THE BOSTON Big Dig tunnel operator was this week doubling up the number of anchors used to support reinforced concrete ceiling panels, in a bid to prevent more from falling onto the road beneath.
A woman was killed on 10 July when a 3t ceiling panel crashed onto the car in which she was travelling (NCE 20 July).
Investigations are focused on the use of epoxy resin to grout anchor bolts into the reinforced concrete slab roof of the Big Dig's I90 connector tunnel.
The collapse occurred when the resin-bonded bolts tore from their sockets in the tunnel roof.
A spokesman for tunnel operator Massachusetts Turnpike Authority (MTA) said that it was 'in the midst of installing redundant connections between the ceiling panels and the roof of the structure.
'We're installing steel-to-steel connections where possible, and where that's not possible we are using mechanical bolts.
The second connection provides redundancy for the first.'
Steel inserts were cast into the tunnel roof over most of its length to provide anchor points for the hanger rods from which the ceiling panels are suspended.
But for as yet unknown reasons, ceiling panel system designer Gannett Fleming decided to use bolts fixed into place with epoxy resin grout at the tunnel's portals and on/off ramps.
This was despite a damning report on the installation of resin bonded bolts in the adjoining Ted Williams Tunnel, published by Massachusetts inspector general Robert Cerasoli in 1998 (News last week).
It was unclear what variety of mechanical bolts was being installed by MTA, but all operate on a similar principle, of creating skin friction between the sides of the bolt and the hole into which they are inserted.
The higher the torque to which the bolt is tightened, the greater the skin friction achieved.
The National Transport Safety Board and Massachusetts attorney general Tom Reilly said that their investigations would be focusing on design as well as construction as contributing to the fatal collapse.