Study On Freeze-thaw Damage Characteristics Of Sandstone Tunnel Surrounding Rock In Cold Region

Recently,the transportation and other infrastructure construction has developed rapidly in China,it is necessary to build numbers of tunnel projects in high altitude and alpine regions.Due to the freeze-thaw cycle,the problem of tunnel freezing damage is becoming more and more serious,especially through the red sandstone tunnel which is rich in groundwater and has developed pores.The surrounding rock is seriously damaged under the freeze-thaw cycle.Therefore,it is significant to study the changes of physical and mechanical properties of red sandstone under freeze-thaw cycles for the safety construction and operation of red sandstone tunnels in cold regions.Based on Huizhou 3#red sandstone in tunnel engineering,the indoor freeze-thaw test,the rock of conventional triaxial test,SEM electron microscope scanning,and numerical simulation methods are adopted to explore the damage of physical and mechanical properties of red sandstone under different freeze-thaw conditions and confining pressures,explore the influence of freeze-thaw action on tunnel confining pressure stability,and provide reference for the prevention and control of freezing damage in red sandstone tunnels in cold regions.The main conclusions are as follows:(1)The changes of physical properties and microscopic pore characteristics of red sandstone under the condition of freezing and thawing:①With the increase of freeze-thaw times,the average quality,compressional wave velocity and coefficient of thermal conductivity and other physical parameters are reduced;②The SEM image shows that with the increase of the number of freeze-thaw times,The cross-microcracks have appeared in the rock,increasing the number of cracks,the width and length increasing significantly,the section edges and corners are obvious,block fracture appears in the rock sample and the damage of microfracture is aggravated.(2)Apparent deterioration mode under freeze-thaw condition and compression failure mode of red sandstone under triaxial condition:① Under freeze-thaw conditions,the crack width gradually increases from initiation to expansion,and the crack has a tendency of penetration along the axial direction.Therefore,the apparent degradation mode of rock samples is crack mode.②The failure mode of triaxial compression test:When the confining pressure is 2 and 4MPa,the failure mode is axial tensile failure,when the confining pressure is 6MPa,the failure mode is "X" type crack shear failure,and when the confining pressure is 8MPa,the failure mode is oblique shear failure.(3)Mechanical properties of red sandstone under freeze-thaw conditions:①Under the same confining pressure,with the increase of freeze-thaw times,the extreme strain and Poisson’s ratio increase.The cohesion and internal friction Angle of red sandstone decrease gradually with the increase of freezing and thawing times,and the shear strength of the rock decreases.① Under the same freeze-thaw conditions,the compressive strength,extreme strain and elastic modulus of red sandstone all increase with the increase of confining pressure,while the Poisson’s ratio of rock samples decreases.When the confining pressure increases from 0MPa to 2MPa,the mechanical parameters change obviously.With the increase of the confining pressure,the change rate of the mechanical parameters decreases.(4)The numerical simulation analysis of the stability of red sandstone tunnel under freeze-thaw cycles:①The total displacement increases with the increase of freeze-thaw times,After 80 freeze-thaw times,the accumulated settlement value of the vault is 52.66mm,which exceeds the allowable settlement value by 2.66mm.The displacement change rate at the arch foot is higher than other monitoring points,the stability of the tunnel is reduced.②Horizontal and vertical stresses at each monitoring point increased with the freeze-thaw cycles increasing.After 80 freeze-thaw cycles,the maximum horizontal stress at each monitoring point is 0.506MPa,which is within the allowable supporting force range of V level confining pressure;After 60 freeze-thaw cycles,the vertical stress at the left and right arch feet is 1.33 MPa and 1.29 MPa,respectively,exceeding the upper limit of allowable supporting force by 0.18 MPa and 0.14 MPa.The supporting structure is in an unstable state,and the stress concentration phenomenon is obvious.The tunnel support structures are at unstable state and the stress concentration phenomenon is obvious.③The plastic zone of surrounding rock is mainly concentrated at the shoulder and foot of the tunnel,the extension range of plastic zone increases with the freeze-thaw times increasing.After 80 freeze-thaw cycles,the ratio of the maximum depth of plastic zone to the hole span is 0.27,which is less than the warning value of 0.75,the plastic zone of tunnel shoulder and foot expands rapidly.Consider the above three aspects,the tunnel is in an unstable state after freeze-thaw 60 times.In order to avoid the tunnel instability caused by excessive local deformation,it is necessary to control the deformation of surrounding rock and improve the support strength at the vault and arch foot.