Subsea tunnel of Dalian metro line 5 is the first large-diameter shield subsea tunnel in karst area in China. The tunnel across the sea is about 2.3 km and in form of a single-tube with double-line

using double-lining with a diameter of 11.8 m. The area which the tunnel through has complex geotechnical and hydrogeological conditions

locally karst development and rich groundwater

and many controlling factors; the design has encountered many difficulties. In this research

the sabsea tunnels which have single-tube with double-line have been compared with those have single-tube with single-line. The selection of large-diameter shield has been carried out under the condition of rock fragmentation and rich groundwater. Through the comparison of numerical simulation

the advantages of shield tunnel with double-lining under complicated geological conditions has been analyzed; and the optimizing design for the waterproof and drainage scheme is carried out combined with the lining form. Complete treatment measures are put forward on shield through the karst development area. The analysis results show that the subsea tunnel in karst area which has the exhaust air duct in form of a single-tube with double-lining is a better solution; and using the double-lining can significantly improve the durability of lining. The research results can be used as reference for similar underwater tunnel.