Development And Application Of Bounding Surface Constitutive Model Considering Strain-rate Effect And Softening Effect

Offshore structures are often subjected to complex loads from wind,waves and currents.In addition to the harsh natural environment,the strength changes of marine soft soil foundations under complex loads directly affect the bearing capacity of marine foundations.Numerical simulation is one of the effective methods to calculate the foundation bearing capacity of marine soft clay foundation.In the numerical simulation of the interaction system between marine soil and foundation,whether the soil constitutive model can reasonably simulate the mechanical properties of soil is the key to study such problems.In view of this need,this paper establishes a bounding surface constitutive model considering the strain-rate effect and softening effect,and development to the finite element software ABAQUS.Finally,the effective stress bounding surface constitutive model of the soft soil was used to model and calculate the bearing capacity and horizontal sliding frictional resistance coefficient of the Mudmat.The main contents of this paper are as follows:(1)Based on the basic theory of the bounding surface model,develop a bounding surface constitutive model considering initial anisotropy of soil.Using Mises threedimensional method and implicit integration method to develop it into ABAQUS finite element software.The simulation of the triaxial test was carried out in combination with the coupled pore fluid flow and stress analysis method in the finite element software.Reasonable consideration of the pore pressure calculation method under undrained conditions,the pore pressure calculation is integrated into the constitutive model of effective stress.The UMAT and VUMAT user material subroutines were developed separately and the results were compared using the general implicit and explicit analysis methods and the UMAT subroutine of the coupled pore fluid flow and stress analysis method.It is proved that the results of the three methods are basically the same in the case of undrained.Compared with the coupled pore fluid flow and stress analysis method that comes with ABAQUS,the self-programmed pore pressure calculation module is more efficient in calculation.(2)Considering the effect of the strain-rate effect on the strength of the soil,the bounding surface model is continuously developed on the basis of the previous one.A strain rate function related to the strain rate was added to the bounding surface to verify the triaxial undrain rate effect test.Then,by summarizing the results of the rate effect experiments of various regional soils,the parameter formula of the rate effect model is fitted.(3)Considering the effects of structural softening and cyclic softening on soil strength,the bounding surface model was further developed on the basis of the previous one.The damage function is set,and the cumulative plastic strain is applied as the main variable of the damage function to correct the hardening function,and the residual strength is set,so that the soil still has a certain strength residue after structural damage and cyclic load softening.The structural surface is set in the structural soil,and the strength of the soil is improved by considering the structural properties of the soil.The simulation of structural soil uniaxial triaxial experiments was carried out to verify the simulation effect of structural softening.The simple shear test and the triaxial test of the cyclic softening were simulated to verify the simulation effect of the model on the cyclic softening.(4)Apply constitutive models to structural calculations.The finite element numerical analysis model of the submarine pipeline infrastructure Mudmat is established,and the bearing capacity coefficient is calculated based on the analysis of effective stress.The calculation results were compared with the Tresca model based on the total stress analysis.Consolidation analysis and subsequent calculation of the horizontal sliding frictional resistance coefficient are then carried out,and the calculated results are compared with the modified Cambridge model to examine the similarities and differences between the model and the modified Cambridge model.Considering the rate effect of Mudmat sliding,the calculation of different rate effect coefficients and softening parameters is used to investigate the effects of different rate effects and softening effects on the sliding friction coefficient.