Pilings needed to secure unstable soil under treatment plant
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 at 4260 Kenthorpe Way in West Linns 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, which will hold up to 2 million stored gallons of water underground and handle up to 38 million gallons each day, also serves as an emergency backup water supply for West Linn.
Along with a new plant, the project involves the installation of a 4-foot-diameter pipeline from the Clackamas River through West Linn and into Lake Oswego. The pipeline, which will be broken into four construction phases, will extend 1.9 miles in West Linn, crossing though both residential and commercial areas.
The West Linn City Council approved permits for the new water plant and pipeline in February, with a group of residents quickly filing appeals with the Oregon Land Use Board of Appeals.
The appeals, however, have not hindered LOT from moving forward on the 30-month project.
Three weeks ago, LOT cut down about 200 trees to make room for the expansion. The project then moved forward with testing the soil surrounding the plant. 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 submissions from contractors 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.Add a comment