Cyclic Triaxial Stress-strain Response Elasto-plastic Simulation For The K₀ Consolidated Saturated Clays And Research On The Axial Resistance Of Pipe In Soft Clay

The stability and the safety issues of the offshore platform and subsea pipeline directly affect the production and the transportation of oil and gas energy.One of the prerequisites to conduct the bearing capacity and stability analysis using the finite element method for numerical analysis is to develop a realistic analytical constitutive of fully saturated soft clay that can objectively reproduce the mechanical behaviours under the combined static and dynamic loadings,and the axial stability of pipe in soft soil is one of the most important aspects to guarantee the normal transformational function of pipelines.Therefore,some improvements have been made on the base of a bounding constitutive model proposed by Dafalias to describe the behaviour of saturated soft clay under monotonic loading and cyclic loading and some efforts are devoted to study the axial frictional properties between pipe and soft clay.Within the framework of critical state soil mechanics and bounding surface plasticity,some improvements have been made over a simple bounding surface plasticity model proposed by Dafalias to incorporate the effect of K₀ consolidation and the effect of anisotropy evoluation during subsequent loading and further describe the behaviour of saturated soil under the combination effect of initial static and cyclic deviatoric stresses.The bounding surface,hardening rule,flow rule and interpolation function of plastic modulus of the model are discussed in details and then the stress-strain relations in triaxial space are put forward.The model adopts a bounding surface function with a parameter reflecting the evolution of anisotropy and the effect of consolidation process on the mechanical behaviour can be represented by the initial value of this parameter.Then,the theoretical determination of this anisotropic parameter for K₀ consolidation conditions of soil is provided.The development of the anisotropy of clays can be depicted by the rotational hardening rule of the anisotropic parameter.A novel rotational hardening proposed by Dafalias for describing the anisotropy development under monotonic loading is generalized into an elasto-plastic model for depicting the behaviours of saturated clay under cyclic loading version to overcome the shortages of others rotational hardening rules,such as the uncompleted physical meaning,nimiety of model parameter and excessive rotation of bounding surface.A modification on the isotropic hardening rule of the traditional critical state soil mechanics is made to take the effect of loading rate on the soft clay mechanical response under cyclic loading.The modification addresses the disadvantages of omission of loading rate of isotropic hardening rule in the traditional critical state soil mechanics and significantly improves the model's capability for describing the behaviour of soft clay under cyclic loading.The newly introduced parameter in isotropic hardening rule can be calibrated by the traditional triaxial tests with different cyclic loading rates.A newly determination of elastic shear modulus and elastic bulk modulus based on experimental results is proposed.The elastic shear modulus and elastic bulk modulus can be obtained by the fitting of stress-strain curve of cyclic loading within a relative small strain range,which solves the shortage of underestimating the value of elastic modulus using consolidation-rebound test curve in traditional critical state soil mechanics.A new plastic modulus interpolation function is introduced into the model to describe cyclic stability and cyclic degradation,which takes the length of the deviatoric plastic shear strain trajectory as the macroscopic measurement of loading process.Merely two newly-added material parameters are required in the interpolation function,which can be calibrated using traditional triaxial tests.The relationship of these two parameters with the static deviatoric stress level and cyclic deviatoric stress level can be established using a series of triaxial undrained tests with different static deviatoric stress levels and cyclic deviatoric stress levels.The rationality of the calibration of these two parameters and the availability of the model are validated by the cyclic K₀ consolidated undrained tests with different confining pressures.The behaviour of isotropic and anisotropic saturated soft clay in available literatures under cyclic loading is simulated by the model for validating its adaptivity and generality.To quantitatively understand the axial frictional response of seabed pipelines in soft clays and supply a basis for the pipeline design,nearly thirty groups of model tests on axial pipe-soil interactions are conducted using full-scale pipes coated with polyethylene in two model test tanks containing soft clays with different shear strengths.The effects of the pipe diameter,the buried depth,the undrained shear strength of soft clays and the loading rate on axial friction properties of the pipe coated with polyethylene are studied.The axial frictional behaviour of half buried pipe in the clay is also preliminary studied.Model tests results show that the axial resistance-displacement responses of pipelines in soft clays are presented with two typical curves,which are the hardening type and softening type.The type of axial resistance-displacement curve is related to the undrained shear strength.The set-up period,the loading rate and the pipe buried depth have no effect on the curve type.Two methods,the total stress method and the effective stress method,on the subsea pipeline stability analysis are briefly introduced and tests results are analyzed using these two approaches to study the effect of undrained shear strength of soft clays,the pipe diameters,buried depths and loading rate on adhesion factor and frictional coefficient.The results reveal that the adhesion factor increases with increasing buried depth of the pipes.The adhesion factor of PE-coated pipes in very soft clays is larger than that in soft clays.The adhesion factor increases with the increasing diameter within the studied scope in soft clays,but this trend is not obvious in very soft clays.The effect of diameter on the frictional coefficient can be neglected and the frictional coefficient decreases with the increasing normalized buried depth?H/D?and with the decreasing shear strength of soft clay.The frictional coefficient increases with the loading rates.The range of the adhesion factor of PE-coated pipes is recommended based on the model test results.The adhesion factor varies from 0.2 to 0.35 for the very soft clay,whose undrained shear strength is less than 10kPa.The peak adhesion factor varies from 0.15 to 0.25 when the undrained shear strength is greater than 10kPa.The lower limit is appropriate for the situation where the equivalent buried depth is less than 0.5 m.The upper limit can be used for the situation where the equivalent buried depth is larger than 1.5 m.When the equivalent buried depth varies from 0.5 to 1.5m,the adhesion factor is determined using the linear interpolation.The residual adhesion factor is approximately 0.75 to 0.95 times the peak value for soft clay.Finally,the method to quantitatively determine the axial friction coefficient is developed by analyzing model test results,which considering the effects of the diameter,the buried depth,the undrained shear strength of soft clays and the loading rate.