5.3 Thermal property of soil
            Thermal property of soil are of great importance in several engineering projects where heat transfer takes place through the soil. These projects include underground power cables, high level nuclear waste repository, hot water or gas pipes and cold gas pipelines in unfrozen ground, agriculture, meteorology and geology. The thermal properties of soil include thermal conductivity (K= 1/ρ), ρ is the thermal resistivity, thermal diffusivity (D), and heat capacity (C). K is defined as the amount of heat passing in unit time through a unit cross- sectional area of the soil under a unit temperature gradient applied in the direction of heat flow. Considering a prismatic element of soil having a cross-sectional area A at right angles to the heat flow q, then K is defined as

5.4

Where, l is the length of the element, T₁ and T₂ are temperature where T₂>T₁.
            The heat capacity C per unit volume of soil is the heat energy required to raise the temperature of unit volume of soil by 1 C. It is the product of the mass specific heat c (cal/g °C) and the density ρ_m (g/cc). Thermal diffusivity is the ratio of thermal conductivity to specific heat. It indicates how materials or soil adjust their temperature with respect to the surroundings. A high value of the thermal diffusivity implies capability for rapid and considerable changes in temperature.

5.3.1 Factors influencing soil thermal resistivity (ρ)
            Fine grained or cohesive soil and peaty soils exhibit high thermal resistivity (ρ) than granular soil. Sand with quartz as the principal constituent has low ρ. The type of clay minerals present in soil also influences ρ. Expansive clay minerals such as montmorillonite would cause the soil particles to be forced apart during swelling action when it comes in contact with water, thereby increasing ρ. Well-graded soils conduct heat better than poorly graded soils because the smaller grain can fit in the interstitial positions between the larger grains thus increasing the density and the mineral-to-mineral contact. The shape of the soil particles determines the surface contact area between particles which affects the ability of the soil to conduct heat. ρ increases with decreasing particle size due to reduced surface contact between adjacent particles.

            The density of soil has an important influence on ρ. The presence of air with its high ρ increases the overall ρ of the soil as compared to that of its solid components. Therefore, a well compacted soil will have low ρ due to low total void volume and better contact between the solid grains. When water is added to the soil, it tends to distribute itself in a thin film around solid grain of the soil. This water film provides a path for the heat and hence bridges the air gaps between the solid particles. Additional water, over and above that required for film formation, serves to fill voids which were initially occupied with air. Since ρ of air is much higher than water, inclusion of water in soil would considerably decrease ρ of soil. The moisture content also has an indirect influence on ρ since higher density can be achieved by adding water to the soil. The ρ of soil is also influenced by temperature, because each of the constituents has temperature dependent ther­mal properties. The ρ of all crystalline minerals increase with increasing temperature, however, the ρ of water and gases exhibit the inverse effect.

5.3.2 Measurement of soil thermal resistivity (ρ)
            ρ measurement of soil could be categorized as steady state and transient state methods. For steady-state method, a known thermal gradient is established in soil specimen with definite shape and length and ρ can be determined based on recording the heat flow through the soil. In transient-state method, known time-rate of energy is applied into soil specimen and the corresponding temperature change with time is recorded and analyzed to determine ρ. The thermal gradient across the soil sample being tested may induce appreciable moisture migration in unsaturated soils there by changing the properties it is attempting to measure. Therefore, selection of appropriate method of ρ measurement should be based on the condition of the target materials. Some of the methods employing steady state and transient measuring principle are discussed below.

5.3.2.1 Steady state method
            In this method, the soil sample being tested should be in steady state when the measurements are made. Attainment of such a state is time consuming after the initial temperature difference has been applied. Also, there is possibility of moisture changes by the time the steady state is reached. The methods based on steady state are described below: