Seattle's new $297 million high school was built on a peat bog. Then the foundation started settling

Months before the long-awaited, brand new Rainier Beach High School opened to students last April, engineers on the $297-million project reported problems with the building’s foundation to the city, records show. The school was settling into the earth more than expected in areas, as much as three inches, and it hadn’t stopped sinking.

“The project is on fairly porous ground, and it did settle more than they expected,” said Tina Christiansen, Seattle Public Schools capital projects spokesperson.

Shortly before the building opened, “it was determined that settlement had stopped,” Christiansen said, and builders dealt with the uneven settlement by releveling the top of the concrete foundation.

While excessive or uneven settlement rarely poses an immediate safety risk, it can require costly remediation, and repairs to elements like cracked walls and floors. The risks depend on factors including the cause and extent of the settlement, said Brett Maurer, a civil engineering professor at the University of Washington.

The effects can be minor, Maurer said, and require only additional concrete to level the foundation surface, the fix employed at Rainier Beach High. “The question is how much settlement is yet to occur,” he said.

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All buildings settle to some degree, but uneven settling can be “more problematic," Maurer said, and “can add stress to structural members and cause distortion within the superstructure and architectural elements of a building." Italy’s Leaning Tower of Pisa is a famous — and extreme —example of this effect, which engineers call differential settlement.

The Rainier Beach High School project carried known risks: The land, which sits across the street from Lake Washington, was previously a swamp. The district built the new school on part of the site where the soil is rich with peat. That soil, made up of tiny fragments of plants and animals from millennia of decay, is especially compressible. It expands when wet, and shrinks when dry, making buildings prone to excessive settlement. Peat soil is relatively uncommon in Seattle, but is found in scattered patches citywide.

In order to build on a peat bog, Seattle Public Schools was required to sign a covenant acknowledging the “unique risks” of the project, including excessive settlement, and releasing the City of Seattle from liability for short- or long-term hazards.

The old Rainier Beach High School, built in 1960, sat on a different part of the site, and not on the portion with peat soil that project engineers identified as a settlement risk, according to a geotechnical report issued ahead of the new construction. For six decades, that wide swath of peat bog lay beneath the school’s athletic fields. By building the new school on a different part of the land, the district was able to keep the old building open during construction, and avoid displacing Rainier Beach’s 800 students to another building or portables.

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In order to support the new, four-story school building on less-stable soil, builders first filled the earth with dozens of columns of compressed, crushed rock, the strategy deemed "most cost-effective," construction records show.

All was going as planned, geotechnical engineers told the Seattle Department of Construction and Inspections last fall. That is until the general contractor, Lydig Construction, noticed unexpected settling of the foundation in July 2023 while installing steel columns.

Over the next seven months, Lydig “measured as much as 3 inches of settlement in the central portion of the structure,” engineers told the city — far more than the one inch of long-term settlement anticipated in the plan.

Three of the 21 monitoring locations were still settling as of November 2024, all in the school’s gymnasium, engineers told the city at the time. Seattle Public Schools, the contractors, and the project design team were discussing the situation, evaluating the cause, and determining possible mitigation options.

Over the next few months, the settling stopped, said Tina Christiansen, district spokesperson, and “no further settlement has been observed since.”

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In a case like Rainier Beach High School, Maurer said, it’s important to measure the rate of compression over time in order to predict how the building might settle long-term.

“But, because building settlements can be a long process occurring over many years, any such prediction inevitably has some uncertainty,” Maurer said.

Christiansen said she was not aware how much settlement was ultimately measured, whether monitoring was ongoing, or whether a cause was determined. The district’s executive director of capital projects and facilities, Richard Best, was willing to answer all such questions, Christiansen said: “Unfortunately, his schedule doesn’t have a clear break in it until November.”

Meanwhile, construction on the project continues. A new performing arts center at the high school is expected to open next year.