强震作用下软土场地桩基负摩阻力振动台试验研究

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

为研究强震作用下软土场地中桩基负摩阻力的产生机理及分布特性,设计开展了软土静力学试验、动三轴震陷试验和软土场地-单桩体系振动台试验,验证了震陷软土场地对桩基的负摩阻力作用,得到了软土的震陷特性、不同输入地震动下桩土体系地震动响应和桩负摩阻力分布发展规律,讨论了强震作用下负摩阻力的产生及发展过程。结果表明:①软土震陷的产生存在一定的屈服动应力,可利用动三轴试验来初步判断震陷引发负摩阻力的启动震级;②水平向地震动下负摩阻力主要产生在桩身的上部,竖向地震动下全桩均会产生负摩阻力;③震陷引发的桩基负摩阻力具有突发性,且微小的桩土相对位移量即可产生显著的负摩阻力,这种瞬时加载可能会对结构造成破坏;④利用桩侧土体和接触面的抗剪强度可初步估算软土场地中桩基可能受到的负摩阻力值。试验成果可为软土场地桩基负摩阻力的判别与计算提供参考,对软土场地抗震设防具有一定的理论和工程实用价值。

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.