Nowadays the most of the energy is producted by electrical Power plant, wiich is bvie based on available conditions as surface or Underground structure.Underground power plants are need to construct big structures. Because of high life and big scale structure, the structure deserve special attention according to design and stability. Underground opening stability is the most common issue in rock mechanic and geotechnical studies. With respect to expansion of various softwares and appearance of advanced computers for iteration calculation, the finite element methods for the stability analysis of the structures are utilized widespread in design and excavation stages. For this reason ,three dimensional softwares based on finite element methods can be used in design and stability analysis to define reinforcements.in this research ,stability of powerhouse cavern and tunnel sets in Bakhtiari dam are investigated at first stage,according to limitations of computers,the relationship between joint spacing and instability of structure with use of 3DEC software is specified,and optimum spacing is selected for reduction of data.according to selected spacing value,the main model is designed with 3DEC software in tn two lithology in dam site of powerhouse wich consist of three blocks.each block consist of inlet.Power house cavern .bus duct tunnel and outlet water tunnels.finally,each block is analysed with stability methods,according to anticipated support system based on empirical methods in each block,the support system is applied and compare the stability of the model with and without support system.
