Norwich City Council called in Harrison Geotechnical to investigate a single dwelling on a Council-owned housing estate that was being re-furbished. A simple trial pit and probing survey indicated major problems of voiding and loose soils. This investigation then grew to cover the whole estate of more than 100 homes. Techniques employed included: trial pits, boreholes, dynamic probing, ground penetrating radar, seismic refraction, magnetometer and borehole tomography.

The investigation and desk study revealed that the houses had been built 50 years previously in an old gravel pit, overlying chalk; and that failure had occurred owing to a mixture of consolidation of the residual soils from gravel extraction and weakening of support owing to solution cavities in the chalk.

The investigation was followed by an extensive remedial programme including demolition and underpinning.