The behavior of underground structures under static loading has been studied since many years ago and a lot of research has been published in this regard. For large caverns, the importance of more rigorous stability studies necessitates analysis of these structures under earthquake induced dynamic loading. The effects of earthquake's magnitude and duration of dynamic loading on safety factor, maximum shear strength and shear displacement in the vicinity of the underground opening were computed in time domain with a distinct element code and the results are compared with the values obtained from the static analysis. In this paper, the extension phase of Masjed Soleiman Power Plant cavern under earthquake shaking has been carried out as a real example. To perform this, four typical earthquakes which have happened in Iran during recent century are used as input wave forms. Although it is generally thought that underground structures provide a safer environment against earthquake, but the two mentioned parameters can affect the safety seriously.
Underground power projects play very important roles in meeting the requirements of present electricity requirements. Several underground structures are being built in regions that are highly prone to seismic risks where their failures could cause severe damages to human lives and properties. It is therefore essential to design underground structures for anticipated earthquakes so that they do not undergo any serious damages. It must be noted that determination of dynamic forces caused by earthquake is very difficult due to complexities of the mixed analysis for accounting the effects of frequency, duration and intensity of the applied forces. On account of this fact, twodimensional dynamic distinct element method was chosen to perform dynamic analysis of underground caverns of Masjed Soleiman Hydroelectric Power Plant, located in South Eastern part of Iran.
The underground caverns of Masjed Soleiman Hydroelectric Power Plant are simplified and depicted in Figure 1. These openings are located in a bedded formation consisting of Sandstone, Claystone, Conglomerate and Siltstone. The average overburden height at the site is about 400 m. The height, width and length of the Power House Cavern are 47.5 m, 30.0m and 110.0m respectively. The dimensions of the Transformer Cavern are 21.0×13.6m×110.0m and is located at a distance of 27.5m at the right hand side of the Power House Cavern.
Geotechnical investigations have been conducted to obtain geological information and to obtain samples for performing laboratory tests to evaluate mechanical properties of the rock mass. Flat jack tests were also conducted to determine the in-situ stress and modulus of deformation. Dilatometer tests were carried out to specify the in-situ modulus of deformation at 22 locations as well. The summary of the rock mass properties are presented in Table 1.
Dynamic studies have been obtained based on four seismic events. The first one (which was the smallest event too) struck the region in September 2004.
