Case study of surcharge embankment performance on soft ground: Peka Peka to Ōtaki Expressway

This paper presents a case study on the performance evaluation of three surcharge embankments constructed on soft, compressible soils as part of the Peka Peka to Ōtaki Expressway project in New Zealand. Embankments, located near Awatea Stream and Mary Crest Basin, were founded on compressible peat and organic silt layers up to 7 m thick, presenting significant challenges related to settlement, stability, and proximity to existing infrastructure. To mitigate the risks associated with soft ground conditions, surcharge embankments were implemented using a staged construction approach supported by instrumentation and monitoring. Performance criteria were established based on NZ Transport Agency requirements. Ground improvement was designed to accelerate primary consolidation and reduce long-term settlement. An observational approach was adopted to manage geotechnical uncertainty, relying on monitored data to refine numerical models and guide construction decisions. Instrumentation and surveying monitored ground response. Finite element analysis simulated consolidation behaviour and calibrated geotechnical parameters. Back analysis aligned model predictions with observed settlement and excess pore pressure data. Results showed good agreement between modelled and measured values, confirming the effectiveness of the surcharge approach. Key findings include the effectiveness of surcharge in achieving >90% consolidation, successful back analysis, and validation of design assumptions using monitored data.