norwich is underlain at shallow depth by abandoned minewor kings made for the extraction of chalk and hints. Early workings took the form of surface pits and quarries, with later working of undergrou nd mines. The latter, often several hundred years old, are liable to c ollapse, and may reach the ground surface. Most of the mines are now i naccessible, although the recent investigation of two such mines has i ncluded 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 a t Earlham Road mine have been found to be well developed, due to inade quate 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 sec ondary contribution from pillar over-stressing.