| Earthquake site investigation proves that site liquefaction is one of the main reasonof bridge pile failure. China faces high seismic hazard and earthquakes have charactersof wide distribution and high intensity. As rapid growth of bridge construction in ourcountry, numerous new and continuing bridges are located in sites with highliquefaction risk. Aseismic design of piles in liquefaction site is always the concern andproblem for researchers on geotechnical earthquake engineering. There exist specificrules for such problem in overseas codes (such as Japan and U.S.A), but thecorresponding part of domestic code is general and lack applicable technical details,cause blindness and arbitrariness in real projects and such projects are in high safety risk.Hence the research on simplified aseismic design method of bridge group piles inliquefaction site will be a great importance in real projects.This paper focuses on such simplified method, and combined analysis of dataobtained from shaking table test and numerical computation. First of all a series ofshaking table tests for pile group in liquefying ground were complied and respond ofpile and soil during liquefaction has been analyzed. Secondly build three dimensionalnumerical model of bridge pile group in liquefaction site and validate its reliability.Finally based on equivalent single pile model and non-linear Winkler hypothesis,simplified method has been given. Main research and relevant results are as follows:Shaking table test of reinforced concrete pile group foundation with high-rise cap,which are buried on clay above and saturated sand at the bottom and excited by ELCentro wave from base, has been complied. Responses of pore pressure, acceleration,soil pressure, pile-soil relative displacement and moment during liquefaction has beenanalyzed. It illustrates that time-history curves of pore pressure, pore pressure ratio,acceleration, moment and soil force under varied seismic waves could be divided intofour parts and peak point comes up on the second part because amplification ofacceleration of base excitation reaches highest in this part.Based on accomplished shaking table test and DIANA software, three dimensionalmodel of bridge pile group in liquefaction site has been built. Mohr-Coulombelastic-plastic constitutive model is applied in clay above and Nishi elastic-plasticconstitutive model for saturated sand at the bottom.20-node hexahedral isoperimetric element is used for clay and20-node hexahedral mixed finite element for sand, whichpore pressure dissipation and redistribution could be considered. Each node has fourdegrees of freedom, where degrees one to three represent translation degrees, degreefour for pore pressure. Behavior of pile-soil interaction is simulated by interfaceelement with zero thickness. Results of computed model and obtained test data havebeen contrasted and dynamic responses of pore pressure, acceleration and moment havebeen analyzed. It proves that they have good constancy and model of three-dimensionalbridge pile group in liquefaction site are testified correct.Simplified method has been concluded for prep accomplished shaking table test.Pile group foundation is simplified to model of equivalent single pile and relativesimplified model of bridge pile group foundation in liquefaction site has been givenbased on non-linear Winkler hypothesis. In this model equivalent single pile issimplified to beam-column element particle system and the cap and super structure is toconcentrated mass. Pile-soil interaction is simulated by macro cell element. By analysisof free-field model, site displacement could be get and it is also the boundary conditionof simplified model. Computed results and data obtained from tests are contrasted andaccuracy of simplified method has proved. Lastly its analysis steps and applicationscope has been concluded. |
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