Authors technical papers have been written discussing the sequence of events and the knowledge acquired during the forensic excavations for the investigation of the causation of the Pinheiros Station collapse on January 2007 (Assis, A. P. 2008 and 2009, Barton, N., 2008 and Sadowski et Alii, 2008), which led to seven deaths. It is the understanding of the authors, all part of the Board of Consultants created by the Construction Consortium to investigate and present a diagnosis for the failure that local geological conditions detected after the accident led to an unforeseen geomechanical behavior.
The Pinheiros underground Station of São Paulo's Metropolitan Subway – line 4 exhibits a 19,5m span, a 16mheight and a length of 47mat the side of a shaft of 40m in diameter, was being excavated by blasting, top heading and two benchs, with the application at the tunnel crown, forepoles, steel arches and 35 cm thick shotcrete reinforced with steel fibers; at the first bench, 15cm-thick shotcrete and wire mesh. On January 12th 2007, at 14:00 p.m., a violent collapse took place and brusquely propagated itself, reaching the shaft, as seen the illustration below (Fig. 1).
Local geological conditions are presented in the paper by Sadowski et Alii 2008. The following plan view indicates the boreholes available during basic and detailed design stages of the project. The major geological structures within the site may be summarized as follows: The foliation, which coincides with the attitude of the Caucaia shearing zone, and consequently with the biotitic zones and the metabasite bands, in N75— 85A-attitude slickenside bodies, with subvertical dips towards NW and SE. At the site of the accident, the rock mass was delineated by these structures, which dipped in opposite directions, forming potential wedges in the station tunnel span.