The Effect of Frost on Soil
Frost is a natural phenomenon that affects not only people’s daily lives but also the stability and characteristics of soil. When temperatures drop below zero, water in the soil’s pores freezes, causing processes such as volume expansion and changes in soil bearing capacity. These changes can have significant consequences for infrastructure, building foundations and the stability of embankments.
At low temperatures, subsurface processes become highly active. This natural phenomenon shapes the landscape, alters the soil and plays an important role in the ecosystem. In geotechnics, understanding the effects of frost on soil is crucial for planning and constructing structures in regions with cold climates.
The impact of frost on soil is a subject studied within soil mechanics. This impact is especially important in areas prone to prolonged low temperatures.
Cuttings and embankments are structures the surfaces of which are directly exposed to the effects of weather conditions. Such structures can be partially protected from frost by selecting appropriate construction materials, protecting slopes and correctly constructing the pavement structure.
The effect of frost on soil can be observed through two processes.
- The first process is freezing, during which ice accumulations form. When water freezes, its volume increases by approximately 10%, taking up more space than its liquid state. In soil, this expansion creates spaces where ice lenses can form. After freezing, the surface of the frozen layer may rise by about 30% of its thickness. Water located in the fine pores of soil is under greater molecular pressure. Freezing induces crystallization forces that generate tensile stresses in the water surrounding the ice crystals, causing liquid water to migrate toward areas where ice has already formed. This leads to the accumulation of larger amounts of ice in specific locations, forming lenses. These lenses, under conditions conducive to freezing, grow continuously and cause significant upward movement of the soil surface.
- The second process occurs during thawing, as temperatures rise. At this stage, the ice lenses melt, leaving cavities in the soil where they had previously been. This phenomenon leads to erosion. It has particularly detrimental effects on roads in the spring, after the ice has melted. As vehicles drive over roadways beneath which ice lenses had formed, the roads become damaged. More precisely, as the ice melts, areas of over-saturated soil with a soft consistency develop. If this occurs in soil under a road, these low-strength zones are compressed under the weight of vehicles, pushing mud to the surface and creating softened potholes in the pavement.
One of the notable researchers who studied the formation of ice lenses was A. Casagrande. He observed that only certain soils are highly susceptible to the formation of ice lenses, and he linked this phenomenon to the grain size distribution of the soil, both in terms of particle size and range. This is determined using the uniformity coefficient (Cu). Casagrande established the following criteria for frost susceptibility: soils not susceptible to frost have a Cu ≥ 15 and fewer than 3% of particles finer than 0.02 mm, or a Cu ≤ 15 and fewer than 10% of particles finer than 0.02 mm. Soils with fewer than 1% of particles smaller than 0.02 mm are not prone to ice lens formation because they develop what is referred to as homogeneous ice — ice confined within the pores of the soil. When this ice melts, water drains quickly from such soils because of their relatively high permeability, preventing the significant accumulation of moisture that could lead to structural issues.
Freezing occurs in all types of soil, but not all soils are equally sensitive to the effects of ice. In coarse-grained soils, such as gravel and sand, ice crystals form within the pores. This phenomenon happens when the freezing front moves rapidly downward. The same process occurs in fine-grained soils if the freezing front also advances quickly. However, when the freezing front moves downward slowly, capillary-bound water from greater depths has enough time to reach the freezing front in its unfrozen state. This water then contributes to the formation of ice lenses.
The impact of frost on soil can be minimized by implementing protective measures against frost damage. Some of these measures include founding structures at depths greater than the frost penetration depth, thus ensuring that the foundation remains unaffected by freezing temperatures, interrupting the capillary rise of groundwater by installing a buffer layer made of highly permeable material or installing a layer of thermal insulation material to prevent freezing within the capillary zone. These measures help mitigate the adverse effects of frost and preserve the stability of structures and soil.
