Vibrations induced by high initial stress release during underground excavations

Abstract

Excavation unloading under high initial stress is a typical dynamic process subjected to the combined effects of different factors. In this study, a mathematical physics method for a two-dimensional circular excavation was developed to investigate the mechanism involved in unloading vibration, which is capable of providing insights into the quantitative relationships between vibration features and correlative factors such as the initial stress, cross-sectional area of the tunnel, unloading rates and unloading paths. Then the dynamic unloading excavation process was implemented in the discrete element program PFC 2D for numerical analysis after verifications against the theoretical results. In particular, the characteristics of unloading waveform under high initial stress were investigated for various ratios of horizontal and vertical in situ stresses and aspect ratios of rectangular tunnels. The temporal and spatial characteristics of the excavation process can also be illustrated. In a practical project which considered the combined action of both the blast and unloading vibration, the finite difference program FLAC 3D was further adopted to investigate the contribution of unloading vibration to the character of the seismogram. Results are presented which indicate the 2D numerical analysis provides satisfactory approximation to the excavation process. While the peak particle velocity (PPV) is in direct proportion to the in situ stress variation, it decreases significantly along with the increase of unloading time. In addition, excavation by drill-and-blast (D&B) method should be taken as a dynamic process of blast loading before unloading in a period of time, instead of instantaneous unloading. Although unloading vibration can be generated and significant damage around the tunnel can be induced in the process, distinct unloading vibration can only be generated under high in situ stress and unloading rate.