UPDATE: NTSB INVESTIGATION INTO BOSTON I-90 TUNNEL ACCIDENT

The National Transportation Safety Board continues to investigate the circumstances of the July 10th ceiling collapse of the Interstate 90 connector tunnel in Boston MA. A portion of the ceiling located in the tunnel's east portal became detached from the roof of the tunnel and collapsed onto a passing automobile, killing the passenger and injuring the driver.

The ceiling is composed of a group of individual pre-cast concrete panels. The standard panel size is 12 feet wide by 8 feet long, and is about 4 inches thick. The panels are bolted to a steel frame to create a modular structure that is attached to the roof of the tunnel.

The ceiling of the tunnel serves as the base for the ventilation duct that runs the length of the tunnel. As such, it is suspended approximately four feet from the structural roof of the tunnel and is held in place by a combination of anchors, tie-rods and brackets. The majority of the anchors in the I-90 tunnel are secured to steel channels that were cast into the tunnel's roof during construction. However, the eastern portal, which was constructed separately from the rest of the tunnel, did not incorporate the steel support channels. In this section, the anchors for the ceiling modules are affixed to the roof by an epoxy adhesive.

Each of the epoxied anchors consisted of a 5/8-inch diameter stainless steel threaded stud, approximately 8 inches in length. During construction, holes were drilled into the concrete roof of the tunnel. Each hole was partially filled with epoxy and a stud was subsequently inserted into the hole. In the area of the ceiling collapse, twenty of the studs had pulled out from the tunnel's roof.

Part of the NTSB probe is focusing on the design of the ceiling panel system and how it compares to other systems, such as the one used in the adjacent "Ted Williams" tunnel. Other factors under examination include the epoxy adhesive used during construction, the construction methods used to apply the adhesive and the construction oversight of the ceiling installation.

"We continue to make progress in this investigation, and seek to learn as much as possible from this tragic accident with the aim of making recommendations to prevent similar accidents in the future," said NTSB Chairman Mark V. Rosenker. "We will keep the public informed with periodic updates."

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