Effects Of Freeze-thaw On Shear Strength And Disintegration Characteristics Of Undisturbed Brown Soil

The Liao-he Plain was an important part of China’s temperate farmland ecosystem,and the brown soil was the main soil type in the region.There was the strong seasonal freeze-thaw cycles of the topsoil layer in the region.Freeze-thaw has the important impacts on soil erodability by influencing the friction and linkage effect between particles,particle size and distribution,structural characteristics and structural stability.In this context,soil samples with five different freeze-thaw cycles(0,1,5,10 and 15)and five different initial soil water contents(10%,15%,20%,25% and 35%)were selected in this study,taking two depths of undisturbed brown soil at depths of 5-7 cm and 25-27 cm as the study objects.Simulated freeze-thaw cycle tests are performed,followed by straight shear tests,disintegration tests,and mercury pressure tests.In order to lay the foundation for revealing the impact of freeze-thaw on brown soils,and to provide a basis for elucidating the mechanism of soil erosion.In this study,the results of the shear strength of the undisturbed brown soil under freeze-thaw conditions showed that the effect of freeze-thaw on soil cohesion would be different as the soil water content increasing,and the internal friction angle was a relatively stable soil property and was not susceptible by freeze-thaw.The freeze-thaw cycles had the over-consolidation effects on the soil samples with the soil water content of 10%,and the cohesion of the soil sample at two depths increased by 74.36% and 60.08%,respectively,under 15 freeze-thaw cycles.The primary freeze-thaw effect had a strong impact on the destruction of soil samples with the soil water contents of 25% and 35%.Strong thawing and sedimentation occurred after multiple freeze-thaw cycles of soil samples with 35% soil water content,with the cohesions increasing by 14.63% and 26.15% after 15 freeze-thaw cycles,respectively.Meanwhile,the friction angles of the two-depth soil samples with 10% initial soil water content were less affected by the first freeze-thaw cycle,and the trends were decreasing after multiple freeze-thaw cycles.The friction angles in two-depth soil samples with a soil water content of 35% were the most stable,varying among 18.57°±(0.88°)and12.86°±(1.14°),respectively.Meanwhile,in the disintegration characteristics,the disintegration process of soil samples could be divided into four stages in turn,of which the exponential disintegration stage was the mainly stage of disintegration.The higher soil water content would inhibit soil disintegration,and freeze-thaw mainly affected the length of time and disintegration rate of the exponential disintegration phase.The final disintegration rates of two-depth soil samples with the soil water contents of 25% and 35% were difficult to reach 20%.The exponential disintegration rates of soil samples at both depths of 10% and 15% soil water were shortened by increasing the pore polarization distribution during the freeze-thaw process.Under the conditions of the soil water content of 25% and 35%,freeze-thaw cyles caused sedimentation and blockage in the soil,which reduced the gas or water exchange capacity,and the total disintegration ratios of the soil sample at the two depths were also reduced.In addition,the 4 μm was the critical value of soil pore change under freeze-thaw conditions,and the proportion of pores in each segment affects the soil erodability parameters.At 10% soil water content,negative pore pressure at the frozen edge caused volumetric shrinkage of smaller pore sizes.At 25% and 35% soil water contents,freeze-thaw resulting in two depths of soil sample total pore volume increased with the increase of >4 μm pore size percentage.At 15% and 20% soil water contents,the total pore volume fluctuation trends were mainly affected by the pore size of >4 μm.According to the results of the redundancy analysis,under freeze-thaw cycles,the pore content of 0.1-4 μm increased,reflecting the increase of the soil internal friction angle,and the pore content of 4-25 μm decreased,reflecting the increases of soil cohesion and total disintegration ratio.The main innovations of the paper were to explore the erodability parameters of the undisturbed brown soil under freeze-thaw cycles and obtain the experimental results with more complex variation laws.The macroscopic property differences such as soil shear strength and disintegration characteristics were explored through the changes in the microstructure as soil pores distribution.The experimental results provide new ideas for soil erosion research.