The modelling of corrosion and fatigue in cast iron water mains buried beneath roadways

Of the vast network of water mains that lay beneath most modern cities, a large proportion of the pipes currently in use are manufactured from grey cast iron and in some instances have been in service for over 100 years. Whilst it has long been believed that the loads induced by heavy vehicles travelling along the roads under which the pipes are buried may contribute to the failure of pipes, surprisingly, little research has been undertaken on the subject. Of the research that has been previously undertaken, most is concentrated on the analysis of the loads induced by the passage of a single vehicle loadings causing fatigue damage within the pipe. A model is developed which attempts to predict the time to failure of a cast iron pipe buried beneath a roadway subject to the combine actions of: (i) time dependent pipe wall thinning due to external corrosion. ; (ii) stresses caused by the internal pressure of the pipe contents. ; (iii) stresses caused by the weight of the soil above the pipe. ; (iv) stresses caused by the weight of vehicles travelling along the roadway under which the pipe is buried. ; and (v) the gradual loss of pipe material strength sue to fatigue damage caused by large numbers of vehicle loadings over an extended period. The model is used to analyse a typical burial scenario and sensitivity testing is undertaken on the input parameters to determine which are the most critical to pipeline life expectancy. A selection of formerly manufactured pipe sections is also analysed to assess which are the most susceptible to failure. An experimental program involving the burial of an instrumented pipe section beneath a heavily trafficked roadway is also detailed. Finally. some recommendations and possible areas for future investigation are presented.