WASHINGTON, D.C. --- The National Transportation Safety Board has determined that the likely cause of last year's fatal ceiling collapse in the I-90 connector tunnel of Boston's Big Dig was the use of an epoxy anchor adhesive with poor creep resistance. Gannett Fleming Inc., which designed the Big Dig tunnel ceiling, is disputing the National Transportation Safety Board's report. Epoxy supplier Powers Fasteners has been indicted for involuntary manslaughter. Epoxy is a polymer and its stiffness depends on time and temperature. If a load is applied suddenly, the epoxy responds like a hard solid. But if the load is then held constant, the molecules within the polymer may begin to rearrange and slide past one another, causing the epoxy to gradually deform in a process called creep. According to the NTSB, the epoxy used in the tunnel had poor creep resistance. In addition, the NTSB said the use of an inappropriate epoxy formulation resulted from a general lack of understanding and knowledge in the construction community about creep in adhesive anchoring systems. The agency emphasized the need to assess the creep characteristics of adhesive anchors before those anchors are used in sustained tensile-load applications.