Influence of biopolymer on gas permeability in compacted clay at different densities and water contents

Reducing soil permeability by natural biopolymers, which absorb water to form viscous hydrated colloids, may sustainably improve earthen structures such as landfill covers. Previous studies mainly investigated biopolymer effects on water permeability of saturated soils. However, performance of biopolymers in unsaturated soil is unclear, especially for gas permeability. This study examined effects of two biopolymers on gas permeability of compacted clay at various soil densities and compaction water contents using a flexible wall permeameter. The density is represented by degree of compaction (DOC). Statistical analysis was also performed for the three replicates of 95% DOC. Soil microstructure was analyzed by mercury intrusion porosimetry (MIP) and scanning electronic microscopy (SEM). Results showed that gas permeability of clay with biopolymers was always lower than that of pure clay with reduction of up to two orders of magnitude. No significant difference was found in the performance between the two biopolymers for most conditions. This is consistent with the statistical results. Reducing gas permeability by biopolymers is due to the reduction in soil macro pores and pore clogging as evidenced by MIP and SEM results. For 75% and 85% DOC, the clay with and without biopolymers showed an increase in gas permeability as compaction water content increased. Until DOC reached 95%, only gas permeability of the clay with biopolymers decreased with an increasing water content since viscous biopolymers stick soil particles and reduce macro pores. Moreover, the permeability reduction by biopolymers was enhanced with an increasing water content as more water is available to hydrate biopolymers and thus facilitate pore clogging. Statistical analysis also supports that the influence of biopolymers becomes more significant with an increasing water content.