ABSTRACT The Siah Bishe pumped storage project is now under construction. The investigation results, the stability analysis and the support concept for the displacement chamber are discussed in this paper. The displacement chamber is arranged in sedimentary and volcanic rocks of Triasic and Jurasic ages. The length of the chamber is 150m and the height is 10m with relatively north-south trend. The results of geo-engineering surveys and statistical joint-graphics of stereograms reveal at least three discontinuity systems in the rock mass. Several transversal sections along the chamber were studied in order to analyze the stability of rock wedges inside the chamber. The frequency and width of bedding parallel shear zones determined the final support concept. The results of stability analyses show distinctly asymmetrical overstressed zones.
1 INTRODUCTION IranWater 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 (Moshanir Consultant Engineer, 2002). 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 Garmrudbar thrust fault separates the Jurassic Formation from the Triassic one. The displacement chamber with the approximately dimensions of L: 50 m, W: 7m, H: 10m lies in the Shemshak and Elika Formation. 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 whole formation is folded and forms the southern flank of an anticline. The folding process caused a shearing of incompetent layers such as thin layers of beds but also between siltstones with different content of fines like clay or fine sand.
2 INVESTIGATIONS The area of the underground displacement chamberwas investigated by the excavation of two test adits in total 320 meters long, by underground drilling of 100 m. The inclination of borehole is 45 degree with south direction, nearly perpendicular to the shear zone plane (Farab and Tablieh Geotechnical Company, 2003). The displacement chamber with a length of about 80mruns nearly normal to the strike of the layers of Shemshak and Elika formations (Fig. 1), mainly in thin and medium siltstone, sandstone and shaly layers of Shemshak over the first 50 m, after that follow limestone of Elika formation at the end of displacement chamber.
3 GEOLOGY OF THE DISPLACEMENT CHAMBER 3.1 Bedding The roof of displacement chamber exposed a rock sequence with a mean dip and dip direction of bedding planes of N015/50 up to station 50m where a main thrust fault crossed displacement chamber.