CHARACTERIZING PORE-STRUCTURAL PARAMETERS OF VOLCANIC ASH SOIL: COMPARISON BETWEEN NONDESTRUCTIVE AND INDIRECT METHODS

Abstract

Mass transport within porous media is governed by their pore networks, which is highly
influenced by pore structure parameters such as pore size distribution, porosity, pore tortuosity and pore
coordination number. Micro-focus X-ray computed tomography (MFXCT) has emerged as a powerful nondestructive tool for the direct visualization and better understand soil pore geometry. In this study, soil
macropore networks (typically, pore diameter ≥ 30 µm) were visualized and gas transport parameters were
measured for volcanic ash soils taken from Nishi Tokyo City, Japan. Especially, the study aimed to identify
the effect of moisture content on pore structural parameters based on MFXCT analysis and compare the
MFXCT derived parameters to indirectly-estimated parameters from soil gas diffusion coefficient and air
permeability such as pore tortuosity-connectivity factor (XG) and equivalent pore diameter (dG) for gas flow.
Both undisturbed and repacked samples were used for characterizing soil pore networks. In MFXCT analysis,
the pore structural parameters such as effective pore diameter (dMFXCT), pore tortuosity (TMFXCT), and
coordination number (CMFXCT) were measured. Results showed the moisture content affected clearly pore
structural parameters, TMFXCT and CMFXCT decreased with increasing soil air filled porosity, however, measured
dMFXCT were independent on the moisture content. For both undisturbed and disturbed samples, the measured
TMFXCT and XG became close and plotted in a narrow range between 1:1.5 and 1.5:1. On the other hand, high
variations were observed between dMFXCT and dG, indicating that the indirectly estimated equivalent pore
diameter from soil gas transport parameters does not correspond to the mean diameter of soil macropore.