Stability of Deep Excavation Slopes in Continuous Surface Lignite Mining Systems

The paper investigates the stability of deep continuous surface lignite mining slopes. As excavation slopes are made relatively steep for exploitation purposes, the risk of slope instabilities is often appreciable, while the adverse consequences of a major failure are usually severe. Despite these, the design of lignite mining slopes against instability is often empirical, because the mechanisms causing such instabilities are poorly understood. Experience shows that slope instabilities are usually governed by sliding along a sub-horizontal, unfavourably sloping interface between a coal seam and an underlying stiff, high plasticity clay or marl layer, very close to the bottom of the slope. The typical mechanism of such instabilities is triggered by the sharp contrast in stiffness between adjacent lignite and clay/marl layers causing different elastic rebound upon removal of the horizontal confinement during excavation. The paper examines the effect of specific parameters on slope stability by a set of parametric analyses. It is shown that the most important parameter is the inclination of the sub-horizontal lignite–clay interface at the base of the slope. The paper also reviews several slopes in lignite mines which remained stable despite movements with relatively constant velocities reaching up to 100 mm/day, while others have failed when velocities accelerated abruptly although much smaller. These cases show that the absolute magnitude of slope velocity is not always relevant in predicting slope instability, while slope acceleration is a better indicator.