# Assessment Framework for the Maintainability of Sewer Pipeline Systems
## 6 Discussion  
Concerning the results obtained and when discussing each category separately to clarify the probability of the occurrence of each defect and its rank, the defects belonging to the structural category are examined first. It can be seen that defect D6 (joint defect), caused by welded joint weakness due to the misalignment of plastic pipes or joints, especially spigot and socket joints for rigid pipes, has the highest probability, with an RII value of 0.733, as reported in Table 5. This is followed by D4, with an RII value of 0.678, which is caused by failed utilities located in the vicinity of a wastewater pipeline. This defect can result in soil disturbances and possible changes in bedding configuration, ultimately leading to wastewater pipe failure. Then there is D3, which occurs due to tampering and damage to sewage works by private and/or unknown persons, with an RII value of 0.677. At the same time, D1, with a primary cause due to excessive loading and poor pipe bedding, which can cause pipe cracks and fractures, and D5, which is caused by improper installation and material, have equal RII values of 0.66. The lowest RII defect was found for D2, which results from excessive external loadings, thermal soil loads, or other mechanical causes, with a result of 0.567. Transitioning to the second category, which includes defects in the operational (hydraulic) category, it is evident that D8 (sediment accumulation) and D10 (the dumping of FOG) carry the highest probability, sharing an RII value of 0.756. D7 and D11 follow them, with RII values of 0.744 and 0.722, respectively. Then, there are D12 and D13, with RII values of 0.711 and 0.678, respectively. The lowest defects were D14 and D9, with RII values of 0.622 and 0.578, respectively. In contrast, the third category (quality defects) has only one defect (D15); see Table 3 for the causes of these defects. In relation to the assessed impacts, it is observed that the influence of defects varies across each parameter. In addition to the difference in rank, the parameters' roles in changing the defects' rankings can also be seen in Table 6. For example, D3 and D4 have the most impact in the structural category, each with a relative importance index (RII) value of 0.692. When moving to the hydraulic category, D7 and D10 have the most impact, with the same RII value of 0.708, while in the quality category, D15 has an impact with an RII value of 0.742; when compared as an overall impact between categories, this category has the highest impact, as reported in Table 6.  
The overall ranking of all defects was calculated by combining the probabilities and impacts using the risk index (RI) values. The three highest defects, D10, D7, and D8, have RI values of 0.535, 0.572, and 0.510, respectively, for the hydraulic category. This is logical since they happen often and, in the case of sewage overflows, will cause traffic jams and the constant involvement of maintenance teams, driving up costs and making the environment more vulnerable to several diseases. The fourth and fifth highest defects were D6 (RI = 0.503) and D15 (RI = 0.487), respectively. However, it is worth noting that there is little variation in the reported RII and RI values in this study, which suggests that the defects assessed are nearly equally important. The overall findings are reported in Table 8.  
It is important to note that most of the causes of these defects, as reported in Table 8, and system failures, which increase the burdens of operation and maintenance, are either design- or construction-related. Therefore, in order to avoid these issues, maintenance teams should be involved in the design phase because their requirements are crucial for minimizing the likelihood of these defects occurring. Employing effective and qualified technical staff at the stage of implementing sewer pipelines and carrying out necessary tests in the presence of a maintenance team will guarantee the system's best performance during its lifecycle and prevent the need for extensive rehabilitation and costly repairs.