Integrated 3D geological and Finite Element modeling of slow rock-slope deformations affecting hydropower facilities

Slow rock-slope deformations are widespread in orogenic belts and pose significant threats to critical infrastructures, due to continuing slow movements and potential evolution to collapse. The analysis of related risks requires realistic models, accounting for the 3D complexity of both large landslides and infrastructures, often hampered by over-simplification of geological aspects.

We propose an integrated workflow for the 3D modeling of a complex system of deep-seated landslides affecting the N slope of Mt. Palino (Valmalenco, Italian Central Alps). The slope was carved by glacial and fluvial erosion in a complex metamorphic sequence including layers of metapelite, serpentinite, gabbro and gneiss with a regional foliation deformed in two folding stages. The slope hosts a hydroelectric power plant and related structures, affected by deformations observed since 1972. Site investigations (field surveys, full-core borehole drilling, seismic surveys) and deformation monitoring (EDM, GNSS, structural monitoring, GB-InSAR) show that the slope is affected by a deep-seated gravitational slope deformation, probably active before the LGM and partially collapsed, and by a system of nested large landslides, including a toe failure up to 200 m deep and two suspended rockslides affecting some of the structures.

We performed an accurate 3D geomodelling to provide sound constraints on the geometry, lithology, and mechanisms of the active landslides. By integrating all available geological data we reconstructed longitudinal and transversal cross-sections in MOVE^TM and performed implicit-surface interpolation in SKUA-GOCAD^TM, eventually obtaining solid objects corresponding to tectono-stratigraphic units that are dissected by the nested landslides. These volumes are populated with their rock mass properties, interpolated from boreholes and surface surveys. The geomodel shows a complex dome-and-basin folded structure, strongly constraining the spatial distribution and anisotropy of weaker rocks (e.g. serpentinites), and thus the geometry, kinematics, rock strength and shear zone properties of active landslides.

Based on the geomodel, we set up a continuum-based 3DFEM elasto-plastic model in MIDAS GTS-NX^TM. Individual solids in the analysis domain were discretized into a 3D mesh of 150000 hybrid finite elements with variable size in the range 20-200 m. Rock masses were considered as Mohr-Coulomb materials with tensile cut-off and post-peak dilatancy, while shear zones were included explicitly. After stress initialization, the model was ran with a Shear Strength Reduction (SSR) technique. Model parameters were calibrated using a quantitative back-analysis approach, optimizing the fit between normalized GB-InSAR measured displacements and computed displacements, projected in the radar LOS. The calibrated model was validated against field evidence and effects on man-made structures, and provided a starting point for forward modeling of the slope response to groundwater perturbations. We considered the effects of groundwater changes for 5 scenarios of perched aquifers, and assessed critical conditions corresponding to different instability scenarios with different impacts on the hydropower facilities.

Our results show that an explicit account for 3D geometrical and geological complexities is key to a realistic modeling of large slope failure mechanisms, their impacts on critical infrastructures and the evaluation of related risks.