Study On Seismic Response Of Liquefiable Foundation-Pile-Tall Building Structure System Under Earthquake

Liquefaction caused by earthquake results in uneven settlement,tilt and collapse for buildings,which has a big impact on building safety.Liquefiable ground and building foundation are the focus of current research,while the superstructure,especially tall building structure,is not paid enough attention.Therefore,seismic response of liquefiable foundation-pile-tall building structure system which prototype comes from a practical engineering is studied in this paper,through a combination of shaking table experiments,theoretical analysis and numerical simulations.Liquefaction mechanism of soil and seismic response rules of pile-structure system are revealed from system's disasters caused by earthquake in the shaking table test,based on which disaster prevention and reduction measures for pile of the tall building on liquefiable ground are developed.Nonlinear seismic responses of soil-pile-structure interaction system are studied in the numerical implementation of Zhang Jianmin's three-dimensional large liquefaction deformation constitutive model of sand.Based on it,influences of important parameters such as damping ratio and permeability coefficient on sand liquefaction and seismic response of structure are applied.The main achievements are as follows:(1)The dynamic change of excess pore water pressure(EPWP)indicates that the upper part of ground is easier to liquefied,and the increase and dissipation of EPWP of sand outside and between pile group could differ due to the difference restraint when earthquake occur,which indicates the complexity of sand liquefaction.The sand EPWP increases rapidly with the earthquake excitation and dissipates gradually after reaching the peak value,and the fundamental frequency of soil decreases gradually,the damping ratio increases gradually and the dynamic response of soil attenuates with soil liquefaction.(2)The frequency of pile group decreases and the damping ratio increases under earthquake,which is caused by the weakening of soil stiffness,the strong nonlinear dynamic stress-strain characteristic of soil and the accumulation of pile damage.The seismic response of the pile group is strengthened under the excitation of Shanghai artificial wave with abundant low frequency components,and the contact pressure of the bottom of the corner piles on both sides has an obvious inverse phase relation,which indicates that the phenomenon of pulling and pressing appears at both ends of the cap during the earthquake.In order to reduce the earthquake damage of pile group of the tall building structure,it is suggested that the pile should be improved in terms of its compressive pulling capacity and soil liquefaction condition,as pile group swings in liquefied ground and is subjected to repeated drawing and stamping action.(3)The tall building structure has a deformation characteristic of bending and shearing,and the inflection point of vibration curve is close to the soil surface.The oscillation acceleration component of the structure is very small,mainly because the dynamic interaction between the tall building structure and the pile group increases the foundation's impedance,and the foundation absorbs some high frequency components of earthquake.There is a positive correlation between the soil liquefaction and the horizontal displacement of the tall building structure.(4)The Zhang Jianmin's constitutive model of sand has strong simulation ability.The numerical results show that the large lateral deformation of soil aggravates the disaster of liquefaction.The damping coefficient has little influence on the pore pressure of sand layer,while the lateral displacement of sand is large when the permeability coefficient is small,which leads to the increase of pile's response.The increase of superstructure counterweight leads to increase of pile head displacement.