Transmission Gully

This PPP project comprises a 27km motorway through steep terrain in a seismically active region north of Wellington, NZ.  The road is being constructed by a joint venture between CPB and HEB.

PSM were engaged to provide the project geotechnical model and design the cuts, embankments, earthworks, liquefaction, soft ground interaction, geohazards and selected high risk bridge foundations.  PSM also provided geotechnical construction phase service support.

More information can be found at the project website:

https://www.nzta.govt.nz/projects/wellington-northern-corridor/transmission-gully-motorway/

This is a technically challenging large scale civil project in difficult ground conditions and terrain.  The geotechnical solutions had to consider the constructability constraints and accommodate large seismic loadings as part of design.  Development of a robust ground model was critical for successful design.  Close interaction with the construction team, and consideration of Safety in Design risks was also a key part of developing the design outcomes. 

As part of our construction phase support services we provide geotechnical temporary works design, which interfaces well with our permanent design role.

A performance based design philosophy was adopted to provide a cost-effective, seismically resilient design within the framework of the PPP project. This design philosophy was accepted by the client (NZTA) and provided a clear definition of the road performance in large earthquake events.

Due to the complex geology, an observational method was adopted to ensure the design is suitable for the encountered conditions. Flexibility was required during construction to respond to changing conditions or adverse ground. In some cases design changes were requested during construction to provide value engineering outcomes for the Builder. Innovative methods were adopted to increase CPS accuracy and efficiency, including drone photogrammetry and digital mapping to hand-held devices.

Our geotechnical design process and construction support has assisted the provision of a seismically resilient motorway in extremely difficult construction conditions. This motorway will provide a ‘lifeline’ into Wellington in the event of a major earthquake.