The influence of rock bridges in block fall processes

Abstract

The detailed study of the detachment surfaces left by rockfalls, together with the 3-D analysis of unstable rock volumes, confirm the fundamental role of intact rock bridges, with different location, that provide an elevated, localised, strength contribution and that are then broken at the moment of the collapse. The back-analysed case histories, carried out assuming a limit equilibrium principle, have ascertained that the limestone rocks involved in rockfalls had mean values of tensile and shear strength ranging from 1.5 to 5 MPa. In the case of overhanging columnar blocks, the stability condition is made possible by the presence of rock bridges with moderate extension (rock bridge area Ap = 400- 3600 cm2, on average) whose size increases for larger blocks and for weaker rock types (τ = 1-2 MPa). These results have been obtained considering the mean strength mobilised by the intact rock bridges, assuming the block weight as the only acting force and neglecting the influence of external forces of different nature (water seepage along the joints, earthquake). According to these assumptions the stability analyses have indicated a very limited extension of the rock bridges (0.2-1.6%) with respect to the whole area of the contact surfaces between block and stable rock mass. Rockfalls involving blocks with base sliding show very poor evidences of rock bridges, which are often very small (length = 10-30 cm) and irregularly distributed over the detachment surface.