Reflection, absorption and filtering of ground vibrations, A study in the use of different arrays of cylinders to attenuate and filter ground vibrations

Abstract

With the increased rate of urbanization comes an increase in disturbing ground vibrations. These can be produced by building operations, different kinds of traffic and in some cases mining and military operations. It is therefore necessary to try and reduce the amplitude of these disturbing, and in extreme cases damaging vibrations. This thesis investigates different wave barrier types in different ground types, with the aim being to find the optimal filter for frequencies below 20 Hz. There are established methods which can reduce the velocity of surface ground vibrations, which include underground walls, trenches and treatment of the ground. These methods, however, can be expensive to implement as they all require extensive groundwork operations which are both disturbing to the surrounding populace and costly. Different arrays of hollow cylinders acting as a barrier in the wavefronts path were therefore studied. This was performed by simulating these barriers in a 3D halfspace in an FEM model to see if any reduction of the velocity level at the surface could be measured below 20 Hz. The model was entirely homogeneous and no attempt was made at changing different ground parameters such as porosity or moisture content. Despite investigating 7 different array types in 3 ground types there was no broadband reduction below 20 Hz, only narrowband reduction at individual frequencies, often above 20 Hz. After these initial investigations the effect of these cylinders was briefly studied for the effect on underground vibrations, where more promising results were achieved. This suggests that in areas where the ground is soft and the surrounding structures require deep foundations, arrays of cylinders could be useful for reducing the amplitude of incoming body waves which would otherwise interact with deep foundations and cause disturbing vibrations in the building. Keywords: Vibrations, reflection, absorption, ground, acoustics, dynamics, rayleigh, waves.