Study On Meso-structure And Mechanical Characteristics Of Turfy Soil In Seasonally Frozen Area

Turfy soil in the seasonal frozen area is a kind of humic acid-rich humus soil which is transformed from the plant residues in the surface swamp environment under the action of oxygen and microorganism under the unique climate,landform and hydrological conditions.Compared with most mineral soils,turfy soils with a lot of plant fibers have the characteristics of large void ratio,high water content,high permeability,high compression,low strength and poor engineering properties.In recent years,people gradually realize that the macro engineering properties of soil are affected by its microstructure and change rule.The research gradually shifts from the macro mechanical level to the micro-level,and attempts to establish the relationship between them.As a kind of unique soil with extremely complicated internal structure,the microstructure of the turfy soil in the seasonally frozen area has a dominant control on the macro strength,deformation characteristics and seepage characteristics of the soil.However,there are few studies on the microstructure of turfy soil.Also,the research on the mechanical properties of turfy soil mainly focuses on the macro scale,while the micro research on the mechanical mechanism of turfy soil is relatively insufficient.The research on the microstructure and mechanical properties of the turfy soil in the seasonally frozen area will not only help us to fundamentally reveal the internal mechanism of the macro mechanical properties and phenomena of the turfy soil but also help us to better solve the geotechnical design and wetland environmental protection problems faced by the engineering construction of the turfy soil area.Based on the field sampling and laboratory test,the micro CT scanner was used to scan the soil sample and obtain the CT image of the internal microstructure of the turfy soil in Jiangyuan town,Dunhua City,Jilin province.Then the CT images were processed by digital image processing.The micropore structure characteristics of the turfy soil were analyzed quantitatively at the end.In this paper,the microstructure analysis of turfy soil is divided into two parts.One is to analyze the laws of micropore structure of original turfy soil with different decomposition degree.The other is to study the changes of micropore structure of turfy soil with the number of freeze-thaw cycles.The DEM numerical simulation technology has a significant advantage in understanding the mesoscale mechanism of soil.In this paper,the three-dimensional discrete element model of turfy soil is constructed by using the particle flow code,and the triaxial compression test of numerical samples of turfy soil is simulated.Then the meso mechanism of mechanical properties of turfy soil under different confining pressure and different fiber arrangement(horizontal,vertical and random arrangement)is analyzed.The main research contents and achievements of this paper are as follows:(1)The micro CT scanning technology was used to scan the original samples of turfy soil to obtain the images of the micro structure of turfy soil.Based on the digital image processing technology,the gray-scale histogram equalization,non local mean filtering and watershed image segmentation of the images of the turfy soil were successively carried out to complete the binary processing of these images.At the same time,the micro structure of turfy soil was visualized in three dimensions,and its micro structure characteristics were analyzed qualitatively.(2)The representative unit which can reflect the micropore structure of turfy soil is determined,and the pore geometry characteristics of two groups of original turfy soil samples with different decomposition degrees are extracted and analyzed.The results show that the two-dimensional porosity of the fault image can represent the threedimensional porosity.Based on the shape factor to classify the pores in the turfy soil,the number of quasi-spherical pores and ellipsoidal pores accounts for the most in the turfy soil,which belongs to the small pore.In contrast,the long column pores and branch pores,although the number accounts for the least,belong to the large pore,which plays a leading role in the permeability characteristics of the turfy soil.Also,the pore size of the high-resolution turfy soil sample is relatively small and has a higher specific surface area.The calculated anisotropy of different decomposition degree of turfy soil samples is lower,which shows that the pore system of small size turfy soil samples tends to be isotropic.(3)The pore topological characteristics of two groups of undisturbed turfy soil samples with different decomposition degree are analyzed,and the permeability is obtained by numerical simulation based on the CT image of turfy soil samples.The results show that the permeability of turfy soil varies with the decomposition degree of turfy soil,and the permeability of low decomposition degree turfy soil is higher than that of high-resolution turfy soil.On the one hand,it is attributed to the larger pore size in the low decomposition degree turfy soil sample.On the other hand,the higher pore coordination number and connectivity density,and the smaller geometric and hydraulic tortuosity,which also means that it has better connectivity,making the pore water flow easier.(4)CT scanning imaging can directly reflect the change characteristics of the pore structure of turfy soil before and after freezing and thawing and reveal the pore evolution law of turfy soil.The results show that with the increase of freeze-thaw cycles,the porosity and the number of pores of turfy soil increase,the percentage of small pores increases gradually,while the portion of medium pores and large pores decrease.Freeze-thaw cycles also increase the complexity of the pore structure and enhance the connectivity of pores.(5)Based on PFC3 D,a three-dimensional model of turfy soil was established,and the triaxial numerical simulation under different confining pressures was completed.By comparing the strength characteristics of three kinds of turf soil under confining pressure,the meso performance of contact force chain network,displacement field,velocity field and coordination number in the numerical model of turf soil is analyzed,which further deepens our understanding of the meso mechanical behavior of turf soil.Numerical simulation shows that confining pressure has a certain influence on the contact force distribution and bearing mechanism of turfy soil.The particle displacement field and velocity field of turfy soil in each stage of loading are axisymmetric outward distribution,which makes the lateral deformation of turfy soil sample mainly,which explains the reason why the shear zone will not be formed in turfy soil sample from the micro perspective.In addition,with the increase of confining pressure,the average normal contact force and interface contact area acting on the fiber surface also become larger,so as to generate greater friction along the surface of plant fiber,mobilize the greater tension in plant fiber,and make greater contribution to the strength of turfy soil.(6)Because of the different arrangement of plant fibers in natural turfy soil,the structural anisotropy of turfy soil will affect its mechanical properties.Based on the three-dimensional particle flow model of turfy soil,the influence of fiber arrangement direction on the triaxial compression test strength of turfy soil was analyzed.By comparing the contact force chain network,contact fabric and coordination number of three kinds of numerical models,we have a deeper understanding of the meso mechanism of structural anisotropy of turfy soil.The results of numerical simulation show that the presence of plant fibers with different orientations has certain influence on the contact force distribution and bearing mechanism of turfy soil.The increase of the normal force and the effective contact area around the fiber body is the largest in the numerical sample of the turfy soil with the plant fiber arranged horizontally,which leads to the largest sliding friction between the fiber and the turfy soil aggregate particles.It is obvious that most of the plant fibers arranged in horizontal direction will be stretched under triaxial compression.When the fiber orientation is in the direction of tensile strain,the effect of plant fiber on the strength of turfy soil is the highest.These findings expand the current understanding of meso mechanical behavior of turfy soil.