Damage Evolution Of Bedded Sandstone And The Transverselyisotropic Under Freeze-thaw Cycles

In order to better understand the damage evolution rules and the transervesely isotropic characteristics of rock mass with obvious stratification development under the action of freeze-thaw cycles in the cold region,and then provide theoretical guidance and reference for the cold region engineering.In this paper,fine grained sandstone and coarse grained sandstone with obvious bedded structure are taken as the research object.Firstly,the freeze-haw cycles test and physical property test of the cycles process were performed on the prepared sandstone samples,including P-wave velocity test,porosity measurement,nuclear magnetic resonance test and three-dimensional morphology scanning test.Then,the mechanical properties of the rock samples after freeze-thaw cycles were tested,including Brazil splitting test,variable angle shear test and the uniaxial compression test.Finally,the test results of porosity,P-wave velocity and nuclear magnetic resonance T2 of sandstone under freeze-thaw cycles were analyzed,and the physical properties of sandstone with different layers under freeze-thaw cycles were explored.The parameters of tensile strength,shear strength and compressive strength of sandstone samples under the action of freeze-thaw cycles were discussed.Based on the physical and mechanical properties of sandstone samples with different layers under freeze-thaw cycles,the transversely isotropic of porosity,NMR T2 spectral area,P-wave velocity and mechanical properties with freeze-thaw times was discussed,and the transversely isotropic of malodorous NMR T2 spectral area with freeze-thaw times of fine.grained sandstone was analyzed.The main conclusions are as follows:(1)the P-wave velocity decreases with the increase of freeze-thaw times.The wave attenuation rate of fine-grained sandstone is lower than that of coarse-grained sandstone with the increase of freeze-thaw times.The attenuation rate of ultrasonic P-wave velocity with the increase of freeze-thaw times is obviously higher than that of ultrasonic wave propagation in the direction of vertical sample bedded.The energy dissipation of the ultrasonic wave in the medium is influenced by density and stratification angle.Densification degree and cementation state of bedded contact surface caused by freeze-thaw cycles are the main reasons that affect the P-wave velocity.(2)porosity shows a "three-stage trend" with the increase of freeze-thaw times,that is,rapid increase-slow increase-apid increase.During the freeze-thaw cycles,the porosity of the vertical bedded samples increased faster than that of the horizontal bedded samples,and the cumulative time of the slowly increasing phase of the vertical bedded samples was lower than that of the horizontal bedded samples.(3)the T2 spectral area of NMR increased with the increase of freeze-thaw times.For fine-grained sandstone,with the increase of freeze-thaw times,the T2 spectral area of the metropolis increases,while the T2 spectral area of the metropolis is in a fluctuating state,with no increasing or decreasing trend.For coarse-grained sandstone,micropore T2 spectral area tends to decrease with the increase of freeze-thaw times,while mecopterous T2 spectral area tends to increase.With the increase of freeze-thaw times,the NMR T2 peak of coarse-grained sandstone has a right-wing trend,and the peak on the left keeps decreasing,while the peak on the right keeps increasing.(4)the tensile strength decreases with the increase of freeze-thaw times.Under the action of freeze-thaw cycles,the tensile strength of the samples with fine-grained sandstone and coarse-grained sandstone in the loading direction perpendicular to the direction of bedded decreased more than those in the loading direction parallel to the direction of bedded.Cohesion decreases with the increase of freeze-thaw times.Under the action of freeze-thaw cycles,the cohesive force of specimens with shear plane perpendicular to the direction of bedded decreases more than those with the shear plane parallel to the direction of bedded.(5)the uniaxial compressive strength and elastic modulus show a tendency of attenuation with the increase of freeze-thaw times;Under the action of freeze-thaw cycles,for fine-grained sandstone samples,the attenuation rate of uniaxial compressive strength and elastic modulus is greater than that of samples parallel to the bedded surface when the loading direction is perpendicular to the bedded surface.The opposite is valid for coarse-grained sandstone.(6)under the action of freeze-thaw cycles,the damage accumulation of sandstone in the stratum has typical transversely isotropic characteristics.With the increase of freeze-thaw cycles,the damage rate of vertical stratification samples is faster than that of parallel stratification samples,and the damage accumulation time of vertical stratification samples is less than that of parallel stratification samples.With the increase of freeze-thaw times,the transversely isotropic of the malodorous T2 spectral area of fine-grained sandstone decreases,while that of coarse-sandstone increases.The transversely isotropic of P-wave velocity,tensile strength,cohesive force and uniaxial compressive strength increase with the increase of freeze-thaw times.Based on the results of this experiment,it can provide an important reference for the evaluation of the damage evolution and the transversely isotropic of the bedded sandstone during the freeze-thaw cycles.