| Because of the frsot heave and thaw collapse of the railway subgrade , obvious difference can be seen between the designed elevation and the actual elevation of the railway track, and the track profile irregularity is one of the important factors affecting running safety and speed of train. The safety of the high-speed train is affected by the frost damage of the railway subgrade in China's north frozen soil zone. So it is very important for us to do some research on frost susceptibility of the subgrade soils under dynamic loading of train and on freezing damage of high speed railway subgrade, which will have great practical value for antifreezing design of high speed railway subgrade and prevention of frost damage of existing subgrade. In this paper, the valuations on frost damage of Beijing-Baotou railway and Baotou-Lanzhou railway subgrade are given on the basis of climatic and geological conditions, engineering character of subgrade soils and frost-heave deformation monitoring of subgrade. The hydraulic loading system is developed and used to study the frost heaving characteristics of the subgrade soils under dynamic loading in closed system and open system . On the base of these work, the conclusions are as follows:1. Because of the the fine-grained fill, the frost deformations of Beijing-Baotou railway subgrade and Baotou-Lanzhou railway subgrade are holistically irregular, and great discrepancies exist among the frost heavings of subgrade at different sections. The maximal frost heaving ratio of subgrade soil is 3.3%.2. During the course of soils frozen, the freezing rate of the soils under static load or dynamic load is greater than that of under load-free, but the maximal freezing depths of soils are almost equal.3. In closed system, the amount and ratio of frost heave of soils under static load are the least, and the amount and ratio under dynamic load are approximately equal to them under load-free. The variation trends of frost heaving ratio of soils under 3 different types of loading are similar, and the frost heaving ratio of soils increases firstly and then decreases with cooling time at the lower cooling temperature. Loading frequency have no siginificent infiuence on the frost heave behavior of soils. The frost heave ratio of soils reach to the maximal at the cooling temperature of-1.5℃.4. In open system, the frost heave ratio of soils under load-free is greater than that of under static load and under dynamic load, and the frost heave ratio of soils under dynamic loading is approximately equal to that of under static load which is equal to the mean value of the dynamic loading. The frost heave ratio of soils is not affected by the change of loading frequency. The frost heave ratio and rate of soils decrease with the loading increasing, and the frost heave rate of soils is inversely proportional to loading basically when thr freezing rate is small.5. In open system the relation between the moisture inflow velocity and the freezing time can be described by v = ae-bt when the freezing rate is small, and the moisture inflow velocity of freezing soils under dynamic loading is approximately equal to that of under static load which is equal to the mean value of the dynamic loading. The moisture inflow velocity decrease with the loading increasing.6. In open system, the amount of moisture migration is the most when the soils under load-free frozen than that of under dynamic or static loading frozen, and greart difference is observed among the water contents of soils under 3 differrent types of loading at the maximum freezing depth.
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