| Laboratory test, finite volume simulation, theories such as thermodynamics have been used to study one-dimensional soil freezing problems without out pressure systematically. Major issues during soil freezing process including water and moisture simulation, mathematical model of frost heave and its calculation, as well as frost heave control and its mechanism were analyzed thoroughly.Harlan's model involving heat and mass transfer has been wildly used to simulate water and temperature distribution during soil freezing process when no obvious ice lens appears, which also means no apparent heave occurs. Problems with constringency and non-necessary conditions in present simulations were pointed out, after that, a new full-implicit scheme based on finite volume method was established, details of which including time step and discretization of special nodes were introduced. Freezing process of silica flour and Zhang ye loam columns were simulated, and results of temperature and moisture were in general agreement with experimental data. The study shows that: water redistribution happens mainly between unfrozen sect and freezing front because water flow in the frozen soil is negligible, and water accumulation behind freezing front is intense while the front is advancing slowly; medium columns with different penetrability differ in sensitivity to ice impedance factor, and columns with higher penetrability such as silica flour column are more sensitive to impedance factor; equivalent capacity model simplified from Harlan model can be used to calculate temperature change when initial water content is low and water current is negligible, but for soil freezing with high moisture content, the effect of water flow should be considered for temperature calculation.In order to establish the mathematical model of ice lens evolvement, the growth process of ice lens during soil freezing course was analyzed firstly. Segregation temperature analog approach which also describe the steady state of ice lens growth was introduced for more general situations by extending the skills of Konrad's SP theory, and this approach is of potential use to establish simple engineering frost heave models. By replacing the actual process of ice lens growth with a series of steady state, which was defined as quasi-steady process, we constructed quasi-steady lens growth model; under the assumption of the model, the growth velocity of ice lens is uniquely determined by segregation temperature, and the relation curve between the two is called growth curve of ice lens; For more general process of ice lens growth, a new model involving coupled heat and mass transfer has been established based on the thermodynamic theory of absorption water film nearby a substrate, particularly the indispensable conditions for ice lens growth were considered, and the necessity of the existence of frozen fringe was analyzed from two different perspectives. After the importance of separation pressure induced by unfrozen water film during the ice lens formation process was analyzed, we proposed a method to calculate average separation pressure, and indicated that when the average separation pressure exceeded the strength of the soil under no out pressure system, new ice lens emerged. A new separate ice frost heave model involving heat and mass transfer was finally established, and the theoretical system and historical system of the model were introduced. The simulations of the model were in general agreements with experimental results from Xu's tests and Konrad's tests, which stimulates further research greatly.For the problem of using different freezing to control frost heave, one dimensional soil freezing tests in open system including two kinds of freezing modes were conducted in three-end refrigeratory cell, and experimental data of temperature field and frost heave were obtained. Comparison of frost heave between two freezing modes indicates that: the advancing velocity of freezing front has a great effect on frost heave, and for the soil we used there was no apparent frost heave during the initial stage of freezing process, in other words frost heave occurred after freezing front stabilized; For intermittent freezing mode of control frozen depth, the curve shape of the frost heave after its appearance is stepwise, which is different from continuous freezing mode, therefore frost heave is greatly prevented, and this intermittent freezing mode is an effective mode to control frost heave. After the final lens is pointed as key lens of controlling frost heave when traditional continuous freezing mode is applied, we explained the mechanism of using intermittent freezing mode to control frost heave by using the qualitative characteristic of ice lens growth curve, we indicated that during the intermittent stage, the segregation temperature of the final lens increases, which causes the state point descending on the ice lens growth curve, and average growth velocity of final lens is greatly constrained, thus frost heave has been controlled effectively. The mechanism of using intermittent freezing mode to control frost heave was analyzed more thoroughly by the simulation of the new coupled model of heat and mass transfer of ice lens growth, we explained the stepwise shape of the frost heave curve, indicated that during the intermittent phase the disappearance of frozen fringe caused the final lens to stop growing, and the cognition is of great importance to further research on frost heave control.
|
PDF Full Text Request