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Complex Residential Hillside Construction


  • Terraced hillside platform with restricted access and damage to existing retaining structures.
  • The property is situated within a Class III Port Hills Mass Movement Zone, approximately 30m south of the Heathcote River.
  • Liquefaction induced settlement within the northern portion of the site was assessed to be within the parameters of a TC3 site, i.e., moderate to significant damage likely.
  • The owners of the site wanted a bespoke architectural build with unique structural features, including a fully cantilevered first floor extending up to 1.9m beyond the ground floor below.

Our Solution

  • Given the complex site conditions, the structural design required close liaison with specialist geotechnical engineers and the architectural designer. The following aspects were incorporated into the design:
    • The dwelling was split into two main areas (pods) to the north and south of the site. Pods were built as independent structures with separate foundation systems linked via a connecting walkway with suitable seismic separation and detailing to accommodate differential movement.
    • New retaining structures were designed across the site and were kept structurally separate from the main pods to limit any future damage and to increase repairability in any future earthquake events.
    • Foundations of the northern pod were designed as an enhanced waffle slab on a compacted gravel raft and had to be capable of meeting several design challenges:
      • Foundation able to tolerate a loss of support over half the footprint of the building.
      • Foundation able to withstand stresses induced from sliding of the ground under half of the foundation’s footprint.
      • Foundations readily repairable via conventional relevelling techniques.
    • A robust portal frame and midfloor system was designed to support the fully cantilevered first-floor of the northern pod.
    • A cantilever pile system was designed to support the southern pod.