Rangitahi Hillside Home – Geotechnical Design for a Steep Site

The Project

This site in the Rangitahi subdivision south of Raglan presented several constraints: a steep slope, expansive soils, and a design brief for a suspended two-storey home with a concrete-floor garage at driveway level. The client wanted internal access between the garage and the main structure, which required a solution that balanced structural demands, ground conditions, and long-term performance.

Structural Engineer: Structural Services Limited

Architect: Red Architecture

Our Approach

We worked closely with the structural engineer from the outset to define the design parameters and understand how the proposed structure would interact with the site. Key considerations included:

• Interaction between the timber retaining wall and the concrete block garage wall

• Settlement risk beneath the garage

• Bearing capacity for bored timber pole foundations

• Long-term slope creep and its influence on timber foundations

• Effects of expansive soils on the foundation design

Geotechnical Solution

The final design included:

• Oversized, deepened timber pole foundations

• A timber pole retaining wall supported by timber props tied back into the foundations

• Lightweight poly fill beneath the garage to reduce load on the lower retaining wall

This approach provided a safe, buildable, and cost-effective solution suited to the site’s constraints.

Outcome

The completed geotechnical design supports the client’s architectural intent and provides confidence in the long-term performance of the structure and surrounding ground.