Traffic was disrupted once again in a key Big Dig tunnel after inspectors found loose bolts in a ceiling panel the same sort of problem that is believed to have killed a motorist earlier this month. The $14.6 billion Big Dig project, the most expensive in U.S. history, buried much of the city's highway network in tunnels. It took over a decade to complete and has since been plagued by leaks, falling debris, cost overruns, delays and problems linked to faulty construction.