Study On The Characteristic Of Frost Heave And Thaw Settlement Of Lanzhou Silty Clay

In this article, silty clay, a typical soil in lanzhou city metro engineering construction, was taken as the research object. First, the conventional geotechnical test was carried out to determine the basic physical indexes of the soil and the soil was proved to have the frost heave characteristic by test. Then、, freezing temperature test was conducted for the soil under the conditions of different moisture content, dry density and salt concentration. Finally, using the freeze-thaw testing system in state key laboratory of frozen soil engineering, single factor test and orthogonal test on the soil of different levels were carried out with the factors including moisture content, dry density, cold end temperature, external load and freezing conditions, which influences the frost heave and thawing settlement characteristic. The forecast model of frost heave and thawing settlement for lanzhou silty clay were also put forward. The research results will provide reliable reference basis for frost heave factor and thawing settlement coefficient for silty clay in lanzhou city metro engineering construction and the application of artificial ground freezing under such layers of soil condition.The main research results are as follows:(1) The mechanism of frost heave and thawing settlement of artificial freezing soil was studied and the factors influencing frost heave and thawing settlement were analyzed, which will provide corresponding theoretical foundation for subsequent test research.(2) Through freezing temperature test, the change rule of freezing temperature was acquired under the condition of different soil moisture content, dry density and salt concentration and the linear fitting formula are established, which will guide the calculation of thickness of freezing wall and determining the excavation time.(3) Through the single factor experiment, the influencing rule of the factors including moisture content,dry density, cold end temperature, external load and freezing condition on frost heave factor and thawing settlement coefficient was gained. The regression equation of the rule was established. Based on the mechanism of frost heave and thawing settlemet conbined with the test results, the change rule of temperature field in the freezing-thaw process and the moisture migration characteristic under closed system and open system via unidirectional frost and thawing were gained.(4) Through the orthogonal test and range analysis, the ranking of influence of the three major facrors including moisture content, dry density, cold end temperature on frost heave factor and thawing settlement coefficient was ganed. The combination of the value of the three factors for the largest frost heave and thawing settlement was given out within the scope of the selected parameters.(5) Based on the analysis of the experimental data, the multiple linear regression equation of frost heave rate and thawing settlement coefficient was established by using SPSS statistical analysis software under the closed system condition. Finally, the frost heave and thawing settlement forecast models of lanzhou silty clay were established and through the comparison between measured and predicted results, the reliability of the models were testified.