A Seawall Update: Jet Grouting, It’s providing a strong foundation

The existing seawall has protected Seattle’s waterfront for more than 70 years, but time and a harsh marine environment have weakened this vital piece of infrastructure, which is currently being replaced.

Cross-section of the existing seawall, including the timber piles and relieving platform

Cross-section of the existing seawall, including the timber piles and relieving platform

The seawall structure is more than just the concrete face at the edge of the water. An estimated 20,000 old growth timbers were driven into the soil to build the old structure. In some locations, such as near the historic piers, the wall is approximately 60 feet wide. The new seawall will utilize a method called jet grouting to stabilize the existing soil behind the seawall face.

Cross-section of the new seawall structure, including jet grout columns

Cross-section of the new seawall structure, including jet grout columns

What is Jet Grouting?

Jet grouting is a soil improvement technique that is the primary foundational element of the new seawall. Jet grout makes soil stronger by drilling into the ground and injecting grout, which mixes with the dirt below to create columns of stabilized soil. The columns are arranged in a honeycomb pattern, and extend approximately 50 to 60 feet below the surface of an excavated pit.

Excavation reveals the relieving platform of the original seawall

Excavation reveals the relieving platform of the original seawall

Excavating down to the existing seawall

Prior to stabilizing the soil, a large pit is excavated down to the relieving platform of the existing seawall. Once excavation is complete, the relieving platform is removed and the tops of the existing timber support structure are identified and surveyed to avoid conflicts with jet grout installation. The timber support structure from the existing seawall will remain in the ground, entombed in the jet grout columns.

Surveyors confirm the location for jet grout column installation near Waterfront Park

Surveyors confirm the location for jet grout column installation near Waterfront Park

Jet grouting continues on the central waterfront

As the Seawall Project works north to south, jet grouting is currently under way near the historic piers and was recently completed near Waterfront Park. When the project is finished, nearly 3.1 million cubic feet of grout and 6,000 columns will be placed in the ground, providing a stable seawall foundation for years to come.

A jet grout rig drills columns near the historic piers]

A jet grout rig drills columns near the historic piers

Learn more!

For more information about seawall construction, visit the Seawall Project website. If you have questions, email (seawall@waterfrontseattle.org) or call the 24-hour hotline 206.618.8584.