Thursday - September 18, 2014

How is the new water pipeline being built under the Willamette River?

A drill rig loading drill pipe for the pilot hole, at the construction site just north of Mary S. Young Park, West Linn

Over 10 miles of large diameter water pipeline is currently under construction in Gladstone, West Linn, Lake Oswego and Tigard as part of the Lake Oswego-Tigard Water Partnership project. Construction methods for the pipeline vary between traditional cut-and-cover trenching and trenchless methods including horizontal directional drilling, micro tunneling and pipe ramming. While much of the pipeline is being installed in streets using typical cut-and-cover trenching, a special technique is used to get across the Willamette River.

The new pipeline between Gladstone and West Linn requires a 0.75 mile crossing under the Willamette River. Crews are using a specialized, steerable, trenchless method known as horizontal directional drilling, or HDD. HDD technology was chosen because it limits disruption and has minimal impact on the environment, compared to cutting a trench in the riverbed. Two drilling rigs are set-up on land at Meldrum Bar Park in Gladstone and Mary S. Young Park in West Linn. Both drill rigs are currently drilling through rock in a shallow underground arc, about 80 feet below the riverbed to meet under the river. Each drill rig is creating an underground tunnel, or a pilot hole. The hole will then be enlarged to four feet in diameter using one or two passes of a large drill bit or a reamer. Once the hole is properly sized, a 36-inch diameter welded steel pipe will be pulled through the hole from Gladstone all the way to the other side of the river in West Linn.

Construction of the water pipe under the river is expected to take about six months to complete. In contrast to the existing, 46-year old undersized 27-inch diameter pipe, the new pipeline will be seismically resistant, and capable of delivering up to 38 million gallons of water a day to customers in Lake Oswego and Tigard for at least the next 75 years. The project is also providing work for local contractors and suppliers and helping the region’s economy by creating more than 2,200 jobs.