Slope stability of deep surface coal mines in the presence of a weak zone

Abstract

Coal and lignite (brown coal) are geo-resources that have been governing energy production for decades. Geomechanical challenges, particularly slope stability, related to surface coal and lignite mines are critical during operation and will also determine the post-coal era for these areas. Several failure incidents (landslides) have been reported in mining areas, typically associated with a sub-horizontal failure surface on a weak -clay or marl -layer or an interface of low strength. This weak zone controls the soil profile in terms of stability and is common in several mines globally. In this work, this soil profile is analysed in terms of slope stability through the probability of slope failure; such a systematic analysis is currently missing from the literature. Initially, several numerical issues are addressed in the simulation of the problem. Moreover, the essential parameters' effect is examined: the height and the inclination of the slope, the inclination, thickness, and strength of the weak zone, and the strength of the overburden soil. Their effect on slope stability is evaluated by combining the probabilistic point estimate method with the finite element method and the shear strength reduction technique. This work can support a preliminary slope stability analysis and expands the knowledge and understanding of slope stability of a weak zone soil profile. It is concluded that in the presence of a weak zone, the inclination and strength of the weak zone and the water conditions are the most critical parameters and control the stability. Article highlights• Slope stability is performed on a soil profile with a weak zone, describing a common type of coal mines' failure in several countries. • Numerical issues of practical importance are addressed on the simulation and analysis of the problem. • The inclination and shear strength of the weak zone together with the water conditions control the stability of the mines' slopes.

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“…For the numerical model, the soil's friction angle and cohesion were φ=28° and c=185kPa (Theocharis et al 2021), while the weak zone's thickness was d z =5m. Note that Mikroutsikos et al (2021) showed that the weak zone's thickness is insignificant for the overall stability.…”

“…They concluded that the weak zone's inclination is the most critical parameter, while the importance of the weak zone's strength and the groundwater conditions was not explicitly examined. In a recent study, Mikroutsikos et al (2021) presented a systematic numerical ana lysis on the effect of essential parameters on slope stability in the presence of a weak zone. They validated that the weak zone's strength and the groundwater conditions are particularly critical, along with the weak zone's inclination.…”

“…led to smaller SF) with an SF error within 3-6%. Additionally, in a recent study referring to lignite mines, Mikroutsikos et al (2021) conducted similar FEM analyses considering the presence of a weak zone. In that case, SF's differences did not exceed 11% (models with benches provided higher SF).…”

“…DOI: 10.1201/9781003188339-47 2 NUMERICAL ANALYSIS 2.1 The effect of water filling on slope stability by the LEM In surface coal/lignite mines, slopes commonly consist of layers of sterile materials and coal/lig nite seams that are frequently (but not always) characterised by similar shear strength. As a result, under certain circumstances, considering the various soil formations as a single material for slope stability analysis is a reasonable assumption (Mikroutsikos et al 2021). Moreover, a bedrock formation with high strength is commonly encountered close to the excavation bottom.…”

Analysis of Faults' Effect on the Stability of Surface Lignite Mining Areas Using the Distinct Element Method

“…Lignite accounted for the country's energy supplies as much as 60% in 200460% in , 49% in 201160% in , and 29% in 201960% in (Eurostat 2020. Greek lignite deposits commonly lie at an average depth of 150 -200m below surface level and consist of lignite layers alternating with other geomaterials, such as stiff to hard clays and marls (Mikroutsikos et al 2021). Mining is performed practically exclusively on open-pit mines, and thus, substantial ground-moving processes have been a constant geotechnical challenge (Kavvadas et al 2020;Zevgolis et al 2019).…”