Study On The Elasto-plastic Damage Constitutive Model Of Xigeda Clay Rock And Its Application In Tunnel Engineering

Xigeda formation strata is a kind of fluvio-lacustrine deposits' half-diagenesis in southwest of China,which has poor engineering geology.The stratum is mainly interbeded with(sandy)clay rock and siltstone or pelitic siltstone.Its carrying capacity is acceptable under the condition of natural low water content,however,is is easy to mud and disintegrated with the strength of surrounding rock obviously reducing in the presence of water,and it's hard to be self-stabilization.With the gradual implementation of the 13th Five-Year Plan,there will be more and more large cross-section traffic tunnel projects.The research of surrounding rock in Xigeda formation strata has just started,and how to study the problem of deformation of the surrounding rock is an important prerequisite for solving tunneling security in Xigeda formation strata.Besides of the field test,numerical simulation is also an effective means to solve the problem of geotechnical engineering.And because of its convenience,as well as the diversity and controllability of the simulation conditions,this method is widely promoted.Furthermore,the choice of constitutive model is an important part of numerical simulation of the geotechnical problems.A suitable constitutive model can make the simulation analysis closer to the real situation,so as to be more with less.Therefore,the paper is based on a few tunnels located in Xigeda formation strata of the reconstructed railsway from Chengdu to Kunming,using the methods of literature research,field test and sampling,laboratory experiment,numerical simulation;theoretical derivation and computer programming to analyse the engineering properties and structural property of Xigeda clay rock,and then study on the suitable constitutive model of it.Meanwhile the following main research conclusions and results are obtained.(1)Through a large number of data research,the properties of Xigeda formation strata are summed up as follows:low-rise natural moisture content;medium sized density compared with the other rocks;a larger void ratio;medium sized compressibility;the higher natural shear strength and compressive strength,but the strength reduce is obvious in the presence of water;the clay rock mainly contains hydrophilic illite and chlorite,which are weakly permeable medium.The analysis of Xigeda clay rock's mineral composition,process of sedimentation and time effect shows that its structural properties are strong.The difference of compressive curve,stress-strain curve and excess pore-water pressure is analyzed by means of the consolidation test and triaxial test of undisturbed soil and remoulded soil,and it shows that the natural Xigeda clay rock has strong structural properties.(2)The traxial test of remoulded soils shows that,the stress-strain relationship of Xigeda clay rock is related to many factors.The confining pressure has a great influence on the peak of deviatoric stress and it's positively related.Moisture content is negative related to the peak of deviatoric stress.While the other experiment conditions are constant,increasing the dry density,doing drained shear experiment and reducing the rate of shear will increase the peak of deviatoric stress.In different stress paths,the conventional traxial test corresponds to the maximum peak value.(3)Through simulating traxial test,we know that,compared with the results of laboratory traxial test,the lateral deformation and excess pore-water pressure variation of the sample calculated by modified cam-clay model are better than those of Mohr-Coulomb model and Drucker-Prager model.The results obtained from simulating preloading test show that,MCC model is more suitable for calculating the variation of foundation settlement,horizontal displacement and excess pore-water pressure than the other two models.The numerical simulation results also show that the 5 of 8 higher sensitive parameters of MCC model in ABAQUS are initial void ratio,the slope of the critical state line,Poisson's ratio,the slope of the isotropic compression curve and the slope of the rebound curve.(4)Based on damage mechanics and the failure criterion of MCC model,the damage variable function is established according to the theory of geotechnical damage proposed by Shen Zhujiang,and then the elasto-plastic damage matrix expressing stress-strain relationship of Xigeda clay rock is deduced.In this constitutive relation,9 model parameters need to be deterrmined,corresponding to 12 experiment parameters.And ultimately the implementation process of the new constitutive model in ABAQUS user material subroutine(UMAT)is given,then the source code is written in FORTRAN language.(5)The feasibility of elasto-plastic damage constitutive model of Xigeda clay rock is verified through the comparison between the uniaxial consolidation experimental results and the numerical simulation results.The deformation of the surrounding rock in Xigeda clay rock formation strata tunneling by the method of three-step temporary inverted arch is predicted by using the new model,and the results show that the elasto-plastic damage constitutive model is more consistent with the actual monitoring values than MCC model.