Some considerations on the stability and design of underground structures during earthquakes

In Japan, many underground structures have been constructed and are in service in spite of complex and poor geological conditions. On the other hand, Japan is famous for big earthquakes resulting a huge national loss, thus the seismic design is very important for every kind of structures. However, the seismic design of underground structures, especially mountain tunnel, has not been well discussed up to now, while mountain tunnels were also severely damaged in the recent earthquakes. Therefore, the seismic design for the underground structures should be discussed as an important and urgent problem. Here the present situation of the seismic design for underground structures ranging from the cut-and-cover tunnels in the shallow underground to the rock tunnels in deep rock mass is briefly reviewed. Then, typical earthquake damages of rock tunnels are presented and problems to establish a proper method of seismic design for the underground structures are discussed.