During an extreme rainfall event, flood flows from Thornton Creek eroded and removed soils from beneath a single-family residence in Seattle, Washington. This created a large, irregular void – running 10-ft. to 15-ft. under the main structure of the house – exposing several piers supporting the house.
The Kessler residence occupied nearly the entire lot, creating a very confined urban site. The stream channel was approximately 10 feet wide with flowing water and mature vegetation overhanging into the stream channel. There were steep slopes leading down to the work site and this, combined with the small work area, provided no access for equipment. All materials and installation would need to occur by hand. The repair would have to restore the bank, fill the void under the house, protect the supporting piers and withstand future flood flows.
Additionally, the stream is a fish bearing stream, requiring the repair to be environmentally sensitive, contain habitat elements and withstand flood flows to provide long-term protection and stabilization of the stream bank. The repair and restoration of the site required permits from both the City of Seattle and State Fish and Wildlife.
Because all materials had to be delivered and installed by hand, this eliminated the use of traditional bank stabilization materials such as large rock. Permit requirements and costs limited additional choices such as poured walls or formal bulkheads. The irregular shape of the void required a fix that was flexible so it could be shaped and formed to match the remaining bank and fit tightly against the piers.
A-Jacks concrete armor units were chosen for this project because they could be easily delivered and installed within the site constraints. They arrived stacked on pallets, facilitating the staging of materials in a standard residential driveway and the separate pieces could be hand carried and assembled.
The interlocking A-Jacks units created a stable structure with sufficient volume and flexibility to fill the irregular void under the house and around the piers. Once interlocked, the concrete A-Jacks units provided the mass and stability to withstand future flood flows. Complete installation took only seven days.