ABSTRACT: Underground facilities built in areas subject to earthquake activity must withstand both static and seismic loading. The seismic stability of large-scale underground caverns, such as underground opening for hydropower house is often affected by faults nearby. In this study, the influence of faults on seismic behavior of underground caverns has been investigated using numerical methods. A parametric study has been carried out to determine the most critical situation of a single fault crossing cavern section with the strike parallel to cavern axis. For such a purpose several cases of faults having different dips and different intersection points with cavern wall were considered. For each case a dynamic nonlinear analysis was performed under an earthquake ground motion applied on bedrock. To assess the seismic effects on stability of cavern, two quantities related to rock mass response, the maximum values of area of plastic zone around underground opening and vertical displacement in cavern roof are selected to be extracted from analyses. The results indicate that the most critical dip angle for a single fault crossing cavern section is ranged between 30° to 50°. In addition, the most critical situation of fault causing large vertical displacement is the case in which the fault intersects the cavern roof.
1. INTRODUCTION
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 Kocaeli, Turkey earthquake. Discontinuity is one of the major features of rock as a construction material. Joints, faults and bedding planes exist inherently in most rocks and affect stability of rock structures. Many static and seismic instabilities of underground caverns ascribed to the influence of faults located nearby. Brekke and Selmer Olsen (1996) concluded in a survey of factors causing failures of underground excavations in Norway, that faults are often a major cause of failures. In particular in recent years, many reports on fault-induced problems considering the stability of underground caverns have become a pivotal issue that engineers and researchers are facing in the current time [1]. However, finding a theoretical solution to survey the stability of underground cavern is beyond the bounds of possibility. In order to construct an economical and reliable support system, it is essential to have a better knowledge of the fault-induced behavior of underground excavations considering the stability aspects. Underground structures have characteristics that make their individual seismic behavior in comparison to most surface structures, most notably their complete enclosure in soil or rock, and their significant length (i.e. tunnels). The design of underground facilities to resist seismic loading, thus has aspects that are very different from the seismic design of surface structures. So using numerical method for investigation of seismic behavior of large-scale underground caverns is inevitable [2]. A significant number of methods have been developed and extensively applied for these problems. However, a convenient method must be problem oriented.