The project management team responsible for building Boston's Central Artery last month called for a thorough investigation of all safetyrelated issues in the route's tunnels after a woman was crushed to death by a ceiling panel on 10 July.
In a statement released on 14 July, the joint venture of Bechtel and Parsons Brinckerhoff said it was 'deeply committed to supporting efforts to investigate the accident, re-open the highway and address public concerns'.
The 3t panel fell from the roof of the eastbound tunnel of the Interstate 90 highway, which passes beneath Boston Harbour. It smashed into a car, killing the passenger, Milena Del Valle.
The US National Transportation Safety Board (NTSB) said it was 'conducting detailed visual examinations and documentation of the remaining anchors of the same section of tunnel where the concrete ceiling panel collapsed'.
But Bechtel and Parsons Brinckerhoff urged for the review to be widened to 'cover the entire range of potential safety-related issues, including structural integrity, structural elements over roadways and public areas, suspended ceilings, ventilation and support systems, fire and life safety, structural fire protection, utilities and lighting, communication systems and the adequacy of operational inspection/maintenance'.
Pull tests were conducted in midJuly to determine the characteristics of the epoxy used to secure the anchors in the ceiling of the tunnel. Investigators said bolts used to anchor hanger rods to the tunnel roof appeared to have pulled from their sockets and 250 other bolts have shown signs of movement.
A ceiling consisting of 4m wide by 2m long, 3t reinforced concrete panels was suspended between 1.5m and 2m below the tunnel roof. The void between roof and ceiling was used for tunnel ventilation.
The suspension system consists of T-shaped brackets bolted to the tunnel roof slab. Holes were drilled into the slab and the anchor bolts grouted into place using epoxy resin.
Hanger bars about 1.5m-2m long were bolted to the brackets. Longitudinal, upside-down T-section beams were then suspended from the rods to support the ceiling panel edges.
Investigations and remedial works could take months to complete, the NTSB said.
The ceiling panel collapse is the latest catastrophe to hit the project.
Following completion in December 2004 a section of tunnel sprang a leak and the scheme ran massively over budget and over schedule - costs soared from £3.6bn to £10bn.
Problems with epoxy grouting were extremely rare, said Alan Clayton, technical service adviser at grout supplier Fosroc. Clayton said epoxy grouting was favoured on civil engineering projects because it was a reliable and tough way of securing anchors: 'It can take higher compressive loads than concrete, but also performs well under tensile and exural loading, ' he said.
But he warned it had to be properly applied. Epoxy resin is not an adhesive, but 'grips' deformations in the socket hole and anchor bolt.
This means that the socket has to have a rough surface, or be underreamed, and that the anchor bolt must have a surface texture - typically a rebar type herring-bone pattern or a conventional thread.
If anchor holes are cored from concrete using a diamond cutter instead of a conventional rock drill the internal surface could be too smooth to provide a key for the resin, added Les Cheriton, a mining and tunnelling expert at chemicals supplier Minova.
There can also be problems if anchor bolts have smooth instead of textured shanks, he said.
But both insisted problems were most likely to stem from improper mixing of the resin with its activating chemical agent.
In its statement Bechtel and Parsons Brinckerhoff commented: 'Supporting concrete ceiling panels by anchoring bolts to the roof with epoxy adhesive is widely and successfully used throughout the public transportation industry.
Determining the causes of this specific failure will require a thorough forensic analysis of design, methods, materials, procedures, and documentation.
'We strongly believe that a comprehensive review - including design, construction, operations, and maintenance - is essential to restoring public condence.'