| Since the reform and opening up, the economic construction of China has been rapiddevelopment. Amount of construction of highways, railways, water conservancy andhydropower facilities carry out. For all the projects, the soil is used as the basis, and definingits physical and mechanical properties under variety of environmental and humen factors isextremely necessary to guarantee the stability and security of the project. The soil is formedby a variety of rock weathering in nature. The nature of the soil cannot be controlled bywhatever human activities. Soil physical and mechanical characteristics vary because ofdifferent formation process. Therefore, different types of soil in different areas will showdifferent mechanical characteristics under the same mechanical conditions. Especially in thepermafrost and seasonally frozen areas, the complex environment impact factors make soilproperties more difficult to grasp.Domestic and foreign experts and scholars have obtained some initial results on the soilresearch. The factors which affect soil physical and mechanical properties have beenidentified including the soil mineral composition, chemical composition, soil salinity,particle size distribution, moisture content, porosity, temperature and load. For compactedsubgrade soil in seasonally frozen soil area, its influencing factors are more complex.Seasonally frozen soil area in China accounts for53.5percent of the land area. For the soilunder the climatic conditions in the region, its physical and mechanical properties willchange, thus it will affect the engineering properties of subgrade in seasonally frost regions.For the subgrade in seasonally frozen area, as the winter freezing and spring thawing, thepavement under the traffic loading is liable to break, frost boiling, mud pumping, etc. Thenthat will bring great harm to transport and economic development and the entiretransportation business will be affected seriously. The micro-structure pictures of compactedsubgrade soils of different plasticity index under different freeze-thaw cycles are collectedusing ESEM (Environmental Scanning Electron Microscope). The micro structuralparameters of soil particles and pores under each state are quantitatively counted andmicroscope mechanism is analyzed.This paper is one important part of the National Natural Science Foundation-funded project plan (50978117). The main content of the paper is as follow:1. Select three soils of different plasticity index, conventional physical properties testcarry out.In order to make the study results to the universality, three kinds of plasticity index soilare selected, they will represent all the soil that can be directly used in roadbed constructionwithout improvement. It will make the reseach results more promotional. Firstly, thephysical properties are obtained including moisture content, density, particle analysis,optimum moisture content, boundry moisture content etc. Soil mineral content and chemicalcomposition are obtained by the chemical element analysis and X-ray diffraction analysis.2. Triaxial compression test on subgrade soils of different plasticity indexSpecimen are prepared indoor to the maximum degree of compaction, then experience0to7time complete freeze-thaw cycle. Triaxial compression test carries out on the specimenunder different confining pressure. The mechanical parameters of three soils experiencingdifferent times freeze-thaw cycles under different confining pressure are obtained. Theresults show that: variation of cohesion c and internal friction angle φ with thefreeze-thaw cycle is not obvious. The variation of Shear strength and elastic modulus withdifferent freeze-thaw cycles under different confining pressure is more obvious: shearstrength and elastic modulus of subgrade increases with the plasticity index experiencing thesame freeze-thaw cycles and under the same confining pressure. Shear strength and elasticmodulus of subgrade experiencing the same freeze-thaw cycles increases with the confiningpressure. Shear strength of subgrade under the same confining pressure decreases with thetimes of freeze-thaw cycles. Elastic modulus of subgrade decreases with the times offreeae-thaw cycles, and decreases significantly at first several times then it will stabilize.Multiple nonlinear fitting is adopted to get the relationship between shear strength, elasticmodulus and confining pressure, plasticity index, freeze-thaw cycles and it shows a goodcorrelation. It will provide references for subgrade design in seasonally frozen region whichlack of relevant information.Frost heaving ratio of three kinds of different plasticity index subgrade soils underdifferent number of freeze-thaw cycles is achieved through the statistics on the geometricsize of test soil samples. After analyzing test data, the regulation between frost heaving ratioand plasticity index, freeze-thaw cycles is obtained. Under the same number of freeze-thawcycles, frost heaving ratio of subgrade soil increases with plasticity index. For the same soil,frost heaving ratio of subgrade soil increases with the number of freeze-thaw cycles. Multiple nonlinear fitting is adopted for test data. The relationship between frost heavingratio and plasticity index, freeze-thaw cycles is obtained and the correlation is good.3. Dynamic triaxial test on subgrade soils of different plasticity indexSpecimen are prepared indoor to the maximum degree of compaction, then experience0to7time complete freeze-thaw cycle. Dynamic triaxial test carries out on the specimenunder different confining pressure. At the initial stage, the dynamic modulus of the soil(including dynamic elastic modulusE dand dynamic shear modulusG d) show a downwardtrend with the increase of the load cycles. As the number of dynamic loads continue toincrease, the dynamic modulus is stable. Take the average dynamic modulus of5000to6000load cycles as the soil dynamic properties of this state. Summed up the laws as follows: thedynamic modulus of the compacted soil in seasonally frozen region increase with theincrease of confining pressure, decrease with the increase of the number of freeze-thaw cycle,increase with the plasticity index. The damping ratio has no obvious rule to follow with thefreeze-thaw cycles, confining pressure and plasticity index. The relationship of dynamicmodulus with confining pressure, plasticity index and the number of freeze-thaw cycles isobtained though data analysis. It can provide a reference for the embankment design andconstruction in the seasonally frozen region.Nonlinear fitting method is adopted to get the test value of the static and dynamicelastic modulus under the same conditions. Dynamic elastic modulus can be deducedthrough the static elastic modulus which easy to measured, it will greatly reduce the dynamictriaxis test work. It will provide fast and accurate dynamic elastic modulus data for roaddesign.4. Microanalysis tests on subgrade soils of different plasticity index experienceddifferent number of freeze-thaw cycles.The specimens are same with those in static and dynamic triaxial tests which arecompacted under the optimum moisture content to the specimens with the degree ofcompaction greater than95%. Then they will experience0to7times freeze-thaw cycles anddried by frozen vacuum sublimation drying method. Field emission environmental scanningelectron microscope (ESEM) is adopted to extract the micro-structure pictures of three soilsexperienced different freeze-thaw cycles. The Image-Pro Plus (IPP) is adopted to analyzeand process the micro-structure pictures. Parameters of morphology selected are counted andstatistical analysis is carried out. The statistical results of microscopic parameters of threesoils particles and pores experienced different times freeze-thaw cycles are gotten including the average diameter, direction angle, the average roundness, abundance and fractaldimension. Variation of each micro-structure parameter with the freeze-thaw cycles isstatistically analyzed and verify the variation of compacted subgrade soil static and dynamicparameters with the the freeze-thaw cycles based on microscopic mechanism.
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