Numerical Stability Analysis Of Tectonically Disturbed And Lithologically Varied Rock Masses Of Collapsed Pressure Shaft Slope In Siah Bishe Dam, North Iran

Siah Bishe pumped storage power plant with capacity of 1000 MW is the first pumped storage power plant that is located in the northern part of the Alborz mountain range in Iran. There are two parallel pressure shafts in the slope which both of them are 450 m long and their diameters are 6 m with 65 degree inclination relatively south-north trend. The pressure shafts slope is highly tectonically disturbed. The site area and especially the area of the pressure shaft are characterized by the presence of three important thrust faults which cut through the whole stratigraphic series. The shafts are arranged in sedimentary and volcanic highly jointed rocks of Triasic and Jurasic ages. In addition, based on several results of site investigations, there are lithologically varied rock masses. These have led to complicated situations and complexities for the project, especially for excavating the pressure shafts. In order to predict and also analyze the stability condition of the shafts, numerical modeling has been used. The results show instabilities within the slope, especially the boundar of Elika and Nesen formations which have highly different permeabilities. The permeabilities diversity has prevented underground water discharge in the boundary. This has led to instabilities in the shafts excavation process. Also, this boundary was severely techtonized; hence, it was very potential to have such huge instabilities. Numerical modeling results show the same instabilities, and they prove that by means of numerical modeling, it is possible to predict and estimate instabilities in geotechnical projects.