Study On Unloading Mechanics And Its Reological Properties Of Rock Under High Strsss

Inasmuch as hydroelectric projects in West China is being encountered challenges of deep depths,extra-high geostress and extra-high hydraulic pressure etc.,the mechanical response of the deep rock masses shows observable distinctions from that of shallow part.It is a key scientific issue how to ensure stability of surrounding rock masses and desirability of lining structure ater excavation.In this point,on the background of excavation and operation of subsidiary tunnels of JinPingâ…¡hydropower station,a representative tunnel section with sandstone strata,chosen as research objective,is investigated.Via methods of experimental researches,theory analysis and numerical simulation,the instantaneous deformation and strength properties as well as its rheological properties of sandstone are explored systematically,and a nonlinear damage viscoelastic-plastic constitutive equation is established which can reflect nonlinear deformation and creep properties of sandstone. Furtherly,a serial of numerical analysis are performed on simulating of mechanical response and its time-depended deformation after excavation of subsidiary tunnels based on the experimental and theoretical results achieved in this paper.It is shown that the computational results are in good accordance with the data observed on site.The main research works included in the dissertation are as following:The typical sandstones were sampled from engineering site and machined as standard specimens of 50mm in diameter and 100mm in length.By using MTS815.03 rock mechanics test system,both conventional uniaxial & triaxial tests and triaxial permeability tests under high confining pressure are conducted systematiacally.Based on the experimental results of uniaxial & triaxial tests,the deformation feature,strength characteristic,brittle-ductile transition laws and failure modes are analyzed in detail. Moreover,the relationship between peak strain,secant modulus and confining pressure are also discussed.By analyzing experimental results of triaxial permeability tests,the varying characteristics,including permeability versus axial strain,permeability versus confining pressure and seepage pressure versus rock strength,are obtained.Therefore,the meaning conclusions can provide foundational scientific evidence for further experimental and theoretical research in terms of rock mechanics.By using MTS815.03 rock mechanics test system,unloading triaxial tests,unloading triaxial permeability tests and SEM fracture scanning are performed on representative sandstone.The deformation characteristic are researched under different test conditions and unloading paths.Combining micro analysis methods,failure characteristic and failure mechanism are discussed.With the experiment of unloading triaxial tests of sandstone, relationship between the deformation feature,strength characteristic and failure mechanism and confining pressure under different unloading paths are discussed,and the equation for damage evolution derived subsequently.By experiments of unloading triaxial permeability tests,the varying characteristics under unloading conditions,including permeability versus axial strain,permeability versus confining pressure and seepage versus rock strength etc, are obtained.By SEM fracture scanning,micro failure mechanism and damage evolution laws,caused by unloading,are investigated lastly.Based on above data of loading and unloading triaxial experiments,constitutive model and strength criterion are investigated.In view of the complete stress-strain curves of sandstone under unloading condition,assuming that the relation between yield strength, peak strength and residual strength is linear,the bilinear elastic-linear strain softening-residual ideal plastic model is put forward.By analyzing strength characteristics under high confining pressure and high seepage pressure,exponential functional Mohr strength criterion is also established,and furtherly,the difference of strength parameters under loading paths,unloading paths and combining with high seepage pressure,are explored.By using TLW-2000 servo-controlling rheology tester,the unloading confining triaxial rheological tests are performed.Detailed investigations are made to the effect of different unloading stress levels on the creep properties,and the variation of the axial strain,lateral strain,plastic strain and strain rate are studied.The test results provide foundational materials for the establishment of sandstone's creep models.Based on the above data of unloading confining pressure creep tests,Burger's model and nonlinear rheological damage Burger's model are selected to describe the properities. By taking the sum square of survival difference as optimization objective,and the least square embedded in Levenberg-Marquardt algorithm is adopted to optimize the fitting parameters of models.And then,nonlinear damage viscoelastic-plastic rheological constitutive model is established.By programming the creep constitutive model,numerical simulations method is achieved in Flac^3D.Numerical analysis about subsidiary tunnels of JinPingâ…¡hydropower station,which emphasize instantaneous and secular deformation behavior caused by excavation,are carried out.The computational results,comparing with the data observed on site,validate that the numerical procedure of rheological model is reasonable and correct.