The City of Mill Creek is proposing to replace an existing damaged 36” corrugated metal stormwater pipe that extends north-south under the Sweetwater Ranch neighborhood perpendicular to 144th Street SE. The original stormwater main was constructed prior to the development of the neighborhood extends to the south, connecting into Holly Pond. In 2018, the stormwater main through the neighborhood was found to be in an advanced degree of deterioration due to a sinkhole and emergency repairs were completed. Since the repairs, another sinkhole was found near the previous repairs. Further inspections determined that the original pipes downstream of the repairs have significant damage. The previous repairs are sound and conveying the flows as expected.
The project will replace the existing stormwater main with new pipes while avoiding steep slopes, adjacent neighboring properties, and utilities, and minimizing impacts to the existing wetlands.
The project will also abandon the existing 36-inch pipe by filling it with flowable grout. The new pipe will be installed slightly to the west, including the installation approximately 444 feet of 36-inch plastic storm pipe with four concrete catch basins using conventional cut and cover trenching techniques.