Under seismic loading

the pile foundations in the liquefiable soil are often destroyed due to the liquefaction of foundation soil. In this process

even if the soil does not reach full liquefaction finally

the strength of saturated sand will be weakened due to the existence of the excess pore water pressure

which will also lead to the decrease of horizontal resistance force of soil. If the influence of excess pore water pressure on horizontal resistance force of soil is not considered

the pile foundations are still designed by adopting the p-y curves in the API standard

and the results will be more dangerous. In this paper

the dynamic cyclic torsional shear tests are carried out for the Fujian standard sands by employing the vertical-torsional coupling shear apparatus

and the dynamic characteristics and weakened parameters of saturated sands in different weakened states are studied. Then the formulas of ultimate soil resistance are derived based on the improved theoretical model of soil wedge at shallow layer. Combined with the theoretical model of flow failure around piles at deep layer

the ultimate soil resistances at different pore pressure ratios at any depth are obtained

and then the p-y curves of pile-soil interaction in saturated sand foundation in different weakened states are constructed. It can be found from the study that the pore pressure ratio

which characterizes the weakened state of soil

has a significant effect on the ultimate soil resistance in pile-soil interaction. With the increase of pore pressure ratio

the weakened degree of soil will be more serious

and the ultimate resistance of saturated sand is smaller. That is to say

the action of the lateral loaded pile on the surrounding soil decreases with the increase of soil weakened degree

and the vice versa.