Study On Mechanical Properties And Damage Constitutive Model Of Mudstone In Complex Loads

Mudstone is a kind of strongly consolidated rock composed of weakly consolidated rock handled by moderate epigenesis.Mudstone has great engineering characteristics such as exquisite crystal grain,inconspicuous bedding structure,water softening and good creeping.And because it has high adsorption,low permeability,good geochemical closure,and self healing and self closed,it is chosen as alternative rock of nuclear waste repository,for example,Callovo-Oxfordian nuclear waste repository(France),Boom nuclear waste repository(Belgium),and Opalinus nuclear waste repository(Sweden),et al.In this paper,mudstone rock samples were collected in field as the research object.Through X-ray diffraction test and self-adaptive full-automatic rock triaxial test,physical properties were analyzed and the evolution law of mechanical parameters of mudstone with confining pressure and osmotic pressure was studied.By finite element software ABAQUS,the elastoplastic model of mudstone is built.The mechanical response of different load combinations was further studied.In the end,elastoplastic model was used at the coupling analysis of seepage and stress in mudstone of nuclear waste treatment reservoir in HADES underground laboratory,Belgium.The research results are as follows:1)Through the X-ray diffraction test of mudstone,it is found that the mudstone is mainly composed of kaolinite and mica,accompanied by certain quartz and other minerals(feldspar,epidote,chlorite,etc.),in which the quartz content is high,belonging to siliceous mudstone,and the degree of consolidation is weaker than shale,and recrystallization is not obvious.2)Through the mudstone triaxial test,it was found that mudstone compressive strength increases along with the increase of confining pressure,and the growth rate of mudstone compressive strength is a certain value.Along with the increase of osmotic pressure the compressive strength decreases.When the confining pressure keeps the condition of lager,the compressive strength is basically affected by the change of osmotic pressure.The reason for this is that confining pressure restrain lateral deformation of mudstone samples,and osmotic pressure have negative effect on effective confining pressure;3)Based on mudstone stress-strain curves of different load combinations,it was found that mudstone elastic modulus increases and the growth rate of mudstone elastic modulus decreases along with the increase of confining pressure.With the increase of osmotic pressure,elastic modulus decreases.With the different confining pressures,the osmotic pressure has greater negative influence on elastic modulus.The reason for this is that confining pressure has squeezing effect on internal micro cracks and microcracks of rock specimens,and osmotic pressure have negative effect on effective confining pressure;4)Combining with the test results and elastic modulus,the change of the cohesive and internal friction angle of mudstone was got under different stress states.With the increase of osmotic pressure,cohesive force of mudstone decreases,which is caused by the argillaceous components in mudstone,and the Angle of internal friction also decreases,which is cause by the weakening effect on the sliding friction between rock particles.5)A isotropic elastoplastic constitutive model of mudstone was independently developed based on ABAQUS,and the corresponding UMAT subroutine was developed,after which model verification was conducted based on the Mohr-Coulomb material constitutive model of ABAQUS material database,verifying the correctness and effectiveness of the self-developed model;On the basis of this validation model,corresponding studies on mudstone hydraulics under different loads were carried out.The results show that along with the same axial pressure and the increasing of confining pressure,the axial deformation of rock samples gradually decreases,the lateral displacement gradually decreases,the Mises stress gradually decreases,and the equivalent plastic strain gradually decreases.6)Based on the above mudstone elastoplastic constitutive model,the seepage effect is further considered to study the seepage-stress coupling process of roadway excavation of nuclear waste repository.The results show along with the effect of seepage-stress coupling,there is obvious stress concentration in the surrounding rocks on the left and right sides of the tunnel,and plastic damage zones appear around the tunnel,the pore ratio of surrounding rock increases,and corresponding seepage coefficient increases.