Research On Unloading Macro-micro Failiure Mechanism Of Weak Interlayer Zone

The"14th Five-Year Plan" statement that large-scale hydroelectric energy bases will continue to be developed and constructed in key river basins such as the southwest area and the upper reaches of the Yellow River.Its hydropower facilities often require the construction of large-scale deep underground cavern as the main bearing structures.The various deformation and damage problems caused by weak interlayer zone(WIZ)in the process of construction and operation of large-scale deep underground cavern is one of the crucial construction problems to be overcome urgently.To better understand the macro-mechanical mechanism of the failure of the weak interlayer zone(WIZ)under high geostress unloading conditions,the revealed weak interlayer zone(WIZ)during the construction of the deep underground cavern on the left bank of a hydropower station in the southwest China has been researched by a series of automatic stress-path controlled triaxial tests,scanning electron microscope(SEM)analysis,as well as the sophisticated quantitative analysis with MATLAB,to reveal the stress path correlation characteristics of the deformation and strength of weak interlayer zone(WIZ)under complex stress paths,and to analyze its multi-scale deformation failure mechanism under different stress paths.Then,a modified equivalent plastic work-hardening constitutive model was proposed(MWHM).And the new model(MWHM)has been applied to the stability analysis in a deep underground cavern through the finite difference numerical simulation software FLAC3D.The results are as below.(1)The high geostress excavation disturbance will make the weak interlayer zone(WIZ)in the initial triaxial stress state in different parts of the underground cavern experience complex stress paths and stress redistribution processes.The stress paths of the indoor test can be reasonably simplified as conventional triaxial loading,increasing axial pressure and unloading confining pressure,constant axial pressure and unloading confining pressure,unloading axial pressure and unloading confining pressure.There are significant confining pressure effects and stress path effects on the plastic deformation and failure characteristics of the weak interlayer zone(WIZ)under high geostress stress.High confining pressure will inhibit the development of hoop plastic deformation of weak interlayer zone(WIZ)under different loading and unloading stress paths.The plastic volume expansion is the most significant under the stress path of unloading axial pressure and unloading confining pressure.(2)The mesoscopic failure characteristics of weak interlayer zone(WIZ)under different paths of high geostress stress all show the decrease of planar porosity,fractured and orientated particles,and the increase of Korcak fractal dimension of particles and pores(Dkg,Dkp).The larger the initial confining pressure,the more significant the changes in the above mesoscopic features.a)The plane porosity of the conventional triaxial loading test is lowered by about 10%compared with that before the test,and the average particle area is reduced by about 40%.The plane porosity of each unloading stress path is significantly larger than that of the conventional triaxial loading test.It demonstrates that springback deformation is more extreme under the unloading stress paths.b)Particle breakage usually occurs during the failure process of weak interlayer zone(WIZ)under different stress paths.After the test,the smaller the average particle area,the more severe the particle breakage.The degree of particle breakage in the stress path used in this paper are as follows:cyclic loading and unloading(202.91μm2)>increasing axial pressure and unloading confining pressure(449.46μm2)>conventional triaxial loading(597.79μm2)>constant axial pressure and unloading confining pressure(608.91μm2)>unloading axial pressure and unloading confining pressure(928.61 μm2).c)particle orientation usually occurs during the failure process of weak interlayer zone(WIZ)under different stress paths.The orientation of the weak interlayer zone(WIZ)is more visible,under the unloading stress path.The direction of the particles is aggregated to 0°～15°.Thereof,the directional characteristics of the stress path of unloading axial pressure and unloading confining pressure are the most significant(i.e.the anisotropy rate was increased by about 22.33%compared with that before the test).The direction of the particles is aggregated in the directions of 0°～15° and 60°～75° due to the bidirectional unloading effect.d)Weak interlayer zone(WIZ)exhibits distinct Korcak fractal characteristics.There is a significant linear regression relationship between the particle Korcak fractal dimension(Dkg)and the average particle area.The smaller the average particle area,the more severe the particle breakage,and the larger the particle Korcak fractal dimension(Dkg).That is,Dkg can be used to quantitatively describe the structural characteristics of weak interlayer zone(WIZ)particles.It can objectively reflect the change of particle structure under different stress paths.(3)Internal variables such as equivalent plastic strain,equivalent plastic work,and plastic volumetric strain of weak interlayer zone(WIZ)are all affected by stress paths,and treating these parameters as hardening parameters cannot reflect the stress-path-independent properties.By correcting the equivalent plastic work hardening parameters,the evolution relationship between deformation parameters and strength parameters of weak interlayer zone(WIZ)with hardening parameters has been proposed.Based on the Mogi-Coulomb yield criterion,a modified equivalent plastic work-hardening constitutive model has been proposed(MWHM),with the associated flow rule used for shear failure and the non-associated flow rule used for tensile failure to determine the plastic potential function.It can better describe the nonlinear deformation process and volume expansion characteristics of weak interlayer zone(WIZ),and has a wide range of applicability.(4)Header file(usermwhm.h)and source file(usermwhm.cpp)of the model written in VC++ language are used to generate dynamic link library file(MWHM.dll)for deep weak interlayer unloading model.It was embedded in FLAC3D to simulate the indoor triaxial unloading test,and compared with the results of the indoor test to verify the rationality of the model.The model is applied to the stability analysis of surrounding rock in deep underground cavern under the influence of weak interlayer zones,the problems such as discontinuous deformation and failure of surrounding rock and stress redistribution caused by weak interlayer zone(WIZ)in the process of layered excavation of deep underground cavern is effectively described.The analysis results can provide important scientific basis for the reasonable construction and support optimization of the project.