Finite Element Analysis of Deteriorated Host Pipe

Abstract

A 2.5 km DN 750 MSCL sewer rising main in Sydney experienced eight failures over 23 years prompting urgent intervention prior to scheduled amplification and replacement. Sydney Water initiated an emergency rehabilitation project to install a flexible reinforced hose liner in six stages over 12 months. The selected Type B liner was designed to resist internal pressures but not external loads during depressurised periods, leaving the deteriorated host pipe responsible for overburden resistance. Investigation revealed failures originated from invert erosion caused by abrasive grit, forming holes that compromised structural capacity. Finite element analysis (FEA) using Strand7 was conducted to assess the host pipe’s residual strength and identify safe deterioration limits. Two primary failure modes were modelled: (1) continuous invert wall loss, and (2) localised slotshaped holes. Soil stiffness, depth of cover, and loading scenarios (dead, earth, hydrostatic, surcharge) were varied across 468 load cases. Results showed that wall thickness loss up to 6 mm could be tolerated over lengths ≤1 m under most conditions, though medium dense sands at greater depths produced near-yield stresses. Slot length tolerance was strongly dependent on soil stiffness, with loose sand governing: instability could occur at slots ≥1.0 m. The study concluded that significant invert deterioration can be tolerated structurally and that Type B lining of deteriorated host pipes is a viable repair solution. A decision framework was developed to guide repair triggers during lining works, recommending further action when average section loss exceeds 4–6 mm over ≥1 m. This approach enables risk-based rehabilitation planning for ageing pressurised sewers.