The use of hyperbolic and Asaoka’s methods to estimate the moisture diffusivity and the hydraulic conductivity of unsaturated soils from tests to determine the SWRC

Centro de Estudios y Experimentación de Obras Públicas (CEDEX), Laboratorio de Geotecnia, c/Alfonso XII, Madrid, Spain

^* Corresponding author: easanza@cedex.es

Abstract

The equation governing the unsaturated transient flow in a soil sample when subjected to suction (ψ) at its base and that of the classical 1-D consolidation are exactly alike (equation of diffusion). The former can be arrived at by Richards’ equation, being in this case the moisture diffusivity (D), instead of the coefficient of consolidation (cv), the governing parameter. D need not be constant, but rather a function of the volumetric water content, D=D(θ), and is defined as the ratio of the hydraulic conductivity, k(θ), over the specific water capacity, C(θ)=dθ/dψ, i.e., the slope of the SWRC. The hyperbolic method has been used for several geotechnical purposes and, most importantly, as an alternative to Asaoka’s method for predicting the final settlement and cv of soft soils undergoing consolidation, improved by preloading. This paper shows that both methods prove to be very useful as a means of obtaining D(θ) and k(θ) at a certain range of θ, provided that a reduced number of water contents at known elapsed times are determined over the medium stage of this transient flow. It is addressed in the paper both by theoretical grounds and on the light of experimental data of 4 soils.