Study of Mechanisms Governing Electromagnetic Alteration of Hydraulic Conductivity of Soils

Abstract

Hydraulic conductivity is a measure of the rate at which water flows through porous media. Because of the dipole properties of water molecules, electric field can affect hydraulic conductivity. In this study, the effect of radio-frequency (RF) waves on hydraulic conductivity is investigated. This is important both for the geophysical measurement of hydraulic conductivity as well as remediation using electromagnetic waves. Bentonite clay and sandy samples are tested in rigidwall, cylindrical permeameters and stimulated using a CPVC-cased monopole antenna vertically centered in the permeameters. The permeameters are encased within RF cavities constructed of aluminum mesh in order to prevent interference from the outside and to confine the RF wave to the medium. Falling-head and constant-head tests are performed to measure the hydraulic conductivity of the clayey and sandy soil samples, respectively. The results show a correlation between the change in the hydraulic conductivity and various characteristics of the RF stimulation. The change is, however, different for sandy and clayey soils.

Electromagnetic Waves’ Impact on Hydraulic Conductivity of Granular Soils

“…Water has high dielectric permittivity (≈80). Oscillations of individual water molecules can induce a net change in the movement and flow of water through a porous medium without altering the properties of the medium itself [10]. When under the influence of an electric field, water molecules can start to reorient parallel to the direction of the electrostatic field [15].…”

“…Applying Darcy's law, v = −k i , where in 3D, i = ∇h = ∂h ∂x i + ∂h ∂y j + ∂h ∂z k . Equation ( 9) can now be rewritten in the matrix form, as shown in Equation (10).…”

“…The computational code was developed based on the central finite-difference method and then used to calculate the spatial variations in the hydraulic head within the soil using the hydraulic conductivity values obtained for the unstimulated tests. Equation (10) was used to compute the flow rate and discharge through the soils for the steady state unstimulated tests. However, this only solved the hydraulic head inside the specimen, while the test was performed in the unstimulated condition.…”

“…The vibration of water molecules enhances different transport mechanisms inside the soil, therefore altering hydraulic conductivity. In another work, Azad et al [10] conducted a study using radio frequency (RF) waves to alter hydraulic conductivity of sandy soils. The hydraulic conductivity increased as much as 14% when an RF stimulation test was conducted on natural sand at a frequency of 153 MHz with an RF-power input of 20 Watts.…”

“…The increase in hydraulic conductivity was justified by the RF energy absorbed by the water molecules, which decreased the viscosity and led to an increase in hydraulic conductivity. Azad et al [10] also tested the effects on clay and noticed a decrease in the hydraulic conductivity of clay [10]. This is important because using electromagnetic waves can reduce soil's susceptibility to liquefaction by reducing the hydraulic conductivity related to liquefaction.…”