# From Climate Conditions to the Numerical Slope Stability Analysis of Surface Coal Mines
###### Abstract  
A new perspective is presented for evaluating the slope stability of coal and lignite mines due to rainfall. The case of Greek lignite mining areas is employed to illustrate the methodology. Initially, past climatic records of rainfall are documented and analyzed; rainfall intensity varies from light (0.8 mm/h) to heavy (up to 9 mm/h). Few extreme phenomena are documented, with the maximum intensity being 17 mm/h. Furthermore, climatic projections of future trends are performed with open-access tools to anticipate possible deviations from the baseline conditions. Although the mean temperature is expected to increase, projections show that the past rainfall range is not expected to change. Finally, the effect of rainfall infiltration on the stability of a typical open-pit lighte mining slope is investigated by finite element analysis. The precipitation range defined by the environmental analysis is used. The SF is practically constant for the lower rainfall intensities (0.8-2.2 mm/h). For the higher intensities of 6.4 mm/h, 9 mm/h, and 17 mm/h, the SF decrease is almost the same (from 2.08 to 1.9), with reduction rates of 8.3%, 8.9%, and 9.3%, respectively. The effect of the critical geotechnical properties--groundwater table depth, unsaturated zone properties, and soil permeability--is also examined for a complete evaluation. Environmental engineering; geotechnical engineering; mining and environmental issues; mine reclamation; climate change; lignite +
Footnote †: journal: Applied sciences  
Intense rainfall and extreme weather events are often associated with landslides and catastrophes on natural or engineered slopes. The analysis and design of such slopes are currently largely based on numerical modeling. Several numerical methods have been developed for slope stability analysis, with two dominating the field: the limit equilibrium method (LEM) and the finite element method (FEM). These stability methods are typically coupled with a finite element analysis considering groundwater evolution during rainfall infiltration. As a result, many works, e.g., [2; 3; 4; 5], use the FEM for groundwater, rainfall, and stability, while others, e.g., [6; 7; 8; 9], combine the FEM and the LEM. The influence of rainfall on the slope stability of open-pit coal and lignite mines has long been identified and considered for design purposes, e.g., [10; 11; 12]. The investigation of precipitation's impact on mining slope stability has been primarily based on observations and empirical correlations between precipitation rates and increasing slope movements, e.g., [13]. This type of slope analysis, although practical, has only incorporated rain in indirect ways, e.g., the rise of the groundwater table rise or the increase of pore water pressures. In contrast, slope stability analysis of cut slopes (civil infrastructure) has progressed tremendously, employing unsaturated soil mechanics and advanced hydrological concepts to incorporate precipitation into numerical modeling, e.g., [2; 5; 14; 15; 16]. However, even in these cases, precipitation has been considered through parametric analyses, with precipitation characteristics usually dictated by general considerations. Differences among the two types of slopes (cut slopes in civil infrastructure versus overburden soil slopes in coal mines) refer mainly to: the enormous height of the mines' excavations, up to 200 m, versus heights up to 30-40 m for cut slopes; the smooth inclination usually utilized for excavations (versus steeper slopes often encountered in infrastructure); the large tolerance in displacements in mining slopes (even in the order of magnitude of meters) versus more strict limitations for typical cut slopes. Additionally, the soil types of cut slopes are frequently different from the excavation materials, including lignite deposits and seams and spoil materials, presenting considerable variability. A novel methodology is presented to evaluate coal and lignite mine slope stability due to precipitation.