In this paper a systematic methodology is developed to estimate economically optimal replacement time interval of a water distribution pipe using the proportional hazards modeling approach. To maximize the full potential of the proportional hazards model (PHM) the pipes in a case study area are redefined so that the pipes have consistent internal and external characteristics along their newly defined lengths. The survival functions of the PHMs are constructed for the six ordered survival time groups of cast iron 6 inch pipes in the area and subsequently used to estimate the intervals of future failure times of a pipe. The recorded and predicted future failure time intervals are used to model the trend of pipe failure using the General Pipe Break Prediction Model (GPBM). The equivalence relationship between the GPBM and the threshold break rate of a pipe is used to estimate the economically optimal replacement time interval of a pipe.

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