Difficulties Ground Condition in Excavation of Longest Inclined Shaft in Iran

The longest pair of inclined shaft is located in the first pumped-storage power plant in Iran. The shafts are about 500 m long with inclination of 65° and with diameter of 6.2 m (center to center distances is 20.5 m). Several access galleries to save time were carried out and excavations in two phases (pilot and then enlargement). 12% of shafts lengths were excavated by blind shaft sinking. Mechanized method was utilized in 60% of pilot shaft (Dia.: 2.4 m), remaining part of shaft pilot was excavated by conventional drilling and blasting method (Dia.: 1.8 m). Enlargement of shafts were done by drilling and blasting method. Geological conditions at the pressure shafts are problematic due to presence of karst, weak rock zone and underground water leakage. This situation has caused some problems like redrilling of raise borer, reamer falling down, and several collapses in pilot and enlargement phase and at intersection of access gallery and shaft. Consolidation grouting, backfill concrete, fore poling and shotcrete by rebar used in order to solve mentioned problems. Pressure shafts excavation were carried out continually from Oct. 2005 to Feb. 2009 (40 months).