Characterization and analysis of a translational rockslide on a stepped-planar slip surface

- Small translational rockslide on a stepped-planar slip surface was documented.
- Ordinary field photographs were used to extract nearly all quantitative data.
- Slope and joint geometry, friction and dilation angles determined from 3D models.
- Stability analyses considered water pressure, cohesion loss, and rockbolt corrosion.
- Inadequate rockbolts, rainfall, and cohesion loss over time caused the rockslide.

In 2014 a small stepped-planar rockslide blocked a road near the town of San Germano Chisone in northwest Italy. Photographs of the rock cut that failed above the road were taken before and after the rockslide occurred. Some photos were acquired with the assistance of a UAV. These photos were used to conduct a forensic analysis of the rockslide. With the aid of photogrammetry software, it was possible to characterize the slope geometry and the larger geological structures that influenced the slope behaviour. A stepped-planar slip surface and the shape of the slabs of rock that failed were defined by discontinuities and these were well documented in the 3D photogrammetry models. The effective dilation angle was estimated from an assessment of the roughness of the discontinuity surfaces forming the slip surface. The stability of the rockslide was analysed as a stepped-translational failure mechanism. A two-block force equilibrium model of the rockslide was developed. With this analytic model, both deterministic and probabilistic methods were used to study the influence of water pressure as well as time-dependent shear strength degradation along the discontinuities and corrosion of rockbolts. The analysis suggests that the rockslide was caused by a combination of inadequate rock support, time-dependent shear strength degradation, and water pressure in the joints resulting from above average rainfall.