NUTS, BOLTS AND HDD: A TOUGH JOB AT A TOUGH LOCATION - CROSSING THE CAPE FEAR

摘要

An existing 30-inch raw water line crossing the Cape Fear River was in danger of failure due to riverbed scour and in urgent need of replacement before peak seasonal demand water requirements in Summer 2009. The solution to this problem was to replace 2,415 feet of 36-inch HDPE pipe connecting it to the existing in-service line at either end. One pipe connection was located in an environmentally sensitive tidal marsh where no drilling operation, bentonite mixing, or spoil discharge was permitted. Given these constraints, from initial design in September 2008, the HDPE line was brought into service May 7, 2009 and from concept to substantial completion, the Cape Fear River HDD crossing was completed in 7 months. Due to the limited contract time and site environmental permitting constraints, the project required innovative engineering and construction approaches. Design began in mid-September 2008 and notice-to-proceed was given to the contractor mid-January 2009. Completion was scheduled May 1, 2009. The geology at the marsh exit location, soft silty alluvial peat deposits with decayed tree stumps and root masses, presented HDD mud-pressure restrictions to prevent inadvertent drill fluid release, or better known, "frac-out". A two-drill approach was used to allow drill fluid handling, spoils removal, and pipe layout from one location. State-of-the-art drill fluid handling techniques were implemented to control, and ideally eliminate, drill-mud discharge into the exit pit. Access and equipment stability in the tidal marsh, a subgrade with no engineering value, and the environmental issues associated with a fragile ecosystem, governed all aspects of design and construction. Close coordination, cooperation, and communication between all parties, owner-designer-contractor, was necessary to get the job done, on time and on budget, leaving only footprints in the marsh.