Engineering Geological Assessment of Diversion Tunnel of Bakhtiari Damsite (Biggest Two-Arch Concrete Dam in Southern Iran)

Abstract

Bakhtiari dam is located on the Bakhtiari river, 120 km away from the north of the Andimeshk city. Upper diversion tunnel of this dam with large cross section (13.7 m excavation diameter) and more than 1 km length is a huge construction. The tunnel is placed in the Sarvak formation carbonate rocks of Bangestan group which passes through seven different geological zones with various specifications (SV1, SV2, SV3, SV4, SV5, SV6, and SV7). Joint studies show two main discontinuit including bedding and a main group of joint (J1) together with random joints (faults and fractures). Most of discontinuities have been filled mainly by calcite or calcite and clay. Data deduced from testing and analysis shows good-to-excellent RQD classes with 75 to 90 values. Based on RMR and Q methods, generally rock masses have good to very good quality with 61 to 95 values for RMR and 10 to 35 values for Q . Based on conducted stability analysis, suitable supports were suggested for tunnel by RMR and Q methods. As a result, it can be concluded that all units have a good stability. Therefore, systematic rock bolting with 40–50 mm unreinforced shotcrete has been proposed for some special place. For rock support, according to RMR method, 3 m rock bolts in crown, 2.5 m spacing and with 50 mm shotcrete in crown has been proposed also 3 m rock bolts, based on Q method, 2.3-2.4 m spacing with systematic Bolting without shotcrete or 40 mm unreinforced shotcrete in some units, has been proposed. According to RMR method, for SV5 zone with very good and excellent quality, local 33 bolting without shotcrete and 3m rock bolts, 3 m spacing and spot bolting according to Q method has been proposed.