Experimental Study On The Shearing Mechanism Of Loess Based On Microstructural Functional Units

Shear damage is a common geological phenomenon in loess areas,and the intrinsic mechanism of its formation and development has been an important part of research in the field of engineering geology.Microstructural characteristics reflect the mechanical process and mechanism of shear deformation.Changes in porosity,particle size and particle arrangement are important indicators to reveal the nature of the shear deformation process,and the particle size distribution of loess particles and aggregates is the main factor to determine the microstructure of loess.In this paper,the concept of shear functional unit is introduced into the study of the shear behavior process of loess as an intermediate structural unit,which facilitates to reveal the shear behavior of loess in depth from a new perspective.Firstly,we classify loess functional units according to their mechanical properties,water stability,and shear function contributions;In addition,we proposed a quantitative characterization method based on image processing,geometric measurements,and statistical means.On this basis,we discussed the residual strength characteristics,shear band evolution,and particle arrangement process of loess during shearing.The research results show that:(1)The shear functional unit stability of different types of loess varies significantly.Particles with high roundness and without their own defects are the most stable,and single particles tend to be more stable than aggregates.There are often "narrow bridges" or cracks in the interior of heteromorphic particles,which are prone to breakage under stress.(2)The pixel distribution of "core + clothes" microstructure units in loess particles on SEM images is in accordance with the Gaussian mixture model(GMM),and the particle size gradation obtained by separating the clothes improves the measurement results which are limited by the change of loess state,and enhances the quantification accuracy of the microstructure units in loess.(3)The residual internal friction angle of loess specimens in different states decreases with the increase of water content of soil samples,while the residual cohesion is the opposite.The residual strength increases with increasing stress level.Small stress fluctuations can be captured in the residual state,which is related to the continuous transformation of shear functional units.(4)Under the shear-induced effect,the specimen is dominated by the sliding shear mode,and shear zone thickness of about 2-7 mm.The fragmentation of unstable units in this process is divided into the disintegration of aggregates and the fracture of the particles themselves,and the main crushing particle size is 0.035-0.09 mm.Particle rearrangement and reorientation make up for the increased proportion of unstable units in the shear process and facilitates the formation of smooth shear surfaces.The study of different functional matrix elements of loess helps us to gain insight into the shear mechanism.The quantitative analysis even provides strong data support to reveal various macroscopic shear phenomena.