Seismic performance of ground improvements on Christchurch Southern Motorway

R. Young, M. Gibson and G. Newby

The Darfield 2010 and Christchurch 2011 earthquake sequence occurred during construction of the Christchurch Southern Motorway, when ground improvements and approach earthworks at two of the major structures had been completed. These earthquakes have provided a means for the designed liquefaction mitigation and earthworks seismic stability to be assessed against known seismic events.

This paper describes the design, construction and performance of dry stone columns installed through stratified alluvial soils, with variable seismic characteristics. At Barrington Street overpass 6 m deep stone columns were installed beneath mechanically stabilised earth wall abutments and at Curletts Road overpass stone columns up to 18 m deep were installed beneath a 7 m high spill-through abutment. The performance of the ground improvements was established by post construction testing and instrumentation monitoring, including profilometers, inclinometers and precise level data. The monitoring has demonstrated the effectiveness of the design based on its performance in recent events. Christchurch Southern Motorway provides a duplication of the existing 3 km southern motorway, a 5 km new build 4 lane motorway over greenfields and, until the future Stage 2 extension of the 4 lane motorway to the south is built, a 2.5 km upgrade of Halswell Junction Road to connect the motorway to State Highway 1 (SH1). The project is being constructed by Fulton Hogan under a design and construct contract for the New Zealand Transport Agency, with Beca as the contractor’s designer.