Loading And Unloading Creep Properties Of Sandstone And Research On A Nonlinear Viscoelasto-plastic Creep Model

Recent years as with the excavations and constructions of the high slopes and large underground caverns,it gradually emerges such problems as the safety during the construction and the stability during the operation,and the long-term stability of projects is closely related to the creep properties of rock.A large number of projects demonstrates that rock creep is an important factor for its large deformation and even instability.In view of this,and in combination with National Natural Science Foundation Project "Study on the damage evolution mechanism and constitutive model of deep surrounding rock's tensile creep under the unloading path",researches carried out in this paper would use a combination method including testing method,theoretical analysis and data fitting on the basis of the sandstone in Chongqing.The deformation and failure characteristics of sandstone under conventional uniaxial and triaxial compression tests were analyzed.Besides,creep properties of sandstone under different loading and unloading paths were studied.Furthermore,the research on a nonlinear viscoelasto-plastic creep model and its correlation of parameters were carried out.The main research contents of this paper were as follows:(1)The uniaxial and triaxial compression tests under different confining pressures were carried out for studying the basic mechanical characteristics of sandstone,and the WDAJ-600 rock creep testing machine was used for the uniaxial test,the RLW-2000 triaxial testing machine was used for the triaxial tests.Based on the test results,the strength,deformation and failure characteristics of sandstone are analyzed,the basic mechanical characteristics were studied too.The basic mechanical parameters were tested out,which laid a foundation for studying on sandstone creep tests.(2)The creep tests under different step loading and unloading paths were carried out for studying the creep characteristics of sandstone,and the WDAJ-600 rock creep testing machine was used for the uniaxial loading creep test,the ROCK 600-50 triaxial testing machine was used for the triaxial loading and unloading creep tests.Based on the creep test results,the creep deformation and failure characteristics of sandstone are analyzed,which laid a foundation for studying on creep constitutive model of rock.(3)Based on the creep test results,it demonstrated that,Burgers model can effectively describe the instantaneous elastic deformation,attenuation and steady creepdeformation of sandstone,and the results fitting at the BFGS method demonstrated Kelvin model had optimization effect on Burgers model.(4)According to the accelerating creep results under different confining pressures,a modified visco-plastic creep model was obtained by introducing the empirical model and combining Kelvin model for its optimization,and it reflected the accelerated creep characteristics accurately.By combining this modified model with Burgers model in series,a new nonlinear viscoelasto-plastic model can be put forward.The new model can effectively reflect the whole process of creep characteristics.(5)Using the BFGS method to identify the model parameters,the results were satisfactory.A comparison between the test and theoretical curves proved the validity of the proposed new model.The changing rules of the fitting parameters in different test conditions were also analyzed.