Dominant geological structures and joint sets are the main and important factors which change the direction and magnitude of in situ stresses. Their importance and role have been examined in this research and finally the direction of stresses has been predicted in the area of Bakhtyari Dam. Studying the manner of geology evolution in the area, the following three main structures within the boundary of underground powerhouse of Bakhtyari Dam were identified: reverse fault F1–3, Siyah kooh anticline and the right hand strike-slip fault F2. Reverse fault F1–3 has become inactive as a result of folding and erosion. Therefore, the present stress field induced by this fault doesn't follow the direction of stresses that have created it. Borehole slotter test proved this issue to be correct. The strike-slip fault F2 has no impact on the direction of in situ stress due to its being far away from powerhouse caverns. Therefore, this fault has not been able to cause any joint or fracture neither to open discontinuities in powerhouse caverns area. Consequently, regarding the extension of joints resulted from folds throughout the area of underground powerhouse.
Below the ground, elements of rock materials have been affected by existing stresses. These stresses either follow the gravity rules and the side pressures or generally are influenced by structural tectonic stresses. Therefore, underground structures can be designed and constructed if the stress conditions and origins are known. Many researchers have studied this subject and determined stress direction and stress magnitude with regard to the structures all around the world (e.g. Hoek [1], Hoek and Brown [2], Hoek and Moy [3], McCutchen [4] etc.). In these researches, theories of stress direction related to structures are predicted. Bakhtyari dam with the height of 315 meters is the highest concrete two-arch dam in the world rank. This dam will be constructed on the Bakhtyari river which is a main branch of Dez river. The dam is located 570 km southwest of Tehran in Lorestan province of Iran. This area is situated in Zagros folded belt and consists of Sarvak formation whose rocks prominently belong to the Cretaceous period. Parallel to this project, a hydropower station with the capacity of 1600 MW electricity will be constructed. It is an underground structure and is about 500 m downstream of the dam axis, near the left bank of the river. This powerhouse includes two caverns, one for generators and one for transformers [5].
The studied site is located in northwest of Zagros folded belt which is composed of numerous high anticlines with steep slopes and deep valleys oriented parallel to Za extending from Bandar Abbas in the south to Naft Shahr in the west [5]. The regional geology involves Garu, Sarvak, Gurpi, Amiran, Tale-Zang and Kashkan formations. Most of the project and the whole parts of dam and hydropower station are located in Sarvak formation [5].
