Study On Frost Heave Characteristics Of Turfy Soil In Seasonal Frozen Area

Turfy soil is a kind of special soil widely distributed in the northeast of China,Qinghai Tibet Plateau,Xinjiang mountain area and other seasonally frozen areas.It is the product of many years' accumulation of incomplete decomposed plant debris in the water rich environment,with the characteristics of high moisture content,high organic matter content and low decomposition degree.The plant debris constitutes most of the skeleton structure of turfy soil.Turfy soil exists in water rich environment,such as swamp.Its existence is not only of great significance for regulating the surrounding climate and maintaining ecological balance,but also challenges the construction of buildings and lines on turfy soil wetland due to its special composition and structure.Because the moisture content of turfy soil in the seasonal frozen area is much higher than that of clay and other soil bodies,it has special causes and structures.The phenomenon of frost heave in winter and thaw settlement in spring must be different from that of clay,sand and other common soil bodies.Therefore,it is necessary to study the frost heave characteristics of turfy soil in the seasonal frozen area in order to provide theoretical basis and design basis for the construction of geotechnical engineering in the turfy soil area.Based on the National Natural Science Foundation Project "study on Frost Heave Mechanism of swamp turfy soil in seasonally frozen area",this paper selects the sample of turfy soil located in Dunhua City,Jiangyuan County,Jilin Province as the research object,and obtains the basic physical properties and characteristic properties data of turfy soil in this area through laboratory test.In the experiment,the moisture content data obtained by low-temperature drying method,the organic content data obtained by burning method,the decomposition data obtained by mass ratio method,and the porosity data obtained by image analysis method are respectively used.Four temperature gradients are set to carry out one-way frost heave test under closed environment for turfy soil,and the following conclusions are drawn:(1)The moisture content of turfy soil is very large,but the frost heave rate is smaller than some common soil.(2)The frost heave rate of the turfy soil is linearly positively correlated with water content,organic matter content and porosity,and linearly negatively correlated with decomposition degree.(3)The five factors that influence the frost heaving characteristics of the turfy soil are sorted according to the degree of influence: water content,temperature,organic matter content,decomposition degree and porosity.SEM images of the turfy soil samples were obtained by scanning electron microscopy(SEM),and the microstructure and pore characteristics of the turfy soil were analyzed qualitatively and quantitatively.Qualitative analysis showed that the microstructure of the turfy soil was loose and there were obvious plant structures in the soil.A large amount of uncompleted plant debris constituted the main framework of the turfy soil.The porosity of the turfy soil was very irregular and pore gaps were large in horizontal and vertical directions.Point-to-point contact and line-to-line contact were the main types of irregular aggregates.SEM images of turfy soil samples were obtained by SEM,and the pore structure characteristics of turfy soil were analyzed qualitatively and quantitatively.Through qualitative analysis,it can be found that the microstructure of turfy soil is loose,there is very obvious plant structure in the soil,a large number of incomplete decomposition of plant debris constitutes the main skeleton of turfy soil.The porosity of turfy soil is large,and the porosity is very irregular.The difference between horizontal and vertical porosity is large.The main contact between irregular aggregates is point to face contact and line to face contact.Professional image processing software Image-Pro-Plus was used to extract four shape parameters,namely,porosity,pore area,pore flatness and fractal dimension,to quantitatively analyze the microstructural characteristics of the turfy soil‘s holes.It was found that there was a significant difference in pore morphology,pore quantity and the fractal dimension between the horizontal direction and the vertical direction of the turfy soil.Horizontal porosity was slightly larger than vertical porosity.The number of horizontal pores was smaller,but the pore area was larger and the fractal dimension of the pore profile was larger,too;the freeze-thaw cycle had little influence on the micropore characteristics of the turfy soil.Finally,frost heaving prediction models of the turfy soil were established respectively,based on BP neural network and Support Vector Machine(SVM).It was proved that both models could accurately predict the frost heave rate of the turfy soil.However,SVM prediction model took less time,required less debugging and training on models,and applied smaller sample size,so SVM prediction model is better.