Dense Gas Flow in Volcanic Ash Soil: Effect of Pore Structure on Density‐Driven Flow

Abstract

Unique physical properties of volcanic ash soils characterize the soil gas transport parameters of gas diffusivity and air permeability. Air permeability controls the density‐driven flow that has been recognized as one of the important phenomena for subsurface dense gas. In this study, one‐dimensional column experiments were conducted to investigate the effects of the pore structure of a volcanic ash soil on the density‐driven flow of a dense gas (isohexane). The results showed that the overall horizontal gas movement in Tachikawa loam (volcanic ash soil) and Toyoura sand (sand) used as reference materials was expressed by Fick's diffusion law. On the other hand, the vertical downward gas movements in Tachikawa loam were considerably enhanced by the occurrence of density‐driven flow, especially at high air contents (30–40%). Pore size distribution and pore structure analysis based on the tube model suggest that a greater volume of large pores (>0.01 cm) and a more continuous pore network led to the greater density‐driven flow in Tachikawa loam than in Toyoura sand.