Geological and geophysical investigations to analyse a lateral spreading phenomenon: the case study of Torrioni di Rialba, northern Italy

We combined geological, geomechanical, geophysical and remote sensing investigations, including persistent scatterer interferometry and bathymetry, to study a slope where four conglomerate towers laterally spread over a shale layer. Electrical resistivity tomography surveys confirm a shale layer that underlies the rock towers with an attitude parallel to the slope. Field mapping reveals that the stability of the rock towers is threatened by weakly cemented conglomerate layers, large eroded zones and karstic weathering due to water circulation. We deem that the most probable failure mechanism would be the toppling of the southernmost tower, promoted primarily by the weak conglomerate layer in its lower section. The plastic shale layer underneath the rock cliff is very likely to promote lateral spreading that may have triggered the toppling of an additional former rock pillar, whose rock blocks were found in the lake during a bathymetric survey. Close- and long-range remote sensing of displacements provide no results that could be interpreted with confidence. Seismic noise recording sessions with 3C low-frequency velocimeters suggest that the dynamic conditions of the towers do not show appreciable variations across the recording sessions, especially considering the two rock towers featuring the most interesting spectral characteristics. However, both the resonance frequencies and the preferential oscillation directions estimated from the seismic noise datasets are consistent with the analytical relationships and will support the design of an effective monitoring strategy.