Study On Damage Characteristics And Mechanism Of Sandstone Under Wetting-drying In Acid Environment

In order to study the damage and degradation characteristics of geotechnical engineering in complex water solution environment,the influence of acid drying-wetting cycle on the stability and safety of deep underground rock and soil engineering is studied.In this paper,the white sandstone of a slope in Shaanxi Province is taken as the research object,and the SEM,uniaxial compression and NMR tests are carried out under different drying-wetting and chemical solution environment.The evolution law and force of fine and micro damage inside sandstone structure are analyzed The damage degradation model of sandstone under the condition of coupling of dry and wet cycle was established.The main research of this paper is as follows:(1)Nuclear magnetic resonance(NMR)tests were carried out to study the macro and micro damage evolution of sandstone structure under different drying-wetting water solution environments.The test results show that:under the action of drying-wetting cycle in acid environment,the mass and P-wave velocity damage degradation effect of sandstone is significant,and the decrease amplitude is larger before 6 dry wet cycles,then the decrease amplitude gradually slows down,and finally tends to be stable;the T2 spectrum curve of sandstone in different pH environments gradually moves to the right with cycles,and the right shift amplitude of T2 spectrum curve of sandstone in neutral environment is smaller;the T2 spectrum curve of sandstone in different pH environments gradually moves to the right.The spectral area increases exponentially with the increase of cycles;the micro pores accounting for more than 98%,and the porosity increases exponentially with the increase of drying-wetting cycle.(2)Uniaxial compression test,acoustic emission test and scanning electron microscope test of sandstone under different drying-wetting cycles were carried out to study damage evolution law of mechanical properties and micro pore structure evolution law of sandstone.The results show that:most of the failure modes of sandstone under different drying-wetring cycle and different aqueous solutions are in line with brittle failure or quasi brittle failure,and the softening effect is significant;the cumulative AE ringing number of sandstone under different acidic drying-wetting cycle decreases exponentially with cycles,and the deterioration degree is slowing down;The uniaxial compressive strength of sandstone under different aqueous solutions reduce with the increase of cycles and the curve tends to be exponential decay.The decline of sandstone in neutral aqueous solution environment is the smallest;the uniaxial compressive strength of sandstone reduce with the enlarge of porosity,and moves like linear decay law;the elastic modulus decreases exponentially with drying-wetting cycle,and with the increase of acidic environment,the more decrease of the elastic modulus.At the initial stage of the cycle,the failure mode is brittle tension splitting.After 16 drying-wetting cycle,the sandstone sample presents multiple shear failure mode,the shear plane is gradually blurred,and some plastic failure characteristics appear.SEM images show that the internal skeleton structure of sandstone is relatively complete,and the surface of sandstone is relatively flat at the initial stage of the cycle.After 16 cycles,the microcracks are interconnected,and the total volume of pores increases continuously.The evolution of pores in neutral environment is relatively slow,and the phenomenon of forming larger pores is less.(3)According to the results of nuclear magnetic resonance(NMR)test,the damage variable is defined by P-wave velocity and porosity.The damage degradation law of rock under different hydrochemical solution environment and drying-wetting load coupling effect is explored,and the influence law of drying-wetting cycle times and different water solution on the damage degradation characteristics of sandstone is analyzed.