Study On Pore Characteristics And Aggregate Stability Of Black Soil Under Freeze-thaw Conditions

Soil structure is the arrangement of primary,secondary and interstitial pore in soil.Soil structure is the basis of maintaining soil function and an important factor affecting the erosion process.The black soil region in Northeast China locates the mid-latitude region.The periodic changes of seasonal melting layer and permafrost layer are very common in late autumn and early spring,which have a strong impact on soil structure.Therefore,in this study we used the typical black soil in Northeast Black Soil Region as the research sample,using CT technology combined with image processing method and Le Bissonais method,the effects of freeze-thaw cycles and soil moisture contents on the stability of undisturbed soil and aggregate pore characteristics were analyzed.Our research provides the insight for further revealing the influence of seasonal freeze-thaw on the structure of black soil,and provides a basis for clarifying the mechanism of freeze-thaw erosion,as well as for reasonable evaluation and utilization of farmland in cold regions.In this study,the pore size of the soil larger than 1.25 mm showed that the soil porosity increased with the increase of freeze-thaw cycles in 15 freeze-thaw cycles,especially for the soil samples after 7 freeze-thaw cycles.The pore number decreased with the increase of freeze-thaw cycles,the increase of pore size showed an increased tendency and the pore size increased with the increase of freeze-thaw cycles.The freeze-thaw action caused a large number of branches in the pore,and the more the freeze-thaw cycles were,the larger the proportion of branches was.When the freeze-thaw temperature difference was constant,the pore characteristics of high moisture content soil mentioned above change more significantly than that of low moisture content soil under freeze-thaw cycles.The study of aggregate microstructures larger than 3.25 ?m showed that with the increase of freeze-thaw cycles,aggregate porosity increased and tended to be stable after 15 freeze-thaw cycles.The proportion of extended pore porosity increases,contrary to the trend of regular and irregular pore.> 100 ?m pore proportion increased continuously.Pore connectivity improved significantly after 7 freeze-thaw cycles and the number of pore nodes increased with freeze-thaw cycles.The number of melting cycles increased,while the number of pore and fractal dimension did not change significantly.High moisture content had a significant effect on the pore structure of aggregates,but the moisture content is 40% showed different regularity of 30% and 50%,which needed to be further explored.In addition,the stability of aggregates under freeze-thaw conditions was affected by the number of freeze-thaw cycles and moisture contents,showed different regularity.With the increase of freeze-thaw cycles,the content of aggregates with particle size less than 5 mmincreased during the freeze-thaw process of undisturbed soil,it was obvious that the phenomenon of large aggregates breaking into small aggregates was obvious.Moisture contents had a certain effect on particle size distribution,the breakage of large aggregates under high moisture content was more obvious.Under the conditions of fast wetting and pre-wetting and stirring treatments,the stability of 5–7 mm aggregates was studied.The results showed that with the increase of freeze-thaw cycles,the content of large aggregate decreased and the content of micro-aggregate increased.The mean weight diameter decreased and the variability decreased with the increase of freeze-thaw cycles.Under the same freeze-thaw cycles,the 50% moisture content had a greater impact effect on the aggregate indexes compared with 30% and 40% moisture content.There was a correlation between pore characteristic parameters and stability.Porosity,>100 ?m porosity,and extended pore reduced aggregate stability.Regular porosity,irregular porosity,<100 ?m porosity,euler number,and fractal dimension improved aggregate stability.The main innovation of this paper is to combine CT technology with non-destructive scanning of soil and interior aggregate,and to use image processing technology to realize three-dimensional visualization of interior soil structure,which makes the study of soil pore structure more direct and quantitative.Pore characteristics and stability are important factors for describing soil structure.It is particularly important to explore the influence of pore characteristics on soil stability.Quantitative analysis of soil pore parameters provides a method for further study of soil structure,and provides the insight for the study of storage and transport of soil moisture and gas by freeze-thaw cycles.