Study On The Creep Properties And Fractional Order Constitutive Model Of Soft Clay

With the rapid economic development,high-speed railway,highway,subway,port and other infrastructure construction are constantly building around the coastal areas in China.These infrastructures are basically built on the deep soft clay layer,and bear long-term load and long-term cyclic load,which causes settlement of soft clay foundation.This dissertation aims to explore the creep characteristics of soft clay under long-term load and long-term cyclic load,and discusses the influence of different drainage modes by laboratory tests,theoretical analysis and numerical simulation.Main research contents and achievements include the three aspects.(1)Triaxial and stage loading creep tests were carried out on the 4th layer of muddy clay in Shanghai,and the effects of confining pressure and drainage conditions on creep properties are studied.The results show that the larger the confining pressure,the smaller the strain,and the smaller the excess pore water pressure and the faster the excess pore water pressure dissipates.Under the same confining pressure,the strain of drained creep test develops rapidly and tends to be stable gradually.The strain of undrained creep test develops slowly at the initial stage and decreases less with time.The step loading creep test results show that the strain increment with the increase of deviator stress,and the instantaneous strain of each stage also increases,but the excess pore water pressure decreases.Under the same confining pressure,the excess pore water pressure of undrained creep test increases rapidly after deviatoric stress is applied,and remains unchanged after reaching the maximum value.When the deviatoric stress is large,the strain increment and strain rate of undrained test is obviously larger than those under drained condition.(2)The results of cyclic triaxial creep tests show that cyclic creep changes and develops within a certain cyclic strain amplitude based on base creep.The peak creep curve and valley creep curve reflect the elasticity of soil,and the base creep curve reflects the plasticity of soil.With the increase of loading times,the elasticity tends to be stable and obvious,and the plasticity gradually weakens.The cyclic strain amplitude decreases with the increase of confining pressure.The cumulative plastic strain of undrained cyclic creep is smaller than that of drained cyclic creep,while the cyclic strain amplitude of undrained cyclic creep is larger than that of drained cyclic creep.(3)Compared with the traditional model,the fractional merchant model can use fewer parameters to show higher fitting accuracy and better prediction effect.The secondary development of the UMAT subroutine of the fractional creep model is realized in ABAQUS.Based on Terzaghi one-dimensional consolidation theory system and fractional merchant model,a one-dimensional fluid solid coupling calculation model of soft clay is established.Considering the time dependence of vertical seepage and soil deformation,the creep characteristics of soft soil foundation under long-term load are studied.The results show that the one-dimensional fluid structure coupling calculation model of soft clay is consistent with the theoretical calculation results,which more intuitively reveals the mechanism of land subsidence under long-term load.The thesis has 58 figures,11 tables and 134 references.