Geotechnical Design Innovations from the Waikato Expressway project

Mark Argent

This technical presentation will provide details of the geotechnical challenges faced by the major New Zealand-based project and innovations developed in response to these challenges.

The Hamilton Section of the Waikato Expressway in New Zealand is being delivered by the City Edge Alliance and has just completed its second earthworks season. The project involves the construction of approximately 22km of expressway standard roadway with 17 grade separated bridges and five major interchanges. Mark was the earthworks design lead for this large infrastructure project which has numerous geotechnical challenges including sensitive volcanic ash soils, liquefaction prone pumiceous sands, and soft alluvial soils. The Alliance delivery model has allowed innovative geotechnical design methods to be explored and developed which have resulted in significant project benefits. These innovations have included:

• Using paleo-liquefaction studies to demonstrate that penetration based liquefaction assessment methods (SPT, CPT) overpredict the susceptibility to seismic loading of the volcanically derived but alluvially reworked Hinuera Formation soils. The reduced risk of liquefaction based on shear wave velocity assessment methods resulted in significant ground improvement savings for the project;
• Leaving topsoil in place below fill embankments built on sensitive, volcanically derived silts and sands. Traditional concerns regarding long-term settlement and ‘seeing what you are building on’ were addressed through the construction of a monitored trial embankment, targeted proof rolling, in situ testing and partial stripping;
• Implementing earthworks conformance testing methods that use a line of optimums approach and shear strength measurements to overcome the challenges posed by highly variable materials. The use of the line of optimums approach was the first of its kind for a road project in New Zealand. This methodology allowed for instantaneous advice to be provided at the time of field testing rather than 3 days later when the material is buried under a further metre of fill;
• Using a light weight deflectometer to assess the conformance of sand fill against design requirements; and
• Building a 3D geological model to interface with the mass-haul planning to achieve optimum earthworks delivery.

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