When Fill Fails

The Ravages of Urban Time

Last week, Sound Transit announced that it had to reduce service on Light Rail because the tracks at Royal Brougham Way (adjacent to the baseball stadium) were sinking and “susceptible to flooding.” The stated reason, as reported in the Seattle Times: the tracks were built on fill that “succumbed over many years.” Succumbed to what: gravity, decay, gribbles? (Gribbles are marine isopods blamed for deterioration of Seattle’s seawall.) Let’s simply chalk it up to time and its ravages across the urban infrastructure, particularly in the area where the tracks run.

Although we refer to the “land” around the Kingdomes, that might not be the best term. The area was a bay of the Whulge (aka Puget Sound) that extended to Tukwila until about 1,100 years ago, when a lahar cascaded mud and muck off Tahoma (aka Mt. Rainier) down the Duwamish River valley and pushed the mouth of the Duwamish to its present location. Underwater at high tide and exposed at low tide, the tideflats became an important harvesting area for Indigenous people. When Europeans arrived, however, the protean nature of the tideflats troubled the newcomers because they could not take advantage of what they thought should have been a good location for development.

A solution arrived in 1876 when former Chief of Police Joe Surber floated a scow to Atkin’s Wharf at what is now First Ave. S. and S. King St. and begin to drive logs into the Duwamish mud with a pile driver. His plan was to pound piles, most likely Douglas fir trunks up to 65 feet long, into the tideflats’ 30 feet of mud. After he completed the line of piles across the tideflats, workers built a trestle, which stood several feet above the highest tides, and then laid narrow gauge rail atop it.

Train service on the Seattle and Walla Walla began on March 7, 1877, though it had to compete with couples who liked to promenade on the tracks on Sundays. Within a year, however, shipworms had destroyed Surber’s pilings and the tracks were unusable. Surber’s solution, pound piles for a trestle closer to Beacon Hill, more or less following modern Airport Way.

City of Seattle map, published by A. Mackintosh, circa 1875

Now that Seattleites realized that they could build over the tideflats, they began wholesale altering, pounding in thousands of piles and building trestles and wharves for industry. They also dumped trash and other debris under the infrastructure in order to create fill, or new land. This was not a new process; in the 1850s, Dutch Ned, an employee of Henry Yesler, had dumped sawdust from Yesler’s mill into the area around First Ave and Yesler Way (then known as Front Street, because it was the waterfront, and Mill Street, for the mill) to raise the land above high tide line.

Bird’s Eye View of Seattle, 1891, Augustus Koch

By 1920, the tideflats had been completely filled. The most significant fill came from a project envisioned by former Territorial governor Eugene Semple. It began in 1895 and involved moving sediment from one part of the tideflats to another, where it was deposited behind a permeable barrier (made of pilings, brush and trees) until the new land was two feet above high tide. Other fill came from a failed attempt to cut a canal through Beacon Hill (1901-1904), the Jackson Street Regrade (1907-1909), and the Dearborn Street Regrade (1909-1912). Only a tiny amount of sediment from the Denny Hill Regrade was used and none for Harbor Island, despite what you may read otherwise. Eventually, about 1,300 acres of new “land” replaced the tideflats; none of the builders attempted to tamp down or stabilize the fill, which is why during earthquakes it tends toward liquefaction, or what has been likened to becoming jello-like, and no one wants to say that their building or tracks is constructed atop jello.

Progress of filling of Duwamish tideflats. Black = made land. Tan = original tideflats landscape. Green = islands in tideflats. Red star = location of Light Rail tracks with an issue. Lower row of images created by Amir Sheikh.

As one might expect in our modern, uncivil society, people have soundly criticized Sound Transit for failing to account for the fill and subsidence. Those people seem to think that ST had no clue about what they were doing but ST did extensive geotechnical work and knew that they were building across a landscape made of trash, sand, silt, and other debris. I was also told that railroads regularly have to fix areas of subsidence, whether on solid land or on fill; that Light Rail is generally heavier than other passenger trains; and that it would be extremely expensive to build a foundation able to withstand all of the expected changes.

If you have walked around Pioneer Square, you have probably experienced subsidence there—tilted sidewalks and parking lots being the most obvious examples but some buildings also have sump pumps to remove water that seeps into their basements, particularly during high tides when water creeps through the seawall and raises the level of groundwater. So, it’s not surprising that the Light Rail tracks are failing. The bigger concern is that Royal Brougham is merely the first of more subsidence issues to come for Sound Transit.