Re-Analysis of Deep Excavation Collapse Using a Generalized Effective Stress Soil Model

This paper re-analyzes the well-documented failure of a 30m deep braced excavation in underconsolidated marine clay using an advanced effective stress soil model (MIT-E3). The collapse of the Nicoll Highway during construction of cut-and cover tunnels for the new Circle Line in Singapore has been extensively investigated and documented. All prior analyses of the collapse have relied on simplified soil models with undrained strength parameters based on empirical correlations and piezocone penetration data. The current analysis use results from high quality consolidation and undrained triaxial shear tests that were only available after completion of the public inquiry. The current analyses achieve very reasonable estimates of measured wall deflections and strut loads using model parameters derived directly from the laboratory tests. The analyses confirm prior interpretations of the failure mechanism but provide a more rational basis for the modeling of soil-structure interaction.