Closed‐System Freezing of Unsaturated Soil

Abstract Recent developments in the theory of soil freezing and frost heaving led to the conclusion that pores directly adjacent to a growing ice lens ought to be filled with water. The literature contains fragmentary reports of heaving of soils that were not saturated with water and which lacked an external source of water. It was thought that this apparent anomaly might be explained by postulating that before lens growth and heaving began, local saturation was achieved by thermal water transfer. Experiments confirm that heaving sometimes occurs in moist closed soil columns, initially at uniform temperature and water content, when the temperature at one end is lowered below the ice point and kept at that temperature while the other end is held at the original temperature. The region just ahead of the advancing ice front does not become saturated in the expected manner, but instead suffers a net loss of water. Water lost from the unfrozen soil enters the frozen soil, and causes its water (ice) content to increase for a considerable distance behind the 0 C isotherm. This transport within the frozen soil exceeded by several orders of magnitude that which can be accounted for as vapor movement through the unfilled pore space and thus, it was concluded, must take place in the liquid phase. Whenever and wherever the degree of saturation reaches about 0.9, a further increase in water content is accompanied by heaving. The frozen soil actively extracts water from the unfrozen soil, supposedly due to attenuation of the iquid water films associated with ice surfaces. The resultant water content gradient in the unfrozen soil induces hydraulic flow toward the frozen soil. During the early stages of the experiments thermal transport in the unfrozen soil is inconsequential compared with this hydraulic flow. Later the rate of ice accumulation diminishes, the hydraulic gradient is dissipated, and the thermal water transport becomes relatively more important, by default. By that time heaving has virtually ceased. Thermal water transport is, therefore, not essential for heaving in moist soils that lack an external source of water.