Stability analysis of underground excavations is one of the most essential studies in geomechanics. With rapid development of numerical programs and innovations in computer hardware to perform fast required calculations for numerical programs, numerical models in analysis of underground spaces have been expanded. FLAC 2D and UDEC are successful softwares for such analyses. In this study, Siah-Bishe right pressure shaft has been modeled by FLAC 2D. Axisymmetry geometry with simple 3D model is used to account for the 3D shape of the shaft. One of the properties of this model is its near real conditions. Using this geometry helps performing stability analysis of shaft in one stage. Due to the length of shaft, various types of geology formations present. After modeling the shaft, the results are compared with closed form solutions presented by Panet (1979). The right pressure shaft is excavated in three steps by raise boring technique which includes drilling a 30 centimeter diameter hole in the first stage. In the second stage, this small hole is enlarged to 2.4 meters by a reamer head and in the third stage the final diameter (6 meter) is excavated. In this project, the first stage drilling was done successfully while enlarging to the second stage caused some collapse in the shaft wall at a certain depth. The aim of this study is to model the observed conditions and to predict the stability problems for the final enlargement stage. Comparing the induced stresses in elements surrounding the excavated area with their strength, a safety factor is determined for rock mass along the shaft. Results obtained from numerical modeling show unstable zones in the middle part of the shaft which corresponds closely with the observed results. Long term stability of the enlarged shaft can be assessed easily by this calibrated model.


Siah Bishe Pumped Storage project is located in Alborz mountain range, 125 km north of Tehran, in the vicinity of Siah Bishe village. The site can be reached by Chalus road, connecting Tehran to the Caspian Sea (Fig 1). This Plant is designed to produce a rated capacity of 4 × 260 = 1040 MW peak energy. Two dams will be constructed in Chalus valley for the water storage. Both dams are designed as concrete faced rockfill dams. Two separate water conveyance systems will connect the upper and lower reservoir. They comprise an upper intake structure, the headrace tunnels, surge tanks, pressure shafts, upper and lower manifolds, tailrace tunnels and outlet structures. A powerhouse and a transformer cavern will be excavated underground to accommodate all machineries and equipment for power generation and pumping. Headrace tunnels are connected to the powerhouse cavern with two pressure shafts (Right and left pressure shaft).

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