Rock Wedge Stability Assessment : A Comparative Analysis of Limit Equilibrium and Discrete Element Methods

Abstract: Rock wedge stability is a common concern in underground excavations. Since the wedge stability influences the support design, it is important to use a wedge stability analysis method that can capture factors like excavation geometry, joint parameters and properties, rock mass properties, rock cover, and stress field as accurately as possible. This thesis compared the Limit Equilibrium Method (LEM) and the Discrete Element Method (DEM) using the software UnWedge (LEM) and 3DEC (DEM). The objectives included studying how factors like excavation and joint geometry, stress field, rock cover, and rock mass properties could be considered by the methods and how that affected the wedge stability and support design. Nine different analysis cases were defined with the aim of capturing wedge stability analysis parameters representable for both the civil engineering and mining sector. The study found that 3DEC allows for more accurate modelling of excavation and joint geometry, considering joint density and full 3D geometries, while UnWedge has limitations in creating intersecting tunnels and does not consider joint density. Only 3DEC, with stress redistribution and a plastic material model, captures rock mass failure mechanisms other than wedge failure. Based on the study, it is recommended to use DEM for situations where the excavation geometry cannot be assumed as two-dimensional with constant cross-section, and, when stress-induced rock mass failure is expected, use DEM with stress redistribution. It is also recommended to use information about joint lengths if available and apply engineering judgement when studying results of wedge volume and support force.