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