What Happened to Bertha (Tunnel Boring Machine)?
Bertha was the world's largest Earth Pressure Balance tunnel boring machine, instrumental in constructing Seattle's SR 99 tunnel. After a significant two-year delay due to a major breakdown in 2013, it completed its boring in 2017, was subsequently disassembled, and the tunnel opened to traffic in 2019. The SR 99 tunnel continues to operate as a critical piece of Seattle's infrastructure, undergoing regular maintenance and occasional closures, with events reported as recently as March 2026.
Quick Answer
Bertha, the 57.5-foot diameter tunnel boring machine, successfully completed its 1.75-mile journey under Seattle in April 2017, despite a two-year delay caused by a major breakdown in December 2013. After its tunneling work, Bertha was fully disassembled and removed by August 2017. The SR 99 tunnel, which Bertha created, opened to traffic in February 2019 and continues to be a vital part of Seattle's transportation network. As of July 2026, the tunnel undergoes regular maintenance, including an emergency closure for life safety systems in March 2026.
📊Key Facts
📅Complete Timeline10 events
Bertha's Assembly Completed in Seattle
The assembly of Bertha, the 57.5-foot diameter tunnel boring machine built by Hitachi Zosen, was completed in Seattle, preparing it for the SR 99 tunnel project.
Tunneling Operations Begin
Bertha officially began its journey to bore the 1.75-mile-long SR 99 tunnel beneath downtown Seattle.
Bertha Halts Due to Damage
After boring approximately 1,083 feet, Bertha stopped due to damage to its main bearing seal system, initially suspected to be caused by striking a steel pipe.
Rescue Pit Construction and Major Repairs
A 120-foot-deep rescue pit was dug to access Bertha's cutterhead, allowing for extensive repairs and partial replacement of the damaged main bearing and seals.
Bertha Resumes Tunneling
Following the two-year repair period, Bertha officially resumed its tunneling operations beneath Seattle.
Further Halts Due to Sinkhole and Barge Incident
Tunneling was stopped again after a sinkhole appeared near the project site and a barge carrying excavated dirt tipped over in Elliott Bay, leading to a temporary suspension of work by Governor Inslee.
Bertha Completes Tunneling Journey
Bertha broke through into a disassembly pit at the north portal, successfully completing its 1.75-mile bore under Seattle.
Bertha Fully Disassembled and Removed
After months of work, the final pieces of the 8,000-ton machine were lifted out of the disassembly pit, marking the complete removal of Bertha from the project site.
SR 99 Tunnel Opens to Traffic
The State Route 99 tunnel, built by Bertha, officially opened to vehicular traffic, replacing the Alaskan Way Viaduct.
SR 99 Tunnel Emergency Closure
Both directions of the SR 99 tunnel were closed for unplanned emergency maintenance on life safety systems, reopening hours later after work was completed.
🔍Deep Dive Analysis
Bertha, officially known as the Hitachi Zosen EPB TBM, was a monumental engineering endeavor designed to replace Seattle's aging and seismically vulnerable Alaskan Way Viaduct with a deep-bore tunnel. Launched in July 2013, the machine, at 57.5 feet in diameter, was the largest Earth Pressure Balance tunnel boring machine ever built. The project quickly encountered significant challenges when, in December 2013, after boring just over 1,000 feet, Bertha unexpectedly halted. Initial speculation pointed to a steel pipe left in the ground, but subsequent investigations revealed that the main bearing seal system was damaged, leading to overheating and a two-year stoppage.
The breakdown triggered an unprecedented rescue operation. Crews had to dig a 120-foot-deep access pit from the surface to reach the machine's cutterhead for repairs and partial replacement, a process that involved lifting a roughly 2,000-ton section. This extensive repair work, which included installing a redesigned seal system, took place between March and August 2015. Bertha resumed tunneling in December 2015, but its troubles were not entirely over. Further halts occurred in January 2016 due to a sinkhole opening near the project site and a barge incident in Elliott Bay, prompting Governor Jay Inslee to temporarily suspend all work citing public safety concerns.
Despite these setbacks, Bertha eventually resumed full operations in April 2016 and successfully completed its 1.75-mile journey on April 4, 2017, breaking through into a disassembly pit at the tunnel's north portal. The completion of the bore was a major turning point, allowing for the focus to shift to the tunnel's interior construction. Following its breakthrough, Bertha was systematically disassembled piece by piece, a process that took several months and was completed by August 2017. Many components were recycled, while some parts, like cutting tools and the control panel, were donated to Seattle's Museum of History and Industry.
The consequences of Bertha's delays were substantial, including significant cost overruns estimated at $223 million and prolonged legal disputes between the Washington State Department of Transportation (WSDOT) and the contractor, Seattle Tunnel Partners (STP), regarding responsibility for the damage and associated costs. The SR 99 tunnel ultimately opened to traffic on February 4, 2019, replacing the Alaskan Way Viaduct and transforming Seattle's waterfront. As of July 2026, Bertha itself no longer exists as a machine, having been fully dismantled. However, the tunnel it built remains operational. The SR 99 tunnel requires ongoing maintenance, as evidenced by an emergency closure in March 2026 for unplanned work on life safety systems. WSDOT also plans for regular inspections and maintenance, ensuring the tunnel's continued safety and functionality.
What If...?
Explore alternate histories. What if Bertha (Tunnel Boring Machine) made different choices?