Integrating the Four-Probe Method and SWCC Device to Measure Electrical Resistivity Anisotropy of Unsaturated Soil

摘要

Electrical resistivity is often utilized to estimate different flow-related properties of unsaturated soils, such as hydraulic conductivity. To be better utilized for such a purpose, the electrical resistivity anisotropy has to be characterized to account for the anisotropic nature of soils. Thus, a testing cell equipped with two individual four-probe arrays was designed and manufactured using the 3D printing technique. The testing cell was integrated with a soil water characteristic curve (SWCC) device, and therefore both the soil resistivity anisotropy and SWCC could be measured simultaneously in the same sample. Numerical simulations using the finite element method (FEM) were carried out to characterize the measurement biases due to the boundary effect from the finite sample size and the influence of the electrode sizes, and then to determine the associated correction factors. The validity of the correction factors was also experimentally verified. Experimental results on the mica samples reveal that the electrical resistivity in the vertical and horizontal directions (i.e., rho(V) and rho(H)) increases as the suction increases or the degree of saturation decreases. In addition, the anisotropic factor, lambda=root rho(V)/rho(H), was found to gradually increase with increasing suction or decreasing water content.