Study On The Variation Of Physical Properties Of Loess Under Freezing-Thawing Cycles

Loess is widely distributed in region of seasonally frozen soil in Midwest China. It's commonly used as one kind of construction materials in this area. But in the lifetime, loess construction suffers destructive active of freezing-thawing cycles inevitably. Strong weathering processes of freezing-thawing changes the structure of loess. It's one of reasons that lead to the loess physical and engineering properties'deterioration. As the rapid economy development, the western development strategy will bring more loess engineering construction in the cold region. Thorough study of the variation of physical properties of loess under freezing-thawing cycles is imperative.Relying on the project of National Natural Science Foundation "The Research on Properties Deterioration Mechanism of Loess Engineering and Life Prediction of Loess Structures" (N0.50779058) and the project of Opened Foundation "The Research on influence and quantification of repeat freezing-thawing cycles on the engineering properties of loess"(SKLFSE200803), this paper researches the variation of physical properties of Q3 loess in Yang-ling under freezing-thawing cycles. Experiments show that:(1) According to the experiment of volume change under closed system, the volume change curves of soil samples are analysized. Soil samples aren't frost heaving and thaw collapsing. In the process of freezing, soil sample frost heave and then shrink. But in the process of thawing, the curves of volume change vary as the initial condition and the times of freezing-thawing change; the higher the initial moisture content, the change of soil volume curve more obvious as the times of freezing-thawing cycles growing; dense soil samples expand at the first and then shrink in the whole process of thawing, but loose soil samples only expand while thawing in the late period of freezing-thawing.(2) The influence of freezing-thawing on the frost heaving and thawing shrinking is analisized. It was found that: the volume change of dense soil samples grows as the times of freezing-thawing cycles and initial moisture content increase; under optimum moisture content, the volume change grows of soil samples as the density rises after iteratively freezing-thawing; and loose soil samples consolidate in the process of iteratively freezing-thawing. Weathering processes of freezing-thawing is stronger in the first five times freezing-thawing and then fade after 30 times freezing-thawing. According to the data of soil deformation, a mathematical model and characteristic parameters are obtained to reflect the relationship between volume change and times of freezing-thawing cycles by data fitting.(3) Based on the data of permeability test after iteratively freezing-thawing, the curves of soil samples'permeability coefficient under different initial conditions are obtained. Destructive effect of freezing-thawing strengthens, and it would take more times of freezing-thawing cycles to tend to be dynamic stability, as initial water content growing; variation of permeability had different tendency as dry density changing in the process of freezing-thawing; after 30 times freezing-thawing the permeability tend to be constant.(4) By taking the freezing-thawing cycles test under open system to study the law of the volume change of loess, the result indicates that: in the process of iteratively freezing-thawing, soil deformation accumulates; destructive effect to the structure of loess by freezing-thawing is strongly in 5 times freezing-thawing; after 10 times freezing-thawing, soil deformation tends to be stable, but destructive effect to surface layer of soil accumulates continuously; on the basis of analisizing the data of volume change, an empirical formula is suggested to reflect the relation of volume change and times of freezing-thawing cycles.