Based on the theory of limit equilibrium,a stress analysis was presented to study the waterburst mechanism in normal geologyical condition and fault disruption zone during crossing undersea tunnel,then crit- ical water inrush pressure formula of undersea tunnel was given.Finally,a calculation and waterinrush risk forecast is made in two deep weathered rocks and sections of Xiang'an Tunnel.In conclusion,in normal geolo- gyical conditon,critical water pressure decreases with the increasing of tunnel radius,in...