Gravity-Induced Fracturing in Large Rockslides: Possible Evidence from Vajont

Abstract

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 gravity-induced 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. Gravity-driven 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.