Pilings needed to secure unstable soil under treatment plant

by: VERN UYETAKE - Workers used a 70-foot drill to dig into the ground near the Lake Oswego water treatment plant in West Linn to test the soil solidity for future expansion.Using a 70-foot drill, the Lake Oswego-Tigard Water Partnership started testing the soil surrounding the water treatment plant last week in anticipation of rebuilding the plant, which sits in West Linn.

Lake Oswego has operated a water treatment plant in West Linn’s Robinwood neighborhood since 1968. In cooperation with the city of Tigard, Lake Oswego wants to expand the plant and run a new pipeline to address the future water needs of both cities.

The plant will hold up to 2 million stored gallons of water underground and handle up to 38 million gallons each day.

Using a 70-foot auger, workers drilled four test holes around the building. Workers judged how long it took to drill 70 feet into the soil and what kind of soil emerged at certain depths.

The water plant is sitting in a liquefaction zone; just 15 feet below the surface, the soil turns into a runny, muddy slew that resembles unset pudding. Then, about 50 feet below that, the soil turns into a hard gravel.

According to the Association of Bay Area Governments, liquefaction happens when loosely packed sandy or silty materials saturated with water are shaken hard enough to lose strength and stiffness. This is a major concern if there is an earthquake, as the stability of the water treatment plant would be at risk. Because of this, LOT will need to install about 1,000 reinforced pilings, separated by about 7 to 8 feet, under the new treatment plant.

A large crane was brought out to the water plant April 3 and the drill was assembled. The test drilling took place April 4, with removal of the equipment on Friday.

The first drilling took 13 minutes, which was an “excellent” rate, according to Tom Lebo, a senior construction manager with Brown and Caldwell. During the second test hole drilling, the auger was completely submerged into the ground. As it was pulled up, a mixture of soil and clay first appeared, and then a brown ooze started pouring out, which was the liquefiable soil. The hole was then refilled with a low-strength cement product to prevent surface water from running into the ground water.

According to Lebo, LOT will use soil and drill rate information when asking for contractors’ bids on the pilings portion of the project. Currently, LOT has 11 contractors prequalified for the project, according to Jane Heisler, LOT communication director. Bids are expected to open sometime in June or July.

Also last week, a temporary wire mesh fence was installed along Mapleton Drive. Soon, sight-obscuring fabric will be applied to the fencing to block the site from neighbors' views.

A permanent security fence will be installed in May or June along with erosion control measures and a "good neighbor" fence.

For more information about the project, visit

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