###### Abstract  
The cover systems in contaminated sites have some problems, including desiccation cracks, which would lead to degradation of the barrier performance. This study presented a systemic laboratory experimental investigation on the liquid-plastic limit, moisture retention, hydraulic conductivity (\(k\)), and gas diffusion barrier properties of amended compacted clay by attapulgite and diatomite for controlling desiccation cracks and migration of water and volatile organic compounds (VOCs). The results showed that the attapulgite could enhance the moisture retention and liquid limit of amended compacted clay. Diatomite could reduce the gas diffusion coefficient (\(D_{\theta}\)) significantly. The compacted clay amended by the dual-additives component of attapulgite and diatomite could enhance the liquid limit, moisture retention percent, gas barrier property, and hydraulic performance compared with the unamended clay. Based on the experimental data obtained, the dosage of additives was targeted to be 5%. The moisture retention percent of dual-additives (attapulgite 4% and diatomite 1%) amended clay increased by 82%, the \(k\) decreased by 25%, and the \(D_{\theta}\) decreased by 42% compared with unamended clay. Scanning electron microscopy (SEM), BET-specific surface area test method (BET), Mercury Intrusion Porosimetry (MIP), and thermogravimetric analysis (TGA) indicated the enhancement mechanism of additives-amended compacted clay. addtives; compacted clay cover; moisture retention; gas diffusion barrier; hydraulic conductivity +
Footnote †: journal: _Sciences_  
0000-0001-9991-9999-2000-0000-0000-0000-0000-0000-0000-0000-0000-0000-0000-0000-0000-0000-0000-0000-00000-00000-0000-00000-00000-00000-00000-00000-00000-00000-00000-0  
the CCC contributes to the role of VOCs/SVOCs and the precipitation barrier. However, desiccation cracks in CCC act as preferential flow paths and affect the barrier performance of CCC, eventually breaking through the cover system [9]. There are various issues resulting in cracks of CCC [7; 10], including differential settlements, extreme drought [11], and dry-wet cycle [9]. The resistance to cracking, i.e., the tensile strength or fracture toughness of the soil, also changes upon drying. Finally, the adhesion at interfaces, which is essential in providing the restraint for desiccation cracks to form, changes with moisture content [12; 13]. [PERSON] et al. [8] showed that the hydraulic conductivity increases nearly two orders of magnitude due to the desiccation cracks of CCC, which would have a great impact on the cover system. [PERSON] and [PERSON] [14] presented that the hydraulic conductivity of cracked soils is typically several orders of magnitude greater than that of intact soils. To restrain the desiccation cracks of CCC, moisture retention additives were added to the soil to improve its moisture retention and desiccation cracks inhibition performance. Moisture retention additives could inhibit moisture evaporation and regulate soil temperature by adsorbing water hundreds or even thousands of times its weight [15]. According to different synthetic materials, the moisture retention additives can be divided into four kinds: modified starch, synthetic polymer, modified cellulose, and other natural compounds and their derivatives, blends, and composites [16]. It is cumbersome to prepare the synthetic polymer moisture retention additives. In addition, it is not suitable for practical engineering because the synthetic polymer would be completely degraded in soil within some years [17]. Therefore, natural materials and their derivatives could be selected as amended materials, namely attapulgite, diatomite, and zeolite. However, the natural zeolite cannot meet the requirements due to its small pore size and being easy to block [18]. Finally, attapulgite and diatomite are proposed as additives for amending CCC. Usually, the migration modes of gases in the soil included advection and diffusion [5]. [PERSON] et al.