Study On Mechanical Properties Of Granites Under High Pressure Conditions And Its Constitutive Models

Because of the peculiar geological structures in west China, most of the deep-lying tunnels for large-scale hydro projects being constructed there have encountered problems of deep depth and extra-high in-situ stresses, so the deep rock masses are observably distinct in physical and mechanical properties from those at shallow part. To deal with the increasing engineering requirements for solving problems occurred in high geo-stress regions, this thesis carries on studies on non-linear mechanical properties and remarkable rheological properties of deep rock masses under high geo-stresses conditions. Based on experiment research, and combining theory analysis with numerical simulation, this paper studies systematically the instantaneous deformation and strength characteristics of hard rocks as well as its long-term rheological properties. Then, 2 macro constitutive models are established respectively according to the experiment results: one is non-linear elastic-brittle-plastic model which can reflect the hart rock's pre-peak nonlinear deformation feature as well as its postpeak strain softening; the other is the nonlinear viscoealstic-plastic model which can reflect the long-term mechanical properties of brittle rock. Finally, on the background of major project in high geostress region, e.g. the DaGangShan power station in Dadu River Basin, a number of finite element simulations are proceeded on the stability of rock masses by applying the experimental and theoretical results achieved in this paper. The main research works included in the dissertation are as following:(1) The typical brittle rock granites were obtained from engineering site and they were prepared as standard specimens of 50mm in diameter and 100mm in length. Both conventional uniaxial & triaxial compression tests and unloading confining pressure tests are conducted on MTS815.03 testing machine. The deformation feature, strength characteristic and failure laws are analyzed in detail. The relation between compression strength, deformation modulus, Poisson's ration and confining pressure is also studied. Besides, the basic physical and mechanical parameters are obtained, which can provide scientific evidence for the creep tests and numerical simulations.(2) Aiming at the problem that the existing elastic-brittle-plastic constitutive equations hasn't been successful in reflect the non-linear deformation features of hard rocks under high geostress conditions, the effect of activity state of the micro-cracks within brittle rock similar to granite on its macroscopic mechanical parameters is studied. The pre-peak elastic modulus and Poisson's ratio on each damage level is defined as the function of major and minor principal stress. Each damage level can be identified by two successive character stresses of major principal stress, which can be determined by conventional uniaxial and triaxial compression tests. Both rock's cohesion and friction angle can be expressed as functions of equivalent plastic strain, which can be determined from conventional tests also. Based on the analysis results, a new non-linear constitutive models is proposed for hard rocks under high in-situ stress conditions, which can an reflect the pre-peak nonlinearity and post-peak strain softening of hard rocks.(3) In order to study the long-term mechanical properties of hard rock in DaGangShan powerstation, triaxial compression rheological tests are carried out on CYL servo-controlling rheology testing machine. A detailed investigations is made to the effect of different stress levels on the creep properties. Furthermore, unloading confining pressure creep experiments are made with standard rock specimens and the variation laws of the axial strain, lateral strain and volume strain during experiments are studied. The test results lay a firm foundation for the establishment of granote's creep models.(4) According to the tertiary creep curves of rocks under high stress, i.e. the specimen represents obvious accelerating creep, a new viscous component is proposed, which divides the traditional Newton's viscous component into 2 parts, i.e., the linear part and the non-linear part. The new component can be in parallel connection with the plastic component to form a new visco-plastic body, which has the ability to predict the steady and accelerating creep. The non-linear visco-elastic-plastic rheology constitutive model can be deduced when the visco-plastic body is in series connection with generalized Kelvin body reflecting rock's decaying creep deformation induced by low stress. The non-linear rheology model effectively describes the mechanics characteristics of three creep stages of the decaying, stable and accelerating creep of hard rock such as granites. The model parameters are identified successfully through the damped least square method according to the creep test curves.(5) The finite element implementation process of the nonlinear visco-elastic-plastic rheology constitutive model as required by the famous FE software ABAQUS is analyzed, and corresponding secondary development is committed to embed the model into ABAQUS through its interface subroutine Creep. Finally, the experimental and theoretical results are applied to the major project in high geostress region, e.g. the DaGangShan power station in Dadu River Basin. A number of numerical simulations are conducted to predict the long-term deformation of the surrounding rock masses as well as its stability.