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EFFECT OF BRITTLE-DUCTILE THRUST WEDGES ON THE LEFT PRESSURE SHAFT COLLAPSE OF SIAH BISHEH DAM, NORTH IRAN

H. Hassani, S. Arshadnejad

Abstract

The Siah Bishe Pumped Storage Dam is located in the northern part of the Alborz mountain range. The investigation results and the effect of brittle deformation of two fault sets in the left pressure shaft of Siah Bisheh Dam are discussed in this paper. The shaft is arranged in sedimentary and volcanic 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 streograms. The site area and especially the area of the pressure shaft is 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.

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