Earthquakes have caused some damage to abandoned quarries in Oya region, Japan and the behaviour of pillars of Oya tuff quarries is therefore of paramount importance for the region. This study reports investigation results of Oya tuff pillars under static and dynamic conditions. Also, the backfilling of underground quarries is considered as a countermeasure of collapse in the long term and during earthquakes. For this purpose, an experimental laboratory program was initiated to investigate the static and dynamic response of Oya tuff pillars. The shock tests demonstrate that the overall stiffness of samples subjected to shock loading is higher than that of samples under static loading. With respect to backfilling, the results show that, the overall stiffness of backfilled samples is higher than that of unfilled and in some cases, the yielding strength of backfilled samples displays strain hardening behaviour, while unfilled samples show strain softening behaviour. Different degrees of backfilling, indicate that partial backfilling is not effective to prevent ground settlement.
Underground structures have been known to be seismically insulated or rather could sustain seismic loads with little damage. However, over recent years the investigations regarding seismic response reveal that underground spaces such tunnels and underground shallow mines could experience significant damages (Wang, et al., 2001; Genis & Aydan, 2008; Aydan, et al, 2010). Unlike in deep mines, where stress-induced failures can occur in a violent manner invoking seismic events, in shallow mines, the type of dynamic loading that could pose a threat to shallow mines is the occurrence of the earthquake. In Japan, there are several cases in which underground spaces have been damaged by the often occurrence of earthquakes. After the 2011 Great Earthquake of East Japan with the magnitude of 9 (Aydan, 2014) embarked on the expedition visiting the affected areas focusing on the various damaged geo-engineering structures, and with special interest to abandoned underground mines and quarries. Aydan, (2014) documented and summarized almost all the accessible damaged underground areas. The records show that the earthquake caused sinkholes in several locations Iwaki in the Fukushima Prefecture, eleven locations in Kurihara, seven locations in Osaki, eleven locations in Higashi Matsushima, three locations in Kurogawa in the Miyagi Prefecture (Aydan & Tano, 2012a). It was later mentioned by the Ministry of Economy, Trade and Industry (METI) that the actual number of the sinkholes is more than 316. Still from the list, it is interesting to note that one of the collapsed mines is the semi-underground mine located in Oya town in Tochigi prefecture and this collapse is not far from the area of the underground room and pillar quarry that is understudy. There is still an apprehension of similar cases reoccurring due to the influence of dynamic loading. A recent study by Seiki, et al., 2016 on seismic responses of long-wall type quarry 2km away from the current study area, has shown that increase in wave velocities results in high strain levels on the roofs walls. With all these past studies revealing the problems associated with dynamic loading in various underground spaces, a dynamic response investigation of pillars serves to be necessary and an essential study that could give fundamental understandings for underground mine stability.
