The two critical frost actions in the soil are the frost heave and the frost boil. Frost heave is the phenomenon in which the water molecules present in the pores freeze during lower temperature which results in the expansion of the soil. Frost boil is the phenomenon of loosening of the soil when the frozen soil starts to thaw. Frost boil occurs after the frost heave process.
The main features of frost action and the methods to prevent this are explained briefly.
Frost Heave in Soils
When the atmospheric temperature falls below the freezing point, the water molecules present in the soil pores freeze and ice is formed. These water molecules are mainly moving from the bottom of the water table to the upper portion of the soil. Formation of ice in the soil pores results in the increase in the volume of the soil. This is because the water to ice conversion brings an increase in volume by 9%. So every pore experiences a heave due to the increase in volume. This is called as frost heave.
The frost heave results in the soil present on the ground level to lift up. This is one of the major issues when light structures are constructed over the ground.
In actual condition, the frost heave occurred is more than what is expected. This is because the formation of ice lenses disturbs the nearby water film of the soil particles. This can disturb the soil suction and overall equilibrium, which in turn draws more water to the top. Hence the frost heave is caused to almost 20 to 30 percent of the soil depth. The soils that are more prone to frost heave action are silts and fine sands.
Frost Boil in Soils
Once the frost heave has occurred, a decrease in temperature results in the thawing of the ice formed. This results in the liberation of free water in the upper layers. The process of thawing is started at the top layers and it moves down with time.
This way the soil in the upper layer is softened and loosened while the bottom layers are in the frozen state. This phenomenon of softening of soil is called frost boil.
The effect of frost boil adversely affects the structures constructed on the ground. This is a major issue observed in highway pavements. Under the action of wheel loads, the soft soil and water get extruded out, thus creating a hole. This can result in breaking of pavement if the condition is extreme. This would result in the exposure of subsoil.
Silty soils are most affected by this. These soil types have lower plasticity index and behave softly for smaller water content.
Prevention of Frost Action in Soils
Frost heave and frost boil bring large maintenance issue in the case of highway and runways. The following measures can be undertaken to mitigate the frost action in soils:
- The most effective method to prevent frost action is to replace the soil that is prone to frost action with coarse-grained soils like gravels or coarse sand. As the water in coarse-grained soil drains out quickly, there is no chance for frost action to occur.
- Providing an insulation blanket between the water table and the ground helps to avoid the migration of water to the top. The insulation blanket is a layer of gravel provided at a thickness of 15 to 30cm.
- Provision of good drainage system prevents the frost action by lowering the water table level and also drains out the water formed after the process of thawing.