Failure assessment of corroded pipes buried in partially saturated soils
Creators: name: Randeniya, Chamal
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 400502
subject: Civil geotechnical engineering
classificationCode: 400510
subject: Structural engineering

description: Pipelines are used to provide variety of services in modern community and have grown rapidly in past few decades due to ever increasing needs of socio-economic aspects. Most of the water pipelines are buried in unsaturated soils where the behaviour of pipes is significantly different when compared to the pipes buried in dry/fully saturated soils. The internal and external loading act on buried pipes in such non-dry soil medium can lead towards undesirable failures as the current approach of pipe failure assessments overlook soil moisture effect (i.e. suction). The risk of pipe failure can be further triggered by aging effects as these pipelines have been laid sometime in the last century or earlier (i.e. highly corroded). As failures of water mains can have negative consequences on economy, society and environment in various ways, accurate prediction of remaining service life incorporating realistic soil and pipe condition can facilitate better asset management by water utilities while providing enhanced service to their customers. <br><br>In this study, buried pipe response under operational loads (internal water pressure and external traffic loads) was investigated using a comprehensive large-scale pipe-soil testing methodology. Pipe deformations as well as soil stresses were evaluated in reference to a cast iron pipeline buried in low plasticity clay under different soil saturation levels. The results obtained from large scale experiments are compared with those from 3-dimensional finite element models that were calibrated against unsaturated soil sample tests conducted in the current study. The calibrated 3-dimensional (3-D) finite element model then formed a basis for detailed investigation of buried pipe behaviour under various loadings in unsaturated soil conditions. Results from large scale tests revealed that the backfill soil saturation can significantly affect the pipe deformation under internal and external loadings.<br><br>The behaviour of buried pipe response in unsaturated soils was then simulated using 3-D Finite element (FE) method with advanced constitutive soil models. The models were first validated using experimental and reported field test data using calibrated soil properties. A series of 3-D FE analysis is used to develop an analytical model for predicting maximum stress in pipes (new and uniformly corroded condition) considering soil saturation effects. Results from the FE analysis reveal that the maximum pipe stress can be lowered by 10-80% depending on the partial saturation condition when compared to dry condition. The proposed formula shows a good agreement with the field data and FE results, so that the expression can be used in calculation of maximum pipe stress when they are buried under realistic (i.e. non-dry) soil conditions.<br><br>Further studies were conducted to investigate the behaviour of corroded pipes subjected to internal and external loadings in partially saturated soil medium. Number of 3-D finite element studies was conducted using advanced soil constitutive models to analyse the behaviour of pipes with various corrosion patch geometries (corrosion patch depth, width and longitudinal length) which were identified on the basis of exhumed pipe sections in Australia. Results of the analyses were rigorously analysed by considering both stress intensity factor and stress concentration factor approach to determine the failure state of buried corroded pipes in unsaturated soils subjected to service loads. Study revealed that corrosion geometry (size &amp; shape) and location can be highly significant in predicting the pipe failures in unsaturated soils. Analysis results were used to develop an analytical model on predicting the maximum pipe stress incorporating corrosion characteristics in addition to pipe and soil parameters. The model can be the backbone in failure assessment of buried pipes which are undergone inevitable corrosion during its service life.<br>