A Case History of Scour Impacted Pipelines in River Crossings Part II — Structural Repairs

Two major water supply pipelines, (48/42 and 51-inch [122/106 and 130 cm] diameter), owned by City of Everett, Washington, at five river and slough crossings (160 to 540 feet [50 to 165 m] wide) where parts of the pipelines had become exposed and unsupported because of scour. Two of the 51-inch [130 cm] diameter pipeline crossings required emergency repair. The underwater emergency repair consisted of repairing the damaged coating, driving steel piles alongside the sections of pipe that could be scoured, encasing the pipe in steel jackets at pile locations, bolting the jackets to the pipe and piles, and protecting the pipe by under-water tube steel fabricated screens and above-water rock-filled polyethylene mattresses. All investigation, scour analysis, design, procurement and fabrication were completed within 6 weeks. The mobilization of construction equipment on four barges at two sites and the emergency repairs at both crossings were performed simultaneously within 20 days to complete the construction prior to the impending flood season. This paper presents the structural evaluation, design of the emergency repairs, environmental aspects of the underwater construction, preparation of the construction documents, and the construction of the structural repairs to the pipeline.