Study On Rheological Properties And Model Of Red Mudstone Based On Initial Damage

Red beds are widely distributed in Southwest China.The red mudstone has poor physical and mechanical properties,and the phenomenon of creep under stress is very obvious.A large number of studies have found that rheology is an important factor causeing long-term deformation of soft rock engineering.In recent years,the deep excavation of high-speed railway cuts and tunnel bases built in the red mudstone strata of central Sichuan have been arched continuously,which has become another problem in the construction of high-speed railways in the red bed area.In order to cooperate with the research on the arch deformation mechanism of red bed soft rock subgrade,based on a typical red bed soft rock deep excavation cutting of a high-speed railway in southwest China,the creep characteristics of red bed mudstone are studied by combining experimental research,theoretical analysis and numerical simulation.The primary researches and outcomes of this text are as below:(1)For the red mudstone samples obtained and processed in the field,the conventional uniaxial compression tests of mudstone under different water contents were carried out to obtain the basic physical and mechanical parameters of mudstone.From the perspective of stress-strain curve characteristics,strength and deformation analysis,and sample failure characteristics,the instantaneous mechanical properties and change laws of red mudstone under different water-bearing conditions were studied.It is found that the stress-strain curve of mudstone has obvious five-stage characteristics.The strength and elastic modulus of mudstone gradually decrease with the increase of water content.The failure characteristics of the specimen are mainly longitudinal splitting,tensile failure and local shear failure.(2)The uniaxial compression creep tests of red mudstone under different water content states were carried out by graded loading and unloading,and the creep test curves were obtained to study the axial creep,creep rate,isochronous stress-strain curve and viscoelasticplastic characteristics.The study found that:under the experimental conditions,the red mudstone is mainly characterized by two stages of attenuation creep and stable creep,and there is no accelerated creep stage.The loading stress and water have a significant effect on the creep deformation of mudstone.With the increase of stress level and water content,the creep deformation and residual deformation of mudstone are gradually increased,and the initial creep rate and steady creep rate are also increasing.Under various loading stress levels,the mudstone generates recoverable instantaneous elastic strain,viscoelastic strain,and unrecoverable instantaneous plastic strain and viscoplastic strain,indicating that the red mudstone has the characteristics of viscoelastoplastic coexistence.(3)Based on the analysis of the characteristics of the creep test curve of red mudstone,the Burgers model is introduced,and the creep test curve is fitted by L-M nonlinear least squares method to obtain the creep parameters at different stress levels.Further on the basis of Burgers model,considering the effect of water damage and time effect of creep parameters,an improved Burgers nonlinear creep model is established,and the creep curve fitting results of the two models are compared.The study found that the Burgers model can better describe the creep characteristics of red mudstone,but the fitting effect of the improved Burgers nonlinear creep model on the creep test curve is significantly better than that of the Burgers model.The nonlinear creep characteristics of red mudstone are better described when considering the water-bearing damage effect,and the creep parameters of mudstone are related to the loading stress level and the water-bearing state of rock samples.(4)Based on FLAC3D software,the Cvisc model was selected for the numerical simulation of uniaxial creep of red mudstone.The numerical calculation results were compared with the experimental results,and the effects of lateral constraint and different shapes and sizes of the model on the creep deformation of the specimen were studied.It is found that the overall variation trend of numerical simulation curve and creep test curve is relatively close.The Cvisc model is more suitable to describe the creep characteristics of red mudstone.However,the relevant parameters need to be adjusted when the test results are used in practical engineering.Moreover,there are some differences in the creep deformation of specimens with or without lateral constraints and different shapes and sizes.