Big Dig Boston: Central Artery / Tunnel Project
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.
$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.