Analysis Of Fluid-Structure Coupling Creep Characteristics Of Red Bed Soft Rock

Red-bed soft rock is widely distributed in our country,studying on mechanics properties of red-bed soft rock is of great significance.In order to study the creep characteristics and mechanism of red-bed soft rock under water-rock interaction,fluid-structure coupling triaxial compression tests and creep tests under stepwise loading were carried out,and a threedimensional nonlinear creep model considering seepage effect was constructed.The triaxial stress-strain curves,failure modes and energy evolution of red-bed soft rocks under different seepage pressures are analyzed.The creep strain-time curve,isochronous stress-strain curve,creep rate curve,creep Poisson’s ratio curve and seepage rate curve of red layer soft rock under different osmotic pressures are obtained.Furthermore,the influences of seepage pressure and stress on creep deformation,long-term strength,Poisson’s ratio and seepage velocity were analyzed.According to the experimental results,the influence of seepage on the creep of soft rock cannot be ignored.The main conclusions are as follows:Seepage leads to the decrease of triaxial strength and elastic modulus,and the increase of Poisson’s ratio.Seepage is damaging to the internal structure of rock.When the seepage pressure increases,the change of the radial deformation is more sensitive than the axial deformation.The failure mode of triaxial compression changes from shear failure to tensionshear conjugate failure.The total strain energy and dissipated energy increase gradually with the increase of axial strain.The elastic strain energy increases with the axial strain before rock failure,but decreases rapidly after rock failure.The seepage leads to the decrease of long-term strength,and the increase of instantaneous deformation and creep deformation.The creep failure is more obvious than the triaxial failure,and the tensile failure is more obvious,while the long-term strength of fluid-structure coupling creep is 60%～70% of the triaxial strength.When the stress level and seepage pressure are relatively small,the Poisson’s ratio of creep increases with the increase of seepage pressure,and the radial creep deformation response lags behind the axial creep deformation.However,at high stress level and osmotic pressure,Poisson’s ratio and seepage velocity increase rapidly,and the duration of accelerated creep is obviously shortened.Through the analysis of the influence of seepage pressure on the seepage velocity,with the increase of the seepage velocity,the seepage velocity changes and fluctuations are more obvious,which further confirms the damage effect of seepage pressure and erosion on the internal structure.In the field monitoring of actual engineering,the rapid change of seepage velocity can be used as the precursor signal to predict the instability.Therefore,the water-rock interaction cannot be ignored in the analysis of mechanical properties and long-term stability of red-bed soft rocks.Based on the Nishihara model,a three-dimensional nonlinear creep model is constructed by introducing nonlinear elements and considering seepage pressure.This model can well describe the creep deformation law of red bed soft rock under different seepage pressure,especially the deformation law of creep failure stage.