Mechanisms of Block Instability at the Toe of a Slowly Deforming Rock Slope

• Published in: Rock mechanics and rock engineering 2024-03, Vol.57 (3), p.1543-1563
• Main Authors: Carlà, Tommaso, Gigli, Giovanni, Lombardi, Luca, Nocentini, Massimiliano, Gracchi, Teresa, Rossi, Guglielmo, Tacconi Stefanelli, Carlo, Raspini, Federico, Fubelli, Giandomenico, Casagli, Nicola
• Format: Article
• Language: English
• Subjects: Civil Engineering; Complexity; Damage patterns; Deformation; Degradation; Design; Displacement measurement; Earth and Environmental Science; Earth Sciences; Environmental hazards; Failure modes; Finite element method; Friction; Geological hazards; Geophysics/Geodesy; Instability; Kinematics; Landslides; Lasers; Mathematical analysis; Mathematical models; Mitigation; Monitoring; Mountain regions; Mountainous areas; Numerical models; Original Paper; Remote sensing; Rock; Rockfall; Rocks; Rockslides; Roughness; Rupture; Scanners; Series (mathematics); Slope; Surveying; Talus; Terrain; Topographic mapping; Topographic surveying; Topographic surveys; Topography; Transportation corridors
• ISSN: 0723-2632; 1434-453X
• DOI: 10.1007/s00603-023-03674-9

Steep alpine rock slopes undergoing deformation may give rise to concurrent landslide hazards of different type and magnitude. The underlying mechanisms of instability are often challenging to investigate due to their inherent complexity; furthermore, they can occur on poorly accessible terrain, preventing the collection of data by means of traditional field techniques or even inhibiting awareness of hazards. This paper focuses upon one such case, in which a major transportation corridor running along the floor of the Aosta Valley (Western Italian Alps) is affected by significant—and until recently unknown—rockfall hazards promoted by a previously collapsed rockslide still deforming slowly at elevations almost 600 m above the road. In particular, two large discrete blocks (volume > 10 3  m 3 ) lie precariously at the toe of the slide and could fall downslope at extremely rapid velocity. The design of countermeasures for the stabilization or removal of these blocks would require the assessment of their mechanical interaction with the bedrock and degree of internal fracturing (i.e., possible pervasive damage within the blocks). We perform this task by first exploring potential kinematic styles and damage patterns at failure according to a series of preliminary finite-element models. We then use detailed displacement measurements from remote sensing and in situ monitoring, in conjunction with repeat topographic surveying from a terrestrial laser scanner (TLS) and a drone laser scanner (DLS), to reconstruct the actual kinematics of the blocks. The results substantiate the hypothesis that instability is primarily controlled by transient degradation of friction on a through-going basal rupture surface. Development of a large tensile fracture in one of the two blocks is inferred to be conditioned by increased non-planarity of the slipping joint in comparison with the other block. We highlight that optimized integration of cutting-edge rock slope investigation tools can help address otherwise unresolved key aspects of complex instabilities in steep mountainous areas. Highlights Two large discrete blocks at the toe of a slowly deforming and highly disaggregated rock slope pose significant concerns to a major transportation corridor in the Western Italian Alps. Knowledge of the underlying mechanisms of instability is required to design mitigation measures, but traditional site characterization is impossible due to difficult terrain. Preliminary numerical modelling,