“Basement Bertha” became the nickname for the colossal Tunnel Boring Machine (TBM) deployed in Seattle, Washington. This machine was central to the State Route (SR) 99 Alaskan Way Viaduct Replacement Tunnel project. Its purpose was to construct a deep-bore tunnel, providing a modern, seismically resilient alternative to an aging elevated highway that served as a main artery through the city.
The Alaskan Way Viaduct Replacement Project
The Alaskan Way Viaduct, a double-deck elevated section of State Route 99, was a prominent feature along Seattle’s waterfront since the 1950s. Decades of heavy use and the 2001 Nisqually earthquake severely compromised its structural integrity, making its replacement a public safety imperative. After extensive debate and considering numerous alternatives, a deep-bore tunnel was chosen in 2009 as the preferred solution for the SR 99 corridor.
The Washington State Department of Transportation (WSDOT) oversaw this ambitious undertaking, aiming to replace the vulnerable viaduct with a two-mile-long, double-decked tunnel beneath downtown Seattle. This project represented a significant engineering challenge due to the urban environment and the scale required for a major highway. The decision to use a large-diameter tunnel boring machine like Bertha allowed SR 99 to remain open during much of the construction, minimizing traffic disruption.
Bertha’s Tunneling Journey and Setbacks
Bertha, measuring 57.5 feet in diameter and weighing 6,700 tons, began its tunneling operation in July 2013. Manufactured by Hitachi Zosen Sakai Works in Japan, the machine was designed to bore a 9,270-foot tunnel through varying soil conditions beneath Seattle. Its journey commenced from a launch pit south of downtown, with an initial target completion date of December 2015.
A significant setback occurred in December 2013, when Bertha encountered an obstruction 1,083 feet into its path. The machine stopped after hitting an 8-inch steel well casing, left from a 2002 groundwater measurement project. This collision damaged Bertha’s main bearing seal system, causing the bearing to overheat and halt operations.
The repair efforts were extensive and complex, requiring the excavation of a 120-foot-deep, concrete-lined access pit to reach the machine’s cutterhead. In March 2015, the damaged cutterhead, weighing 2,000 tons, was lifted to the surface for repair and partial replacement. After nearly two years of repairs, Bertha resumed tunneling in December 2015. The machine continued its advance, with intermittent delays, including a sinkhole incident in early 2016 that briefly halted work again.
Tunnel Completion and Bertha’s Decommissioning
Bertha completed its tunneling mission on April 4, 2017, breaking through into a disassembly vault at the north end of the project site near Seattle Center. This breakthrough marked the culmination of a 1.7-mile journey beneath the city. The tunneling machine had installed 1,426 concrete rings, forming the structural lining of the new SR 99 tunnel.
Following its breakthrough, Bertha’s disassembly began. The immense machine, weighing 8,000 tons, was cut into pieces for removal from the tunnel. The cutterhead was cut into sections and transported to a steel recycler.
Much of Bertha’s steel was melted down and recycled, while some components, such as electric motors, were slated for potential reuse. The entire disassembly process took several months, with the final pieces of the TBM removed from the site by August 2017.
The Legacy of the SR 99 Tunnel Project
The SR 99 tunnel project, and Bertha’s role, represents a monumental achievement in civil engineering. The completed tunnel now provides a vital north-south transportation route beneath downtown Seattle, enhancing safety and mobility. It replaced an aging, earthquake-vulnerable viaduct, improving the region’s infrastructure resilience.
The project also transformed Seattle’s waterfront, allowing for the eventual demolition of the viaduct and the development of new public spaces. The experience gained from designing, building, and operating Bertha, particularly in addressing complex geological challenges and unforeseen setbacks, offers valuable lessons for future large-scale tunneling endeavors. The SR 99 tunnel stands as a testament to human ingenuity and perseverance in tackling formidable engineering feats.