The M Street Trunk Rehabilitation Project will repair about 2.6 miles of old concrete sewer pipes in Auburn. We will also repair 45 maintenance holes and over 100 sewer connections to homes and businesses. This maintenance helps ensure reliable sewer service in your neighborhood and our region. 

The M Street Trunk is a large King County sewer pipe running through Auburn. The pipe carries untreated wastewater from homes, businesses, and schools in your neighborhood. Wastewater is the water that goes down your drain when you flush the toilet, take a shower or use the sink. It goes down your drain to pipes headed to the South Treatment Plant in Renton. It is then cleaned and safely released into the Puget Sound. To learn more about the cleaning process, visit the King County Wastewater Treatment Division’s (WTD) website. 

We understand construction can be disruptive. Here are ways we are reducing impacts to our community. During construction, we will use a temporary pipe to keep sewer services working. A temporary pump will lift sewage into a diversion pipe so the underground pipe can be repaired. You will be able to use your sinks, toilets, and showers normally. Noise and odor control equipment will also be used at critical points in the sewer diversion system. 

The M Street Trunk was installed in 1963. Over time, gases from sewage in the pipe corroded the pipe walls, which makes the pipe less reliable. The hydrogen sulfide gas coming from sewage in the pipe is a normal and expected byproduct. We will repair the pipe by installing a liner inside that will protect against further corrosion. Relining the pipe is the cheapest and least disruptive way to extend its service life. Learn more about how our clean water investments are funded on King County’s website.   

The project’s design and construction are divided into two phases. The first phase of construction was completed in late 2025. The second phase will begin as early as May 2026. The project schedule is separated into four work zone areas based on the sewer pipe size. These are Zones 18, 24, 30, and 36. Construction will take place during the summer months when sewer flows are lower.  

In 2025, we completed repair work on 0.6 miles of pipe on M Street in Zones 18, 24, and a small section of Zone 30. This early work will help us complete the pipe repair work in 2026. It also allows us to coordinate with the City of Auburn's planned road widening project on M Street beginning in 2026.  

In 2026, we will repair about 1.9 miles of pipe in the remaining portion of Zone 30 and the full length of Zone 36. This work will take place between the intersection of 2nd Street Northeast and Pike Street Northeast and 23rd Street Northeast, near the Auburn Municipal Airport. Once this work is complete, we will begin restoring driveways, streets, and sidewalks. Restoration work is expected to finish in June 2027. You can view active construction updates on our interactive map on our project webpage.  

King County Wastewater Treatment Division will be using two methods for relining the M Street Trunk: cure-in-place piping and spiral-wound lining.  

Cure-in-place piping (also known as “cured-in-place pipe lining” and abbreviated CIPP) is an efficient way to repair sewers. It requires no digging, reducing any impacts on neighbors and commuters. CIPP involves inserting new, flexible lining into the sewer pipe. The liner is then inflated to match the existing pipe size. Once inflated, the liner is hardened with heat or ultraviolet (UV) light to set it in place. The lining then forms a smooth, water-tight seal on the inside of the existing pipe. CIPP will be used for Zones 30 and 36.

Spiral-wound pipe lining is an efficient way to repair sewers. It requires no digging and can sometimes be done while water is still flowing through the sewer. This method uses a flexible material that can be wound in a spiral around the inside of an existing sewer pipe. Imagine wrapping a ribbon around your finger, overlapping it just enough as you wrap it down your finger so that none of your skin is showing. That is what the spiral winder does inside the pipe. The edges of the lining material are often self-sealing, so it forms a water-tight bond as it is wound down the length of the pipe. Spiral-wound technology was used for Zones 18 and 24 during the 2025 construction season.