Research On Correlativity Between Strength And Microstructure Of Red Sandstone Weak Sandwich Under Dry-wet Cycle

In recent years,extreme climate events have been increasing frequently in China due to global climate change.The frequent occurrence of extreme weather events has a negative impact on the geological environment,so that rock and soil mass were placed in a dry and wet alternate environment for a long period of time.The influence of the climate has intensified the weathering effect,caused damage to the rock and soil structure,reduced the geosynthetic self-stability,and changed the physical and mechanical properties of the rock and soil mass.Therefore,extreme weather events have a negative impact on rock and soil mass,especially rock and soil structures with special engineering properties.As a result of adverse effects on the stability of red sandstone,the red sandstone slope is frequently destabilized.The red sandstone weak interlayer plays a role in the stability of the slope,and the research on the change rule of strength and microstructure of red sandstone weak sandwich under the condition of dry-wet cycle has a certain guiding significance for the stability analysis of red sandstone slope under extreme climate conditions.In this paper,the red sandstone interlayer of Xiangtan area in Hunan was taken as the research object.The SEM test,XRD diffraction test,direct shear test,and direct shear creep test were used to research the law of shear strength,creep properties and microstructural changes of the weak interlayer of red sandstone under dry and wet cycle conditions.In the microscopic analysis,the microstructural parameters were quantified.Through correlation analysis and stepwise regression,the microstructural parameters that affect its mechanical properties were obtained,and the mathematical relationship between the main microstructure parameters and the strength was constructed.The content and results of this paper were as follows:(1)Through the direct shear test,the shear strength of the red sandstone soft interlayer gradually degrades to 18.68%after 1 to 5 cycles of wet and dry cycles,and the cohesion force decreases significantly,however,the internal friction angle was less affected by the effect;After 5 times of wet-dry cycles,the change of the internal structure of the weak sandwich tended to be balanced by the influence of the wet-dry cycle and the shear strength was stable.The number of wet-dry cycles and the cohesion force were approximately exponential functions.(2)Through the direct shear creep test under wet-dry cycle conditions,the wet-dry cycle reduces the creep strength,and the creep characteristics are more obvious with the increase of the number of dry-wet cycles and the decrease of vertical stress.(3)The results obtained by XRD diffraction test and SEM test showed that the strength attenuation of weak intercalation is caused by clay minerals under the conditions of dry-wet cycle.Because the clay minerals have the characteristics of water loss and shrinkage,the microstructure of the clay minerals wss destroyed,the number of particles was increased and the particle size was reduced.(4)The microstructure parameters such as average area,average diameter,average perimeter,and orientation probabilistic entropy decrease with the increase of the number of wet-dry cycles;microstructure parameters,such as the number of particles,the average shape factor,the fractal dimension of the morphology distribution,increased with the number of wet and dry cycles;the microstructure parameters and the change of shear strength were regular.(5)Stepwise regression analysis was used to select the average perimeter and shape coefficients from seven microstructural parameters as two microstructural parameters that mainly describe the change of cohesion,and the mathematical relationship between microstructure and cohesion was established.