Floating Concrete Bridges of Washington State: History, Design, and Future Tunnels

In this Practical Engineering episode, Grady explores how Seattle’s geography demanded a new kind of crossing: a bridge that floats on hollow concrete pontoons. The story starts with the 1940 opening of the Lacey V Murrow Bridge, the first floating concrete highway, and follows the evolving engineering choices used to accommodate boats, wind, and water. The video recounts dramatic failures like the Hood Canal Bridge sinking in 1979 and a partial sinking of Murrow in 1990 during rehabilitation, then surveys modern developments such as testing light rail on the Homer M Hadley Bridge. It also looks ahead to bold ideas like floating tunnels that could shorten deep water crossings. Washington State has become a center for floating infrastructure, with ongoing research impacting mobility across water.

Introduction and Context

The video opens by describing Seattle’s early geography, where Puget Sound to the west and Lake Washington to the east created pressure for a reliable cross-lake route. Lake Washington’s depth, plus a 100 ft layer of soft clay and mud beneath the water, made traditional piers impractical and costly. In 1921, Homer Hadley proposed a radical concept: a bridge that would float on mass concrete pontoons, essentially riding the surface like a ship. This idea gained traction with New Deal era funding, leading to the construction of the Lacey V. Murrow Bridge, which opened in 1940 as the first floating concrete highway of its kind. The bridge became a symbol of engineering ingenuity under constraint and set the stage for Washington’s floating infrastructure network.

Design Concepts and Challenges

Floating bridges must manage navigation, water movement, and weather while carrying a roadway and vehicles. The Evergreen Point Floating Bridge uses elevated approach spans to let ships pass underneath. The original Murrow Bridge employed a retractable center span that opened a navigable channel but introduced traffic interruptions and awkward curves. Across the region, other floating bridges incorporate movable sections, hydraulic lifts, and bearings that distribute movements across the structure. The video explains that pontoons are subdivided into sealed chambers with leak detection and pumps to keep the system safe, and that the concrete mix, curing, and temperature control are carefully engineered to minimize cracking and leaks.

Historical Lessons: Failures and Safety

The narrative moves through two major sinking events that shaped policy and practice. In 1979, the western half of the Hood Canal Bridge lost buoyancy after rain and wave intrusion filled open hatches, necessitating a four-year ferry service. In 1990, during rehabilitation of the Murrow Bridge, water entered open chambers after watertight doors were removed to store runoff, causing partial sinking and cable damage on the adjacent Hadley Bridge. These incidents highlighted the importance of rigorous design verification, robust watertight separation inside pontoons, and careful handling of runoff during construction and maintenance.

Modern Developments: Rail and Real-World Testing

Today, Washington State continues to push floating infrastructure forward. Sound Transit has begun testing light rail on the Homer Hadley Bridge, addressing new challenges such as stray electrical currents on rails and the complex movement of a floating deck. Engineers devised insulated track blocks and drip caps to reduce conduction and moisture path issues, and a specialized track bridge system helps keep trains aligned as the structure moves with wind, lake level changes, and loading. The video notes that more work remains before riding a floating link rail across the lake, but the progress illustrates how floating roads and rails can adapt to dynamic environments.

Future Prospects: Floating Tunnels and Beyond

Looking to the future, the video discusses the possibility of floating tunnels. Immersed tube tunnels are common, but the idea here is to suspend or tether sections in the water column with negative buoyancy pontoons or positive buoyancy anchors, potentially shortening deep-water crossings and reducing environmental disruption. Norway has even proposed a fjord-crossing tunnel using such floating concepts, which would represent a first of its kind. The presenter emphasizes that engineering advances emerge from working with local conditions, weather, tides, and water depth, and that floating infrastructure can unlock new possibilities for mobility in select locations.

Broader Impacts and Takeaways

The video places Washington State among the world’s floating-bridge leaders, noting that four major floating bridges in the region form the backbone of a unique engineering ecosystem. It frames floating bridges as a niche but potent solution for crossing large, water-filled expanses where solid ground is unavailable or impractical. The talk ends by reflecting on how a blend of creativity, careful material science, and rigorous maintenance continues to push the boundaries of what is possible in the field of civil engineering.