Study on the gas permeability of unsaturated compacted loess and its underlying micro-mechanism

Aiming to investigate the gas permeability of unsaturated soil and its underlying micro-mechanism, the gas permeability coefficients of compacted loess under various initial states, along wetting/drying, and constant stress ratio compression paths were examined in this study by utilizing a modified gas permeation device. The Mercury Intrusion Porosimetry(MIP) technique was employed to further examine the microstructure changes of compacted losses, thereby analyzing the micro-mechanism of air permeation. The test results indicate that the gas permeability coefficient(<italic>k</italic>_eff) of compacted loess varies within the range of 10^－12 to10^－15 m² in response to the increasing compaction saturation degree(<italic>S</italic>_r0). At low dry densities, <italic>k</italic>_eff exhibits an initial rise followed by a decline as <italic>S</italic>_r0 increases, whereas at high dry densities, a nonlinear decrement is observed. Wetting significantly reduces <italic>k</italic>_eff of compacted loess by up to three orders of magnitude, with a rapid decrease after the wetting saturation degree reaches 0.65. Conversely, drying improves gas permeability, but its impact is much less significant than that of wetting. Under constant stress ratio compression, <italic>k</italic>_eff decreases exponentially with increasing stress, and the decrease is more pronounced at lower stress ratios. The MIP test results reveal that the macro porosity first increases and then decreases with the increase of <italic>S</italic>_r0. Wetting has a minor effect on the pore size distribution curve(PSD), while drying can increase macro porosity. Under similar state, the as-compacted soil exhibits more macropore structures compared to the after-wetting soil, while less macropore structures compared to the after-drying soil. The constant stress ratio compression results in a reduction of macropores, with a greater reduction at lower stress ratio. According to the variation of <italic>k</italic>_eff, the gas permeability of unsaturated compacted loess depends on the amount of the interconnected macropores, the more macropore structures, the better pore connectivity, and the better gas permeability. A pore structure paramesed, and a micro-scale air permeation model considering the pore structure parameter is established. The applicability of the proposed model is then verified through experimental data.