Study On Hydration And Freeze-thaw Coupling Damage Mechanism And Strength Prediction Model Of Sandstone

The rock engineering usually experience daily and seasonal freeze-thaw cycles in cold regions.The freeze-thaw action will cause the internal damage and strength loss of rock mass,which has an adverse influence on the stability of these rock engineerings.Sandstones,as one typical sedimentary,are widely exposed on the surface of rock engineering.Because sandstones often contain some clay minerals,they will be softened in the presence of water,and the freeze-thaw cycles will further intensify the damage degree of these sandstones.Water content is a key factor influencing the hydration softening and freeze-thaw damage degree of sandstone.Therefore,it is necessary to systematically study the hydration and freeze-thaw coupling damage mechanism of sandstones considering the influence of water saturatons.In this study,first the mechanical and acoustic emission tests of typical unsaturated red sandstone are carried out in the absence of freeze-thaw cycles.The influence of water saturations on the loss law of the mechanical strength,the deformation and acoustic emission characteristics of sandstones were obtained.Then,a novel stress-strain constitutive model has been proposed considering the influence of water saturations.Furthermore,the hydration softening mechanism of sandstones containing clay minerals was revealed.In addition,several freeze-thaw experiments were conducted on four different sandstones with different water saturations.The change of P-wave velocity,porosity and uniaxial compressive strength were continuously monitored under freeze-thaw.Then,the failure form,stress-strain relationship characteristics and strength loss law of these sandstones under freeze-thaw were obtained,and a strength prediction model of unsaturated sandstones under freeze-thaw was developed.Based on above research,the hydration and freeze-thaw coupling damage mechanism of sandstone was revealed.In this study,some main conclusions can be drawn:(1)Saturation has a great influence on UCS,TCS,BTS,PLS and DSS of red sandstone.At low saturation of 9.23%,the mechanical strength decreases faster.In addition with the increase of saturation,the cumulative acoustic emission ringing count shows a downward trend.When the saturation is 60%?80%,the reduction rate of mechanical strength and cumulative acoustic emission ringing count of sandstone slowed down and reached at a constant.Therefore,the critical saturation causing obvious softening damage of the red sandstone is between 60% and 80%.(2)The water bearing sandstone has an obvious compaction hardening stage,which satisfies the exponential hardening law.Considering this compaction hardening and effect of water saturtions,a novel constitutive model was proposed based on the micro element strength theory.In this model,the elastic modulus and peak strength were found to be linearly and inversely correlated with water saturations.The novelty of this constitutive model was that it had reflected the compaction hardening and described the stress-strain relationship of unsaturated sandstones.(3)The stress-strain relationship of four typical sandstones with different saturations after suffering freeze-thaw cylces were obtained.The teset results shows that with the increase of freeze-thaw cycles,the freeze-thaw damage becomes more serious,especially for the sandstones with higher water saturations.In addition,an interesting finding was that the ratio of transition stress to the peak stress is almost independent of the saturation,freeze-thaw number and the type of the sandstones.The trainsition stress at the yield point and compaction point are about 81% and 33% of the peak stress,respectively.Only the stress ratio at the pore compaction point increases slightly with the increase of freezethaw times,from 12.8% to 15%.(4)The damage of sandstones under freeze-thaw condition was caused by the coupling action of water softening and freeze-thaw deterioration.The water softening is caused by the physical softening and chemical erosion,and the existence of clay minerals will lead to the further enhancement of water softening.The essence of freeze-thaw deterioration is caused by the frost heaving pressure generated during waer-ice phase transition.The tensile stress around the pores may be produced and leads to the expansion of pores during freezing.Therefore,an irreversible damage is accumulated under repeated freeze-thaw cycles.The critical saturation for causing a significant decrease of UCS is also between 60% and 80%.(5)A new prediction model of freeze-thaw strength was proposed,which has the advantages of both classical exponential decay model and multiple linear regression model.Compared with the experimental results,it was found that the model was simple and practical.Only the initial physical and mechanical parameters(porosity,elastic modulus and Brazilian tensile strength)of sandstone can be used to predict the uniaxial compressive strength during the freeze-thaw process,which provides a reference for the stability evaluation of rock engineering in cold regions.