Physical And Mechanical Properties And Failure Characteristics Of Sandstone Under Freeze-thaw Cycles

With the vigorous development of infrastructure construction in my country,the problem of freezing damage in some geotechnical engineering has gradually become prominent.Especially in the process of geotechnical engineering construction in high latitude and high altitude areas,the engineering rock mass is prone to freeze-thaw damage due to the influence of the environment and seasons.In the vertical shaft construction using the freezing method,the surrounding rock will also suffer short-term freeze-thaw damage.Freezing and thawing will lead to changes in the physical and mechanical properties of rocks,which will seriously affect the construction safety and stability of the project.In this paper,the typical sandstone in northern China is taken as the research object,and the physical and mechanical properties and acoustic emission characteristics of saturated sandstone are studied through freeze-thaw cycle test,conventional mechanical test and acoustic emission detection test.By means of scanning electron microscopy(SEM)and mercury intrusion(MIP)experiments,the microstructure changes in sandstone after different freeze-thaw cycles were analyzed.The damage and deterioration characteristics of sandstone under the action of freeze-thaw cycles are also discussed.The main research results of the paper are as follows:(1)The effect of freeze-thaw cycles on the general physical properties of sandstone was studied.After 40 freeze-thaw cycles,the sandstone surface appeared debris spalling.The saturated water content of sandstone increased exponentially with the increase of freeze-thaw times,and increased from 0.52% to 0.79% after 60 freeze-thaw cycles.The porosity of sandstone increased rapidly in the range of 0 to 30 freeze-thaw cycles,from 1.39% to 1.96%,and then stabilized.(2)Microstructure experiments were carried out to study the microscopic physical properties of frozen-thawed sandstone.The SEM test showed that as the number of freeze-thaw cycles increased,the microstructure of sandstone developed into a honeycomb shape,the degree of cementation between rock particles gradually decreased,and the structure tended to be loose.The rock particles fell off and denuded,and the tiny pores inside were gradually connected and developed into large pores.The MIP test showed that after 60 freeze-thaw cycles,the proportion of micropores(d?1 ?m)inside the sandstone decreased to 26.17%,and the proportion of macropores(d?100 ?m)increased to 51.89%.(3)The strength characteristics of freeze-thawed sandstone were studied through indoor mechanical tests.With the increase of freeze-thaw times,the strength parameters of sandstone gradually decreased.The tensile strength of sandstone decreased to 5.13 MPa after 60 freezethaw cycles,an attenuation of 38.65%.The triaxial compressive strength gradually decreased with the increase of the number of freeze-thaw cycles.With the increase of confining pressure,the loss rate of triaxial compressive strength of sandstone gradually decreased after 60 freezethaw cycles.Verification found that the Mohr-Coulomb strength criterion better describes the relationship between the principal stresses in frozen-thawed sandstones.(4)The deformation characteristics of freeze-thawed sandstone were studied through uniaxial and triaxial compression tests.The elastic modulus of sandstone under various confining pressures had a negative exponential function with the number of freeze-thaw cycles.The cohesion and internal friction angle of sandstone showed a decreasing law firstly and then slowly with the increase of freeze-thaw times.With the increase of freezing and thawing times,the sandstone damage became more severe,and the freezing and thawing sandstone with confining pressure lower than 7.5 MPa mainly showed splitting damage.The failure mode of sandstone which experienced multiple freeze-thaw after exceeding 7.5 MPa gradually turned into shear failure.(5)The characteristics of acoustic emission during the failure of frozen-thawed sandstone under load were studied.With the increase of the number of freeze-thaw cycles,the activity of the acoustic emission signal in each stage of the sandstone compression test increased,and the characteristic of the failure omen was that the active period of the acoustic emission signal was prolonged,and the quiet period before the failure was shortened.The peak value of the acoustic emission ringing count in the sample destruction stage showed an upward trend,and the accumulative acoustic emission ringing count increased first and then decreased.It was shown that the freeze-thaw damage inside the sample gradually intensified,and the development of the internal microstructure changed significantly.(6)The coefficient was introduced to study the degree of freeze-thaw weathering of each group of sandstones.Comparing the freeze-thaw weathering coefficients,it is concluded that the sandstone has good frost resistance after 10 freeze-thaw cycles,and the sandstone has poor frost resistance after more than 10 times of freeze-thaw.The introduction of the comprehensive influence coefficient showed that the confining pressure has an inhibitory effect on the freezethaw damage,and the higher the confining pressure,the more obvious the inhibitory effect.An empirical formula for estimating the triaxial compressive strength of sandstone after different freeze-thaw cycles was obtained by linear fitting.