Experimental Study And Simulation On Subgrade Soil After Freeze-thaw Cycles

Seasonally frozen soil area is about 54% of the total area in China. The freezing issue of subgrade brings about great harm to the development of transportation and economic. Subgrade is an important part of road engineering. The strength and stabilization of subgrade play an important role in ensuring safe condition of road. In seasonally frozen area, due to more complecated reasons, the freezing issue of subgrade causes greater harm to road. Besides, by reason of lacking experiences in subgrade design and construction, the research about freezing behavior of subgrade soil is not only meaningful in construction, but also of great value to research.Previous studies on the subgrade soils are generally limited to a certain kind of soil after freeze-thaw cycles. To make test results more representative, three kinds of different subgrade soil with different plasticity index was selected. One kind of soil is clay, the other two is slity clay. They are subgrade soils always used in construction. In order to find common mechanical properties of soils after freeze-thaw cycles, the triaxial tests were carried out. What's more, simulation of traxial test was applied in order to provide reference for prediction of the strength of subgrade soil after freeze-thaw cycles. Conclusions obtained from research are discussed below.1. The frost heaving of height of clay is similar to silty clay with higher plasticity index. The lowest frost heaving of them is similar, but clay has higher frost heaving when they comes to the highest frost heaving. That means clay is more sensitive to frost heaving. The silty clay with lower plasticity index is less sensitive to frost heavint than other two kinds of soils and its highest frost heaving is close to the lowest frost heaving of other two kinds of soils. Clay also shows the most sensitive to frost heaving in the freezing of volume.2. Thickness degree and mineral composition of soils affect the frost heaving. The thickness degress of soils play an important part in the previous freeze-thaw cycles and the mineral composition of soils starts to affect the frost heaving with the increase of freeze-thaw cycles. Moreover, the content of clay minerals, such as montmorillonite, plays the most important role.3. Experimental results show that the shear strength of soils increases with the increase of confining pressure. The stength of clay is similar to silty clay with higher plasticity index: the strenhth of them increases a little after the first freeze-thaw cycle; then strength starts to decrease and stablizes after the forth freeze-thaw cycle. On the other hand, the silty clay with lower plasticity index behaves differently:its strength starts to decrease just after the first freeze-thaw cycle and behaves more stably. The study of variation of shear strength under the same confining pressure indicates that the silty clay with higher plasticity index is more sentitive to confing pressure; the silty clay with lower plasticity index is less sensitive to freeze-thaw cycles and its strength tends to stabilize after the third freeze-thaw cycle.4. The cohesion variation of three kinds of soils behaves similarly to the variation of shear strength. The clay and silty clay with higher plasticity index also behave similary between the variation of friction and variation of shear strength. However, the variation of friction of silty clay with lower plasticity index has nothing to do with the variation of shear strength.5. Research results show that both clay and silty clay with higher plasticity index reach 0.7% of the frost heave after the first freeze-thaw cycle and the shear strength of them increases a little at that time; the shear strength of them reduces to the lowest value when the frost heave of them increases to 0.9% at the first time. We can conclude that 0.7% of the frost heave is maximum the intensity point for clay and silty clay with higher plasticity index and 0.9% of the frost heave as minimum intensity point.6. According to the similarity of variation of modulus, author apply function to fit the modulus of three kinds of soils. Results compared with experimental data indicate that the fitting error is within 15%. Based on the fitting of modulus, author apply Flac3D to simulate traxial test of soils under seven freeze-thaw cycles and record the variation of stress and displacement during simulation. Results compared with experiment show that the simulation error is almost within 15% and the simulation has higher accuracy for soil with higher plasticity index.