1. Department of Civil Engineering, Teesside Polytechnic Middlesbrough TS1 3BA, UK
2. Department of Geology, University of Sheffield Sheffield S1 3JD, UK
3. Engineering Geology and Reservoir Properties Research Group, British Geological Survey Keyworth, Nottingham NG12 5GG, UK
4. Department of Geology, University of Nottingham Nottingham NG7 2RD, UK