To rehabilitate a drinking water system approaching the halfway mark of its projected 50-year lifespan, the Miami-Dade Water and Sewer Department (MDWASD) is using plastic pipe and its own crews.
Buildup on the 2-inch galvanized steel and cast-iron pipe had reduced the inside diameter on 500 miles of water mains to that of a pencil; and pressure, which should have ranged from 58 to 60 psi, was often in the low double digits.
Surprisingly, no one complained.
“People don't realize they're not getting enough water out of the shower because tuberculation slows it down gradually,” says Luis Aguiar, chief of the department's Water Distribution Division. “When we get a call it's usually because the flow is down to 1.5 cubic feet/second and the pressure is below 20 psi. It's like they have no water at all.”
Miami is full of driveways decorated with colored and tinted concrete, which lead to alleys and streets that homeowners won't allow the department to rip up — even to provide better service. Most water lines are in alleyways behind the houses, so Aguiar looked for a minimally invasive replacement method. He decided to burst the pipes and slip in 2-inch high-density polyethylene (HDPE) pipe that's expected to last 50 to 100 years.
The department is one of the nation's largest public utilities, providing service to more than 2.4 million customers. Approximately 330 mgd of water are drawn from the Biscayne aquifer for consumer use. MDWASD has more than 7,000 miles of water lines. The 20-year replacement program began in 1999.
To ensure control over the operation, Aguiar kept the job in-house, preparing the crew of five with on-the-job training on the TT Technologies Grundoburst 30 TX static pipe-bursting machine he bought for the project.
Once an area for reclamation is determined, the smallest trench possible is dug and a bladed cutting wheel, pulled by a hydraulic bursting unit, splits the old pipe. Then an expander spreads and displaces the split pipe into the surrounding soil while pulling in the new pipe.
“Splitting galvanized pipe is different from eliminating the newer cast iron,” Aguiar says. “Galvanized pipe reduces our efficiency, but we make it happen. Because we're doing the work in-house, our crews get inventive. We designed our own cutter head in the shop, and it's doing a great job.” The crew simply attached the homemade cutter head to the machine in places where the bladed cutting wheel was unable to split the galvanized pipe.
Though the work is considered maintenance, it's funded through the department's capital-improvement program as a “blanket project” that runs between $250,000 to $500,000 a year depending on need. The existing pipe is being “replicated,” not replaced, in a new, nearby trench, sparing the department of the surveying, planning, and budgeting required for full-replacement projects.