Brown and Caldwell’s creative engineering design replaced an aging 2-mile underwater interceptor in Lake Oswego, Ore., with an unprecedented buoyant, flexible gravity sewer constructed of HDPE pipe. The $100 million project — that also includes 1.5 miles of pile supported pipe, 3 miles of cured-in-place rehabilitated pipe and two new pumping stations — allows the city to replace an undersized, corroding, earthquake-vulnerable system with a seismically sound, lower-cost, low-impact system designed to meet the city’s needs for another 100 years