###### Abstract  
In this paper, the performance of a gas barrier that consisted of polyacrylamide (PAM)-modified compacted clayy soil was experimentally explored. The moisture content and water loss characteristics of the tested soils were adopted as indicative indices of water retention capacity (WRC). The gas permeability (\(\mathrm{K_{p}}\)) and gas diffusion coefficient (\(\mathrm{D_{p}}\)) of the modified compacted clays were evaluated via gas permeability and gas diffusion tests. The test results showed that the moisture content of the modified compacted clay samples subjected to drying tests increased with increasing polyacrylamide content. \(\mathrm{K_{p}}\) and \(\mathrm{D_{p}}\) decreased with increasing PAM content. Compared with 0.2% PAM content, the \(\mathrm{K_{p}}\) of the sample with 1.0% PAM was reduced by ten times, and the \(\mathrm{D_{p}}\) was reduced to \(\sim\)35%. Compared to the unmodified clay, the liquid limit of the PAM-modified clay increased by 45-55%. Comparison of the liquid limit tests between this study and previous studies revealed that the liquid limit ratio of the zwitterionic polyacrylamide (ZP)-modified soil was much higher than the other material-modified soils. The results of this study are useful to facilitate the application of modified compacted clays as gas barrier materials at industrial contaminated sites. compacted clay; polyacrylamide; gas barrier performance; gas permeability; gas diffusion coefficient +
Footnote †: journal: _Sci._  
Footnote †: journal: _Sci. barrier in contaminated sites. However, the bentonite in GCL is usually sodium-activated calcium bentonite in China, and the barrier performance is not as good as sodium bentonite GCL. This is because the high-quality sodium bentonite resources in China are scarce. Previous studies [11; 12] found that the gas permeability of the sodium bentonite GCL decreased with increasing moisture content under different pressures. Studies have shown that the gas permeability changes after hydration from 1.0 \(\times\) 10\({}^{-18}\) to 1.0 \(\times\) 10\({}^{-11}\) m\({}^{2}\)[13; 14; 15]. After drying, the gas permeability of the GCL can range between 0.03 and 0.21 m\({}^{2}\)[16]. The VOC/SVOC gas migration from contaminated soil through GCL into the air is mainly controlled by pressure and concentration gradients. The advection governs gas flow caused by differential pressures, and the gas advection indicator is reflected by the gas permeability [17]. Gas migration due to a concentration gradient is controlled by the gas diffusion coefficient [18]. [PERSON] et al. [19] demonstrated that when the apparent degree of saturation (ADOS) of the sodium bentonite GCL increased greater than approximately 65%, both the gas diffusion coefficient and gas permeability of the GCL were considerably reduced. The ADOS is defined as the gravimetric water content (\(w\)) of a GCL at a given time, divided by the maximum gravimetric water content (\(w_{\text{ref}}\)) that the same GCL reaches during hydration under the same applied stress conditions [19]. It can be anticipated that the gas diffusion and gas permeability of CCC increase significantly with the decrease in both gravimetric water content and ADOS [20]. Extremely low moisture content will lead to VOC gas migration into clay by diffusion and advection more easily, thus impairing the CCC gas barrier efficacy. Low moisture contents in compacted clay can cause fissures due to water loss [21]. Ultimately, a crack network may develop from the fissures, becoming the preferential and dominant pathway for the upward advection of VOC or SVOC gas released from the unsaturated contaminated soils. Through laboratory tests, [PERSON] et al. [22] found that the hydraulic conductivity near soil cracks increased sharply as compared to intact soil that is not intact, confirming the existence of dominant flow paths after the soil cracked. Adopting modified materials in CCC is one potential approach to suppress cracking caused by water loss and improve the gas barrier performance.