Fill Failure - Urban Geology
As you stand here the ground feels solid beneath you. Cobblestone, sidewalks, and pavement are all readily in view. This street corner feels no different beneath your feet than millions of other street corners in thousands of other cities - except it is. As you explore this Earthcache you will discover that the land you stand on is far from solid. As a reminder, an Earthcache does not have a physical container, rather you must instead read the description, make some observations on site, and then send your answers to the Cache Owner in order to claim credit for this cache! With that in mind... let's begin!
Anecdotal Awareness
This earthcache lesson was actually started by an innocuous (and seemingly more and more common) phenomenon - the Sound Transit Light Rail was delayed. In the late summer/early fall of 2023 Sound Transit announced interruptions to service as it did work on the Royal Brougham tracks as they were prone to flooding. This led to some googling (as I sat waiting for my train to come) and eventually this earthcache.
This all used to be mud
It turns out the reason that the Royal Brougham area floods is because it's not built on ground - well not on solid ground at least. Although we refer to the "land" in SODO and near the water quite frequently, that might not be the best term. The area was a bay of the Whulge (aka Puget Sound) that 
extended to Tukwila until only about 1,100 years ago when a lahar (Take a visit to GCA99G4 for more information on lahars) cascaded mud and muck off Tahoma (Mt. Rainier) down the Duwamish River valley and pushed the mouth of the Duwamish to its present location. This new "land" was underwater at high tide and exposed at low tide. These tideflats became an important harvesting area for local Indigenous peoples but when Europeans arrived however, they found the protean nature of the tideflats troubling as they could not take advantage of what they thought should have been a good location for land development.
In 1876 initial attempts were made to "ford" the mudflats by driving pilings into the mud and building tresseled train tracks to cross it. This was abandoned less than a year later as erosion left the pilings unusable. However this gave early Seattlelites a taste for building over the tideflats and they quickly began wholesale alterations to the region. Thousands of piles and building trestles for wharves and local water based industry were created. They dumped trash and other debris under the infrastructure in order to create fill, or new land. This process was not entirely new however - right where you're standing in the 1850s, Dutch Ned, an employee of Henry Yesler, had dumped sawdust from Yesler's mill into the area near the intersection of modern day First Ave. and Yesler Way in an effort to raise the land above high tide.
Where are we today?
By 1920, the tideflats had been completely filled. The most significant fill came from a project envisioned by the 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 material came from a failed attempt to cut a canal through Beacon Hill, the Jackson street regrade, and the Dearborn Street regrade. Eventually a ground total of 1,300 acres of new "land" replaced the tideflats.
The Red Star above shows where the lightrail repairs are being made.
This might have been the end of the story if not for one major construction oversight - none of the builders of this fill project attempted to tamp down or stabilize the fill; rather, developers proceeded to build right on top of the new "land." This has created a unique geological hazard - during earthquakes this non-solid landmass tends towards liquefaction. A geomorphological process, liquefaction takes place during earthquakes and other seismic events and causes the normally solid feeling substrate to take on the appearance and characteristics of a thick liquid - like Jello!
A visual description of liquefaction.
Questions and Observations
While Royal Boughman is a bit farther away, this Earthcache brings you to Pioneer square because there is evidence of this fill right here at your feet (Whereas you'd have to be a light rail technician or engineer to access the tracks and see this first-hand at Royal Boughman).
- The area beneath your feet is built on non-stabilized fill. What visible evidence of this do you see around you? (Hint: look towards your feet).
- Do you think this area closer to the water is more or less stable in an earthquake. Why?
- Take a photo of you at GZ and include it in your log! If you prefer not to show your face, you may instead include a photo that includes your caching name or a personal item!
References
- https://streetsmartnaturalist.substack.com/p/when-fill-fails
- https://www.aspectconsulting.com/blog/2020/1/30/seattle-workforce-housing
- Pioneer Square - Seattle's Oldest Neighborhood by Mildred T. Andrews