Research On Construction Technology Of Large Section Tailrace Tunnel Under Unfavorable Geological Conditions

The tailrace main tunnel of Liyang Pumped Storage Power Station has the characteristics of large buried depth,large cross-section and relatively unique geological conditions.Due to the limitation of engineering geology and natural conditions,the engineering construction technology is extremely difficult.Two main tailrace tunnels are the most complex parts of the geological conditions of the whole power station.The surrounding rocks of type IV and V account for more than 95% of the surrounding rocks of the whole tailrace tunnel.The surrounding rocks of type III are very few.The main geological problems are faults,dikes,abundant groundwater,etc.Especially in faults and dikes,groundwater is a prominent combination of adverse geological conditions.The two main holes pass through the weak fractured zones composed of F136,F137 faults and 6# altered dikes with lengths of about 93 m and 199 m respectively.The geological characteristics of this large fault zone are not only structural development,serious rock fragmentation,weathering,dissolution,soft structure,containing groundwater,and easy to collapse.The stability of surrounding rock during construction is prominent,which directly affects the safety,investment and progress of the project.In order to ensure the quality of the main tailrace tunnel under the premise of safe and rapid construction,this paper summarizes a complete set of bad geological conditions of large cross-section tailrace tunnel construction experience and technology.In view of the special geological conditions of the main tailrace tunnel,the geological conditions of the untapped surrounding rock are determined by using advanced geological advanced prediction technology,which provides reliable reference for the concrete construction.On the basis of the conventional support method "NATM",the method of "short footage,weak blasting,strong support and reserved core soil" is put forward to ensure the safety of the initial support of the cavern.The use of nano-steel fibers and nano-imitation steel fibers shotcrete technology ensures the stability of surrounding rocks in unfavorable geological sections such as fault fracture zone and water-rich zone.The surrounding rocks can be quickly sealed in the collapse section and the spread of cavern collapse can be effectively controlled.In order to adapt to the complex geological conditions of the main tailrace tunnel and the excavation and support construction methods,the layered lining technology of the tunnel is summarized to ensure the stability of the overall structure of the tunnel and the completion of the project on schedule.The problems that should be paid attention to and solved in the process of construction technology are also put forward,which has played a certain reference role in the construction of similar projects.