Abstract

A metro project that was constructed in the most difficult elevation possible, with constantly changing rock–to–saprolite–to–soil–to–rock conditions, due to São Paulo metro operator requirements, suffered the predicted consequences of severe overbreak and slow progress. On two occasions there was break-through to surface. This paper describes one of these events that involved a set of adverse circumstances that tragically converged in time and location. On January 12th 2007, the following dramatic accident occurred. Nearly the whole of one of the station caverns of 40 m length and 19 m span suddenly collapsed. Despite extensive drilling around and even within the cavern centre, a misleading top–of–rock elevation was indicated, giving an assumed average 3 m of rock cover above the arch of the cavern, beneath about 18 m of saprolite, soil and sand. Heavy lattice girders at 0.85 m centres and steel–fibre reinforced shotcrete of minimum 35 cm thickness were used as primary support. Subsequent excavation through all the collapsed rock and soil during 15 months of investigations revealed a previously undiscovered, 10–11 m high ridge of rock with adversely oriented steep sides, caused by differential weathering of the foliated gneiss and an amphibolite band. A secondary planar joint set, a major bounding discontinuity, and probable elevated pore pressure from a cracked storm drain, constituted an unpredictable set of adverse conditions at an adverse location beneath a road, causing the death of seven people when sudden collapse occurred. Lessons learned the hard way confirmed the prior opinions of several prominent consultants who had called for either shallower, or deeper construction, either options in order to avoid frequently changing mixed-face conditions, which create a range of unnecessary difficulties.

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