Earthquake-induced rockfalls and rockslides are a major component of landslide hazard in Central Italy as they posed major threat to people, buildings and infrastructures in the last 50 years. During the recent 2016 earthquake sequence, many landslides have been systematically collected and documented. In the framework of the Sapienza grant in 2017, several surveys in the area hit by the 2016 earthquakes and analyses of landslides allowed recognition of further instability phenomena. Moreover, the analysis of related data posed some questions about instability mechanisms, which deserve further investigations and analysis. Main findings of this preliminary reconnaissance work has been made available on the web through freely downloadable reports and published in peer-reviewed journals.
The proposed research project can be considered a continuation of the 2017 Sapienza grant. In this view, two large rockslides and a rock toppling phenomenon have been identified for investigation and analysis. Particular attention will be focused on the peculiar structural features that characterize the selected landslides. To this aim, innovative procedures will be carried out such as the high-resolution UAV photogrammetric surveys that will allow, in addition, to get a precise geometrical 3D model of the rockslides. Site investigation through geophysical methods is a very important part of the proposed project, whose objective is twofold: to define the geometrical characteristics of landslide bodies and to characterize the low-strain elastic stiffness through seismic velocity. The outcomes of these investigations will provide the input data for numerical modelling of the unstable slopes under dynamic actions.
Insights on the main factors predisposing instability in different structural and geotechnical contexts will be finally provided and therefore will be ultimately useful to improve procedures for assess landslide susceptibility and for mitigating seismic risk in mountain areas.
Rockslides are deeply influenced by site-specific structural features. This aspect augments the peculiarity of each rockslide and makes more difficult to find classes of rockslides that can be modeled similarly. Therefore it is quite worthy to study cases for which the structural framework is directly observable from the surfaces exposed by the detached blocks and at the same time the actual seismic action that produced the failure is estimable.
The three landslides will be analysed seeking for the particular structural features that characterize them. To this aim the data collected through the investigation techniques will be analysed with innovative procedures. In particular the high-resolution photogrammetric imaging dataset from UAVs could be used not only to get a precise geometrical 3D model of the rockslide but also with the following intents:
- to identify the prevalent original mechanism that detached the rockslide from the rock mass; in particular data on local orientation of the planar surfaces are important to reconstruct the original discontinuity orientations that formed the sliding surfaces.
- to distinguish pre-existing discontinuities from new fractures that developed during the rockslide and possibly to recognize the mode of failure that involved the new fractures (rock bridges); this is an important issue in order to estimate the cohesive contribution to the resistance of the sliding mechanism.
- strictly related to the previous point is the attempt to distinguish the original volume that was mobilized by the inertial forces of the seismic event from the eventual further volumes that were involved only in the catastrophic phase of the rockslide because hit by the first volume.
Site investigation through geophysical methods is a very important part of the proposed project, conducted also through innovative procedures and taking advantage of long-term scientific collaborations with several national research centres. The objective of geophysical investigations is twofold: to define the geometry and shape of the landslide body and to characterize the low-strain elastic stiffness through seismic velocity. To this regard, horizontal profiles of seismic velocities potentially can provide details about the improving mechanical properties of the rock mass with depth from the surface of the cliffs and therefore could be employed to better characterize the structure of the rock mass. These data are important to define the stiffness of subgrade reaction that a deformable formation can provide respect to overlying rigid blocks (a typical geotechnical condition in layered clayey and arenaceous flysch deposits). On-site seismic characterization is needed to complement dynamic laboratory testing in order to fully characterize the seismic behaviour.
The laboratory test campaign will include several usual mechanical tests on intact rock specimens and rock joints. Nevertheless tests of new conception will be carried out to investigate some peculiar behaviour not yet well analysed in the literature. In particular the behaviour of rock bridges along slide surface for cyclic and dynamic loads are quite unknown even they provide a significant contribution to the shear resistance. In this perspective cyclic shear tests and shaking table tests on joints in calcareous rocks in presence of rock bridges could be performed. Specific objectives are the estimate of both the peak strength and how the strength drops as displacements develop after the peak conditions.
In the modelling phase of the project, distinct elements and finite difference methods will be employed to simulate the mechanical and loading conditions of the observed case histories. Nevertheless simpler models could be developed in order to focus on single aspects of the phenomena that can be easily described through only few variables. Simple models, although not always applicable to real slopes, can show the influence of different factors thus enhancing the general comprehension of a failure mechanism.
The results of this research have the potential to increase the ability of engineers and researches to predict and prevent rock slope failures. This will mitigate the economic consequences and social diseases to the transportation network and economic loss of property and could even save lives. The main goal is to disseminate the results of this research to as many audiences as possible and to increase interest in the topic. It is anticipated that publications on leading peer-reviewed journals will result from the research activities.