Dr E. D. Yardley C Eng, PhD, MIMMM, MIMechE is a retired Consultant Engineer, who has more than 30 years' experience of development, testing and practical application of equipment in the coal mining industry.Dr L. R. Stace Ceng, PhD, FIMM is Associate Professor, School of Civil Engineering, University of Nottingham and has some 20 years past experience in the coal mining industry.