Experimental investigation of ice lens formation in frozen soils

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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.