Wave propagation in rock and the influence of discontinuities

University dissertation from Lule├ą tekniska universitet

Abstract: This thesis concerns wave propagation in rock as a tool for determination of rockproperties and as a consequence of activities, such as trains. Using waves as a tool often means that rock properties are determined or the interior of a sample is studied without being damaged. Another use of waves is to measure the velocities in rock samples shaped as cubes, spheres or cores with plane and parallel end surfaces in order to determine if the rock is anisotropic; an important property to for example for the evaluation of stress measurements. The preparation of such samples are rather time consuming and costly, especially if many measurements have to be carried out. To overcome this obstacle diametrical measurements on drilled rock cores have been evaluated as a possible method to detect anisotropy. Measurements have been performed on metal cores, isotropic and anisotropic rock cores as well as rock cores containing microcracks. The results show that the technique is able to detect anisotropy caused by both geological composition and microcracks having a preferred orientation. However, in order to be detected the anisotropy must be parallel or sub-parallel to the core axis. Furthermore, diametrical measurements on cores retrieved from the rock mass beneath a drift showed that the anisotropy decreased while the P-wave velocity increased with increasing distance from the drift floor. Microcracks with a preferred orientation were developed either during excavation or by the increased stresses around the drift. Waves as a consequence of activities are generally considered as something negative, for example, vibrations radiating from underground railways. In densely populated areas these vibrations reach nearby buildings and the residents as ground-borne noise and/or vibrations. Reliable predictions to ensure that residents will not be annoyed are a necessity when planning a new railway or constructing new buildings along an existing route. Numerical analysis is a natural part of the prediction models for train-inducedvibrations. In general these analyses treat the ground as homogeneous and isotropic. To determine if such an assumption is valid wave propagation through discontinuous rock masses have been studied using numerical analyses. The results of the analyses show for example that discontinuities can significantly increase the vibrations locally on the ground surface above a dynamically loaded tunnel. Properties having the greatest impact on wave propagation are the shear and normal stiffness of the discontinuity, the number of discontinuities and their internal distance, angle of incidence and the frequency of the wave. This study shows that discontinuities under certain conditions have an impact on the propagation of train-induced vibrations. Zones with a non-zero thickness show some other interesting phenomena, for example: they can result in channelling of waves resulting in higher velocity-levels at the ground surface where the zone daylights but also as a wave trap or filter. If the uppermost part of the rock mass has properties different from those of the host rock mass, generally amplifies the peak particle velocity on the ground surface especially in the horizontal direction.

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