## 2 Materials and Methods
### Slope Stability Simulation Using Roccience Software Program  
The slope stability simulation was conducted using limit equilibrium analysis with Roccience Slide2 software, a two-dimensional slope stability program that evaluates the factor of safety or failure probabilities within a given slope geometry. The base slope form, external loading, groundwater, and reinforcements can be modeled for different cases in simulations and analyses. The software program analyzes slopes by determining the stability of slip surfaces using various methods. In this study, we considered the Bishop Simplified, GLE/Morgenstern-Price, Janbu Simplified, and Spencer methods of slope stability analysis in the calculation of the factor of safety of the slope [1]. Since the main purpose of the simulation was to compare the performance of synthetic geotextiles to that of coir geotextiles as quantified in terms of the factor safety of the slope, a fictitious slope geometry (as shown in Figure 5) was created and used for the analysis. A slope angle of 45 degrees was considered in the slope geometry, as this was considered a steep slope [30]. The fictitious slope has a base of 80 m and a maximum height of 40 m.  
Figure 4: Test methods used to determine the mechanical properties of nonwoven coir geotextiles. (**a**) Grab-breaking test. (**b**) CBR puncture test. The retained soil of the slope, colored in red, was set to be silty sand, and the backfill or the reinforced soil is colored in orange. The groundwater table is represented by the blue line from the left to right of the slope geometry. The properties of the soil used in the analysis are shown in Table 1. All the soil parameters are assumed values and were set constant in all cases for slope stability simulations. With the grab-breaking strength of each type of geotextile identified, the reduction factors listed in Table 2 were then applied to attain the allowable tensile strength used in the analysis. The thickness of the synthetic geotextile was the same as that of the nonwoven coir geotextiles in order to assess the effect of the different treatments on the nonwoven coir geotextiles in the stability of the slope. All the grab-breaking strength values were multiplied by 3 mm, the original thickness of all samples before treatment so that the strength was expressed as load per unit width. As for the nonwoven synthetic geotextile, a sample with the nearest ultimate tensile strength to that of the nonwoven coir geotextile was considered. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Property**} & \multicolumn{2}{c}{**Soil**} \\ \cline{2-3}  & **Silty Sand** & **Backfill** \\ \hline Unit Weight (kN/m3) & 18 & 16 \\ \hline Strength Type & Mohr-Coulomb & Mohr-Coulomb \\ \hline Cohesion (kPa) & 22 & 2 \\ \hline Angle of Friction (°) & 35 & 35 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Soil properties used for the slope stability analysis. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{} & \multicolumn{2}{c}{**Reduction Factor**} \\ \cline{2-3}  & **Coir Geotextile** & **Synthetic Geotextile** \\ \hline Creep & 2 & 1.6 \\ \hline Installation & 1.2 & 1.1 \\ \hline Deterioration & 1.4 & 1.1 \\ \hline Factor of Safety & 1.5 & 1.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Reduction factors applied to geotextiles [23; 31]. Figure 5: The geometry of the slope used for the slope stability analysis with Roccience Slide2. A summary of the procedures is presented in Figure 6. The geotextiles were subjected to the treatment processes. One set of geotextiles was subjected to merecization treatment, another set of geotextiles underwent treatment by bleaching, and one set was not subjected to any treatment, serving as control specimens. After the treatment process, each group of treated geotextiles and the untreated group underwent exposure, namely no exposure, field condition simulation, and groundwater exposure.