The giant pile group foundation of the Su-Tong Bridge located on the Yangtze River is selected to be the subject investigated. The seismic excitation is regarded as stationary stochastic process considering the uncertainties of real seismogram. A stochastic seismic response method

in which the complex response technique is employed

is applied to study the dynamic responses of this pile group foundation with a large pier cap. The dynamic nonlinearity of soils is considered through the equivalent linearity method. During a seismic motion

the displacements along depths of pile shaft appear triangle because of inertia force of pier and weak restraint of soft soil. The displacement of soil is related to the distance between soil and pile foundation. The effects of pier-pile-soil interaction on displacement response of the soil surrounding the pile foundation are significant. The effected range is about 1.5 times width of pile foundation. The internal force responses of piles indicate that the bending moments and shear forces of pile head and upper portion of pile shaft are larger relatively. The shear force of outer piles is marked larger than that of center piles. In addition

based on the principle of intensity damage

the dynamic reliabilities of the pile foundation are analyzed under four different yield shear force levels.