Aspects of the regional tectonic and structural geological model of the transmission gully project in Wellington, New Zealand for rock cutting slope design

The rocks of the Torlesse Composite Terrane in New Zealand are highly anisotropic and as such local to district scale geological structure patterns tend to orientate with respect to crustal anisotropy. Accordingly, a study of the tectonic setting and regional scale faulting is an important tool when evaluating structural domains for rock slope design in quarries and cuttings for road, rail and land development projects.

This approach has been adopted for rock cutting design at Transmission Gully in New Zealand which is a project to build a new 27 km, four lane motorway along SH1 north out of Wellington. The Transmission Gully alignment is dominantly located in Torlesse rocks and crosses several regional scale faults. Structural data for slope design was predominantly derived from borehole imaging. To guide engineering geological evaluation of local structural domain models a regional tectonic and structural geology study was undertaken to assist in predicting highway cutting scale structural patterns and guide interpretation of the borehole dominated structural database. This paper summarises key aspects of the study and illustrates how this approach is integral to managing design and construction risk.