Impact of Soil–Pipe Interaction and Ground Settlement on the Structural Reliability of Buried Water Supply Pipelines

Abstract

Buried water supply pipelines are routinely exposed to differential ground settlement from consolidation, groundwater drawdown, backfill compaction and nearby excavation, which induces longitudinal bending rarely considered in design. This paper quantifies how soil–pipe interaction and the magnitude and shape of a ground settlement trough govern the structural reliability of a continuous welded mild steel water main (323.9 mm outside diameter, 6.35 mm wall). The pipeline is modelled as an Euler–Bernoulli beam on nonlinear, asymmetric Winkler springs whose bearing, uplift and axial resistances are derived for loose sand, dense sand, soft clay and stiff clay, and is subjected to a Gaussian settlement trough together with an internal pressure of 0.6 MPa. Deterministic analyses show that first yield of the pipe wall is reached at maximum settlements of 153–203 mm for a trough width parameter of 5 m, and that narrow troughs are far more damaging than wide ones. Monte Carlo fragility analysis yields lognormal fragility curves with median capacities of 170–228 mm, and first-order reliability analysis shows that the reliability index falls below a target of 3.0 once the mean settlement exceeds about 55–75 mm (for example, β = 2.24 in dense sand and 2.84 in soft clay at 80 mm). The settlement magnitude and trough width together account for more than 90% of the importance factors, whereas soil strength uncertainty, yield strength, wall thickness and pressure contribute less than 10%. The results provide settlement thresholds and design charts for reliability-based assessment of water mains.