The Creep Characteristics Research Of Mudstone Under Different Unloading Paths

Under the action of external load,rocks shows not only elasticity and plasticity,but also rheological characteristics which are related with time.Rock rheological phenomenon can be observed everywhere in the engineering practice,so rock damage caused by rheology cannot be ignored.For example,after the excavation and support of high slope,the rock mass of it may still cause deformation or even damage.The foundation of building may settle with time,and eventually lead to the buildings damage.This paper is based on National Natural Science Foundation Project,a high slope of an expressway in Guizhou Province as the engineering background.Using Rock 600-50 PLUS multifunctional rock test system of three axis to explore the creep properties of the mudstone collecting from the high slope under two different unloading Conditions.Established the creep model,using Origin to fit the creep curve obtained from the test and using inversion to validate the rationality of parameters.Specific research contents and conclusions are as follows:(1)The mudstone were acquired from a high slope of an expressway in Guizhou Province,and were processed into standard specimens indoor.The basic mechanical parameters such as elastic modulus,Poisson's ratio,compressive strength,angle of internal friction and cohesion were obtained by uniaxial and three axis compression test.(2)Two different stress paths,as keeping the confining pressure and changing the axial pressure or Simultaneous unloading of axial and confining pressures in equal proportion step by step,were adopted to carry out three axis creep test of the mudstone.Through the analysis of creep test curves,it is found that mudstone mainly consists of two stages: attenuation creep and steady-state creep.For constant axial stress unloading,when the deviatoric stress level is low,the mudstone shows decay creep,and when the deviatoric stress level is higher,the mudstone shows decay creep and steady creep.For equal proportional unloading,when the deviatoric stress level is higher,the mudstone shows decay creep,and when the deviatoric stress level is low,the mudstone shows decay creep and steady creep.The long-term strength of mudstone under different axial pressure is obtained.When the axial pressure is 10 MPa,the long term strength is between 76.2MPa and 86.2MPa,when the axial pressure is 20 MPa,the long term strength is between 111.1MPa and 116.1MPa.The change regulation of creep rate andcreep variable of mudstone under different stress states are analyzed.(3)Comparing the creep rate and failure mode of mudstone under two different unloading paths,it is found that the creep rate of mudstone under the equal proportion unloading path decreases faster.The failure mode of rock samples under the two unloading paths is compression-shear failure,but the creep rate decreases slower under the constant axial pressure unloading path,and the accumulation of internal damage is relatively large,so the failure mode is more fragmented.(4)Through the analysis of the creep curve characteristics of mudstone under three axis unloading test,the Burgers model is established as creep model of the sandstone.Using the LM algorithm of Origin to fit the creep curve,the creep parameters of Burgers model under different stress states are obtained,and the final creep parameters of mudstone under three axis loading and unloading test are acquired and validated by parameters inversion,providing a reference for the analysis of long-term stability of the high slope.