FUTURE USE of the controversial New Austrian Tunnelling Method (NATM) is to be reviewed by the Catalan government after a major collapse on Barcelona's Metro extension.
Two apartment blocks were undermined and collapsed, leaving a surface cavity between 25m and 30m in diameter and 32m deep. More than 50 families were left homeless following the incident on 27 January.
Client Generalitat de Catalunya (GDC) said that while the use of NATM would not be banned outright, in future additional excavation supports would be mandatory.
NATM is an open faced method of tunnelling using temporary support from sprayed concrete or rock bolts. Although used successfully around the world it has been associated with several high profile problems including the 1994 Heathrow Express collapse.
Work on the metro extension was being carried out by a joint venture contractor of FCC, Comsa and Copisa, with design by the Generalitat's own public works company Gisa and supervision by consultants Tec-Cuatro and Geocontrol.
Client GDC this week confimed that the problem was due to a 'hidden' fault in the relatively mixed old weathered slate and very old metamorphic silaceous ground where tunnelling was under way.
'It was a vertical fault and therefore had remained hidden from the borehole surveys done for the ground investigation, ' said a GDC spokesman.
Contractors will continue to use NATM on the remaining extension for Line Five but engineers will insist on the use of additional support and monitoring.
Two attempts to control the void with grouting over two days failed. The tunnel will now be filled with grout.
GDC's engineering department is carrying out a detailed investigation into the design, construction and supervision of the work. Initial conclusions are expected in two weeks.