A finite difference method for measuring soil thermal diffusivity in situ

Abstract

SUMMARYA method is devised for measuring soil thermal diffusivity in situ. It is based on direct experimental simulation of the finite difference approximation to the one-dimensional heat conduction equation.The method does not require the soil to be homogeneous except between the three thermometers that are used, at depths z + d, z and z -a! Nor need the energy input curve be sinusoidal. However, it must be fairly smooth for the finite difference approximation to be accurate. Experimental results for London Clay are presented, obtained using thermometers at depths of I, 6 and I I cm to give a mean thermal diffusivity of 0.0074 cmz/s at a depth of 6 cm. This value is consistent with other estimates of diffusivity for clay soils.The method is capable of automation, and should be suitable for use on engineering sites, at low cost. The method is capable of generalization to other linear diffusion equations containing one independent parameter. The same limitation also applies to its application to constitutive or geometrical non-linear one dimensional diffusion equations, and each equation requires individual study to assess feasibility of use of the method.The method in effect uses the usual finite difference approximation, not to prepare a numerical solution, but to design an experiment carried out essentially within the finite difference 'molecule'. The measured parameter of the diffusion equation is the usable product of the method.