Bridges

The Christchurch Southern Motorway will provide a vital link in the State Highway network to the south of Christchurch. It has a total length of 10.5 km and comprises a 3 km duplication of the existing Southern Motorway, a 5 km greenfield extension and a 2.5 km upgrade of the existing Halswell Junction Road to provide a temporary link to State Highway 1 to the south. Currently under construction, Stage 1 of the motorway will be completed in 2013. This paper describes the design of sustainable and economical bridges in the face of significant seismic demands, combined with weak and liquefiable ground below bridges and approaches and, how through ground improvement and structural design, these challenges have been met. The importance of providing designs that perform under the design earthquake and that can sustain significantly larger events without collapse has been emphasised through recent events in Christchurch. Structures include both overpasses and underpasses, with seven major bridges within the project. Typically, the bridges utilise precast beams with integral piers and abutments. Ground improvement comprises stone columns under approach embankments and mechanically stabilised walls. The design considers the performance of the structures and soil interaction in relation to the phasing of seismic loads and compatible displacements of specific elements.