Abstract: The water inrush risk encountered in Qingdao Metro tunnel crossing submarine slightlyweathered tuff with 30~60 m burial depth and F5 fault and fracture zone with 500 m width should be solved. The doublemode TBM is adopted, the open mode of TBM is adopted for submarine hard rock section, and the soilpressure balance mode of TBM is adopted for F5 fault and fracture zone based on comprehensive comparison and selection on construction period, geology and groundwater. According to the hydrogeological environment under special working conditions, selection and optimization of TBM are carried out as follows: (1) The shield tail brushes are added to improve the equipment′s antiseepage ability; the shield plate at the tail of the shield is installed to ensure the grouting effect. (2) The advanced geological drilling rig and double grout grouting equipment are equipped to achieve advanced drilling and grouting reinforcement. (3) The heavy alloy cutters are adopted to reduce the frequency of tool replacement and the size of excavation face is increased to reduce the risk of TBM jamming due to complex geology. (4) The steel grille at the opening position of the cutter is added to prevent large amount of hole slag from damaging the cutter. (5) The holes on the pressure separator are added to realize nopressure mode front drilling and grouting reinforcement. When crossing fault and fracture zone, the belt conveyor and related equipment are removed, the holes in the pressure separator is sealed, the screw conveyor and foam system is installed to change the open mode to the earthpressure balance mode, so as to improve the safety of the double mode TBM. Through the research of optimization of equipment, process test, mode conversion of TBM and soil pressure balance, the construction of auxiliary guide tunnel for Dayang StationNorth Qingdao Station of Line 8 crossing complex hydrogeology of sea area is successfully completed.

Key words: complex geology of sea area, double mode TBM, equipment selection, metro, TBM mode conversion