Historical reports indicate that underground structures have different behaviors against earthquake with the same magnitude. This finding infers that the earthquake magnitude alone is not sufficient for determination of earthquake intensity. In other words, peak ground acceleration (GA) that was calculated by attenuation relations is not enough criteria for determination of "design base earthquake" time history because, in addition to GA, seismic loads frequency affect on the earthquake intensity. The aim of this study is to evaluate the effect of seismic load frequency on the behavior of underground structures. For this purpose, effective parameters on the frequency content of seismic loads were identified and two time histories with different frequency content were compiled. Further, Isfahan-Shiraz tunnel, a railway tunnel in southern part of Iran, located in a disturbed zone was analyzed for two separate time histories that have the same GA and different frequency content. These analyses reveal that the frequency content of seismic load has significant effect on the earthquake load energy, stress distribution and underground structure stability. Furthermore, loads with higher frequency attenuate faster whereas, generally, loads with low frequency influence more, to compare with high frequency loads, on the structure stability.
Underground facilities are an integral part of the infrastructure of modern society and are used for a wide range of applications, including subways and railways, highways, material storage, and sewage and water transport . Underground facilities built in areas subject to earthquake activity must withstand both seismic and static loading. Historically, underground facilities have experienced a lower rate of damage than surface structures. Nevertheless, some underground structures have experienced significant damage in recent large earthquakes, including the 1995 Kobe, Japan earthquake, the 1999 Chi-Chi, Taiwan earthquake and the 1999 Locale, Turkey earthquake . Prediction of ground motions resulting from earthquake is very difficult and it is almost impossible to determine characteristic of ground motion until earthquake actually occurs. Every earthquake causes some unique motions the characteristics of which depend on several factors including disruption mechanism of fault at earthquake source, the wave's propagation media and geological features of earthquake site. Disruption mechanism of fault is complicated and the nature of energy transfer between earthquake source and structure is indeterminate therefore, investigation on all of ground motion characteristics is not possible for common engineering application . For this reason, probable earthquake magnitude is the only parameter used for earthquake risk evaluation for a given special region. Whereas, the effect of seismic load frequency is not considered. It should be noted that the frequency does not play an important role in assessing the shear strength of intact rocks, as shown by Burden , Jamison and Kim  and Ray et al. . Further, Chen Fang et al.  considered the dynamic response and failure behavior of rocks subjected to dynamic loading with the frequency ranging from 0.5 to 5 Hz and found that the obtained strain rate increase with the increase of load frequency.