The residual excess pore pressure ratio is used as the basic parameter for the evaluation of liquefaction potential and seismic design of hydraulic structures.For the purpose of clarifying the approach of calculation of residual excess pore pressure in United States code,the cyclic stress ratio causing liquefaction,the equivalent cyclic stress ratio and the factor of safety against liquefaction are discussed.An example of gravity quay is provided to illustrate the calculation of dynamic earth pressure including residual excess pore pressure.The results show that this method can solve the estimation problem of soil excess pore water pressure ratio.