Modeling the Freezing in Coarse Grained Sands on a Microstructural Level

The mechanical properties of frozen soils depend on the structure of and the interaction between four phases: solid particles, ice crystals, unfrozen water and gas. Laboratory investigations on coarse grained sands show that the thermal conductivity of the various phases control the advance of the freezing front and the growth of the ice crystals on a microstructural level within the soil for different temperature conditions. The consequence is that the structure of the frozen soil changes until the soil reaches steady state thermal conditions. One-dimensional freezing tests further showed changes in the growth of the ice crystals in saline pore water due to solute rejection during phase change of the water. Based on these laboratory investigations numerical modeling was performed using finite elements. The numerical modeling allowed for investigating the effects of the four phases and their heat conductivity ratios on one- and two dimensional freezing. The modeling further confirmed that depending on the temperature gradient and freezing rates, different structures within the frozen soil are to be expected. Even though these numerical investigations were solely based on thermal modeling, i.e. changes in stress due to freezing were ignored, qualitative conclusions can be drawn about the variation in strength and creep behavior of a coarse grained frozen soil based on its thermal history. Depending on the volumetric content of the components and the heat conduction ratios between the four different constituents a freezing rate exists at which the freezing front advances uniformly to freeze the sample. On the other hand, rapid freezing results in a heterogeneous structure of the frozen soils and pockets of unfrozen water must be expected, which will weaken the partially frozen soil.