Experimental investigation of ice lens formation in frozen soils

地面冻结是一种环保的施工技术，用于暂时增加地基的刚度和强度，并提供水密性。土壤冻结的一个主要风险是孔隙体积的变化以及孔隙水冻结和融化引起的土体变形。从物理角度看，冻结过程中的体积变化有两个不同的来源：1)与未冻结水相比，冻结水的体积增加了；2)冻结过程中冰透镜的形成。冰透镜的形成和传播在很大程度上归因于冰在存在热梯度的情况下主动排出嵌入土壤颗粒的倾向。这就导致了冰和土相的分离，以一系列离散的纯冰带的形式散布在冻土层中。尽管研究界做出了相当大的努力，但其控制机制尚未完全了解。本基础研究的重点是冰透镜在冻结过程中的形成和传播以及诱导升沉的实验研究。主要目标是：1。开发一种新颖的、高度先进的测试装置，以高分辨率追踪冰透镜形成的空间和时间，用于不同易受冰透镜形成影响的土壤。2. 研究结构退化和开裂对冰透镜形成过程中土壤敏感性的影响。3. 对比观察三种不同温度边界条件下所研究土壤的冰透镜形成和体积土变形。4. 探讨核磁共振（NMR）测定的土壤冻结曲线（SFC）与部分饱和未冻土的土壤保水曲线（SWRC）的相似性。5. 在接近冰点的温度下，确定部分冻结和未冻结土壤的抗拉强度，并将这种抗拉强度与冻结过程中的裂纹萌生联系起来。

Ground freezing is an environmentally friendly construction technique used to temporarily increase the stiffness and strength of the subsoil and provide water tightness. A major risk of ground freezing concerns the change of the pore volume and the associated soil deformations that can be induced by pore water freezing and thawing. From a physical point of view, the volume changes during ground freezing have two different sources: 1) increase of volume of the frozen in comparison to the unfrozen water, and 2) the formation of ice lenses during freezing. The formation and propagation of ice lenses is largely attributed to the tendency of ice to actively expel embedded soil particles in the presence of a thermal gradient. This leads to the separation of the ice and soil phases in the form of a series of discrete bands of pure ice interspersed by frozen soil layers. Despite the considerable efforts of the research community, its controlling mechanisms are not yet completely understood. This fundamental research focusses on the experimental investigation of the formation and propagation of ice lenses during freezing as well as on the induced heave. The main goals are: 1. To develop a novel, highly advanced test setup to trace ice lens formation spatially and temporally with high-resolution for soils with different susceptibility to ice lens formation. 2. To study the influence of structure degradation and cracking depending on the soil susceptibility to ice lens formation. 3. To comparatively observe ice lens formation and volumetric soil deformations in the investigated soils for three different temperature boundary conditions. 4. To investigate the similarity of the soil freezing curve (SFC) determined by means of Nuclear Magnetic Resonance (NMR) tests with the soil water retention curve (SWRC) of the partially saturated unfrozen soil. 5. To determine the tensile strength of partially frozen and unfrozen soils at temperatures close to the freezing point, and relate this tensile strength to crack initiation during freezing.