Cox proportional hazards modelling of blockage risk in vitrified clay wastewater pipes

Older wastewater systems often contain vitrified clay pipes and these pipes are responsible for a significant proportion of blockages. From previous research and site experience, it is known that age, installation period, pipe length, diameter, depth and joint type affect the risk of blockage. Using statistical analysis we extend this list of variables to include soil type, road/rail proximity and road type. The analysis is based on blockage data for 43,976 vitrified clay pipes over a 7 year period. A Cox proportional hazards modelling (PHM) approach using established and new explanatory variables finds that pipe depth, pipe length, pipe gradient, pipe diameter, submergence in water, joint type, pipe installation decade, soil type, road proximity to the pipe, pipe purpose, land use code and road type are associated with risk of blockage. Pipes with the highest risk of failure due to blockage are identified and their locations and risk level are visually represented on a colour coded map. This analysis approach can be used by asset managers working in cost-constrained environments to appropriately target inspection, plan maintenance and replacement programs.