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Numerical simulation of the impact of shallow underground cavities on the surface seismic motion

Olympios Petros

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Summary

In general, local site conditions, such as stratigraphic, geomorphic (bedrock) and topographical (surface geometry) are very important in the propagation of seismic waves and their impact on the structures at the ground surface. More complicated geometries, such as the existence of near-surface underground cavities, increase the complexity of these phenomena. In this context, the present diploma thesis focuses on the study of the impact of small-scale underground voids on the seismic motion at the ground surface. The purpose of the study is to examine whether the existence of underground cavities or other geomorphological anomalies of relatively small dimensions near the surface are capable of affecting the measurements recorded by accelererographs during strong earthquakes.Initially, basic concepts are described that help to understand the problem at hand, such as the basic characteristics and propagation of seismic waves. Emphasis is then given on describing the adopted numerical simulation methodology, which has been verified utilizing an analytical procedure. In particular, ABAQUS finite-element software has been used to perform the dynamic analyses, whereas SeismoSignal software is employed to generate the spectra in order to compare the results. The parametric study examined the main factors affecting the complex phenomenon under consideration. A detailed description of the simulations is provided regarding the geometry of the cavities, the properties of the materials and the characteristics of the imposed base excitations.Different types of cavities at different depths have been studied, having either small dimensions or shaped like trenches or tunnels. Furthermore, comparisons were made for different seismic shear-wave velocities, VS, of the soil layer. Comparisons were also made for the case where the cavity is either empty or filled with a soft material having smaller VS compared to the Vs of the surrounding soil layer. Lastly, two Ricker pulses with frequencies 2 Hz and 6 Hz have been used as base excitations, as they have a suitable frequency content for the examined problems.On the basis of the derived results, it can be concluded that, under certain conditions, the underground cavities may influence the propagation of seismic waves near the surface. As this can either be beneficial or detrimental on the seismic distress of adjacent structures, a reliable identification of their presence and a quantitative assessment of their role is deemed necessary.

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