Thermal diffusivity is a parameter describing the ability of soil to transfer heat throughout its volume.
Used for calculating Soil Ambient Temperature and for cable Cyclic Rating Factors.
If no data is available on the soil then a standard value to use for soil thermal diffusivity is 0.5 x 10^-6 (m^2/s) which corresponds roughly to a soil having a thermal resistivity of 1 K.m/W and a moisture content of around 7 % of dry mass.
Can be estimated from soil thermal resistivity according to Table II from Standard IEC 853-1  below:
Can be calculated according to the below equation (IEC 853-1) :
 IEC 60853-1, First edition 1985, Calculation of the cyclic and emergency current rating of cables Part 1: Cyclic rating factor for cables up to and including 18/30 (36) kV