Flexural failure phenomena affecting continental Pleistocene deposits along coastal cliffs (Croatia)

Abstract

This note describes the rockfall processes involving some coastal cliffs with moderate lengths (30-50 m) and elevations (4-5 m), constituted by alternating loose sediments and cemented levels (calcareous breccias) dating to Upper Pleistocene and widespread along the western coast of Lussino/Lošinj Island (eastern Adriatic Sea, Croatia). The rockfall phenomena are due to the collapse of thick layers (20-70 cm, essentially) made of angular matrix-supported calcareous breccia forming overhanging rock slabs. The cantilever of rock slabs increases with time owing to the coastal erosion processes caused by the wave action impacting against the base of the rock scarp. Rockfalls, provoking the collapse of horizontal slabs of considerable size (block volumes: 0.1-2.5 m3), are caused by a typical flexural failure mechanism involving cantilevered rock beams. Many geomechanical models, considering the real size and the geometry of failed slabs, have been carried out. The results of these analyses, based on the rock beam model, demonstrate that at rupture only a small part (28%, on average) of the restrained cross-section of the slab is constituted by intact rock (rock bridge), whereas the remaining part is characterised by a fracture or a joint partially crossing the slab. The presence of the fracture strongly influences the stress state acting on the rock bridge. The calculated values of tensile stresses agree very well with characteristic intact rock strength (T0=3.8 MPa) measured by means of PLT laboratory tests only for models considering a partially resisting cross-section (i.e. a fractured rock beam). Therefore, fracturing process is a decisive phenomenon in preparing rockfall-type slope failures.