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Mint Farm stone column

Around 2011 when construction crews began building the Mint Farm Water Treatment Plant in Longview, crews built rock pilings into the ground as part of the structure's foundation. The columns were needed to support the weight of the facility and prevent soil liquefaction in areas of soft, sandy soils.  

Two recent construction projects in Longview-Kelso turned the notion of “dirt cheap” on its head when architects learned poor soil conditions will require costly foundations.

The Kelso School District will have to spend about $5 million more than budgeted to build deeper foundations for two new elementary schools with it’s voter-approved bond. And state highway planners have been forced to reconsider their plan Longview’s Oregon Way-Industrial Way intersection project when estimates for the original design came in $50 million — nearly 60 percent — above early projections.

“What we found over a series of projects in the Kelso-Longview area, but specifically on this (intersection) project, was the geotechnical holes we drilled brought back a soft, silty soil — and it goes pretty deep,” said Frank Green, WSDOT’s regional assistant administrator for development and delivery.

Sandy or silty soils can liquefy during earthquakes, Green sad. That means the dirt moves and resettles, causing the buildings sitting atop them to shift or sink.

Though the building won’t “get lost or sink completely,” just a few inches of shifting can cause a building to crack or make a bridge’s foundation unstable, Green said. It can also sever water, sewer or gas lines, which could cause safety hazards, said Longview Public Works Director Jeff Cameron.

To prevent buildings from suffering the consequences of liquefaction — and to meet state building code requirements — projects must use deeper foundations, said Tim Barney, president and lab manager for Pacific Testing and Inspection in Centralia.

Barney’s company provides soil testing and inspection services for projects in Lewis and Cowlitz county, including projects like the Washington Way Bridge, Kelso's West Main changes, and the Kelso High School turf football field.

Those projects familiarized the company with the general soil conditions in this area — and why those conditions might call for deep foundations to keep structures safe when the next major earthquake hits the region.

“When you drill for deeper foundations, you get into more bedrock. You get into material that’s not influenced by weathering. You get into denser glacial material or basalts that support heavier loads and aren’t affected as much (by seismic activity),” Barney said.

The schools and the intersection aren’t the first two projects to face challenges of soft soils in this area. Builders have been working with the sandy soil conditions here for decades, Cameron said.

He pointed to the Longview Wye (Interstate 5 exit 36) interchange, which was reconstructed about 10 years ago. The project was ultimately scaled back due to poor soil conditions and unexpected costs, he said.

The sandy, porous soils that make up the Kelso-Longview lowlands are “alluvial fill.” They are made up of sand and gravel washed down the Cowlitz and Coweeman rivers over millennia. The layer of soft dirt built up after “hundreds of years of runoff,” Cameron said.

“Anywhere that a river empties down from the mountains, it’s carrying materials with it. If there’s a flat spot on the bank of the river or near the mouth, the soil will deposit there,” Cameron said.

So why have the long-standing soil conditions thrown a financial wrench into construction plans for the schools and the intersection? The answer lies in the depth of the layers, which vary from site to site, officials say.

Out near the Mint Farm the layer extends to depths of nearly 175 feet, but in the northern parts of Longview — near the rises that make up the Columbia Heights area — many areas only have 2 feet of sand, Cameron said.

Builders can’t know the depth of sands at a particular site until conducting a soil study, where testing companies like Barney’s drill into the ground and pull up layers of the earth. Because these studies can cost tens of thousands of dollars, most projects base their initial cost estimates using information from nearby sites or geologic maps, Barney said.

Cameron said the city usually holds off on soil studies until funding is approved. And it is “standard protocol in school construction” to wait for the study until after voters approve a bond, said Michele Larsen, Kelso School District spokeswoman.

Most project budgets have “contingency” built in to account for poor soil conditions and any unplanned costs they might reveal, Larsen said, but there’s always a risk that actual costs might exceed the contingency allowance. And that just happened to be the case for the school and the intersection projects, officials said.

“It’s kind of the unknown, what’s under the ground, until you do the borings,” Green said. “When you get those geotechnical borings back and see what they mean, that’s when you see the costs might increase.”

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