Use of electrical resistivity tomography to evaluate processes in soils

Electrical resistivity tomography (ERT) is a non-invasive imaging technique capable of evaluating the distribution of electrical properties within a cross-section or volume of soil specimens. The technique provides an alternative for monitoring processes in soils that otherwise do not yield mechanical or density contrasts.; This research presents the development and testing of 2D and 3D ERT imaging systems to map processes in soils and other porous media. The 2D ERT testing cell consisted of 16 electrodes placed along the perimeter of a circular section while the 3D ERT testing cell is composed of 64 electrodes distributed along four planes of the cylindrical testing cell. The 3D ERT testing cell also included 16 bender elements that were used to monitor S-wave propagation parameters to complement the electrical resistivity data.; Two different tests were monitored with the ERT systems. The first test involved monitoring chemical diffusion in a 2D soil media by injecting 6 mL of 0.2 mol/L NaCl solution into tap water-saturated Ottawa sand. The ERT system successfully monitored the progression of the chemical diffusion over time. The second test images the injection of sodium pyrophosphate (SPP)-modified bentonite slurry in a sand specimen. Researchers at Purdue University developed this methodology with the intent to reduce liquefaction potential of loose saturated sands. Three tests were conducted to characterize the injected material and the injection processes: (i) electrical impedance analysis were conducted on the SPP-treated bentonite slurries to evaluate the reduction in resistivity with SPP concentration and time, (ii) viscosity measurements of the SPP-treated bentonite slurry were conducted to characterize the mechanical behavior of the slurry and show how increasing SPP concentration decreases and then regains slurry viscosity and (iii) the ERT system and bender elements were used to monitor the injection of bentonite slurry treated with 1% SPP in loose sand specimens. Results show the complementary nature of ERT and bender element data to successfully evaluate the injection process and the effect on the sand-saturated system.