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.
“Basically, it's just labor and materials,” says Dan Mathews, assistant superintendent for the department's Water Transmission and Distribution Division. “Our labor is a fixed cost, and the materials are small change compared to doing an open cut. Restoration is also minimal. And because we do it ourselves, we're experts. This means the crew is quick and efficient.
“Plus, they care. This is a dedicated crew, not only in that they just do this one job, but because they are also responsible professionals.”
The crew consists of five employees doing the job from beginning to end, including digging, running the machine to split the old pipe and feed in the new pipe, tying in the new service to the home, and replacing the soil, sod, and any other patching. On a typical day, the crew installs 300 to 500 feet of new pipe.
“Our largest pull was 550 feet,” Aguiar says.
When crews run into gas and other utility lines, they bypass the lines as easily as possible without disturbing them. “Because we're not in there with heavy equipment, we can control the path and the speed of the installation,” Aguiar says. “That gives us a real advantage.”
“Because it's being replaced in 500-foot coils, even long runs can be one, joint-free continuous piece,” says Bruce Kuffer, PE, manager of polyethylene market development for the Plastics Pipe Institute (PPI). “When connections are made to homes, or pipe sections added, they are literally fused together, which makes the system leak-free. This seamless, jointless system provides a longtime solution.”
The average temperature in Miami is in the mid to high 80s. With the coils sitting out in the supply yard exposed to intense sunlight for long periods of time, it's important that the pipe have proper UV protection.
Aguiar chose Endot Industries' Endo-Trace pipe because of its minimum five-year Florida Sunlife Ultraviolet Protection Rating and its tracer wire, which will make it easier for crews above ground to find the buried pipe. The product is supplied by HD Supply Waterworks of Thomasville, Ga.
According to PPI, splitting an old pipe and inserting HDPE pipe costs one-third to two-thirds less than traditional dig and replace, although Miami-Dade declined to disclose local figures. Because of the longevity of the new pipe, savings generally may be as much as 300%.
To fuse sections of the plastic pipe, the ends are inserted into the clamps of a fusion unit and trimmed. Then they're held against a heat plate, which is removed when the proper melting point is reached. The ends are then pushed together and held until the joint is cool.
In Miami-Dade service runs to the property line where the meter is placed. Connecting to the main is done by electrofusing (applying an electrical current that melts and then fuses the connecting tee to the curve of the pipe) side-saddle tees from Central Plastics. Electro-fusion differs from heat fusing in that it uses electricity — not heat — to fuse the pipe ends.
“We make the connection to the meter and water is running typically within a few hours,” Mathews says.
“Water pressure is immediately restored,” Aguiar says. “For our customers it's like they're seeing sunshine for the first time. And that's a good feeling for us.”
— Cooper is a New York-based business writer.