Shaking table tests on negative friction of piles in soft soils under strong earthquake motion

The negative friction of piles caused by seismic settlement of soft soils will produce the downward loads on pile foundation and cause serious damage to the foundation and structures. In order to study the formation mechanism and distribution characteristics of negative friction of pile foundation in soft soils under strong earthquake motion, the static tests and dynamic triaxial tests on soft soils and the shaking table tests on single pile in soft soil field are carried out. The negative friction on piles caused by seismic settlement of soft soils is proved. The seismic subsidence characteristics of soft soils, the dynamic response of pile-soil system under different input ground motion and the distribution development law of negative friction resistance of piles are obtained. The generation and development of negative friction under strong earthquakes are discussed. The results indicate that: (1) There is a certain yield dynamic stress in the formation of seismic subsidence in soft soils. The dynamic triaxial tests can be used to preliminarily judge the initial magnitude of negative friction induced by seismic subsidence. (2) Under the horizontal ground motion, the negative friction resistance is mainly generated at the upper part of the piles, while under the vertical ground motion, the negative friction resistance is generated at the whole pile. The development law of negative friction resistance is different in these two cases. (3) The negative friction resistance of the pile foundation caused by seismic subsidence of soft soils is sudden. At the initial stage of ground motion loading, the small relative displacement of the piles and soils can produce significant negative friction resistance, so attention should be paid to the damage effect of such instantaneous loads on the foundation and superstructure. (4) The potential negative friction resistance of pile foundation in soft soils can be estimated preliminarily by using the shear strength of pile-side soil and contact surface. The above results may provide reference for the discrimination and calculation of negative friction resistance of pile foundation in soft soil sites, and have certain theoretical and engineering practical value for seismic fortitude of soft soil sites.