Tunnel crown collapse was being seen as a possible failure mechanism in the mid-January, fatal cave-in of a shaft being built for the new Pinheiros metro station in São Paulo, Brazil, as T&TI closed for press.
Seven people were suspected to have been killed, all having been on the surface prior to the collapse of the east sidewall, some in vehicles. Six bodies were recovered in the initial days after the accident.
No tunnellers were reported injured when working underground.
The tunnelling works beside the shotcrete-lined shaft had moved on to bench excavation for platform tunnels prior to the collapse, T&TI understands. Construction of the platform chamber requires excavating 45m either side of the base of the 42m diameter shaft. The chamber meets the single-bore running tunnels, which will be 14.5m wide by 9.5m high for the new Line 4 extension to the metro.
The top headings of the NATM-driven tunnels holed through into the heading excavated for the platform zones in early December, which enabled bench excavation to get underway. Located not far from the east bank of the river Pinheiros, the tunnel and shaft are being excavated in alluvial deposits, sediments and gneiss – and the headings had been opened in rock.
However, on 12 January the sidewall of Pinheiros shaft suddenly collapsed with tragic results. The cave-in took nearby construction plant and trucks, but the tower crane near the edge remained standing on its deep foundation stalk amidst the debris. Some buildings closeby had to be evacuated or demolished.
No official explanation was immediately given for the failure of the shaft. The state government of São Paulo has appointed its Institute of Technological Research (IPT) to lead the investigation. It is believed IPT will be seeking the help of Brazilian and international engineers to study the shaft and tunnels before reporting on the fatal collapse.
IPT’s investigators have yet to begin examining the site due to the continued work of the emergency services – themselves delayed by heavy rain. Consequently speculation has been rife as to the cause of the collapse.
Views being given for the possible cause, or causes, of the shaft failure have ranged from the crane’s load over-stressing the ground, heavy rain weighing on undrained soil and unforeseen ground conditions, to the root reason being faulty construction, either of the shaft or tunnel.
Line 4 is being built on a turnkey basis by a consortium of five local firms – Odebrecht, OAS, Queiroz Galvao, Andrade Gutierrez and Camargo Correa. No statement on the cause was available from the contractor.
Pinheiros station is due to open in 2009 and the full Line 4 service is scheduled for 2012.
