Ensuring Reliable Service with Asset Management
- Ethan Heijn, Trish Carney - Hazen and Sawyer
The Miami-Dade Water and Sewer Department in Miami-Dade (FL) serves more than two million customers, oversees three water treatment facilities, and manages a distribution system that encompasses 7,559 miles of pipes.
On March 2, 2010 at approximately 2:00 AM, a Miami-Dade Water and Sewer Department 54-inch diameter concrete water transmission main ruptured, creating a sinkhole 10 feet deep by 40 feet wide in a major intersection and flooding adjacent houses with a foot of water.
The ruptured main provided the water supply for the City of Hialeah and north Miami-Dade County, but the Department was able to maintain distribution system pressures. The ruptured portion of the main was isolated and subsequently dewatered and repaired by removing two damaged pipe sections and replacing them with adapters, a pipe length, a short, and a closure piece.
In the aftermath of the repair, the Department retained Hazen and Sawyer to validate inspections of the failed pipe and assist in planning relative to condition assessment and repair/replacement options for other prestressed concrete cylinder pipe (PCCP) in operation throughout the Miami-Dade County system.
During the initial repair, while a one-mile section of the pipeline was isolated and dewatered, the Department had contracted with a specialty inspection company to obtain an electromagnetic survey of the isolated section of pipe that included the failed pipe segments. This survey involved sending an electromagnetic signal transmitter through pipe and recording the return signal as influenced by prestressing wires, and provided data on the approximate number and location of wire breaks. Of 256 pipe segments inspected, 46 were characterized as having broken prestressing wires and 17 of the 46 damaged pipes appeared to have broken wires throughout the pipe segment. In total, 24 pipes were recommended for immediate repair.
This initial electromagnetic inspection was followed by a second electromagnetic inspection of a 16.3-mile section of pipe, after the section that ruptured was placed back into service. Once completed, this second inspection revealed that 126 of the 4,505 pipe segments had at least five wire breaks and 40 of 126 damaged pipes had broken wires throughout the pipe segment. Seventy-three pipes were recommended for immediate repair, including the 24 identified during the first round of testing.
Once the pipeline had again been isolated and dewatered for repair of the most severely distressed pipe segments, Hazen and Sawyer conducted a visual and sounding inspection of the pipeline interior to confirm the electromagnetic survey results and verify that the correct segments had been identifi ed for repair. The visual inspection included looking for cracks in the inner or outer coating of the pipes and tapping the inner core with a hammer to detect delamination of the concrete core from the steel cylinder.
Longitudinal cracks and audible delamination were encountered on two of three pipes characterized as most severely distressed, and Hazen and Sawyer confirmed the condition of the third most severely distressed pipe by wire continuity testing. For the wire continuity testing, the pipes characterized as severely distressed were identified from above ground using a measuring wheel and a receiver to locate a sonde placed inside the pipe; the tops of the pipes were excavated and exposed; and the outer mortar was carefully removed to expose the wires. The electrical continuity between wire wraps was then checked to identify broken wires and it was found that the actual wire breaks exceeded the numbers predicted by the electromagnetic testing.
Assessing Repair Options
Following verification for repair, we turned our attention to investigating alternatives to prevent future failures and addressing the following questions:
• What is the condition of the remaining PCCP pipe in the system?
• How can these pipelines be inspected to determine their condition?
• What preventive maintenance program is required to maintain the integrity of the system?
• What cost-effective repair alternatives are available?
In order to answer these questions, Hazen and Sawyer worked with the Department to develop an Infrastructure Assessment and Replacement Program (IAARP). Through this program, 15 miles of pipe are inspected per year via electromagnetic methods that can be conducted while the pipeline remains operational. The prioritization for these inspections is determined by a variety of factors, including operating pressure, diameter, age, land use, operational criticality, repair history, and the date last inspected. Inspections are scheduled for every three years for known distressed pipes, every five years for 48-inch and above, and every 10 years for pipes smaller than 48-inch.
As part of this program, we developed a PCCP pipe inventory of the Department’s water system including calculated theoretical pipe failure rates during the next 5-, 10- and 100-year periods. We also reviewed the advantages and disadvantages of repair and replacement alternatives, the factors to consider when selecting a rehab method (including structural condition and proximity of distressed pipe, hydraulic adequacy, and surface conditions), and associated costs of each repair option.
The project has enabled the Department to proactively monitor the state of their prestressed concrete cylinder pipes in order to properly prioritize repairs, select the best repair option, extend the useful life of their assets, and save money by avoiding emergency repairs.
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