That Time A Bridge Sank Due To A Sublime Roadworks Fail

We drive across all kinds of bridges every day. Whether it be an ordinary bridge over a small creek, a grandiose suspension bridge like the Golden Gate Bridge in San Francisco, or the 24-mile-long Lake Ponchartrain Causeway, most bridges have one thing in common: they’re usually supported, in part, by a series of pilings driven into the bottom of whatever body of water they cross.

However, one bridge across in Seattle is quite a bit different. The Lacey V. Murrow Memorial Bridge, which carries Interstate 90 across Lake Washington, doesn’t have any pilings tethering it to the ground. This is because, at its deepest point, Lake Washington is well over 200 feet deep. It’s not exactly practical to fabricate 200-foot-long concrete pilings and then transport them to the construction site. Luckily, the conditions of Lake Washington allowed for a simple, yet ingenious solution. The answer was to build a bridge that floated over the lake.

The concept of a floating bridge is fairly simple: instead of using pilings, the bridge relies on a series of concrete pontoons to support the bridge deck. In essence, the bridge behaves like a giant boat, relying on a series of cables anchored to the bottom of the lake to keep it in place. The design is ideal for Lake Washington because there are no strong currents or high waves, and the lake itself is very deep. Not surprisingly, when engineers in Washington state wanted to connect the city of Seattle to Mercer Island, the natural decision was made to build a pontoon bridge to do the job. In 1940, the Lacey V. Murrow Memorial Bridge opened to the public.

Named after the former director of the Washington State Highway Department, the Murrow bridge became a critical component of the interstate highway network. For 50 years, the bridge saw an inordinate amount of vehicle traffic, eventually necessitating the construction of an adjacent bridge to alleviate the traffic problems. In 1990, the bridge was set to undergo routine maintenance when a series of basic engineering mistakes resulted in a somewhat embarrassing disaster.

The plan to rehabilitate the bridge consisted of widening, then resurfacing the bridge deck. To tear up the old pavement, construction crews used a technique called hydrodemolition. This consisted of using high-pressure water cannons to remove the asphalt surface from the bridge deck. While this procedure is fairly common, the water used in the process becomes contaminated and must be disposed of properly. Incredibly, the decision was made to temporarily store this wastewater inside the empty pontoons that kept the bridge afloat.

In defense of the engineering crews, the pontoons had been designed to hold a certain amount of water to safeguard against leakage; and the wastewater could be discharged into the pontoons so long as the water levels were carefully monitored.

However, on Thanksgiving Day in 1990, it was clear that somebody had made a serious blunder. A storm had pummeled the city of Seattle over the weekend, causing rain and lake water to accumulate on the bridge. Normally, this wouldn’t be a problem, but the hydrodemolition crews had left the doors to the bridge pontoons open. This caused water to enter the pontoons, and caused the water levels in some of them to exceed the critical levels. By the time crews realized that there was a problem, the crews couldn’t pump the water out of the pontoons quickly enough. What happened next came as no surprise: the Lacey V. Murrow Bridge sank to the bottom of Lake Washington.

A thorough and complex investigation by some of the world’s leading failure analysts arrived at a remarkably simple conclusion: floating bridges will sink when you fill their floats up with water. Fortunately, this engineering blunder resulted in no deaths or injuries, but the damage to the bridge was estimated at $69 million. In 1993, the Murrow bridge was rebuilt, and it continues to be in service today, thanks in no small part to the fact that the bridge pontoons haven’t filled up with water.