Experimental Investigation On The Effects Of Water Saturation And Freeze-thaw On The Damage Failure And Acoustic Emission Characteristics Of Sandstone

Rock masses in cold regions are inevitably subjected to freezing and thawing,and the resulting rock damage rupture is an important factor affecting the stability of rock masses.Most of the existing studies on rock freezing and thawing have been focused on the changes of physical and mechanical properties,and little research has been done on the effects and mechanical mechanisms of rock damage caused by freezing and thawing.Due to the high porosity and some clay minerals of sandstone,it softened under the action of water,and the freeze-thaw cycle further aggravates the degree of damage.In this paper,freeze-thaw cycle tests and uniaxial compression tests were carried out on three different types of sandstone,and the whole process of uniaxial compression of rock samples was monitored by acoustic emission（AE）system.The influence laws of water action and freeze-thaw cycles on physical and mechanical properties of sandstone were obtained,focusing on the change laws of stress threshold and acoustic emission characteristics of sandstone,and revealing the damage mechanism of sandstone hydration and freeze-thaw damage.The main conclusions of the study are as follows:（1）The action of water has a significant softening effect on all three kinds of sandstone.After water saturation,peak strength,peak axial strain and elastic modulus decreased,longitudinal wave speed and Poisson’s ratio increased.With the increase of the number of freeze-thaw cycles,the internal fractures of sandstone develop and the quality changes,and the longitudinal wave velocity,peak intensity,elastic modulus and Poisson’s ratio of sandstone all show a decreasing trend.Under the action of water and freeze-thaw cycles,the total energy（U）,elastic energy（U_e）and dissipation energy（U_d）at the peak stress of the sandstone decrease,the ratio of elastic energy to total energy（U_e/U）decreases,and the energy storage capacity decreases.The brittleness of sandstone was evaluated based on stress-strain curves and energy,and the results showed that the brittleness of sandstone was significantly weakened after water saturation.With the increase of the number of freeze-thaw cycles,the sandstone brittleness shows a continuous trend of weakening.Water-saturated and freeze-thaw action are important factors affecting rock brittleness.In terms of the magnitude of variation of each index,water-saturated and freeze-thaw have a greater effect on the white sandstone,mainly because the clay mineral content of the white sandstone is about 7%,and its fragments are sub-angular-sub-rounded and uniform in size,which are more prone to misalignment and slip between rock particles under uniaxial compression.（2）The crack volume strain method（CVS）was selected to calculate the stress thresholds of three types of sandstone,and the crack initiation stressσ_ci,damage stressσ_cdand peak stressσ_fwere obtained for the sandstone under the action of water-saturated and freeze-thaw,and the corresponding axial crack initiation strainε_aci,axial loss strainε_acd,axial peak strainε_af,circumferential crack initiation strainε_cci,circumferential damage strainε_ccdand circumferential peak strainε_cpwere determined.After water saturation,the normalized initiation stress of red sandstone and white sandstone decreased,the normalized initiation stress of yellow sandstone increased,the normalized damage stress of all three kinds of sandstone decreased.For the strain threshold,the difference between normalizedε_acd/ε_afandε_aci/ε_afand the difference between normalizedε_ccd/ε_cpandε_cci/ε_cpare reduced.With the increase of freeze-thaw,the normalizedε_aciand normalizedε_acdof sandstone showed an increasing or stable trend,while the overall changes of normalizedε_ccd/ε_cpandε_cci/ε_cphad no obvious pattern.Under the action of water saturation and freeze-thaw,the pore water inside the rock as a medium for stress transfer weakens the stress concentration around the pore,which may cause new cracks to be less likely to be produced;while the clay minerals in sandstone are softened by water and the degree of interparticle cementation is weakened,which makes it easier to produce new cracks under external loading.The interaction of these two competing mechanisms is the main reason for the variation of rock stress and strain thresholds.（3）Based on the analysis of AE parameters,it was found that the AE activities of the three kinds of sandstone specimens were significantly weakened after water saturation,and the peak of AE counts and AE cumulative counts decreased,while the differences in AE counts under different freeze-thaw counts were related to the differences in the properties of different sandstone.The AE b value increase after water saturation for red sandstone and white sandstone,while the AE b value decrease after water saturation for white sandstone.It indicates that more small-scale ruptures are produced when the red sandstone and white sandstone specimens are damaged after water saturation,while more large-scale ruptures are produced when the yellow sandstone specimens are damaged.With the increase of freeze-thaw cycles,the AE b values of red sandstone,yellow sandstone and white sandstone showed“V”,inverted“V”and“N”type respectively,reflecting the change of fracture scale caused by freeze-thaw action.Based on the AE RA value（ratio of rise time to amplitude）and AF value（ringing count/duration）,the uniaxial compression rupture forms of the three kinds of sandstone are dominated by tensile rupture under dry,water-saturation and different freeze-thaw cycles.The proportion of tension rupture decreases and the proportion of shear rupture increases after saturation and freeze-thaw cycles,which is more likely to produce intragranular fracture.（4）Damage to rocks under freeze-thaw cycles action is caused by the combined effect of water softening and freeze-thaw deterioration.Water has softening,dissolving and water wedging effects on rocks.The essence of freeze-thaw deterioration lies in the fact that:during the freezing process,the volume expansion of ice produces frost heave force,and different mineral particles deformation unharmonically when the temperature changes.The resulting tensile stress promotes the further propagation of primary microcracks and generates new microcracks.During the thawing process,more water enters the interior of the rock,clay minerals and other cements soften,dissolve,transport and diffuse in contact with water,and the interparticle bonding force is weakened,which makes it easier for crack expansion and penetration to occur under external loading.Therefore,the freeze-thaw cycle will cause the deterioration of rock properties.