Thermal Stabilization of Soil -A Ground Improvement Technique

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Thermal stabilization of soil is a ground improvement technique. The concept, method and applications of thermal stabilization of soil is discussed. It has been observed that heating or cooling shows certain marked changes in the soil properties. Many types of research were conducted on the same and many impressive results were observed that is found relevant for soil stabilization. Heating and cooling have been extensively used as soil improvement techniques. Whatever be the mode of thermal stabilization we opt for it has the following needs:

• Thermal evaluation of heat flow
• Heating or refrigeration system has to be designed
• Strength and stress-strain -time properties of the soil have to undergo performance analysis

The below figure represents the ultimate strength of three soils with temperature as a function.

Fig.1.Compressive Strength of three soils as a function of Temperature (After Sayles, 1966)

Concept of Thermal Stabilization of Soil

Like seepage or consolidation analysis of soil, a heat flow analysis can also be carried out. The transfer of heat in soil occurs by conduction, convection (free, forced, by thawing) and radiation. The most predominant mechanism of transfer is through conduction, which takes place in three constituents of soil which are soil solids, water (which may be in the form of a liquid, ice or vapor) and pore air. The phenomenon of heat conduction is influenced by soil thermal properties which are its thermal conductivity, latent heat of fusion, the heat of vaporization of soil water and the heat capacity of the soil. The behavior of heat flow in soil is mainly governed by the latent heat of fusion of water on freezing and heat of vaporization of water on heating above 100⁰C. The latent heat of fusion can be defined as the heat amount that must be added to the unit mass of a substance to change it from liquid to solid or solid to liquid without any change in the temperature. Now, the heat of vaporization is defined as the heat required to change the substance from liquid to vapor. The latent heat of fusion (L) of  Water to ice = 333 x 10³ J/kg (143.3 BTU/lb) Heat of vaporization (V) of Water = 2260 x 10³ J/kg Freezing or thawing latent heat for soil water, L_s Heat of vaporization of soil water V_s Where 'w' is the water content and is the dry density (kg/m³). The heat capacities of ice, water as well as soil minerals can be given as follows C_i = 2098(J/kg)/K C_w = 4286 (J/kg)/K C_m = 710 (J/kg)/K The heat capacities of unfrozen (C_u) and frozen soils (C_f) are given as The thermal conductivity of soils can be determined by several methods, some of which are Kernsten's empirical equations, Johansen's method etc.

Thermal stabilization of soil from Heating

It is noted that higher the heat input per mass of the soil (which should be treated), the greater would be the effect. A small increase in temperature will cause a strength increase in fine-grained soils due to the reduction of electric repulsion between the particles, pore water flow due to variation in thermal gradient and due to the reduction in moisture content because of increasing evaporation rate. Therefore, it is found that it is technically feasible to stabilize fine-grained soils by heating. The following mentioned statement gives the temperature and the corresponding possible change in soil properties.

• 100⁰C Cause drying and a significant increase in the strength of clays. This results in decrease in the compressibility of the soil.
• 500⁰C Cause permanent changes in the structure of clays results in a decrease of plasticity as well as moisture adsorption capacity.
• 1000⁰C Cause fusion of the clay particles into a solid substance like brick.

It has been experimented and found that heat was able to change an expansive clay into an essentially none- expansive material. When liquids or gas fuels are burned in boreholes or an injection of hot air into 0.15m to 0.20m diameter holes, resulted in the formation of 1.3 to 2.5m diameter stabilized zones after continuous treatment for 10 days. Heating, therefore, would cause permanent changes in the soil making the soil harder as well as durable. So, treatment would cause overall decrease in

• Compressibility
• Increase in cohesion
• Increase in Internal friction angle
• Increase in modulus of elasticity

These effects take place at a temperature range from 300^OC to over 1000⁰C. Soil melts at a temperature of 1250 to 1750⁰C. The melting point of the soil can be reduced by addition of fluxing agents such as Na₂CO₃. The amount of fuel energy that has to be generated to obtain high temperatures is found to very costly. Ingles and Metcalf (1973) could propose an estimation equation in order to estimate the cost of thermal stabilization by heating as Where w= moisture content in percentage, T = burning temperature, C_f = unit heat capacity of the fuel, where 35% is for open burning and 70% for closed burning and F= fuel used per unit volume of the soil.

Table.1: Minimum heat requirement from different applications of stabilization by heating

Applications of Thermal Stabilization of Soil

The usage of heating as a mode for thermal stabilization of soil is mainly employed in U.S.S.R and eastern Europe. It is used in

• Landslide stabilization
• Improving soil undergoing collapsing
• Mat foundation construction
• Forming vitrified piles in place
• Reduction of lateral stresses acting ton retaining walls
• The method opted may be
• Combustion to electrical
• Investigations of microwave drying
• Soil fusion by laser beam

Reference:

1. Foundation Engineering Handbook, By Hsai-Yang Fang
2. Ground Improvement Techniques (PB), By Dr. P. Purushothama Raj

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