The Theory Of Finite Deformation And Its Application In Geotechnical Engineering

Now,with quickly developing of minerals exploration,energy development, communications and transport,urban construction,undergrounding engineering,the dimensions of these projects are more and more tremendous and problems involved in rock and soil mechanics are more and more complex.Many geotechnical engineering and projects can not be solved by the conventional theory of small deformation,so the theory of finite deformation(large deformation) and its application attract more and more researchors attention,and it is becoming a hot topic.Founded by "research on forecasting settlement and construction process of drilling and blasting method for a group of tunnels and large span tunnel in the condition of complexly geological structure",and taken a certain tunnel as an engineering example,the hard core of this paper is the theory of finite deformation applied in geotechnical engineering,and the emphasis is studying on the theory including elastic and elasto-plastic theory.It is combined Mohr-Coulomb yield criterion common used in geotechnical engineering with the theory of finite deformation,and putted forward the strain softening model.The model is implemented by user subroutine,and verified by applied it to the horizontal subject in the last.From the research,the following conclusions were achieved:1.Studying on the elastic finite deformation,relationships of all kinds of common stresses and strains are obtained.So the relatioship between stress and strain,stress ratio and strain ratio can be ascertained.2.Studying on the elasto-plastic finite deformation,plastic deformation of it is correctly expressed.The balance equations and boundary conditions of it are obtained by using variational principle.Combining additive decomposition of the streching and the deformation gradient,the method of solving plastic deformation of it is also obtained.Based on the above methods,relevantly finite element method is putted forward.So elato-plastic analysis of finite deformation is implemented which is appropriate to geotechnical engineering,adopted Mohr-Coulomb yield criterion.3.Turned perfect complete stress-strain curve in uniaxial compression test into Cauchy stress- logarithmic strain curve,so the tangential modulus can be acquired which is available for analyzing large deformation.Internal state parameter- equivalent plastic strain is introduced,and taken cohesion force and modulus of material as a function of equivalent plastic strain,but Poisson's ratio and friction angle as constants,that is strain sfotening model.4.Based on the result of material mechanic properties,constitutive relationship,yield criterion and parameters variant regular patterns,by using universally FE software ABAQUS, writing user subroutine USDFLD and embedding it into ABAQUS,the constitutive relationship of strain sftening model is implemented in order to apply it in practical projects.5.Based on the result of above,taking a certain tube as a verification project,its construction process has been respectively simulated by small deformation method,finite deformation method and taken into account the effect of strain softening.That provides scientific foundation for forecasting,and controlling settlement for a group of tunnels and large span tunnel in the condition of complexly geological structure.By comparing the result of numerical simulation to measurement in situ,that is indicated the results of finite deformation is more coincided with the fact,and verified its feasibility to apply finite deformation theory in large deformation.So it is concluded that the strain softening model can be applied in similar project and provided scientific foundation for it.The unique points of view in the dissertation are as follows:Firstly,the plastic deformation of the finite deformation was exactly expressed by using the relationship between Cauchy stress and logrithmic strain,and corresponding ratio, combined additive decomposition of the streching and the deformation gradient.Then the elasto-plastic analysis of the finite deformation is implemented by taken Mohr -Coulomb yield criterion.Secondly,based on the plastic deformation of the finite deformation,and introduced internal state parameter,equivalent plastic strain,a quantitative relationship between cohesion force,defomation modulus and equivalent plastic strain are derived.So,the strain softening model is obtained which is appropriate to geotechnical engineering.Lastly,the model is implemented by user subroutines writed in Visual Fortran,and embedded in ABAQUS.It is efficiently simulated the course of strain softening,through modelling the test of standard specimen of rock.Because the settlement of tunnel was very large,the model was applied and good results were obtained in the tunnelling process.