# The influence of different characteristics on the probabilities of failure of sewer pipes

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## Abstract

Sewer assets face a great challenge for being underground assets and for experiencing constant deterioration and aging. Reactive decisions are no longer technically nor economically viable, and hence asset managers are migrating to the implementation of proactive strategies. Despite this, managers are making decisions that are poorly justified, based mostly in intuition. This is why it is important to improve the means to implement a more objective and justified proactive approach. Risk-based decisions are a proactive strategy that may help the prioritisation and optimisation Sewer assets face a great challenge for being underground assets and for experiencing constant deterioration and aging. Reactive decisions are no longer technically nor economically viable, and hence asset managers are migrating to the implementation of proactive strategies. Despite this, managers are making decisions that are poorly justified, based mostly in intuition. This is why it is important to improve the means to implement a more objective and justified proactive approach. Risk-based decisions are a proactive strategy that may help the prioritisation and optimisation of maintenance and inspection strategies. It requires for its implementation the estimation of probabilities of failure and their consequences. In this thesis, the analysis is oriented on the failure probabilities on sewer systems, especially on understanding the influences of different characteristics of the pipe (explanatory variables) on these probabilities. The outcome of the thesis answers the following research question: “What is the influence of different characteristics on the probability of failure of the sewer pipes?” Two sub-questions were stated to guide the research and help answering the main research question: 1.What defects can lead to sewer structural failure? 2.What physical characteristics make sewer pipes more prone to fail/deteriorate? For the analysis the Dutch municipalities of Breda and Almere facilitated the results of several years of CCTV inspections of their sewers. Due to the lack of availability of collapse records, this study considered a proxy of collapse, which is loss of watertightness on the pipes. Furthermore, defects that potentially cause loss of watertightness are used as a proxy of the loss of watertightness on a pipe. Defects are obtained from the CCTV reports. The first sub-question was answered based on the proxy failure under study. It was found in the literature review ten defects that potentially cause loss of watertightness, which were classified according to the coding of the standard NEN 3399. These defects are: cracks (BAB), break (BAC), defective connection (BAH), intruding sealing material (BAI), displaced joint (BAJ), porous pipe (BAN), soil visible through defect (BAO), void visible through defect (BAP), infiltration (BBF) and exfiltration (BBG). The second sub-question was answered based on some explanatory variables that were available in the dataset of the inspections, which included: sewer system type, materials, shape, diameter, length, and pipe’s above ground material. To answer this question, a descriptive statistical analysis and two survival methods were implemented: a non-parametric and a semi-parametric model. The non-parametric model consists of an extended version of the Nelson-Aalen estimator of the cumulative hazard (ENE) and its derivative the Extended survival estimator (ESE). Implementing ESE, each characteristic was analysed using the aggregate information of the defects that potentially cause loss of watertightness, and each defect individually. This was done to identify the influence in the failure probabilities (the probabilities of occurrence of defects that potentially cause loss of watertightness). The semi-parametric model that was used is the Cox proportional hazard function, used to estimate the risk ratio associated with one unit increase in one of the characteristics under study. Results of the ESE model showed that: The defects displaced joint (BAJ), infiltration (BBF) and defective connection (BAH), are the ones that have more incidence on the loss of watertightness for the two municipalities. Based on the information of aggregate defects it was observed for both municipalities that the median survival probability is past 14 years. Also, the analysis showed that stormwater sewers have a lower survival probability than foulwater sewers, that PVC pipes have a higher survival probability than concrete pipes, and that shorter pipes have a higher survival probability than longer pipes. In the case of the diameters, for the aggregate defects and defects displaced joint (BAJ) and defective connection (BAH), smaller diameters have a higher survival probability than larger ones. But this tendency is the other way around for defects like cracks (BAB), break (BAC) and porous pipes (BAN), where the diameter has a higher survival probability when are larger than when they are smaller. Characteristics shape and above ground material were only analysed for Breda. It was observed that egg-shaped pipes have a lower survival probability than circular shapes, and that pipes with green fields and floor tiles above them have a lower survival probability than pipes that have above asphalt and pavement. For both municipalities, the results of ESE showed that material is that characteristic that influences the most the probabilities of failure. Results of the Cox proportional model showed that: Almere’s results met the proportionality assumption and showed that the characteristics sewer system type and length are the one that influences the most the failure probabilities. Breda’s data is no appropriate to be used with this model, as it does not meet the proportionality assumption. Recommendations of analysing Breda with an extended Cox are given, as this version of the model allows to use time-dependent variables. of maintenance and inspection strategies. It requires for its implementation the estimation of probabilities of failure and their consequences. In this thesis, the analysis is oriented on the failure probabilities on sewer systems, especially on understanding the influences of different characteristics of the pipe (explanatory variables) on these probabilities. The outcome of the thesis answers the following research question: “What is the influence of different characteristics on the probability of failure of the sewer pipes?” Two sub-questions were stated to guide the research and help answering the main research question: 1.What defects can lead to sewer structural failure? 2.What physical characteristics make sewer pipes more prone to fail/deteriorate? For the analysis the Dutch municipalities of Breda and Almere facilitated the results of several years of CCTV inspections of their sewers.

Due to the lack of availability of collapse records, this study considered a proxy of collapse, which is loss of watertightness on the pipes. Furthermore, defects that potentially cause loss of watertightness are used as a proxy of the loss of watertightness on a pipe. Defects are obtained from the CCTV reports.

The first sub-question was answered based on the proxy failure under study. It was found in the literature review ten defects that potentially cause loss of watertightness, which were classified according to the coding of the standard NEN 3399. These defects are: cracks (BAB), break (BAC), defective connection (BAH), intruding sealing material (BAI), displaced joint (BAJ), porous pipe (BAN), soil visible through defect (BAO), void visible through defect (BAP), infiltration (BBF) and exfiltration (BBG).

The second sub-question was answered based on some explanatory variables that were available in the dataset of the inspections, which included: sewer system type, materials, shape, diameter, length, and pipe’s above ground material.

To answer this question, a descriptive statistical analysis and two survival methods were implemented: a non-parametric and a semi-parametric model.

The non-parametric model consists of an extended version of the Nelson-Aalen estimator of the cumulative hazard (ENE) and its derivative the Extended survival estimator (ESE). Implementing ESE, each characteristic was analysed using the aggregate information of the defects that potentially cause loss of watertightness, and each defect individually. This was done to identify the influence in the failure probabilities (the probabilities of occurrence of defects that potentially cause loss of watertightness).

The semi-parametric model that was used is the Cox proportional hazard function, used to estimate the risk ratio associated with one unit increase in one of the characteristics under study.

Results of the ESE model showed that:

The defects displaced joint (BAJ), infiltration (BBF) and defective connection (BAH), are the ones that have more incidence on the loss of watertightness for the two municipalities.

Based on the information of aggregate defects it was observed for both municipalities that the median survival probability is past 14 years.

Also, the analysis showed that stormwater sewers have a lower survival probability than foulwater sewers, that PVC pipes have a higher survival probability than concrete pipes, and that shorter pipes have a higher survival probability than longer pipes. In the case of the diameters, for the aggregate defects and defects displaced joint (BAJ) and defective connection (BAH), smaller diameters have a higher survival probability than larger ones. But this tendency is the other way around for defects like cracks (BAB), break (BAC) and porous pipes (BAN), where the diameter has a higher survival probability when are larger than when they are smaller.

Characteristics shape and above ground material were only analysed for Breda. It was observed that egg-shaped pipes have a lower survival probability than circular shapes, and that pipes with green fields and floor tiles above them have a lower survival probability than pipes that have above asphalt and pavement.

For both municipalities, the results of ESE showed that material is that characteristic that influences the most the probabilities of failure.

Results of the Cox proportional model showed that: Almere’s results met the proportionality assumption and showed that the characteristics sewer system type and length are the one that influences the most the failure probabilities. Breda’s data is no appropriate to be used with this model, as it does not meet the proportionality assumption. Recommendations of analysing Breda with an extended Cox are given, as this version of the model allows to use time-dependent variables.