Proposed amendments to geotechnical requirements of NZTA Bridge Manual

Work is currently underway to update the NZTA Bridge Manual (BM) based on recent
advancements in bridge design practice in New Zealand and worldwide, and also the NZTA’s experience on the performance of bridges during the Canterbury earthquakes. In the current 2nd edition of the BM various geotechnical design requirements are included in a number of different sections of the Manual. In the new 3rd edition, a new section, Section 6 “Site Stability, Foundations, Earthworks and Retaining Walls” now combines most of the geotechnical design requirements and deals with geotechnical design of bridges and other highway structures; natural cut and fill slopes as well as mechanically stabilised earth structures and retaining walls. The proposed amendments include the use of unweighted peak ground accelerations (PGAs) for the assessment of soils’ potential for liquefaction and lateral spreading; recommendations on the assessment of liquefaction potential of soils and soil – structure interaction under seismic conditions; design strategy for bridge foundations on liquefiable sites; soil testing techniques; categorisation of retaining structures and slopes in terms of their importance level; more detailed minimum post-earthquake performance standards; no collapse requirement under a greater than design event seismic loads; maximum allowable seismic displacements for walls and slopes and additional design requirements for geosynthetic reinforced soil structures. Early next year this work will be further complemented by results
from a NZTA Research Project to develop design guidelines for bridges and other highway structures located on sites prone to liquefaction and lateral spreading, which is currently underway.