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Sewer Pipe Failure Probability Curve Development
Seattle Public Utilities

Seattle, Washington

BIS staff developed and applied a method for using inspection and failure data to create failure probability curves for SPU’s concrete and vitrified clay sewer pipes. These curves express the likelihood of a failure as a function of the age and length of the pipe.

The methodology is based on a maximum likelihood statistical approach. In addition to the base failure probability curves, we also developed correlations between failure probability and installation characteristics such as steepness and soil type.

These curves will be used by SPU in their risk-based planning models to project pipe replacement or lining requirements, and also to justify expenditures on an economic basis, which is a key part of their asset management program.

These curves replace the “industry standard” curves used in the past, and may have a significant effect on SPU’s future spending.

The white paper detailing this study can be ordered here from the National Clay Pipe Institute.

Date Completed: 2006

Creation of failure probability curves for concrete and VC pipes. Supplementary correlations between failure rate and soil type.
Terry Martin
Strategic Asset Manager
(206) 615-1744
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