Structurally-controlled instability, damage and slope failure in a porphyry rock mass

Rock slopes fail through structurally-controlled mechanisms, global circular failures, or complex mechanisms depending on structural patterns and rock mass damage. Here, structural geology, rock mass characterisation, Terrestrial Laser Scanning (TLS), ground-based radar interferometry (GB-InSAR) and Finite Element modelling are integrated to explore relationships between structure, damage and global slope failure at Mt. Gorsa (Trentino, Italy). There a porphyry quarry has been excavated in complex, strongly anisotropic rhyolitic ignimbrite rock masses. The slope was affected by a major rockslide in 2003 and undergoes continuing instability. Site investigations and GB-InSAR monitoring revealed that the 2003 failure was a roto-translational rockslide involving about 400,000 m3 of disrupted rock. Structural analysis of TLS and field data shows that the slope is affected by widespread structurally-controlled mechanisms (sliding, toppling, strain localization in kink bands). The non-obvious relationships between structurally-controlled and global roto-translational slope failure mechanisms are investigated by characterising rock mass damage in different slope sectors. A new approach to quantify rock mass damage by mapping the Geological Strength Index and interpreting its topographic signatures in TLS point clouds is presented. A persistent geological marker is systematically mapped in TLS point clouds, and correlations between attitude variability statistics and rock mass damage are established, providing an efficient assessment tool. Rock mass damage increases in kinematic domains affected by structurally-controlled instability (GSI = 35-40) and is maximum in areas of ongoing global instability (GSI = 15-20). The 2003 rockslide occurred inside a damaged rock mass zone with GSI < 35-40, also suggested to be a threshold condition for the onset of global slope displacements by GB-InSAR data. Finite-Element numerical modelling allows integrating available data and observations. It is suggested that rock mass damage induced by local, structurally-controlled slope instability provides the required conditions (loss of structural pattern, block size reduction, cohesion loss) for transition to equivalent continuum behaviour and global slope failure