1. Lecturer, School of Engineering and Computing Sciences, Durham Univ., Durham DH1 3LE, U.K. (corresponding author).
2. Dean, School of Engineering, Mathematics and Computer Science, City Univ. London, London EC1V 0HB, U.K.
3. Marie Curie Research Fellow, Plaxis BV, Delftechpark 53, 2628 XJ, Delft, Netherlands.