Siphon Drilled Under Spokane River

Story by Carl Molesworth | September 28, 2010

The city of Spokane's South Riverton Avenue Siphon project is giving the contractor, Westway Construction Inc., an opportunity to explore the capabilities of horizontal directional drilling.

The South Riverton Siphon is an existing 12-inch-diameter pipe crossing of the Spokane River, which is about 400 feet wide at this point. The west manhole is located about 300 feet south of Mission Park near the river bank, and the east manhole is located in South Riverton Street near the intersection with Hogan Street. Because the sewer main on the east side of the river is bigger and about 4 feet lower in elevation than its west side counterpart, it drains west side sewer flows to the treatment plant farther east.

The project involves the installation of two 14-inch-diameter high density polyethylene sanitary sewer pipe siphons and a communications line under the Spokane River near Mission Park to augment the existing sewer system and alleviate overflow issues on the west side of the river. The second siphon provides redundancy (safety) and will allow better maintenance access to each siphon, according to information provided by the city of Spokane.

The project originally was designed with cofferdams to hold back the river water and allow open-trench pipe installation in the stream bottom. However, the water in the Spokane River was found to be deeper than anticipated when the project was bid, according to city documents. The use of cofferdams and open trenching would have more than doubled the increased costs and would have further impacted the stream bank vegetation, city officials said.

At that point, the contractor suggested switching the pipe installation to HDD construction, said Jason West of Westway Construction.

"All involved agreed that HDD was a cost-effective solution to the problem with far less environmental impact to the Spokane River," West said.

Four-Step process

Westway, based in Airway Heights, Wash., had some experience in smaller HDD projects but nothing of this magnitude, West said. So the company hired Kinnan Engineering Inc. of Camas Valley, Ore., which does a lot of oil field projects, to subcontract the HDD work for the siphon.

Kinnan brought in a Vermeer Rockfire directional drill, Tri-Flo slurry processor and a Vacmaster Soil Vac SPV800 vacuum system to do the job. They set up on the east bank of the river.

West described the process that was involved:

  • The Westway crew exposed utilities running parallel to the river and installed sleeves under them to direct the drill head through the area at the correct angle.
  • The Rockfire drilled 9-inch-diamater pilot holes under the river, and then the DD100B reamed them out to 18 inches in diameter.
  • HDPE piping was delivered as 50-foot sticks to the west side of the river for fusion and installation of the pulling heads. Then the pipes were pulled back across the river. Bentonite used to lubricate the bores and flow the cuttings was recycled in the Tri-Flow and reused.
  • Once the three pipes had been installed, the crew could set the collection vaults and manholes to tie the system together.

Westway had bid the project for $526,551, but the unexpected depth of the river caused the city to add $50,000 to the job and another $50,000 to its administrative reserve. The contractor started work at the beginning of October 2007 and is scheduled to finish this month.

West said he has been very pleased with the work of subcontractors Kinnan Engineering and Aztec Electric, along with the cooperative attitudes of Ken Brown and Steve Sather, with the city of Spokane's project management staff for construction.