Infrastructure Management and Deterioration Risk Assessment of Wastewater Collection Systems.

摘要

Proper collection and transmission of wastewater within pipe systems is of utmost importance to minimize public health problems and environmental contamination resulting from discharge of untreated sewage. However, due to reactive management strategies, overall condition status of wastewater systems has reached a sub-standard level. Moreover, budget restrictions due to competing needs are preventing sewer agencies from addressing inspection and repair/renewal of every deficient pipe. Therefore, sewer agencies are in dire need of establishing risk assessment tools in order to optimize use of limited resources by prioritizing inspection and/or renewal needs of sewer pipes. The objective of this dissertation is to develop such a risk assessment tool at an individual pipe level by combining the probability of failure values determined by statistical deterioration modeling of sewer pipes and consequences of failure values determined by examining the geographical, physical, and functional attributes of sewer pipes in the light of expert opinions that reflect the relative importance of these attributes.;In order to determine probability of failure values, three statistical methods, namely ordinal regression (proportional odds model), multinomial logistic regression, and binary logistic regression, were employed in successive steps. Five ordinal regression models were generated by using logit, probit, negative log-log, complementary log-log, and cauchit link functions. Proportional odds assumption of ordinal regression was tested for each model; however, due to unsatisfactory results, ordinal regression was excluded from further analysis. Based on the percentage of correct predictions, binary logistic regression was deemed to be the most suitable method for predicting probability of failure.;Consequences of failure values were determined based on a weighted scoring system. This method was selected due to uncertainties associated with direct costs of sewer failures and the intangible nature of social and environmental impacts of sewer failures. Expert opinion was obtained from a local sewer agency to evaluate geographical, physical, and functional attributes of pipes in terms of consequences of failure.;Three methods were employed in order to assess risk of failure: multiplication, risk matrices, and fuzzy inference systems. The multiplication method helped differentiate pipes with similar probability of failure values but different consequence of failure and vice versa. However, this method failed to distinguish pipes having low probability of failure accompanied with high consequence of failure from pipes having high probability of failure accompanied with low consequence of failure values. Use of risk matrices overcame the aforementioned limitation of the multiplication method by allowing different levels of risk values to be assigned to different combinations of probability and consequence of failure values. On the other hand, the discrete nature of the variables in risk matrices caused inaccurate representation of risk values near boundary levels of probability and consequence of failure values. Finally, fuzzy inference systems were used to represent the fuzziness in probability, consequence and risk of failure variables; and to assign risk values based on fuzzy rules. Based on the outcomes, the use of fuzzy inferences led to a better representation of failure risk of sewer pipes.