Building on the edge

By - , Build 129

Designing a building on a slope? You need to be aware of the increased distance now required between the bottom of a building’s foundation and an adjacent bank sloping away from the building.

Figure 1: Minimum separations between the bottom of a building’s foundation and an adjacent bank (developed from Figure 3.1 in NZS 3604:2011).

Clause 3.1.2(b) of NZS 3604:2011 Timber-framed buildings has new requirements for the distance between the bottom of a building’s foundation and an adjacent bank sloping away from the building.

NZS 3604:2011 now considers existing ground conditions and has increased the distance foundations must be away from the edge of the bank to prevent the foundation being exposed by:

  • localised slip failures in the bank
  • erosion or frittering of the bank.

Minimum distances on banks

The minimum separation requirements from a building to the steeply sloping part of the bank are shown in Figure 1 for three different ground conditions. While the drawings show a foundation wall, the edge distances also apply where the perimeter of the building has an edge-thickened slab or anchor or braced piled foundations.

Other requirements given in NZS 3604:2011 for buildings close to the top of a bank are that:

  • dimension H on Figure 1 is not more than 3 m
  • the slope of the land away from the bottom of the bank is not more than 5°
  • the ratio H/V is greater than or equal to:
    • 1 for rock
    • 2 for clay or sandy clay
    • 3 for other materials.
  • the ratio L/V is greater than or equal to:
    • 2 for rock
    • 3 for clay or sandy clay
    • 4 for other materials.
Figure 1: Minimum separations between the bottom of a building’s foundation and an adjacent bank (developed from Figure 3.1 in NZS 3604:2011).

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Figure 1: Minimum separations between the bottom of a building’s foundation and an adjacent bank (developed from Figure 3.1 in NZS 3604:2011).

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