Evaluation of Large-Diameter Interconnection to an Existing 96-in. PCCP
The City of Houston was faced with a need to replace an approximately 50-year-old 42-in. PCCP line with a history of leaks, which carries water from a critical 96-in. transmission line with no redundancy to seven smaller partner cities to the south. This replacement required a new interconnection to the existing 96-in. line without impact to water service by maintaining an internal pressure of at least 55 psi throughout the work. To respond to this request, the design team performed two levels of evaluation, first considering the trade-off between risk, cost, and temporary service interruption for options including cut-in TEE, single full-diameter tap, multiple smaller taps connected to a header, and extending the new water line an additional 1.5 mi to enable a connection at an existing stub-out within the treatment plant. For the option with multiple hot taps, the required removal of prestressing wires and concrete outer core while the 96 in. line with unknown condition is in service warranted detailed analysis. Finite element modeling was performed to evaluate the safe maximum line pressure during the time the prestressing wires and concrete outer core are being removed for potential 24-in. or 30-in. taps.