Unified measurement system with suction control for measuring hysteresis in soil‐gas transport parameters

[1] A unified measurement system with suction control (UMS_SC) was developed for measuring soil water characteristics curves (SWCC) and gas transport parameters under alternating drying and wetting cycles. The new system consists of a diffusion chamber, sample ring, porous plate, tensiometer, moisture sensor, oxygen electrodes, and air pressure gauges. The SWCC and gas transport parameters [gas diffusion coefficient (Dp) and air permeability (ka)] for two different porous materials, Toyoura sand and granulated slag, were simultaneously measured under drying and wetting cycles. The SWCC and gas transport parameters measured by UMS_SC were consistent with recent models and independently measured data on exactly the same materials using standard experimental setups from literature. For an applied water suction head of less than 50 cm and corresponding water saturation of around 0.3–0.5, the UMS_SC data documented hysteretic (nonsingular) behavior in both measured Dp and ka under repeated drying and wetting cycles. The hysteretic behavior was insignificant for water and air contents but large for both gas transport parameters when applying suction, and hysteretic effects were larger for air permeability than for gas diffusivity. Additionally, hysteresis in the percolation threshold (soil-air content where gas transport ceases due to interconnected water-filled pores) for both gas diffusion and air permeability was insignificant for both materials. These findings should be taken into account when developing models for diffusive and convective gas transport and their parameters in variably saturated porous media.