Investigation and calculation of frost heave considering specific boundary conditions of ground freezing

常规的实验室冻胀试验是通过从样品的顶部到底部冻结圆柱形土壤样品来进行的（参见道路建设）。与这些试验不同的是，在城市建设项目中应用的地面冻结通常从土体内部开始，并朝着土壤表面的相反方向发展。这首先会导致孔隙水体积膨胀9%，此外，对于易冻土壤，还会形成冰透镜体，从而导致表面额外的土壤隆起。到目前为止，还没有研究考虑向上冻结方向。因此，在这些条件下冰透镜体的形成是未知的。对于冻土的冻胀计算，必须考虑冻结过程中复杂的物理过程。这就需要建立一个考虑孔隙-热-水-力耦合作用的模拟模型，以确定和描述水平冻结管冻结引起的易冻性土的冻胀特性。本研究项目的一部分是模拟，不同于传统的测试，冻结方向朝向地面顶部的冻胀试验。冰透镜体的形成发生在冻结区上方，在冻结区中，流向冻结线的水流不仅受到真空的影响，而且受到重力的影响。与冻结方向向下的试验相反，冻土的隆起方向与水流方向相反。实验测试验证了这对冰透镜体的形成有多大影响。通过这些试验，研究了土壤样品中水分的时空分布。在参数研究范围内，将确定相关参数的影响。此外，还将进行标准的冻胀试验，以比较产生的隆起以及形成的冰透镜体，从而消除由于冻结方向而产生的差异。此外，将开发一个多相数值模型来模拟各种物理效应和定义的初始和边界条件下的冻胀。建立了基于多孔介质理论的多相流模型，并利用相场法（PFM）对该模型进行了扩展，以模拟多孔介质中冰的形成和从孔隙水到冰的相变。TPM-PFM模型考虑了由于冰的形成而减少或阻止的孔隙流体的流动以及由于从水到冰的相变期间体积增加而导致的固体骨架的有限变形。通过室内的冻胀试验，对考虑上向冻结和下向冻结的冻结过程数值模型进行了标定和验证。

Conventional frost heave laboratory tests are performed by freezing a cylindrical soil sample from the top towards the bottom of the sample (cf. road construction). In contrast to these tests, the ground freezing applied in inner-city construction projects usually starts from inside the soil mass and grows in the opposite direction towards the soil surface. This causes at first a 9 % volume expansion of the pore water, and in addition for frost susceptible soils, the formation of ice lenses which results in additional soil heave at the surface. Until now there are no investigations which consider the upward freezing direction. Thus, the formation of ice lenses under these conditions is unknown. For the calculation of frost heave, the complex physical processes during freezing must be taken into account. This requires a simulation model, which focuses on the poro-thermo-hydro-mechanical coupling phenomena.The aim of this project is to determine and describe the frost heave behavior of frost susceptible soils caused by ground freezing with horizontal freezing pipes. Part of this research project are frost heave tests which simulate, different to conventional tests, a freezing direction towards the top ground surface. The formation of ice lenses occur above the frozen zone where the water flow towards the frost line is affected not only by the arising vacuum but also by gravitation. In contrast to tests with a downwards freezing direction, the heave direction of the frozen soil occurs in the opposite direction of the water flow. The experimental tests verify how far this influences the formation of the ice lenses. With these tests, the temporal and spatial distribution of water in the soil sample is investigated. Within the scope of a parameter study the influence of relevant parameters will be determined. In addition, standard frost heave test will be carried out to compare the resulting heaves as well as the formed ice lenses in order to carve out differences due to the freezing direction. Furthermore, a numerical multiphase model will be developed to simulate the frost heave with the various physical effects and under defined initial and boundary conditions. A theory of porous media (TPM) based multiphase model is formulated and extended with the phase field method (PFM) to model ice formation and phase change from pore water to ice in porous media. The TPM-PFM model considers the flow of the pore fluid reduced or prevented as a result of ice formation as well as the finite deformations of the solid skeleton due to an volume increase during the phase transition from water to ice. The frost heave tests are used to calibrate and validate the numerical model to calculate freezing processes with upwards and downwards freezing.