São Paulo cavern-shaft collapse viewed as a trap-door problem.

Abstract

This paper refers to the numerical simulation of the conditions that have lead to the collapse of the shaft-cavern collapse in São Paulo, Brazil (2007) constructed with the Conventional Tunnelling Method (CTM) or the New Austrian Tunneling Technique (NATM). The Pinheiros station where the incident has occurred, is located in an area known as the Caucaia Shear Zone, resulting in a highly fractured medium (four main families of discontinuities, i.e. two subvertical and two dipping towards the tunnel walls).The main observed lithologies were biotite gneiss and granite gneiss. According to the Bieniawski classification, the following rock mass classes were observed: II,III,IV (partially corresponding to saprolite), and V (partially corresponding to residual soils). The shaft-tunnel construction is simulated by virtue of the 3D finite differences code FLAC3D™. Special emphasis is given on the appropriate quantitative description of the geological conditions. A kinematic cohesion-friction softening model of the discontinuous rock mass is used. It was found that failure is manifested with the evolution of shear bands starting from the corners of the cavern periphery as this approaches the shaft while retreating. Before this, another cylindrical shear band initiates from the bottom of the shaft and also propagates upwards to connect with the former and hence producing the final collapse.