Study on the fabric characteristics and evolution mechanism of the mechanical behavior of weakly cemented Xiyu conglomerate
The weakly cemented Xiyu conglomerates are widely distributed in the northern and southern foothills of Tianshan Mountains in Xinjiang.The material composition of Xiyu conglomerates is complex,and their mechanical properties are poor.In order to scientifically understand the evolution laws of mechanical properties of weakly cemented Xiyu conglomerate,its grain fabric characteristics and macro mechanical properties were systematically studied by means of laboratory test and in-situ large-scale true triaxial test.The results indicate that:(1)The material composition of weakly cemented Xiyu conglomerate is relatively high in the three particle groups of crushed stones,coarse gravels,and medium gravels.And the particle size distribution of weakly cemented Xiyu conglomerate is poor.The inter-granular contact between large and coarse particles such as crushed stones and gravels involves either particle suspension or particle support.The inter-granular cementation is mainly pore-type cementation.(2)Due to the influence of particle arrangement,weakly cemented Xiyu conglomerates exhibit obvious anisotropic deformation characteristics.The deformation modulus is highest in the vertical direction,lowest in the direction of the particle's major axis(nearly perpendicular to the Tianshan Mountain),and moderate in the direction of the particle's mid axis(nearly parallel to the Tianshan Mountain).However,with the increase of stress level,the weakly cemented Xiyu conglomerates tend to be isotropic.(3)Under triaxial stress state,weakly cemented Xiyu conglomerate shows strain hardening characteristics in the yield stage.After reaching peak strength,it shows typical plastic flow characteristics.(4)The shear strength of weakly cemented Xiyu conglomerates have obvious nonlinear characteristics,loading and unloading stress path effects and intermediate principal stress effects.The higher the stress level,the smaller the internal friction angle and the greater the cohesion.The internal friction angle of Xiyu conglomerate under unloading condition is higher than that under loading condition,while the cohesion under unloading condition is lower than that under loading condition.Under true triaxial loading,the peak strength increases with the increase of intermediate principal stress,and the effect of the intermediate principal stress on the shear strength parameter,cohesion,is more obvious.(5)The failure of weakly cemented Xiyu conglomerate under triaxial stress is mainly compression-shear failure.After failure,a steep main fracture zone is formed,and the main fracture zones are connected by tensile cracks.The test results provide an important basis for systematically understanding the macro mechanical properties and their evolution laws of the weakly cemented Xiyu conglomerates.
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