Big Dig update – From the Boston Globe: “Safety margin for tunnel was smaller than most, US says.” In other words, the safety margin was zero as there were no redundant systems to hold up the ceiling tiles in case of bolt failure: “The Big Dig tunnel ceiling that collapsed in July was designed with a smaller margin of safety than other tunnel ceilings around the country, leaving nothing to prevent heavy concrete slabs from falling on a passing car when ceiling bolts fell out, according to a preliminary report by federal investigators obtained by the Globe.”