Research On The Dynamic Seismic Response And Practical Design Method Of Piles In Typical Liquefiable Soil Of Jiangsu Province

In the past few years, a lot of earthquakes happened in China. Under the effect of earthquakes, the liquidation of liquefiable soil layers would have great negative impact on piles. How to consider the earthquake effect properly, predict the influence on piles reasonably and analyze the dynamic response of piles accurately is an important problem that should be resolved timely. Based on ABAQUS, this thesis studies the dynamic response of piles embedded in liquefiable soil layers in Jiangsu provience, discusses the interaction rules of pile-soil-structure and analyzes similarities and differences of pile’s dynamic response with different parameters of piles, soil layers and superstructures. Finally, a design flow is proposed based on the simulation results. The main achievements are as follows:(1) The theoretical and experimental researches on pile’s seismic response are totally reviewed. With literature works summarized above, the problems this thesis aims to resolve are preliminarily analyzed.(2) The main engineering geological characteristics of Jiangsu Yellow River flooded area are summarized. Through indoor dynamic tri-axial test and in-situ SCPTU test, some dynamic parameters of soil are achieved. Based on the test data, an empirical equation between shear wave velocity and main CPTU test parameters is proposed.(3) Based on the engineering geological conditions of supporting project, a numerical calculation model including liquefiable soil layer is established and differences between calculation results of quasi-static analysis and time-history analysis are compared. It suggests that the result of time-history analysis corresponds more to the reality, but the result of quasi-static analysis is relatively small due to the ignorance of soil displacement. According to the result, the formula of calculating bending moment on pile tip in quasi-static analysis is revised.(4) The influence on pile-soil-structure interaction from four fields of parameters (piles, soil layers, superstructure, connection between piles and bearing platform) is discussed. Under given soil parameters and working conditions, the bored pile has an optimal diameter range which is 0.6 m～0.8 m. For the PHC pile, simulation result shows that its seismic performance can be effectively enhanced by adding a certain length of core concrete or steel ring. The strength of core concrete is suggested to be C45 and the thickness of steel ring is recommended to be 3 mm ～8 mm. The existence of shallow liquefiable soil layers less than 10 meters depth has a great impact on superstructure and would let the response spectrum have a second peak in the long-period range (>2s) which could have negative effects on tall buildings. However, the deep liquefiable soil layer (10 m-20 m) can act like crash pad to the superstructure, which can mitigate the damage caused by erathquake. The inertia force generated by seismic oscillation would only have obvious impacts on piles when the level number of structure is higher than 10, but the existence of basement can effectively reduce the internal force of piles by 15%-20%. Study also reveals that the internal force of solid-jointed piles is much greater than that of hinge-jointed piles and the differences mainly happened in a range of 10 meters from the top of pile (approximately 15 times of pile diameter). But the hinge-jointed form may have a negative impact on the superstructure’s seismic performance. CP method and FT method can both let piles and bearing platform connected in a semi-rigid form. A discussion on CP method suggests that the coupling stiffness can be from 2500 kN/m to 4000 kN/m. Meanwhile, two simplified mechanical model called displacement-control model and force-control model in FT method are proposed. The displacement-control model can be used when soil conditions are unsatisfactory, whereas force-control model should be used when soil layers are in good condition.(5) The technical approaches of liquefaction treatment, seismic design of piles and base isolation to mitigate pile damage are summarized. Finally, a seismic design flow suitable to piles in Jiangsu Yellow River flooded area is proposed.