Creep Characteristics And Constitutive Model Of Overconsolidated Clay

With the exploition of underground space and the development of foundation engineering practice in China,more and more overconsolidated soil cases are encountered,such as unloading and reloading process during underground structure construction,preloading(vacuum)treatment of soft soil ground,etc.The deformation of overconsolidated soil owes to both stress impact and time-dependent effect.More and more engineering practices show that the post construction settlement/deformation of overconsolidated soil cannot be ignored.Therefore,it is of great significance to properly describe the engineering behaviour including the time-dependent deformation characteristics of overconsolidated clay for guiding the construction,operation and maintenance of this kind of foundation.In this paper,a series of laboratory triaxial creep tests were carried out to study the creep characteristics of overconsolidated clay soil.Based on the test results,a creep model and a constitutive model suitable for overconsolidated clay soil are established.The main contents and conclusions of the study are as follows.1.CU tests were conducted on clay samples of different overconsolidation ratios.The results show that the relationship between the maximum shear strength of the sample and the overconsolidation ratio(OCR)is approximately positively correlated when OCR=1.00～3.00.And the effect of soil overconsolidation ratio(stress history)on the effective stress path of triaxial undrained shear is shown as that the greater the overconsolidation ratio,the steeper the triaxial undrained shear effective stress path curve will be.By comparing the CU test results of clay samples of different OCR before and after shear creep,it can be seen that shear strength of soil can be increased by triaxial creep,and the undrained stress path curve of the overconsolidated clay after creep is steeper than that before creep.2.Isotropic loading creep tests were carried out on clay samples of different OCR.The results show that when the OCR is no more than 2.00,the axial strain and volumetric strain of the sample are approximately inversely proportional to the overconsolidation ratio.The larger the OCR is,the earlier the secondary consolidation starts.However,the larger the OCR is,the smaller the amount of secondary consolidation deformation(creep)develops.3.The triaxial loading creep tests were carried out on samples of different OCR.The results show that under the same deviatoric stress,The greater the OCR of soil,the smaller the excess pore pressure generated within the sample during loading process.During the creep stage,the greater the OCR,the smaller the axial strain and deviatoric strain of the specimen develops,while the volumetric strain evolution shows that throughout the whole loading process,a shringkage trend was observed in normally-consolidated soil specimen and a dilation trend happened in overconsolidated clay shortly after creep started.For the creep stage,no matter the total stress creep analysis method or the effective stress creep analysis method is used,the creep curves of the samples of different OCR show nonlinear relationship with time,and the larger the OCR is,the smaller the creep evolution will be.4.The hyperbolic empirical model was tried to fit the creep test results.The results show that the hyperbolic empirical model is suitable for describing the creep characteristics of overconsolidated soil.At the same time,according to the fitting results,the correlation analysis of the model parameters As and BS shows that the variation of As has no obvious effect from OCR of the sample,while the model parameters BS is linearly increased with OCR of the sample.5.By deriving the constitutive model of overconsolidated soil based on the Hvorslev surface,the definition of some key parameters in the model is verified on basis of the test data.The mathematical model is established by taking use of MATLAB platform,which proves that the overconsolidated soil constitutive model based on the Hvorslev surface can be well applied to the overconsolidated soil.Finally,based on the overconsolidated soil constitutive model,the hyperbolic empirical model is inserted to modify the delay hardening parameters,and therefor an overconsolidated soil constitutive model with consideration of time effect is proposed.