In South Portland, a sewer trunk line built almost 90 years ago runs beneath Interstate 5, a hillside neighborhood, and two long-buried timber trestles. The Carolina Trunk Rehabilitation Project in Portland, Oregon is a $79.3 million effort to replace the old sewer lines. The pipe is failing, and city officials have warned that if it goes, the result could be a sinkhole on I-5 or raw sewage in the streets.
Stiver Engineering designed the temporary gravity bypass flume that kept flow moving through the tie-in, the point where the new alignment meets the old one and one of the most delicate moments in any sewer replacement.
Why It Matters
The existing trunk was built before the interstate, before the surrounding streets, and before the neighborhood was built on top of it. Repairing it in place would mean displacing residents and digging beneath private property. The new alignment moves the line into public right of way instead, but getting there means threading pipe past steep slopes, a live highway, buried trestles, and streets barely wide enough for one lane of traffic.
The project team, led by the City of Portland’s Bureau of Environmental Services along with construction manager general contractor James W. Fowler Co. and design partner WSP, had to solve for all of that at once. Unexpected groundwater conditions ruled out the original deep microtunneling approach. The design shifted to a shallower auger bore approach with custom vortex drop structures, all of it engineered to fit within tight casings between a highway, a wetland, and two buried trestles. Every shaft on the project came with its own constraints, from soil nail walls and utility conflicts to a business that had to be relocated during construction.
Where Stiver Engineering Came In
Every sewer replacement project reaches a point where the new grade and the existing grade have to cross, and someone has to keep sewage flowing while that switchover happens. Stiver Engineering designed the temporary gravity bypass flume used at that stage of the Carolina Trunk project, allowing crews to excavate down to the old line, connect the bypass, and safely demolish and abandon the section being replaced without interrupting service.
It’s a narrow scope inside a much larger project, but it’s also the kind of detail that determines whether a construction sequence actually holds together in the field. A bypass that isn’t sized or engineered correctly can stall a schedule fast, especially on a site with as little room to maneuver as this one.
Why This Kind of Work Matters
Projects like the Carolina Trunk Rehabilitation are a reminder that large infrastructure programs are built from a series of smaller, precise engineering decisions. Bypass and tie-in design doesn’t make headlines the way a tunnel crossing does, but it’s just as critical to keeping a project like this on schedule and keeping a system running while it’s rebuilt underneath a city.
Stiver Engineering supports projects through construction engineering design, dewatering system design, and shoring and tunnel design for underground infrastructure projects across the country. If your team is planning a sewer, water, or conveyance project with a complex construction sequence, contact Stiver Engineering to talk through your scope.
