Some Considerations on Seismic Stability of Oya Underground Quarry

Following the 2011 Great East Japan Earthquake, it has become important to evaluate seismic stabilities of infrastructures in Japan as well as the structures utilizing underground spaces. Oya tuff,which is one of the famous building stone and is classified into pumice volcanic tuff, has been extracted from underground quarries around Oya town, Utsunomiya City, Tochigi Prefecture, Japan. Up to now over 200 underground quarries are developed and abandoned. The numbers of people who consider those abandoned underground quarries as space resources and the demand to utilize publicly are increasing. The authors have been involved with the evaluation of the stabilities un-supported underground spaces for public utilization. The authors first compared the stability condition obtained from numerical analysis with field observations and measurements and concluded they should be stable in short term. In this study, the seismic response of some underground quarries are evaluated through numerical analysis.The authors consider the influence to seismic wave on simplified two-dimensional model to clarify the fundamental response of underground quarries to seismic shaking. It is found that the eigen frequency of underground structures analysed may range between 3.5'Hz and 5.0 Hz. And then, a series numerical analyses were performed on the response of the underground quarries during two types of large seismic loading. Although computational results showed that the corners of undeground openings at roofs or floors and pillars may be damaged and have possibility of some spalling, they may not be critical for the stability of the undergound structures. The results confirm that active underground quarries, in which Oya tuff has been still extracted, survived the Great East Japan Earthquake without any major damage. However some caverns near the ground surface were either damaged or collapsed. This paper presents the outcomes of dynamic analyses on the stability condition of the some underground quarries during quarrying and seismic loading.