Abstract
In the past decade, ambient vibration measurements found numerous applications on unstable rock slopes and developed into a powerful tool for site characterization of slope instabilities. In this study, for the first time ambient vibration measurements were applied to a rock mass strongly disturbed and damaged by subsurface explosions. The site above the ammunition storage chamber at Mitholz (Switzerland) is especially interesting because the subsurface geology below the seismic array is well known, including the location of the caverns, and the degree of degradation caused by the subsurface explosions in 1947 of around 40 t TNT of ammunition. Measurement data were analyzed using current state-of-the-art seismic single-station and array methods, focusing on surface-wave dispersion analysis, wave field polarization, wave amplification using site-to-reference spectral ratios and analysis of normal mode behavior. The results allow for calibrating the elastic properties of a 2D numerical rock mechanical model which was used to simulate the stability of the disturbed rock mass during seismic loading. Therefore, ambient vibration measurements can contribute not only to a better understanding of the subsurface, but also for the assessment of earthquake risk.
Highlights
Underground storage of ammunitions in natural or excavated caverns offers a number of advantages, for example, release of valuable land above ground for other uses, improved safety, because the rock mass is able to confine and reduce the effects of accidental explosions, a constant temperature and decreased exposure to weathering (Geneva International Centre for Humanitarian Demining, 2008)
Kleinbrod et al (2019) observed the ground motion on different rock mass instabilities and differences between types of sites whose seismic response is mainly controlled by the depthdependent shear wave profile favoring the propagation of surface waves, and sites whose response is mainly controlled by volumes separated by deep fractures and characterized by normal mode behavior
Measurements presented in this paper support our hypothesis that a rock mass disturbed from subsurface explosions shows a similar signal in ambient vibration data as unstable rock slopes do
Summary
Underground storage of ammunitions in natural or excavated caverns offers a number of advantages, for example, release of valuable land above ground for other uses, improved safety, because the rock mass is able to confine and reduce the effects of accidental explosions, a constant temperature and decreased exposure to weathering (Geneva International Centre for Humanitarian Demining, 2008). Kleinbrod et al (2019) observed the ground motion on different rock mass instabilities and differences between types of sites whose seismic response is mainly controlled by the depthdependent shear wave profile (depth-controlled) favoring the propagation of surface waves, and sites whose response is mainly controlled by volumes separated by deep fractures (volume-controlled) and characterized by normal mode behavior. Towers with their extreme geometry represent a special case of volume-controlled sites featuring strong amplification and directionality.
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