A Study On The Mechanism Of Microstructure Of Frost Heaving In Subgrage Soil In Seasonal Frost Zone

Seasonal frozen soil is one kind of frozen soils which keep frozen states for one half of month to a few of months. In china, the area of seasonal frost soil is about 513.7×10~4km~2, which accounted for 53.5% of our country. One-third of the country was deepness frozen soil which distributes in the three provinces of Northeast China, Inner Mongolia, Gansu, NingXia, north of XinJiang, QingHai and west of Sichuan. Thus, the development and utilization of seasonal frost soil is very important for China's economic construction and it has profound impact on china's exploitation of natural resources So study on seasonal frost soil has great significance.In order to develop Northeast of china rapidly, it will need to build more and higher highways in these areas. However, there is an important problem which had troubled the highway construction in these areas, it is roadbed freeze damage. Roadbed frost can induce road damage, which disturb traffic, bring a tremendous economic losses and damage to the country and the people. For the sake of prevention and cure the damage that caused by roadbed frost heave, it is urgent to study frost heave of roadbed soil to settle this problem in seasonal areas. However, in our country ,existing information of the road works in cold areas was mainly directed against the permafrost regions, and studies on problem of road damage that caused by roadbed frost heave in seasonal frost zone are relatively shallow and scattered ,specially on the highway. Therefore, studies on roadbed soils frost heave in seasonal frost zone have very important significance and urgency on the project.At present, though many studies on seasonal frost heave soil have been done, there still have many problems. Firstly, emphasis was put on the macro factors that impact frost heave of roadbed soil, such as properties of soil itself, temperature, moisture and moisture migration issues, power which drive water in soil movement, migration coefficient ,and quantities of migration, and so on. Secondly, many mathematical frost heave models have been established are based on macro factors considered on the basis of coupling model, such as the numerical simulation model for coupled moisture and heat of frozen soil under different freezing conditions; the couple model of heat, water and stress fields of saturated soil during Freezing; the model of water, heat, and stress fields of frozen soil; the couple model of three fields of unsaturated soil. In these models, the main considerations are freezing temperature, properties of soil, water content, load and so on. However, studies on microscopic mechanism of frost heave of roadbed soil are relatively fewer, the model also contains little micro-structure parameters. In fact, there are ample facts shows that the engineering characters of soil work to a large extent by the micro-system control. Soil macro acts, in essence, is a series results of micro-structure changes. Therefore, it's necessary and meaningful to study microscopic mechanism of frost heave of sub-grade soil, to compare changes of microstructure parameters during freeze-thaw cycles and analyze the reasons for any changes, and to evaluate relationship between frozen-heave factor and micro structure parameters. Micro-structure parameters which have far-reaching impact on frost heave were selected, and frost heave model including micro-structure parameters was established which are added to the existing research.In view of the above, the paper studies seasonal frozen soil in Chang-Chun to research micro structure change in freeze-thaw circle, applied with grey relation analysis method and rough set theory to estimate the soil micro structure effect on its frost heave, discuss correlations between micro structure parameters and frozen-heave factor. Choose micro parameters which have higher associated coefficient, and finally based on selected micro parameters, set up frost heave model with BP neural network and indicated seasonal frozen soil heaving mechanical in micro structure level.Following study was done in this paper:Firstly, soil physical, chemical quality trials were done by indoor tests. The particle component of roadbed soil researched mainly is silt and clay , which is most advantage to frost heaving and supplied good condition to film water storing and translating; mineral component includes raw mineral quartz and feldspar; clay mineral; water soluble salt and organic content is less; total amount of easy solution salt is less than 0.1%,which has less effects on soil frozen heaving; soil specific surface area is general little, with which basic ion exchange capacity varies proportionally; temperature has less effects on specific heat of soil frame.According to results of orthogonal experiments of frost heave in different frozen temperature, water contentand compactness, with temperature declining, soil sample shrinks in some level firstly and then heaves significantly; soil frost heave raised when its water content inclining; and compactness raising intents heaving effect.Secondly, qualitative describing and quantitative analyzing on roadbed soil frost heaving were done, 18 structure parameters including size, shape, direction of micro pore and structure unit body were studied. Micro structure exhibits flocculent, agglomerating and two type crossing mainly. Diameter of unit body distributes from 2~10μm mainly; shape fractal dimension is 1.0838~1.2956; abundance ratio is 0.4~0.9. Particle shape is oblate, and presents no obvious direction. Pore aperture lines in three interval, that is less 1μm, 1~2μm and 2~5μm, pore form fractal dimension is between 1.197~1.342, pore distribution is relatively uniform. Abundance ratio lines between 0.3~0.8, that is to say pore presents oblate mainly. And pore presents no fixed direction.In a frost-thaw circle under the condition of different frost temperature, compactness, water content, micro pore and unit body in soil both changes. The connectivity of micro-pore improves when soil samples are freezing or have been frozen. In a frost-thaw circle, the pore size is the largest in frozen progress and the smallest before frozen. Compared with before freezing, small pore content (aperture less than 1μm) decreases and other pores which aperture lines in 1-2μm, 2-5μm and 5-10μm increases when freezing. More lower temperature, more increasing amplitude. Pore shape fractal dimension lines between 1.211~1.355, that means around large pore there gather much small pore; abundance ratio lines between 0.5~0.6 increases in middle of frozen and after frozen, different increasing amplitude in different frost process; Pore distributes in some fixed direction. Pore's anisotropic radio is more sensitive to outside frost condition such as frozen temperature, compactness and water content.In the middle frozen process, soil structure unit comes to aggregate more easily, and after thaw, aggregation phenomenon fades. Particle content which aperture lines in 2~5μm and 5~10μm that silts increases. Micro unit abundance is from 0.4 to 0.9, that means shape of soil structure element is oblate mainly, and the lower temperature, the more oblate. Unit morphologic fractal dimension is 1.0636~1.216, less than original data in some times, that unit shape simplifies and less small particle aggregates around large particle.Unit assemblage extending angel focuses on 70-90 degree relatively, that presents strong directionality effect.Thirdly, associating macro frost-heave factor with micro structure parameters, grey relativity method was used to analyze the relations between micro structure parameters (pore's and structure unit) and frost heave. The computation data shows that the relativity coefficient of mean abundance, mean circularity, mean shape coefficient, shape distribution fractal dimension, mean anisotropy, directional probability entropy, direction fractal dimension of soil pore and structure element was more than 0.8, which indicated that those factors were significantly relative to soil frost heave. And that pore anisotropy raised with frozen temperature declined, which means pore anisotropy was more sensitive to frozen temperature and the total direction of soil micro pore was more significant to frozen heave factor.Rough set theory was applied to reduce relative effect factors including 10 groups of pore parameters and 3 groups of structure unit parameters. In the progress of reducing 10 groups of pore parameters, each factor was found in set, which indicated that micro pore effects soil frost heave deeper and only 4 structure parameters were retained in computation. Those means that when frozen, as water store space and transportation path, micro pore effects frost heave more significant than micro structure element and play mast role.In this paper 8 parameters which effect most significant on soil frost heaving, including pore's mean anisotropy, shape distribution fractal dimension, mean abundance ratio, direction fractal dimension; structure element's mean anisotropy, shape distribution fractional dimension, mean shape coefficient, description probability entropy, associated with 3 frozen condition factors, 11 effect factors were used to set up BP frost heave model based on 3 layers adaptive decreasing neural network. The model exhibits the effect of micro pore and structure unit and exactly describes and predicts frost heave of roadbed soil. It is a feasible method to research complex nonlinear micro mechanics of frozen roadbed soil in seasonal areas. According to this method, the error of model prediction result is between 0.05~0.2mm, which is relative less,and shows that the established model can predict frost heave accurately.