## 2 Materials and Methods
### Observational Method Approach Adopted in the Present Study  
The exploitation and reclamation of a landfill is a long-term process that requires applying interdisciplinary approaches. However, the crucial part determining the effectiveness of the process is the geotechnical structures incorporated within the landfill site (namely, reinforcing structures, barriers, cut offs). The landfills can be considered as earth engineering structures filled with anthropogenic material with associated protection systems such as liners, drainage, and reinforcements. The engineering works on a landfill require the application of certain rules when waste layering, compaction, or even reinforcing structures designed for steep slopes on embankment-type landfills. Due to the long-lasting reclamation process and its complexity, so-called observational methods are becoming \begin{table}
\begin{tabular}{c c c} \hline \hline
**Mechanical Properties** & \\ \hline Tensile strength at 2\% strain (kN/m) & 19.0 \\ Tensile strength at 5\% strain (kN/m) & 33.5 \\ Peak tensile strength (kN/m) & 55 \\ Yield point elongation (\%) & 11.2 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Mechanical properties of uniaxial geogrid. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Waste Material** & \(\mathbf{\Gamma}\) [kN/m\({}^{3}\)] & \(\mathbf{\Phi}\) [\({}^{\circ}\)] & \(\mathbf{C}\) [kPa] & **Testing Methods** \\ \hline Solid waste & 9.0 & 20 & 25 & trial loading, CPT, WST \\ Solid waste + sand & 12.0 & 25 & 23 & trial loading, CPT, WST \\ Old waste & 14.0 & 26 & 20 & back-analysis, CPT, WST \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Shear strength parameters for municipal solid waste obtained from diver’s approaches. Figure 2: (**a**) Geometry of uniaxial geogrid and (**b**) landfill slope reinforced with geogrid. more popular in landfill engineering practice [5; 6]. An observational method in geotechnics is a continuous, controlled, and integrated design process. It is considered an execution process control tool, monitoring system, and a review method allowing the introduction of an initially defined modification during the working plan execution. The objective of the observational method is the reduction of investment expanses with the simultaneous increase in safety level of the investment. The method could be applied in the early stage of design or after completing certain tasks if the benefits derived from its use were identified at earlier stages of the investment process [21]. The observational method is always a combination of a common geotechnical site investigation, theoretical modeling, and plans for unpredicted scenarios. Using only monitoring results, the structure safety is assured to not be enough according to the international standards [22; 23]. The key is to identify the potential failure mechanism. The investigation needs to be designed carefully so the state limits are monitored. They always need to be comprehensively conducted and executed in a strict manner. The effectiveness of the observational method mainly depends on the flexibility of the management system. It is necessary that the alternative plans are validated and applied at every stage of the investment process. Drawing the limit boundaries in the observational method before engineering works execution is a challenging and complex task. It requires the use of sophisticated estimation models depending on the analyzed process. The key factor here is developing an effective control program and comprehensive monitoring. The method, when applied in geotechnical engineering, mainly covers monitoring implementation and setting the boundary levels, and, when exceeded, allows appropriate decisions to be made regarding further stages of the investment. It allows execution works or repetitions of analyses to be improved and amendments to the project to be made. In the observational method, there is a rule of threshold settings. These are the boundaries determining the timescale for taking appropriate action.