REPLACING A 1940'S CAST IRON PIPE CROSSING A MAJOR RIVER

When breaks began to occur in an aging 24-inch cast iron subaqueous force main, the utility owner looked for a cost effective replacement. The solution was a combination of pipe materials and installation technologies that could be constructed quickly, while keeping the existing line in continuous service.The cornerstone of the replacement pipeline was a 1,700 linear foot horizontal directional drill (HDD) using 36-inch outside diameter high-density polyethylene (HDPE) pipe. In order to resist the pulling forces during installation, a wall thickness of 3.273 inches (SDR 11) was selected. This resulted in a nominal inside diameter of 29.5 inches. The HDPE pipe was sufficiently flexible to negotiate the tight vertical radii dictated by the available working space on each shore of the river. The pipe was installed more than 40 feet below the mudline of the existing shipping channel and more than 70 feet below the entry and exit points. The HDD crossing was estimated to be 40 percent cheaper than dredge-and-cover construction, and avoided dealing with potentially contaminated river sediments.After the HDD was in place and tested, the “land” portion of the pipe was constructed using ductile iron pipe encased in loose-wrapped polyethylene. A special flange adapter was fabricated to transition between the HDPE and DIP pipes. Much of the “land” pipe was in a busy city street, and some was up to 16 feet deep to safely pass beneath the labyrinth of existing utilities. Extensive dewatering and traffic control were required. The “land” pipe also included a 30-inch direct-bury magnetic flow meter.The final phase was to make the connections to the existing force main on each shore. The connection point on the south shore was made at a relatively new section of ductile iron pipe, and a portion of the existing pipe was abandoned and filled with concrete. On the north shore, the connection was 16 feet deep, and had to be made to a portion of the line that was constructed of concrete pressure pipe. The concrete pipe lay directly beneath a 48-inch storm drain, which even during dry weather was partially full of tidal water. The storm drain had to be temporarily plugged, drained, and removed to accomplish the force main connection below it. The concrete pressure pipe was pulled apart at the nearest joint and connected to the new DIP with a fabricated adapter. The existing line between the tie-in point and the river was lined with a cured-in-place “sock” that was used to hold the concrete fill.The new pipeline was successfully completed and has been in service for approximately 12 months.