# Experimental Characterization of the Engineering Properties of Landfill Compost-Biocover
## 3 Results and Discussions
### Hydraulic Behavior of Compost Material  
It can be seen from Figure 10a that the hydraulic conductivity of compost material shows a gradual decrease with an increase in water content for the dry optimum. Minimum hydraulic conductivity occurs at a water content of 86%, slightly above the optimum moisture content, similar to [42]. Beyond this point, k initiates to increase with water content. This hydraulic behavior of compost materials can directly be associated with the adsorption of water on solid surfaces, i.e., electrostatic forces. Though water is continuously added to the sample on the dry optimum side, k decreases gradually. The added water is used to increase the dry unit weight and degree of saturation of the specimen (Figure 7), which compels the compost particles closer to each other. Alternately, the free air space squeezes (Figure 8), and due to this, the attractive interfacial force between water cations and colloid anions gradually increases due to high solid-water interactions. Alternately, hydraulic conductivity decreases, or in other words, at low water content, the diffuse double layer of ions surrounding the particle is not freely developed. The cation concentration increases with increased water content, which therefore diffuses toward the colloid surface to equalize concentrations [43]. As in this study, the ions equalization has taken place at a water content of 86%. Figure 10: _Cont._  
The hydraulic conductivity increases beyond this water content, i.e., 86%, as the water is added to the compost specimens. At this point, the compost material has almost reached close to saturation, as can be seen in Figure 10b. Due to a higher degree of saturation, the added water dilutes/reduces the concentration of soil solids per unit volume, providing a gradual decrease in dry unit weight and increase in porosity as well (Figure 6), or in other words, beyond the optimal water content, the diffuse double layer of ions surrounding the clay particles expands significantly, which consequently reduces the interfacial attractive forces between solid particles and water molecules, as the repulsive forces of water molecules dominate the attractive forces between water molecules and solid colloids, and due to which, the swelling of the compost materials occurs, as seen from the porosity profile (Figure 6), which therefore opens easy paths for water to flow through the pores due to less interfacial attractive force, so k starts to increase. Furthermore, it can also be seen from Figure 10a that similar to other materials, k of compost material is relatively high for the dry optimum segment in comparison to the wet optimum segment, which is because on the dry optimum side, the compost has lower water content/degree of saturation, it seems to have a flocculated structure with continuous voids and due to this it provides higher hydraulic conductivity than the wet optimum, for which the compost has a higher water content/degree of saturation. However, the structure appears to be dispersed, providing a more tortuous path, so the hydraulic conductivity is less, as in Lambe [42] and Seed and Chen [44]. The studies stated that the degree of particle orientation changed with changes in moisture content of compacted Boston Blue clay and kaolinite soil minerals. Compaction induced changes in the structure of the cohesive soil, which influenced the hydraulic conductivity of the clayey soil [42]. Generally, the hydraulic conductivity of the studied material varies between 7 \(\times\) 10\({}^{-6}\) and 2 \(\times\) 10\({}^{-9}\) cm/s. This trend is in agreement with the data published in [45; 46; 25] for landfill covers, and it also falls in the range of natural clayey soil (1 \(\times\) 10\({}^{-6}\)-1 \(\times\) 10\({}^{-8}\) cm/s) [43]. The test data imply that compacting compost to low hydraulic conductivity can be accomplished for a wide range of water content.