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Case Studies on the Application of Structural Reliability 151


           undertaken to identify the most significant factors among 15 random variables
           that affect the pipe behavior and failure. Among all variables, sensitivity indexes
           of the corrosion parameters (k and n) were highly remarkable. This indicates that
           corrosion is a very important factor for the design of underground pipelines with
           longer lives. High values of contribution for these two variables means that the
           sensitivity of these variables is more dependent on the actual value of their coeffi-
           cient of variation. In such cases, more concern should be taken in order to deter-
           mine relevant parameter values.



              5.4 Case Study 4-Concrete Sewer Pipes in the UK

           In this section the two model approaches outlined in Chapter 4 (i.e., first passage
           probability method and gamma distributed degradation model) are used for the
           reliability analysis and service life prediction of a concrete sewer pipeline in the
           UK. The results are verified by a Monte Carlo simulation and the weakness and
           strength of each method is discussed (Mahmoodian, 2013; Mahmoodian and
           Alani, 2013, 2014).
             Two conditions are considered for the reliability analysis and service life pre-
           diction of the sewer pipeline in the UK. Firstly, the analysis is carried out based
           on an individual limit state function (i.e., considering one failure mode), and sec-
           ondly, to have a more representative assessment of different failures to test reli-
           ability. The merit of this approach is the consideration of all possible corrosion
           induced failure modes that exist in practice. The effectiveness and contribution of
           the different variables (on the failure probability) provides new insight to the sen-
           sitivity analysis of concrete sewers.
             In an individual failure analysis scenario, loss of concrete cover was assumed
           as the criterion for failure of the sewer. Four failure modes including flexural,
           shear, cracking, and cover loss were assumed as possible failure modes for multi-
           failure mode analysis. These modes of failure, which are set into the two catego-
           ries of serviceability and ultimate strength limit states, were then put into a sys-
           tem configuration consisting of a combination of series and parallel systems.


           5.4.1 INDIVIDUAL FAILURE MODE ASSESSMENT

           The two developed methodologies in Chapter 4 for time-dependent reliability
           analysis of pipes are applied on CCTV data from surveys of concrete sewers in
           Harrogate in the United Kingdom. Harrogate is a spa town in North Yorkshire,
           England. The town is a tourist destination with a population of approximately
           75,000. The study considers concrete sewers with a 500 mm diameter and 25 mm
           of internal concrete cover.
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