To cater for future growth in Auckland Watercare Services Ltd is constructing the Hunua 4 Watermain. The watermain runs for 28 kilometers from South to Central Auckland. One of the highest risk crossings in the alignment is a crossing under the Southern Motorway (on SH1), New Zealand’s busiest road.
GHD and CH2MBeca (CBG) have delivered the design and monitoring as well as a quality role within a NEC3 contract framework. As the designer of the permanent works, CBG provided the preliminary watermain alignment, and geotechnical investigation. The final vertical alignment and temporary works design including tunnelling design were undertaken by the main contractor, the Fulton Hogan John Holland Joint Venture (FHJHJV) and its subcontractor Bothar Boring Ltd.
The ground conditions, high groundwater, the proximity of buried and above ground services, and SH1 combined to make this a high risk crossing.
One of the greatest risks to utilising trenchless technology is machine and equipment selection. The design process should always be collaborative between designers, geotechnical specialists and specialist contractors. An integrated planning and design approach was undertaken and minimised the risks for this crossing although some issues still arose during construction.
This paper details the planning process, outcomes and the lessons learned.