Stability of a rock slope overlying a weak clay: the difficult case of Balze di Verghereto (Italy)

A common slope instability problem is the presence of hard rock lithotypes (such as limestones or sandstones) overlying weak rocks or soils (such as clays or shales). In this geological setting, hard rocks tend to create steep slopes or cliffs that become unstable because of the low shear strength of the underlying weak material. Landslides can take the form of slow, gradual spreading of rock blocks, or they can occur suddenly, in the form of a quick catastrophic collapse. These catastrophic landslides typically consist of deep rotational failures in the weak rocks induced by the weight of the rock mass above. The presence of groundwater at the contact between the two lithotypes and the generation of tensional cracks within the more brittle rock are additional factors that can influence the triggering of these hazardous landslides.

At the same time, gentle slopes below cliffs are ideal for human settlements. The physical presence of a cliff can act as a natural barrier, protecting the site from external threats and helping to preserve the site. Moreover, natural water springs or water-bearing strata are usually present in these sites due to the presence of permeable rock masses over less permeable materials. For these reasons, many cultural heritage sites are in these peculiar geological conditions. It is therefore important to assess the risk of landslides at cultural heritage sites and take appropriate measures to reduce the risk of damage and ensure visitors' safety.

In this study, we analyze the case of the Balze di Verghereto village located in the Nothern Apennines of Italy (Forlì-Cesena Province). This small historic village is built directly at the foot of a sandstone cliff and sits upon a heterogeneous clay formation. The site was affected by several landslides in the last century, and the main concern is now the collapse of the rock slope due to a deep rotational slide. Slope stability analysis of these phenomena are challenging for many reasons:

• Geological materials are difficult to characterize, especially in the case of overconsolidated-fissured clays and weakly-cemented rocks.
• Numerical instability can occur because of the presence of two geological materials characterized by very different mechanical properties (strengths and deformability).
• The abruptly stepped morphology, produced by the presence of different lithologies, complicates the generation of the grid model used to represent the slope.

To face these problems, stability analyses were conducted using different strategies. In particular, we compared finite-difference modeling performed by FLAC 3.4 2D software adopting different constitutive laws (elastic-plastic and anisotropic ubiquitous-joint), boundary conditions (fixed vs free boundaries), and the presence or absence of interfaces. Numerical simulations were then compared with general limit equilibrium analyses conducted using various potential shapes of slip surfaces (circular, composite, and trapezoidal).

The results show that the collapse of the rock slope is unlikely, but clearly highlight the difficulty of the prediction. Beyond this result, the study provided an understanding of the advantages and disadvantages of different approaches for the analysis of a slope in this peculiar geological setting.