How much over budget was the Big Dig?

The construction cost rose from a 2.8 billion US dollar estimate in 1985 to 14.8 billion in 2007, an overrun of about 429% against the original estimate. Roughly 20 to 25% of the increase was construction-cost inflation across the 16-year build; the rest was scope expansion, unforeseen ground conditions, and remediation of quality and safety failures.

Why did the Big Dig cost so much?

Four main drivers: massive scope expansion (the 1985 estimate excluded the Ted Williams Tunnel, the I-93 north reconfiguration, the Charles River bridges, and surface restoration); unprecedented urban underpinning of live buildings, an active subway, and South Station; construction-cost inflation across a 16-year build; and quality and safety failures including the 2006 ceiling collapse and substandard concrete.

Big Dig Boston: Central Artery / Tunnel Project

Q: How much did the Big Dig cost?

The Big Dig (the Central Artery/Tunnel Project) finished at about 14.8 billion US dollars in construction cost in 2007, up from an original 1985 estimate of 2.8 billion. Counting interest on the Massachusetts bonds that financed the state share, the total cost is projected to exceed 24 billion dollars by 2038. It is the most expensive highway project in US history.

The Central Artery / Third Harbour Tunnel Project (CA/T) was the most expensive highway project in US history. Original 1985 cost estimate of 2.8 billion USD; final 2007 cost of 14.8 billion USD; total including interest on Massachusetts bonds is projected to exceed 24 billion by 2038.

How much did the Big Dig cost?

The Big Dig finished at about 14.8 billion US dollars in construction cost in 2007, up from an original 1985 estimate of 2.8 billion, an overrun of roughly 429%. Counting interest on the Massachusetts bonds that financed the state share, the total cost is projected to exceed 24 billion dollars by 2038, making it the most expensive highway project in US history.

$14.8B

final construction cost

$24B+

total inc. interest on bonds (projected 2038)

+429%

construction overrun vs 1985 estimate

Why the cost grew so much

Massive scope expansion. The 1985 estimate did not include the Ted Williams Tunnel, the I-93 north reconfiguration, the Charles River bridges, or the surface restoration of the old elevated artery footprint. By 2000 all four had been added.
Unprecedented urban underpinning. The project required underpinning live high-rise buildings, an active subway, and the South Station rail terminal. Unforeseen geological conditions added EUR 1bn+.
Inflation across a 16-year build. Construction-cost inflation 1991-2007 was significantly above general CPI; about 20-25% of total overrun was inflation.
Quality and safety failures. The 2006 ceiling collapse, leaks, and use of substandard concrete added cost in remediation and litigation.
Originally a 70/30 federal/state split that became 50/50 after federal caps. Massachusetts had to absorb the unanticipated state share via bond issuance, hence the projected 24bn including interest.

Sources

Massachusetts Department of Transportation (MassDOT) CA/T archive
Federal Highway Administration project records
US Government Accountability Office reports on CA/T (GAO/RCED-00-141 and follow-ups).
Greiman V. (2013). Megaproject Management: Lessons on Risk and Project Management from the Big Dig. Wiley.

Big Dig Boston: Central Artery / Tunnel Project

Why the cost grew so much

Sources

Related