# From Climate Conditions to the Numerical Slope Stability Analysis of Surface Coal Mines
## 2 The Methodology and the Case Example of Greek Lignite Mines  
The present study presents a methodological framework including the assessment of past and current rainfall, future rainfall projections, and numerical analysis of the effect of rainfall on the slope stability of a case example. Figure 1 presents an overview of this work's methodology. This flowchart focuses on the slope stability of surface coal mines at the center and includes all aspects as conceptualized herein. Table 1 summarizes the main parameters of each aspect for some previous characteristic studies, analyzed in the following: \(\mathbf{g}_{\text{a}}\) is a parameter of the soil-water characteristic curve; \(\varphi^{\prime}\) and \(c^{\prime}\) are the effective friction angle and cohesion; \(\mathbf{k}_{\text{s}}\) is the saturated permeability; H is the height of the slope; q is rainfall intensity; \(\text{H}_{\text{w}}\) defines the groundwater table's depth. The upper part of Figure 1 describes the present work's approach to evaluating precipitation, while for the other aspects, the most critical parameters are included in a parametric analysis. Unlike previous works where precipitation was estimated based on global data [2; 3; 5; 9; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25] or was derived for a specific period as a particular time series [3; 14; 26; 27; 28; 29; 30], the way to assess the precipitation range is through past events and climate projections of the investigated area. This approach provides a clear advantage in terms of how the rainfall range is calculated. Greek mining areas are utilized to quantify the precipitation range because it is area-specific, and the methodology followed needs specific to the areas to be implemented. Figure 1: Flowchart presenting an overview of this work’s methodology. The lower part of the flowchart describes the slope (geometry, groundwater, soil, and unsaturated zone properties). As briefly mentioned in the introduction, the geometry of surface coal and lignite mines differs from embankments, cut, or natural slopes. Previous studies have mainly focused on engineered slopes less than 30 m high, with one work studying a slope reaching 40 m [5]. Additionally, three works have analyzed natural slopes of more than 50 m, but these present case studies and not systematic analyses of this type of slope [3; 27; 28], thus ignoring methodological issues and not evaluating the effect of the various parameters. Moreover, the groundwater conditions, crucial for the stability, differ in surface mine slopes; in the literature, a narrow range is commonly employed  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**References** & **g\({}_{\mathbf{a}}\)** & **\(\varphi^{\prime}\)** & **c\({}^{\prime}\)** & **k\({}_{\mathbf{a}}\) (cm/s)** & **H (m)** & **q** & **H\({}_{\mathbf{w}}\)** & **Comments** \\  & **(m\({}^{-1}\))** & **(c\({}^{\prime}\))** & **(kPa)** & **k\({}_{\mathbf{a}}\) (cm/s)** & **H (m)** & **q** & **(mm/h)** & **(m)** & **Comments** \\ \hline Cai and & 1.06–7.09 & 25 & 8 & \(10^{-4}\)–\(6\times 10^{-3}\) & 10 & 5–50 & \({}^{*}\) 0 & Cut \\  & [PERSON] [2] & & & & & & & \\ \[PERSON], & & & & & & & & \\ \(\quad\) & [PERSON], & & & & & & & \\ \(\quad\) & [PERSON], and & & & & & & & \\ \(\quad\) & [PERSON] [20] & & & & & & & \\ \[PERSON] and & & & & & & & & \\  
for the depth of the groundwater table, usually evaluated as horizontal or with a small inclination. In surface mines, the water reaches the toe of the slope, and inside the slope, the steady-state flow of the water varies significantly due to varying initial conditions and drainage wells. These are considered in the analysis that follows through a steady-state groundwater flow with largely varying groundwater depth. Finally, soil properties, including the unsaturated zone, are case- and site-specific. Due to the lignite basins' creation process, it is common to observe fine-grained materials overlying and mixing with the lignite layers and seams.