NZ Geomechanics News Issue 110 December 2025 p. 86-96
A new highway to the north of Auckland in New Zealand features four, up to 35m long, single span bridges. The project used geogrid reinforced soil (GRS) abutments to support the integral bridge bank seats, rather than steel soil reinforcing elements. The use of geogrids for abutments is unusual in NZ and Australia as ’extensible‘ reinforcements are not generally permitted under the NZ Transport Agency (NZTA) guidance. The project received permission to use geogrid reinforced soil abutments on the above project and this paper describes design considerations and observations from construction and monitoring of instrumentation. The design was undertaken using North American design guidance from FHWA and AASHTO, but with NZ specific seismic design provisions. One bridge was founded on cohesive (alluvial) subsoils that were prone to consolidation settlement in excess of 200mm and preloading was used to meet project timelines and strict displacement limits. The designers reduced post-construction settlements by applying a preload at the top of the bridge abutment comprising five vertical multi wire strand anchors tensioned to a combined load of 1,050 tons. Additionally, 3 metres height of soil preload was placed behind the anchors.
