Gravity-induced fracturing in large rockslides: Possible evidence from Vajont

Large rock slope failures can be characterised by considerable gravity-driven fracturing that affects unstable rock masses before final collapse. These newly formed fractures determine the rupture of intact rock and the formation of new discontinuities related to gravity. In large rockslides, fractures of non-tectonic origin can be associated with other features inducing rock mass damage such as folding, faulting, and rock mass disintegration. Joint orientation measurements (N = 1,204) carried out on the 1963 Vajont landslide demonstrate the presence of gravity-induced fractures that are added to those of tectonic origin. The described gravityinduced joints essentially formed during the prehistoric slope instability. In fact, they are related to the 3D geometry of the prehistoric slide and are characterised by an orthogonal and a parallel azimuth if compared to the sliding direction of the ancient slope movement. Gravitydriven joints formed through coalescence with pre-existing rock discontinuities, thus providing the prehistoric three-dimensional rupture surface and enabling the kinematic release of the ancient unstable mass. Gravity-driven joints are newly formed fractures caused by tensile and internal shear stresses. The fracture joints caused by gravity can be identified in large rockslides characterised by en masse sliding motion (as the Vajont case history) or within unstable rock slopes characterised by strong rock mass damage prior to failure. In the latter case, gravity-induced joints can be considered as important geomechanical evidence that the rock mass suffered progressive fracturing, indicating any possible future slope collapse.

Microseismicity within a karstified rock mass due to cracks and collapses as a tool for risk management

Seismometer arrays have been widely applied to record collapse by controlled explosion in mines and caves. However, most underground failures are natural events, and because they can occur abruptly, underground failures represent a serious geological hazard. An accelerometric array installed on 4 September 2008 has been used to manage the geological risk of the Peschiera Springs drainage plant of Rome's aqueduct, which is located in the Central Apennines approximately 80 km from Rome, Italy. The plant occupies a karstified carbonatic slope that is extensively involved in gravitational deformations, which are responsible for underground failures such as cracks and collapses. To distinguish among different types of recorded events, an automated procedure was implemented based on the duration, peak of ground acceleration (PGA) and PGA variation in the recordings of the plant's accelerometric stations. The frequencies of earthquakes and micro-earthquakes due to underground failures are, in general, well correlated. Nevertheless, many underground failure sequences can be directly associated with the continuous deformations that affect the slope. The cumulative Arias intensity trend derived for the underground failures combined with the failure and earthquake frequencies enabled the definition of a control index (CI) that identifies alarming or emergency conditions. The CI can be used as a tool for managing the geological risk associated with the deformational processes that affect the drainage plant