The Macro-meso Creep Properties Of Jurassic Red Sandstone Subjected To Cyclic Seepage Pressures

During the operation of the reservoir,due to the needs of flood control and power generation,the water level of the reservoir will fluctuate greatly periodically.Under the condition of periodic water level fluctuations,the seepage field and stress field in the slope of the reservoir bank influence each other,and the creep characteristics of the rock mass in the anti-slide pile embedded section will change under the coupling action of the seepage field and stress field,thus affecting the anti-skid effect of anti-slide piles.Therefore,it is of great practical significance to carry out the evaluation of the long-term prevention and control effect of the landslides-piles system.This paper takes the red sandstone in the Three Gorges reservoir area as the research object,and studies its macro-and meso-creep characteristics and permeability evolution characteristics by using laboratory tests,theoretical modeling,and numerical simulation,and establishes an ideal landslide-anti-sliding slope based on numerical methods.The pile structure model discusses the long-term deformation characteristics of the anti-slide pile system of the landslide considering the cyclic seepage pressure.The main research results are as follows:(1)The short-term mechanical behavior and permeability characteristics of red sandstone were studied through triaxial seepage-stress coupled tests.Studies have shown that under the lower stress environment of the landslide in the reservoir area,the confining pressure has a more obvious influence on the stress-strain curve,while the seepage pressure has an insignificant influence on the stress-strain curve;the initial elastic modulus and peak strength of red sandstone increase with the confining pressure.The peak strength of red sandstone decreases with the increase of seepage pressure.Seepage pressure has a significant effect on the initial elastic modulus of red sandstone.The permeability of red sandstone is not obvious in the initial deformation and elastic deformation stages,but the permeability increases rapidly after reaching the peak strength.The initial permeability is closely related to the confining pressure and seepage pressure.The initial permeability increases linearly with the increase of seepage pressure,and decreases exponentially with the increase of confining pressure.(2)The seepage model of the pipe network is improved,and the microscopic characteristics of red sandstone deformation and failure under the coupling action of seepage and stress are studied based on the particle flow program PFC^2D.Research shows that the stress-strain curve of red sandstone can be divided into three stages according to the characteristics of microcracks and stress-strain curve.The initiation and propagation of microcracks began to appear from the second stage.The effective confining pressure will have a significant impact on the number of microcracks after rock rupture.Stress concentration will appear on the macroscopic fracture surface of the rock after failure.The stress concentration will have an impact on the seepage.The position of the stress concentration will reduce the aperture of the flow pipe of the model and affect the seepage.However,as the confining pressure increases,the stress concentration becomes insignificant.The permeability evolution of rock is closely related to microcracks,and tensile cracks have a greater impact on permeability than shear cracks.The flow velocity at the shear zone is usually large.(3)The creep characteristics and permeability evolution characteristics of red sandstone under constant seepage pressure and cyclic seepage pressure were studied through triaxial seepage-creep tests.The research results show that the red sandstone shows three distinct creep stages during the high-stress creep process,namely the decelerated creep stage,the steady-state creep stage and the accelerated creep stage.The instantaneous strain of red sandstone is positively correlated with deviatoric stress and seepage pressure.The axial creep curve of red sandstone under cyclic seepage pressure is similar to the creep curve under constant seepage pressure,and the volumetric strain shows periodic fluctuations with the periodic change of seepage pressure.Under the action of cyclic seepage pressure,the volume creep strain and creep strain rate of red sandstone both increase significantly.With the periodic fluctuation of seepage pressure,the permeability of red sandstone also shows obvious periodic fluctuations.The long-term strength of red sandstone under the action of cyclic seepage pressure is lower than that under the condition of constant seepage pressure.(4)By coupling the stress corrosion model and the improved pipe network model,the red sandstone creep damage characteristics under the condition of seepage are studied based on PFC^2D.The research results show that the rock creep curve and the microcrack curve have similar changing trends.In the instantaneous creep stage,the microcracks grow rapidly.As the creep enters the decelerated creep stage,the growth rate of the microcracks decreases.When the creep enters the steady-state creep stage,the number of microcracks nearly keep constant.Under cyclic seepage pressure,the microcracks generated by the rock after all levels of deviatoric stress creep are greater than the constant seepage pressure.The effect of the cyclic seepage pressure causes the effective confining pressure of the rock to change periodically,which will further aggravate the rock damage.When the rock creeps under a lower stress level,due to the small number of microcracks,the model permeability is mainly controlled by the compression of the flow pipe between particles,and the permeability decreases.When the stress level is high,the model has more microcracks.The number of microcracks is large,and the permeability is mainly controlled by the growth of microcracks.(5)A nonlinear creep constitutive model of red sandstone is established based on the theory of fractional calculus.The improved elastic element can use a set of parameters to consider the instantaneous deformation under different stress levels,greatly reducing the model calculation parameters.The effective stress principle considers the influence of seepage pressure on the effective confining pressure,thereby introducing seepage pressure into the creep model.Based on the theory of elastoplastic mechanics and adopting the Drucker–Prager yield function,the one-dimensional seepage creep model in this paper is extended to the three-dimensional model,which provides a theoretical basis for the engineering application of the model.(6)Based on the secondary development module in FLAC^3D,the derived nonlinear creep model is integrated into FLAC^3Dsoftware,and the ideal landslide anti-slide pile structure model is established,and the anti-slide pile and the surrounding rock mass under the condition of periodic seepage pressure are studied.The research results show that with the increase of creep time,the resistance of the rock mass around the pile gradually decreases,and the maximum rock mass deformation appears in the rock mass at the top of the embedded section in front of the pile.Increasing,the creep deformation and creep rate of the rock mass decrease.The rock mass displacement under the action of periodic seepage pressure is obviously greater than the rock mass displacement under the action of no seepage pressure.As the creep time increases,the maximum axial force of the anti-slide pile decreases.With the increase of creep time,the bending moment of the anti-slide pile in the embedded section gradually decreases.The maximum value of the bending moment of the anti-slide pile under the condition of periodic seepage pressure is higher than that of the anti-slide pile under the condition of no seepage pressure.