The Siah Bishe Pumped Storage Dam is located in the northern part of the Alborz mountain range in Iran. The investigation results and the effect of brittle deformation of two fault sets in the right pressure shaft of Siah Bisheh Dam are discussed in this paper. The shaft is arranged in sedimentary and volcanic highly jointed rocks of Triasic and Jurasic ages. The length of the shaft is 450m and its diameter is 6m with 65 degree inclination relatively south-north trend. The results of geo-engineering surveys using boreholes and statistical joint-graphics of stereograms reveal at least two sets of faults in the rock mass. Several transversal sections along the shafts were studied in order to analyze the effect of faults in shaft collapse. The findings from the surface and underground mapping and investigation boreholes are described and interpreted in a geological map of pressure shaft slope including sections and stereograms. The site area and especially the area of the pressure shaft are characterized by the presence of three important thrust faults that cut through the whole stratigraphic series. The Main Thrust Fault (MTF), fault Nr. 2, both dipping to the north and fault Nr.3 that dips to the south. East of the site, faults 2 and 3 border a large ‘pop-up’ structure while the Main Thrust Fault occupies a lower tectonic position. The results show that the collapse could be a local occurrence which is limited to a critical, fractured and water-saturated rock zone at the boundary between MTF and fault 3. Stability of the shaft was analysed with Unwegde 3.0 software. Result of rock bolting of the shaft was not suitable because of depth of the tip wedges are longer than maximum allowable rock bolt length. The lowest safety factor occurred on wedge number 4 (SF=0.873). But using of shotcrete as a support shows that safety factor of the wedge is increased to 3.219, when shotcrete thickness is 20 cm.
Iran Water and Power Resources Development Company was entrusted 1983 with the design of Siah Bisheh pumped storage scheme. The waterways of the plant are now under construction. It is located in the northern part of the Alborz Mountain, at a distance of 80 km from the Caspian Sea [1]. The pumped storage plant is situated in layers of the Jurassic Shemshak, Triassic Elika and in the strata of Permian age called Dorud, Ruteh and Nessen formation. The Main Thrust Fault (MTF) separates the Jurassic formation from the Triassic one [2]. The shaft lies in the Elika, Nesen, Routheh and Doroud formations. These formations consist of shaly, slightly sandy siltstone, sandstone and thin layered limestone and intrusions of igneous rock such as spilitic basalt partially bedding parallel orientated. The investigation results and effect of brittle-ductile thrust wedges on the left pressure shaft of Siah Bisheh Dam are discussed in this paper. An extensive investigation program was thereafter initiated in April/May 2006 by authors with the aim to investigate the geological conditions of shaft area in more detail.
