In 2013, the City of Miami Beach, Florida, conducted a condition assessment on the existing 54-in. sanitary sewage force main (FM). After years of satisfactory service, it was discovered that the sole sanitary prestressed concrete cylinder pipe (PCCP) serving the City was deteriorated and potentially at risk of failure.
The culprit, hydrogen embrittlement, a phenomenon that has plagued prestressed bands in Interpace-era pipe. This FM is the sole means of wastewater conveyance for the City. Failure would prove catastrophic to the City: discharging millions of gallons of raw sewage along the urbanized corridor; impacting residences and commercial operations; and contaminating the pristine and ecologically sensitive waterways and beaches.
Based on data indicating concerns about pipe reliability and the consequential and devastating economic impacts that could result from its structural failure, including beach and hotel closures, the City determined the construction of a new redundant FM as a priority.
The City selected the design-build delivery method in order to accelerate the effort, solicit innovations and mitigate all construction risks associated with the project. A significant criterion was the mandate to utilize a trenchless method of installation to minimize social and business impacts from construction. Additionally, direct replacement of the sanitary FM was not viable as this was the City’s primary and only sanitary sewer and in place replacement (open cut) would prove extremely expensive and disruptive to the community.
The City is located on a relatively narrow north-south orientated barrier beach. Sewage is generally collected in east-west systems which then discharge to the north-south orientated FM that in turn discharges from the southern land tip of the City to a treatment plant through a tunnel. Heavy development over the past two decades has resulted in the only viable project alignment existing along a heavily-trafficked and utility-congested corridor. The project’s urban nature and the City’s high tourist value presented unique challenges, along with the typically trenchless, unfriendly South Florida geology.
The City of Miami Beach worked with David Mancini & Sons and ISCO Industries to construct a redundant 54-in. high density polyethylene (HDPE) FM using an innovative design-build approach. This line was constructed 40 ft below historic Euclid Avenue using horizontal directional drilling (HDD) technology.
The City’s experience with HDD was previously limited to smaller diameters and shorter drill lengths. Directionally drilling the redundant FM involved approximately 3,300 ft of 54-in. DR 17 HDPE weighing approximately 225 lbs per linear foot. The City of Miami Beach had never attempted a directional drill on this scale. The installation of approximately 750,000 lbs of HDPE required creativity, experience and teamwork.
A 54-in. IPS DR-17 bimodal HDPE pipe was chosen with technical assistance, pipe fittings and fusion services provided by ISCO. ISCO’s technical support helped to maximize the construction efficiency and minimize the social impact. HDPE was chosen for this project not only because of its low life cycle cost and monolithic features, but also because of its lightweight and ductile properties, which makes it the ideal material for the HDD construction method.
The pipe fusion on the project was completed utilizing one of ISCO’s McElroy 2065 MegaMc fusion machines operated by one of its certified technicians. The highlight of the fusion operation was joining two 1,650-ft strings of pipe during the pullback process. HDPE pipe is manufactured with a material density of 59.28 pcf and has a minimum wall thickness of 3.18 in. The product satisfied the required 80-year service life, with its characteristic long-term strength, ductility, and its resistance to corrosion, providing equal and better value as the FRP.