Influence Factors Of Rayleigh Wave Propagation Characteristics And Soil Dynamic Response In Offshore Sites

The innovation of ocean engineering science and technology is an important component of the strategy of marine potestatem put forward by the 18^thNational Congress of the Communist Party of China.In order to study the propagation and attenuation characteristics of Rayleigh waves and their influencing factors of Marine sites,as well as the dynamic response law of seabed sites induced by Rayleigh waves and their influencing factors.Starting from the kinetic theory of saturated two-phase media,based on the dynamics equation of two-phase medium proposed by Men and Zienkiewicz,combined with elastic half-space boundary conditions,a mathematical model of Rayleigh propagation in an ocean site is established.In particular,the effects of dilatancy(shear shrinkage)volume deformation characteristics,soil porosity and seawater seepage under elastic small deformation state are considered.Innovative physical methods are used to set boundary conditions,interconnections and deformation coordination between air-sea-soil layers.The propagation and attenuation characteristics of Rayleigh waves are discussed in three models based on different boundary conditions a seabed site with seawater overly and uneven water-soil interface,an ocean site with seawater overly and flat water-soil interface,and a flat site without seawater.The sites are classified to study the propagation characteristics of Rayleigh waves in rock sites,sandy sites and soft soil sites.And discussed with the influence factors,such as water and soil interface fold degree,porosity,permeability coefficient,Poisson's ratio and elastic modulus of soil layer,soil particles elastic modulus on the influence of Rayleigh wave propagation and attenuation curve.Based on the deduced two-phase medium motion equation for Rayleigh wave propagation in an offshore site,the responses of mechanical parameters such as displacement of soil and seawater,stress in soil,pore pressure and volumetric strain induced by Rayleigh wave are studied.The responses of various mechanical parameters under the influence of different locations of the sites,depths,porosity,permeability and Rayleigh wave velocities are discussed.In addition to summarizing the influence of various influencing factors on the response law of mechanical parameters of the site,it can also provide reference for Rayleigh wave geological exploration and soil parameter inversion.Moreover,in order to study the influence of Rayleigh waves on the dynamic response of free offshore sites in practical engineering sites,and verify the rationality of the site displacement response caused by Rayleigh waves derived from the formula.The finite element model with infinite element boundary condition was established,and the soil constitutive model was improved by calling Abaqus subroutine and Matlab programming.The equivalent linear viscoelastic constitutive model combined with infinite element boundary free site seismic response model was realized,and the attenuation process of soil shear modulus was considered.Chichi wave,Kobe wave and Rayleigh wave were input at the water-soil interface in the center of the site.The results were compared with each other and summarized with the calculation results of elastic constitutive conditions and theoretical formula of site response.The result of research shows that the propagation and attenuation characteristics of Rayleigh waves and their influencing factors need to be discussed in a specific material.Among them,the elastic modulus and Poisson's ratio are the prerequisite to determine the properties of the propagation medium.The change of Poisson's ratio and elastic modulus will change the influence of permeability coefficient,porosity and other influencing factors on the propagation and attenuation characteristics of Rayleigh waves.Therefore,the setting of basic soil parameters such as Poisson's ratio and elastic modulus is also the premise to discuss the influence of other factors on the propagation and attenuation characteristics of Rayleigh wave.Even for the same stratum with different elastic modulus or Poisson's ratio,the influences of porosity and permeability on the phase velocity and attenuation coefficient of Rayleigh wave with wave number are also inconsistent.Under the conditions of sandy soil,Poisson's ratio and wave number,different permeability coefficients have different effects on the phase velocity of Rayleigh wave.In sandy soil,the influences of permeability coefficient on Rayleigh wave phase velocity are different under the same Poisson's ratio and different wave numbers.According to the phase velocity curve with Poisson's ratio of 0.38 and 0.3,it can be seen that before a specific wave number,the smaller the permeability coefficient,the higher the phase velocity,and after this specific value,the larger the permeability coefficient,the lower the phase velocity.The change of Poisson's ratio will make the front end of the curve of phase velocity and attenuation coefficient with wave number change in low wave number region move as a whole.In the rock layer,the change of elastic modulus of soil particles will also make the curve of phase velocity change with wave number move as a whole,and the attenuation coefficient will change obviously.However,no matter what formation conditions,the influence of permeability coefficient on Rayleigh wave propagation and attenuation characteristic curve is limited.The influence is significant only in low wave number and low frequency region,and the influence of permeability coefficient gradually decreases with the increase of wave number.In addition,the undulation degree of the water-soil interface and different models also influence the propagation and attenuation curves of Rayleigh waves.In this study,the phase velocity of Rayleigh waves at the flat water-soil interface is relatively low.The wave velocity at the flat site without seawater is lower than that at the flat seabed site with seawater.In the study of dynamic response of seabed soil induced by Rayleigh wave,soil parameters such as elastic modulus,permeability coefficient,porosity,water-soil interface and site location all affect the dynamic response of each parameter of seabed soil.The characteristics of the seismic wave,such as the velocity of Rayleigh wave,also affect the dynamic response amplitudes of mechanical parameters of soil layer and the influence range along the soil depth.Under different wave velocities,the response amplitudes of mechanical parameters of soil layer and the influence ranges along soil depth are different.Moreover,the additional volume strain of soil is positively correlated with the change of pore water pressure.In addition,the dynamic response amplitude of liquid is much larger than that of soil,which is consistent with the law obtained through numerical simulation.Different from the law that the response values of each parameter of soil layer calculated by the formula decrease with the increase of soil depth.The maximum lateral response of displacement,velocity and acceleration with depth calculated by numerical simulation is not located at the water-soil interface,but at the soil depth with a certain thickness.Both mathematical formula modeling and finite element modeling have their advantages and disadvantages.For the dynamic response analysis of the site,the finite element numerical simulation is more intuitive and can also take into account the dynamic constitutive characteristics of soil.Through the combination of formula derivation and numerical simulation,the influencing factors of Rayleigh's propagation and attenuation characteristics and their influencing laws during Rayleigh wave propagation in Marine sites are summarized,as well as the influencing factors and their influencing laws of the dynamic response of Marine free sites during Rayleigh wave propagation.The conclusions can not only provide basis for seismic fortification of offshore engineering site,but also provide reference for geological exploration and stratigraphic parameter inversion.