1. Research Assistant, Institute of Geomechanics and Underground Technology, RWTH Aachen University, Aachen, Germany,(corresponding author)
2. Managing Director, Professor (Emeritus), ZAI Ziegler und Aulbach Ingenieurgesellschaft mbH, Aschaffenburg, Germany,
3. Professor, Institute of Geomechanics and Underground Technology, RWTH Aachen University, Aachen, Germany,