Investigation of excavation-induced brittle damage in hard rocks using large-scale true-triaxial experiments and 3D micromechanical DEM modeling

• Applicant: Privatdozent Qingsheng Bai, Ph.D.
• Subject Area: Applied Mechanics, Statics and Dynamics
• Term: from 2021 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 491064630

Deep underground excavations frequently witness brittle failures (or even rockburst) that seriously endanger excavation stability. Therefore, firstly the need for an improved understanding of mechanisms and triggering of brittle failure is becoming evident. To accomplish this, we will plan systematic tests and numerical simulation in investigating excavation-induced brittle failure in hard rocks with various stress regimes typical for depths of up to 2000 m. To take the entire damage and spalling process into account, large-scale (cubes with 30 cm edge length) true-triaxial tests will be conducted where the excavation face is included. Therefore, the 3D stress state (magnitudes and orientations) and its evolution in the near-field of excavations are duplicated. Several tests will be carried out to investigate the damage initiation stress (DIS) as a function of far-field stress and excavation shape. The mechanical analyses will be combined with advanced acoustic emission (AE) analysis of the excavation-induced damage, in particular with respect to microcrack development, moment tensor inversion, stress tensor inversion, crack size and orientation, focal mechanism, AE clustering, and the associated velocity field. We will also check whether pattern recognition techniques (PRT) can be used to interpret damage mechanisms. Once this is confirmed, we will develop a PRT workflow to distinguish different damage modes and micro-crack mechanisms. With the assistance of digital image processing, we will develop a 3D grain-based DEM rock model (digital twins) to reproduce the true-triaxial experiments, which will explicitly duplicate the damage / spalling process at the grain size level and uncover the underlying mechanisms.Particularly the role of micro-heterogeneity (e.g., grain shape, grain size distribution, pre-existing cracks, grain-scale mechanical parameters, and micro-contact heterogeneity) in respect to DIS and the associated brittle failure mechanism will be evaluated. The developed micromechanical model enables us to investigate constellations going far beyond the laboratory experiments. The gathered data will help us to develop a damage initiation criterion involving the complete 3D stress tensor and its rotation during the excavation process. Considering also microscopic heterogeneities, we want to predetermine whether a brittle failure has to be anticipated and to develop potential solutions for safe project design.

• DFG Programme: Research Grants
• Co-Investigator: Professor Dr. Heinz Konietzky