Willamette Water Supply Project

By John Meissner | December 9, 2022

Excavation Shoring

The Willamette Water Supply Project is a $1.6 billion water conveyance pipeline project in Oregon.  Pipeline construction began in 2016, and when substantially complete in 2026, will provide fresh drinking water, capable of meeting current and future needs, while ensuring resiliency, protection of public health, and quick recovery during natural disasters and emergencies.  Improvement of surrounding infrastructure along the pipeline alignment is an added benefit with much of the work taking place within public right-of-way.  The pipeline begins in Wilsonville, where raw water is pulled from the Willamette River and pumped to a treatment plant in Sherwood.  From there, clean drinking water is pumped to storage facilities on Cooper Mountain, then gravity fed to service areas in Hillsboro and Beaverton.  In total, the pipeline stretches over 30 miles.

Pipeline design and construction was divided into many segments, each with its own set of bespoke contract drawings, specifications, and geotechnical data, often prepared by different engineers of record from segment to segment.  The segments were put out to bid separately, with a handful of contractors vying for each.  This variability combined to form a unique and complex project, often with different approaches and methods of construction from segment to segment.

 

The majority of the pipeline consists of 48”-66”-diameter welded steel pipe, installed underground at depths generally ranging from 15’-30’ below ground surface.  Open trenching with conventional excavation shoring was the preferred method of installation where site conditions permitted.  At crossings such as railroads, creeks/wetlands, and roadway intersections where open trenching was not feasible, various methods of tunneling were employed to install pipe without disruption at grade.

 

D.H. Charles Engineering provided design consulting services to many of the contractors on the project.  Our scopes were primarily focused on excavation shoring and safety and included design of many different types of shoring systems such as hydraulic jacks, trench shields, slide rail, sheet pile, and soldier pile, as well as stability analyses of slopes.  We also provided tunneling engineering including jacking force estimates, analysis of casing and carrier pipes, and thrust block design, as well as other miscellaneous construction site engineering including design of crane bearing pads, ladder anchorage, utility supports, and traffic bridging.  We also provided on-site support.

 

This project presented a lot of unique challenges.  With numerous contractors involved, and varying requirements from segment to segment, organization and communication were extremely important.  We also had to be mindful of the preferred construction methods of different contractors, and the shoring equipment with which they were most comfortable and productive. Some contractors would strictly use trench shields, while others would drive piles – two very different approaches to accomplish the same goal of laying as much pipe per shift as possible.  Another substantial challenge came from the geotechnical conditions.  Some areas were solid bedrock that required blasting before trenching.  Other areas were the opposite with widespread soft clay.

 

Conversely, this project was extremely rewarding because the work was in our backyard, with the purpose of improving the communities where we live.  A bonus was working with many great local contractors whom we have had relationships with for years, while also forging new relationships with contractors who came to the area just for this project.

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