Simplicity Waiatarua, Remuera

Understanding the ground conditions in a challenging geological setting

The site sits on basalt flows that extend to the southeast. Extensive borehole drilling confirmed the presence of basalt underlain by natural alluvial deposits and East Coast Bays Formation rock. Like all basalt flows, the material is inherently variable across the site. This variability presented real challenges for foundation design: basalt can be extremely stiff in places, and within any given flow it can contain voids, ash, tuff, rubble zones or intact rock. The performance of foundations through each of these materials differs considerably.

Stage 1 foundation design

The approach for Stage 1 used piles extending into the basalt to provide axial compression and tension capacity, as well as lateral resistance. High lateral stiffness in the basalt tends to attract high structural loads. To address this and achieve consistent stiffness across each building, the upper sections of piles were over-bored. Grouting of the basalt was also adopted to manage the risk of piles founding on voids or weak materials. This is a reasonably common design approach where piles are required in basalt. The first two buildings (Stage 1) were successfully constructed using this method.

Rethinking the solution

During construction, an opportunity to value-engineer the remaining foundations in Stage 2 was identified. Working under significant time pressure, Tonkin + Taylor investigated ground conditions, developed a robust alternative design approach, and then further optimised and simplified it in collaboration with structural engineers Holmes Consulting.

Holmes designed the Stage 2 building structures so that the foundations could be decoupled from the supporting stratum, removing the need to resist tension entirely. This change unlocked the use of founding on shallow improved ground rather than tying the building to the ground with piles. The solution involved excavating material above the basalt, cleaning off the rock surface, and compacting hardfill back up to founding level. Geotechnical risk around depth to basalt was managed through targeted investigations completed before work began. The quality of the basalt was then assessed directly from the exposed surface as excavation progressed.

A simpler build with better outcomes

The Stage 2 design was substantially simpler to construct. Work below the slab became routine earthworks, with no piling plant or specialist ground improvement equipment required on site. Once earthworks were complete, slab construction was straightforward. Without pile penetrations to manage, waterproofing and reinforcing bar placement were both faster and easier.

The result was meaningful savings in both programme and cost, along with a reduced risk profile. The simplicity of the solution was itself a key part of what made it successful.