## 2 Materials and Methods
### Processing and Application  
The presented study conducted a stability analysis to estimate the effects of rapid drawdown and seepage conditions on the Alavian earth-fill dam as a case study. The purpose of the application is to investigate the earth dam's stability during drawdown conditions depending on factors such as soil permeability, drawdown rate, and drawdown ratio. Also, the transient drawdown corresponds to different rates and ratios of drawdown relative to the permeability in the dam's core material, which is considered in the calculations. To this end, a coupled transient seepage and deformation analysis was performed to conduct a stability analysis of the studied dam by using both LEM and FEM models. In this regard, the GeoStudio package and PLAXIS software were used to evaluate the problems. As part of the processing of the modeling, three different stages were conducted to provide the earth dam's stability in simulations. In the first step, the geometry of the dam specifies the exact design plan based on the information in Figure 1 and Table 1, which is called the geometrical model. Secondly, specify the boundary conditions and perform the material behavioral model for describing the failure criterion. The modified Cam-Clay model, one of the most-advanced nonlinear elastoplastic material models, was chosen for the core material in order to compute the stress-induced pore-water pressure as realistically as possible. For shell materials, the Mohr-Coulomb model was selected.  
\begin{table}
\begin{tabular}{l c c c c} \hline
**Parameters** & **Unit** & **Shell** & **Core** & **Filter** \\ \hline Hydraulic conductivity & m/day & 1.736 \(\times\) 10\({}^{-8}\) & 7.523 \(\times\) 10\({}^{-9}\) & - \\ Genuchten fitting’s \(\alpha\) coefficient & 1/cm & 12.52 & 28.57 & 12.52 \\ Genuchten fitting’s n coefficient & - & 5.60 & 1.213 & 5.60 \\ Genuchten fitting’s m coefficient & - & 0.295 & 0.175 & - \\ Volume compressibility coefficient (m\({}_{v}\)) & m\({}^{2}\)/MN & 1.0 & 1.0 & 1.0 \\ Saturated unit weight & kPa & 21.3–23.0 & 17.2–20.1 & 21.3–23.0 \\ Unsaturated unit weight & kPa & 19.5–20.0 & 15.6–17.9 & 19.5–20.0 \\ Consolidation coefficient (C\({}_{v}\)) & cm\({}^{3}\)/s & 0.0057 & 0.0055 & 0.0061 \\ Poisson’s ratio & - & 0.22 & 0.28 & 0.22 \\ Elasticity modulus & MPa & 30–80 & 10–30 & 80 \\ Shear modulus & MPa & 12.3–33 & 4.1–11.8 & 33 \\ Lame coefficient & MPa & 9.7–25.8 & 0.0053 & 25.8 \\ P-wave/Constrained modulus & MPa & 34.3–91.4 & 0.0772 & 91.4 \\ Friction & Degree & 35 & 20 & 35 \\ Cohesion & kPa & 0 & 50 & 0 \\ Critical state ratio (M) & - & 1.20 & 1.13 & 1.13 \\ Logarithmic hardening modulus (\(\lambda\)) & - & 0.220 & 0.225 & 0.225 \\ Isotropic swelling index (\(\kappa\)) & - & 0.085 & 0.077 & 0.077 \\ Unified classification & - & GW–SW & CL–CH & GW \\ \hline \end{tabular}
\end{table}
Table 2: Geotechnical properties for the Alavian dam’s seepage and slope stability analyses.  
Figure 1: The cross-sectional layout for the Alavian Dam.  
The material properties are assigned to the model according to Table 2, and the models are unscrambled for static stability analysis based on different rapid drawdown conditions. The results are used to provide the graphical charts to present the F.S variations versus the rapid drawdown parameters. Finally, in order to verify the models and the analysis, the estimated pore-water pressure during the real drawdown of the reservoir is compared to the real in situ measured pore-water pressure data from the dam. This data was gathered in 4 months from 1 June 2007 to 20 September 2007, by conducting onsite instrumentations.