# From Climate Conditions to the Numerical Slope Stability Analysis of Surface Coal Mines
## 5 Numerical Analysis of Slope Stability of Lignite Mines under Extreme Rainfall Events
### Effect of Rainfall on Slope Stability  
A typical 50 m deep pit excavation is considered with a mild slope of 14\({}^{\circ}\). This geometry represents a deep excavation for civil infrastructure but not so deep for coal and lignite mines. This height is over the limits of the literature regarding the parametric analysis of artificial (cut or embankment) slopes (see also Section 2). A parametric study was performed for the critical parameters of groundwater depth, soil, and unsaturated zone properties (see Figure 1 and Section 1). The groundwater was considered through a steady-state flow, and the effect of its depth was investigated. The effect of the unsaturated zone was examined through the parameter \(\rm g_{a}\), as analyzed in the previous section. Finally, the soil's permeability, the main soil property related to infiltration, was parametrically investigated. The chosen saturated permeability lies in the range of the dominant overburden material of Greek open-pit lignite mines [31]. Figure 7 illustrates the FEM model, including the geometry, the hydraulic characteristics, and the discretized mesh. Two-dimensional (2D) plane strain and drained conditions were used. Vertical boundaries were at a 2H distance from the slope's crest and toe, respectively. The horizontal bottom boundary was at a distance, H, from the slope's toe to minimize boundary conditions; H stands for the slope's overall height. A fine mesh was used for FEM, and mesh density was increased in critical areas. The initial hydraulic conditions (before the rain starts) were established by a steady-state flow, defined by two boundary groundwater heads: the water is located at depth \(\rm H_{w}\) at the left border and the bottom of the excavation. Figure 7: FEM pit slope model’s geometry and hydraulic conditions. Rainfall intensity was simulated by imposing appropriate boundary flux velocities. Different rainfall intensities were used, representing a wide range. A low-to-medium range from 0.8-6.4 mm/h (19.2-153.6 mm/day) and two extreme cases, 9 mm/h (216 mm/day) and 17 mm/h (408 mm/day), are employed based on the climatic measurements and projections analyzed and discussed in the previous sections. The time duration of the rainfall for the simulations is one day (24 h). For the reference case, the groundwater head, H\({}_{\text{w}}\), was 20 m, and a medium rainfall intensity was chosen of 3.6 mm/h (86.4 mm/day). All the physical and hydro-mechanical parameters of the reference model's soil material are provided in Table 2. The fundamental parameters employed in the numerical model, including the geometry (height and angle) and the soil shear strength, have been validated to represent actual cases of Greek lignite mines. The parameters that have not been extensively validated are considered in the parametric analysis that follows to obtain the range of the results. The possible spatial and temporal variability of the basic parameters was not considered in this work to retain the practical and general nature of the framework. Nevertheless, if the soil variability is known, this could be involved in the methodology by applying the appropriate changes in the numerical models. The Mohr-Coulomb constitutive model with a non-associated flow rule was used. Four critical parameters were varied to evaluate their effect on stability: the rainfall intensity, the initial groundwater head, H\({}_{\text{w}}\), the soil's saturated permeability, and the SWCC parameter, g\({}_{\text{a}}\). Table 3 shows the examined values; all other parameters are kept constant. Typical parameters affecting slope stability, such as the soil's strength, are not investigated. They are well-known to affect slope stability but do not relate to the influence of rainfall infiltration.