# A Novel Technosol Formulation for Sustainable Landfill Top Covers Using Non-Hazardous Wastes
## 3 Results and Discussion
### Geotechnical Properties  
Effective landfill topsoil covers require a balance of key properties: mechanical strength, long-term stability, minimal infiltration, and the ability to facilitate lateral drainage. While WFS and WAS individually might not fully meet all these requirements, their strategic combination can achieve the desired particle size distribution, leading to the necessary geotechnical properties. The results of the evaluated geotechnical parameters are presented in Table 2.  
Grain size distribution significantly impacts water retention and permeability, which in turn influence compaction, drainage, and cover soil stability [39]. The waste materials used in the formulation of the protective layer showed distinct grain size compositions (Figure 3). WFS was coarse-grained, retaining over 80% of particles by weight on the #200 sieve (0.075 mm), while WAS consisted of fine-grained particles, with almost 90% of fine particles by weight. The fine-grained fraction varied among mixtures, ranging from 20% to 35%. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Geotechnical Parameter** & **Waste Foundry** & **Washing Aggreg.** & **Mixture 90:10** & **Mixture 80:20** & **Mixture 70:30** \\  & **Sands (WFS)** & **Sludge (WAS)** & **(WFS/WAS)** & **(WFS/WAS)** & **(WFS/WAS)** \\ \hline Fine-grained fraction 1 (\%) & 17.5 & 89.4 & 20.6 & 26.4 & 34.8 \\ Liquid limit (LL) & 26 & 60 & 28 & 27 & 33 \\ Plastic limit (PL) & no plastic & 29 & no plastic & no plastic & no plastic \\ Plasticity index (PI) & 0 & 31 & 0 & 0 & 0 \\ USCS classification 2 & ML & CH & ML & ML & ML \\ Optimum moisture & 13.40 & 26.95 & 13.57 & 15.36 & 15.45 \\ content (\%) & 1.74 & 1.50 & 1.77 & 1.75 & 1.74 \\ Maximum dry density & 1.74 & 1.50 & 1.77 & 1.75 & 1.74 \\ (g/cm\({}^{3}\)) & 2.50 & 2.58 & 2.47 & 2.50 & 2.50 \\ Relative density of & 2.50 & 2.58 & 2.47 & 2.50 & 2.50 \\ particles (G) & 0.44 & 0.72 & 0.40 & 0.46 & 0.46 \\ Free swelling (\%) & 0 & 0.25 & 0 & 0 & 0 \\ Swelling pressure (kPa) & 0 & 0 & 0 & 0 & 0 \\ CBR index & - & - & 23.2 & 8.4 & - \\ CBR swelling (\%) & - & - & 0.58 & 0.76 & - \\ Permeability coefficient & - & - & \(1.48\times 10^{-6}\) & \(1.00\times 10^{-6}\) & - \\ (cm/s) & - & - & 40 & - & - \\ Cohesion (kPa) & - & - & 30 & - & - \\ Friction angle (\(\phi\), degree) & - & - & 30 & - & - \\ \hline \hline \end{tabular}  
* Material passing through the #200 sieve size (0.075 mm); 2 USCS classification (ML: silty sands with slight plasticity; CH: inorganic clays of high plasticity).  
\end{table} 
Table 2: Geotechnical parameter values of the tested samples, including percent finer, Atterberg limits, optimum moisture content, maximum dry density, relative density, void ratio, swelling potential, California bearing ratio (CBR), permeability coefficient, and shear strength parameters. The blend 90 WFS/10 WAS maintained a moderate fine-grained fraction, balancing coarse and fine particles, which could enhance both compaction and permeability. WFS is known for its ability to granulometrically stabilize clayey soils, making it a suitable protective cover material in landfill settings [40]. Selecting a moderately fine-grained blend like 90 WFS/10 WAS could improve compaction and alleviate settlement concerns. However, mixtures with higher fine-grained fractions, such as 80 WFS/20 WAS and 70 WFS/30 WAS, may require additional consideration to ensure effective compaction and proper drainage. This strategic choice aligns with sustainable landfill cover design principles by promoting long-term stability [41] and minimizing maintenance requirements. High-plasticity materials in landfill top covers can lead to significant volumetric changes due to moisture fluctuations, often resulting in shrinkage and cracking. Materials with high liquid limits and plasticity indices exhibit increased settlement potential and volume changes. Among the tested materials, WAS displayed the highest liquid limit (LL = 60), indicating substantial plasticity and susceptibility to volume change during wetting and drying cycles.