Continuum Damage Modeling of Rocks Under Blast Loading

Abstract

Rock fragmentation by blasting has been in practice in civil and mining industries for centuries. The controlled use of explosives is considered the dominant approach for the purpose of breaking the rock material in any hard rock mining. The fracturing process of rock material, when subjected to blast loading, is a complex phenomenon and requires substantial study. Obtaining desired results in any rock blasting project demands for extensive understanding in two separate engineering fields. The first field of study should focus on the mechanics of dynamic fracturing of brittle rock material in response to blast loading. Given the inherent nature of the blast phenomenon, care should be given in the rock mechanics studies, and the analysis should be handled considering the dynamic behavior of the material. The second field comprises the study of the behavior of stress waves in brittle materials, the controlling parameters of wave attenuation, and the effect of stress wave interactions on dynamic fracturing of rock mass. In this thesis, the strain rate dependency of dynamic tensile strength of Laurentian granite is investigated by the aid of different experimental methods i.e. Hopkinson bar experiments and Split Hopkinson Pressure Bar (SHPB) experiments. The obtained results were combined and implemented as Dynamic Increase Factor (DIF) in the RHT material model using LS-DYNA numerical code. Using the modified RHT material model, two distinct rock blasting problems are studied numerically. First, the effect of stress wave interaction on the resulted rock damage and fragmentation is investigated using a range of initiation delay times. Different scenarios of wave superposition are examined and an optimum delay window is introduced based on the elastic stress wave theory. Second, the effectiveness of current destress blasting practices in burst prone deep underground excavations is studied. The capability of the conventional destressing patterns in alleviating the burst potential is explored by damage investigation and stress studies ahead of a tunnel face before and after destressing. A new destressing pattern is introduced which is successfully capable of transforming the stress states ahead of a tunnel face, reducing the rockburst potential.