Research On Nonlinear Dynamic Response Of Underground Structure Based On TTS Sand Model

With the development of underground structures,its dynamic response has received increasing attention.The nonlinear stiffness of the soil,the hysteretic behavior and the possible liquefaction under cyclic loading,all have important influences on the dynamic interaction of soil-underground structure.In this paper,constitutive research,numerical simulation,and centrifuge experiment were carried out to comprehensively study the nonlinear mechanical properties of soil and the nonlinear dynamic response of underground structures.Firstly,the stiffness characteristics of different kinds of hyperelastic models are investigated to select a suitable model for soil.The normal-shear coupling effects of the selected hyperelastic model are studied,including the maximum shear stress ratio and the at-rest earth pressure coefficient of the soil.Then,based on the Granular Solid Hydrodynamics(GSH)theory,the granular entropy is introduced to describe the fluctuation of soil particle motion.Through a quantitatively describtion for the energy dissipation in soil,the Tsinghua thermodynamic soil model for sand(TTS-S)is proposed.A calibration method for all parameters is proposed based on the experimental data.The apparent stiffness behaviour of this model under different stress conditions are studied and compared with the classical soil mechanics theory as well as experimental data.At very small strain condition,the model can properly show the influence of confining pressure,density and non-isotropic consolidation state.As the strain increases,its stiffness decays.A smaller confining pressure leads to a stronger nonlinearity.In addition,the critical state under different stress paths and the liquefaction under cyclic loading are also studied.Dynamic centrifuge tests were carried out to have better understanding on the performance of batter piles in liquefiable ground.Results reveal that for the symmetrically placed battered piles,the bending moment of the front pile is greater than that of the rear pile,caused by the liquefaction induced lateral deformation,and attains its maximum value at the middle of the pile.The presence of lateral spreading led to a faster accumulation of pore pressure,greater acceleration in the soil,and greater settlement both during and after shaking.However the maximum bending moments of batter piles in lateral spreading soil are similar to those in level liquefiable soil.The design of battered pile foundations in lateral spreading ground with the presence of the overlaying nonliquefiable layer should be considered in case.The relatively permeable and flexible overlaying nonliquefiable layer,such as dense sand layers,can reduce the potential of liquefaction and lateral spreading of the ground and its interactive forces with piles,thus exerting beneficial effects on the dynamic response of batter pile foundations.However,in other cases with the presence of relatively impermeable and rigid overlaying nonliquefiable layer,such as the slightly cemented sand layer,this nonliquefiable crust can apply a larger lateral load on the pile during the ground lateral spreading,resulting in a higher bending moment,which is obviously detrimental to the battered piles.The total settlement in the mildly sloping shaken ground is greater than that in the level shaken ground.A modified method is proposed for the prediction of the settlement of sloping shaken ground based on the present experimentation and the Ishihara's procedure,which works only for the level ground situation.The TTS-S model was implemented into the FLAC3 D program and verified by the static and dynamic numerical simulation.During the numerical simulation for dynamic centrifuge test of the tunnel in dry sand,the influence of the tunnelsand interface was analyzed.The calculated accelerations of soil layers and the bending moment in the tunnel agree with the measured data and are independent from the interface parameter.The hoop force of the tunnel is significantly affected by the interface,and agree with the solution of the quasi-static solution under different interface condition.