frost action in soil - freezing and thawing - pdf

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frost action in soil - freezing and thawing - pdf 

FROST ACTION - freezing and thawing

The freezing and thawing of water present in soil and the resultant effects on soil and on structures founded in this soil is known as frost action. Frost action consists of two processes : 

1. Frost heave,
2. Frost boil. 

1. Frost Heave 

Frost heave is defined as rise of ground water surface due to frost action. When the atmospheric temperature falls to the freezing point, the water present in the capillary fringe may freeze, leading to the formation of ice. When water is converted into ice, there is an increase of about 9% in the the volume. Therefore, in a saturated soil, the void volume above the level of freezing will increase by the same amount, representing an overall increase in the volume of soil of 2.5% to 5%, depending upon the voids ratio. 

frost action examples

For example, if the voids ratio is 40%, the expansion of the soil if saturated, will be 0.09 x 40 = 3.6% , which means that there will De a heave of 3.6 cm in every 1 m thickness of soil strata in the capillary fringe. In certain circumstances, a much greater increase volume can occur due to formation of ice lenses within the soil 

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The temperature at which water freezes in the pores of the soil depends upon the pore size - smaller the pore size, lower the freezing temperature. Initially, water freezes in larger pores, remaining unfrozen in the smaller pores. As the temperature falls below zero, higher soil suction develops and water migrates towards the ice in the larger voids where it freezes and adds to the volume of ice. Continued migration gradually results in the formation of ice lenses and a rise in the ground surface. Only silts and fine sands are prone to frost action because these soils have large capillary rise due to relatively fine particles. Also because of relatively fair permeability, water can easily flow in these soils. In contrast to this, coarse grained soils have very little capillary rise and hence frost heave in these soils is very limited. Similarly, in clayey soils, though their capillary rise is very high, their permeability is very low resulting in limited water migration and hence limited frost heave. However, the presence of fissures can result in an increase in the rate of migration. A well graded soil is reckoned to be frost susceptible if more than 3% of the particles are smaller than 0.02 mm. A poorly graded soil is susceptible if more than 10% of the particles are smaller than 0.02 mm. 

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The depth or boundary below the ground surface upto which water may freeze is called frost line. Following are the conditions for the formation of ice lens and marked frost heave. 

1. Soil to be saturated in the beginning and during the freezing period 
2. Proximity of ground water reservoir so that water may be pulled up in the zone of freezing. 
3. High capillarity of soil so that the soil possesses high capacity to pull water.
4. Moderate permeability of the soil, so as to allow unobstructed flow of water through it. 
5. Gradual dropping of seasonal temperature, so that temperature of soil drops below freezing point, and this reduced temperature to persist for a longer duration. Also, there should be gradual rate of change of temperature with depth. 

2. Frost Boil 

If the temperature rises in the soil in which frost heave has occurred, the frozen soil mass thaws and free water is liberated. When thawing takes place, the soil previously frozen will contain an excess of water with the result that it will become soft and its strength is reduced. These effects are known as frost boil.

Silty soils are more susceptible to frost boil than sands and clayey soils. In sands, the released water due to thawing drains out quickly. However, silty soils have more plasticity index compared to clays and hence the softening effect due to excess water will be more severe in silts than in clays. 

Factors affecting Frost action & Frost boil in Pavement & Building

1. Settlement of structures resting on the ground surface 
2. Settlement of highway pavements
3. Formation of pot holes due to extrusion of soft soil and water under the dynamic action of wheel loads. 
4. Eventual breaking of pavement and ejection of subgrade soil in a soft and soapy condition, under moving wheel loads. 

Frost Action in Pavement & Building Preventive measures : 

Ill effects of frost can be prevented or minimized by adopting the following preventive measures : 

1. Founding buildings below the frost depth to avoid possible frost heave 
2. Placing insulating blankets of 15 to 30 cm thick layers of sand and gravel on the ground surface above the frost susceptible soil strata, to prevent deep frost penetration. 
3. Interposing a pervious gravel blanket between the frost line and highest water table, to prevent considerably the capillary saturation of frost zone. 
4. Providing proper drainage to lower the water table and to allow quick escape of water during thawing. 
5. Removing the frost susceptible soil upto the frost depth and substituting by less susceptible soil.