Public safety workers have called the walkway safety handrails in the Big Dig tunnels "ginsu guardrails", because the squared-off edges of the support posts have caused mutilations and deaths of passengers ejected from crashed vehicles.

A grand jury has been convened to investigate whether a Big Dig tunnel where a woman was killed in July was improperly designed or constructed, a spokesman for Attorney General Thomas F. Reilly said.

Four concrete slabs, each weighing at least three tons, fell from the ceiling of a Big Dig tunnel here on Monday night, crushing a woman to death in a car.

Safety repairs in part of a Big Dig tunnel where a woman was killed last month are complete, and the tunnel will open pending federal approval, Gov. Mitt Romney said.

Massachusetts officials said Thursday that more potential problems had been found in a Big Dig tunnel: 242 areas where loose bolts have created gaps between the tunnel roof and a metal frame that supports concrete ceiling panels.

Before we look at some of the techniques used in the construction of these Big Dig tunnels, let's review why Boston officials decided to undertake such a massive civil-engineering project in the first place.

Several such tiles fell from one of Boston's Big Dig tunnels Monday night and crushed a woman to death in her car.

How could it be that the Romans built aqueducts 2,000 years ago that are still standing today, while the ceiling on the Big Dig tunnel came down in two years?

Gov. Mitt Romney said Friday that the section of a Big Dig tunnel where a woman was killed by falling concrete in July would take much longer to repair than anticipated because of additional problems discovered last month.

Larger, heavily-used tunnels, such as the Big Dig tunnel in Boston, Massachusetts, may have a dedicated 24-hour manned operations center which monitors and reports on traffic conditions, and responds to emergencies.