Geogrid Reinforced Soil Abutments in High Seismic Environments – Case Study from New Zealand

Abstract

Load bearing geogrid reinforced soil (GRS) abutments are becoming an economic and popular solution for supporting single-span bridges on major transportation projects in New Zealand, particularly where shallow ground provides adequate bearing capacity. While their use has been pioneered in low-seismicity regions using international design guidance (e.g., AASHTO, FHWA), New Zealand’s unique seismic setting presents significant design challenges. The revised National Seismic Hazard Model indicates that many areas face Ultimate Limit State (ULS) events with Peak Ground Accelerations (PGAs) approaching approximately 1.0g, necessitating a critical reassessment of international design practices developed for much lower seismic accelerations. This paper addresses key design challenges in these high-seismic environments, including elevated bank seat pressures and the requirement for robust ground improvement to counter strength loss by seismically triggered soil liquefaction. Furthermore, the paper introduces the innovative concept of incorporating ‘ fusible elements’ into the design to promote favourable failure modes during extreme seismic loading. Through case studies of completed GRS abutments, the authors present design outcomes, discuss performance, and explore potential failure mechanisms, and thus providing a framework for robust seismic design in New Zealand and elsewhere.