Numerical modeling of seismic wave propagation in underground mines.

Abstract

The phenomenon of rockburst damage localization, which is not well understood, has been observed in deep underground mines. Analysis of seismic wave propagation in underground mines is of great interest for improved understanding of the dynamic rock failure problem. This thesis aims at making a contribution for improving understanding of the seismic wave propagation in deep underground mines. Advanced numerical modeling tools are used and new modeling techniques are developed to attain this goal. In this thesis, research is emphasized on the ground motion around excavations due to seismic wave propagation that results from a fault-slip seismic event in the far-field and the near-field. It is found that moment tensor point source model seems to be suitable for the source representation in the far-field and the non-point source model (such as kinematic rupture source model) seems to be suitable for the source representation in the near-field. The modeling results confirm that ground motion is influenced by many factors such as target-source distance, slip direction, spatial location, and geometrical and geological conditions. Influence of wavelength-to-excavation span (/D) ratio on the wavefield is investigated to gain insights of ground motion behavior under both quasi-static and dynamic loading conditions. It is revealed that PPV (peak particle velocity) values increase as the /D ratio increases and the amplification effect increases as the /D ratio decreases. The loading condition maybe changed from the dynamic loading to the quasi-static condition when the /D is larger than 30. Strong dynamic loading should be considered when the /D ratio is small (less than 10, with a shear wavelength less than 50 m and an excavation span greater than 5 m) for most underground excavations. A method is proposed to estimate the quality factor (a measure of energy loss per oscillation cycle) for shear waves propagating in underground hard rocks so as to gain insight into the influence of internal attenuation on seismic wave propagation. A proper shear wave quality factor can be obtained by comparing modeling results with that from a scaling law, even if there are no high quality data for quality factor back analysis. Furthermore, the influence of different geological structures on seismic wave propagation is studied. It is shown that wave propagation patterns around an excavation can be altered and PPV amplification and shielding effect can occur near the excavation boundaries amongst other reasons due to heterogeneities such as tunnels, open and backfilled stopes, and dykes in underground mines. Finally, a coupled numerical procedure, which couples FLAC and SPECFEM2D, is developed to consider the excavation effect on ground motion. The FLAC model considers the excavationinduced stress change and rock mass failure, and passes the input data to SPECFEM2D by invoking FISH scripts. In addition, a new nonlinear velocity model that considers the influence of confinement and rock mass failure on wave velocity is presented. This nonlinear velocity model and the coupled numerical technique are used to model a simple stope excavation problem. It is found that there is a large difference in the wavefields and ground motions between the results from the uniform and non-uniform velocity models. A relatively stronger amplification is observed in the low confinement zones and on the excavation surface in the non-uniform velocity models. Because stress redistribution and rock mass failure around an excavation are considered, a realistic non-uniform velocity field can be obtained. The proposed coupled numerical procedure offers a method to improve the understanding of the site amplification effect and ground motion near excavation boundaries. This thesis presents some insights with regard to seismic wave propagation due to fault-slip seismic events in underground mines. If seismic wave propagation in underground mines can be modeled properly using techniques such as these presented in this thesis, then it is possible to conduct forensic analysis after a large seismic event so as to explain one of many factors that caused rockburst damage localization. Alternatively, the modeling approach may provide valuable inputs for decision-making with regard to strengthening high risk areas to prevent rockburst, thus improving mine safety.