Geotechnical aspects of the Narrows Bridge duplication

Aspects of the Narrows Bridge Duplication Design are presented including a general introduction to the Geology and seismic risk of the Perth City area. An overview of the original bridge geotechnics and design (late 1950s) is given, in addition to a discussion of what effects the site reclamation works for the original bridge have had on the design of the Duplication. Characterisation of the subsurface profile (strata and materials) for the Bridge Duplication Design is described, together with comment on the site investigation. Soil analysis models used for design and typical foundation details are presented, together with consideration of pile design loads including earthquake and liquefaction. Details of the pile load testing are given together with pile load test results. Comparison is also made between the geotechnical investigations for the original bridge and Duplication.

One of the geotechnical issues addressed for the Duplication is the execution, reporting and review of the geotechnical monitoring before and during the project construction period. Concern had been expressed about possible ground movements and potential detrimental influence on the existing bridge foundations, associated with the proposed piling and bridge construction works. The contract required monitoring of settlements and lateral movements, as well as land survey of the existing bridge, to ensure that the new bridge was constructed in a manner that did not have any significant impact on the existing structure. Inclinometers, settlement gauges, and Sondex settlement monitoring systems were surveyed over a period of nearly two years to enable continuous assessment of the impact of the various phases of construction on the existing structure. The paper discusses issues concerning the nature and performance of the monitoring systems, the data obtained and an overview of the ground and bridge deflections that were observed.