The “Big Dig” was the largest, and most expensive, highway project of its kind in the United States.
It included 7.8 miles of highway (161 lane-miles in all), about half of which tunnel under the city and its harbor. The Big Dig placed over 3.8 million cubic yards of concrete and excavated more than 16 million cubic yards of soil. The project was the result of more than 30 years of planning and 12 years of construction. However, the megaproject was subject to escalating costs and time delays.
The Big Dig is also famous for cost increases. Its initial estimated cost was $2.56 billion. Estimates increased to $7.74 billion in 1992, to $10.4 billion in 1994, and, finally, $14.8 billion in 2007—more than five times the original estimate. The reported reasons for the cost escalation included inflation, the failure to assess unknown subsurface conditions, environmental and mitigation costs, and expanded scope. Mitigation alone required 1,500 unanticipated, separate agreements.