Transition from rock mass creep to progressive failure for rockslide initiation at Mt. Conero (Italy)

The full description of rock mass behaviour involved in slope deformation, through its onset and progression until ultimate failure, is still a challenge in numerical simulations. The slope deformation that involved the coastal sector of Mt. Conero (Italy), leading to the rock avalanche of Portonovo, is an impressive example of a multistage creep evolution involving a jointed rock mass towards general failure. A landslide mass for the last event up to 5 M m3 has previously been estimated from geomorphological analysis. This volume is not consistent with the size of the scar or the landslide deposit, suggesting a history of failures. Moreover, field evidence of pre-failure strain suggests progressive failure of the slope leading up to failure. In this study, a numerical back-analysis was set up using the finite difference method in a stress–strain approach, which applied an uncoupled solution to simulate the creep process and the progressive failure of the slope, as well as the accumulation of damage. The modelling outputs support the hypothesis that the landslide event was marked by two distinct episodes, each with volumes of approximately 20 M m3, at 56 and 16 ka, which is consistent with the relative dating constraints. Additionally, a numerical back-analysis of the rock mass viscosity was calibrated and validated on the size of the scar area of the Portonovo landslide and on the morphological and structural evidence. The adopted strain-softening models caused significant differences in strain distribution and slope failure evolution. The proposed modelling demonstrated the importance of the creep process in controlling slope evolution and the reliability of the tested analytical approach for the analysis of progressive brittle rock failure, including the three-stage evolution of time-dependent viscous processes.