GEOTECHNICAL EXPERTS this week expressed surprise that there were no visible anchors or struts supporting the walls of a 40m diameter, 30m deep shaft that collapsed in S ão Paulo, Brazil, last Friday.
Seven passers by were swept into the shaft as the wall sheared away and earth poured into the shaft. They are assumed dead.
The shaft was sunk as part of construction of S ão Paulo Metro's Line 4 or 'Yellow Line', from the city centre to the western suburbs. The shaft was at the location of Pinheiros station, one of 12 along the line's 12.8km length.
Contractor for the project is a joint venture known as the Yellow Line Consortium, consisting of Brazilian rms Odebrecht, OAS, Queiroz Galv ão, Camargo Corrêa and Andrade Gutierrez. Client is the S ão Paulo Metropolitan Transport Secretariat.
It appears that the walls of the shaft had been lined with sprayed concrete, applied in 2m deep rings as excavation advanced.
One geotechnical engineer said that on small diameter excavations it was possible to design sprayed concrete linings so that they were in hoop compression, but that on a structure measuring 40m across this would be impossible to achieve.
'You would expect there to be ground anchors or waling beams and struts to provide support to the lower levels. There's no evidence at all of either in the debris at the bottom of the hole, and there are no telltale anchor heads visible through the concrete lining, ' he said.
The consortium on Monday said that torrential rains over the past month had destabilised ground around the shaft.
Another geotechnical expert said that saturation of the ground could have doubled earth pressure on the structure.
But deputy governor of S ão Paulo Alberto Goldman said that heavy rains occur annually and should have been foreseen by the shaft's designers.
Efforts have so far focused on trying to recover the seven lost people and on stabilising the