Site Analysis

Understanding all the features of a site, using and protecting the best, and minimising the impact of the worst.

Landslides, slumps and erosion

Landslides, slumps and soil erosion can undermine a building structure.

The risk is likely to be high where the site:

  • has been substantially altered through earthworks or removal of vegetation
  • has a river or beach frontage
  • is at the top of a cliff
  • is on a faultline
  • is sloping and a in high rainfall area where the soil readily becomes saturated
  • has had mining activity in the past
  • is in a geothermally active area.

Visual signs of slipping and slumping

Visual signs of potential slipping or slumping include:

  • soil cracking parallel to the top of a bank
  • a hump in the soil at the base of the bank
  • where power poles, trees or fence posts are on a lean
  • a hollow in the centre of a flat area of ground
  • an undercut bank
  • a bank cut steeper than the angle of repose for the soil type see the table below.
Angle of repose Soil type
30 Soil
33 Sand
25-40 Dry sediment

In some parts of the country, expansive clays may also pose a risk to the stability of the building as the clays will shrink and swell between wet and dry parts of the year.

Gathering information

Observe the site for visual signs of past or potential landslides, slumps or erosion. Talk to neighbours who may be able to provide information about the history of the site regarding slips or soil erosion.

The local council should have information on past landslides, slumps and soil erosion within its boundaries. Aerial photos can give an indication of areas that may be at risk from slipping or soil erosion.

A land information memorandum (LIM) may contain information about slips and soil erosion.

Consider the potential impact of slips or slumps on stormwater and sewer systems. There is provision under section 72 of the Building Act 2004 for the council to refuse to grant a building consent if the land is at risk of a natural hazard, such as erosion, flooding, subsidence, or slippage, or if the building work itself is likely to accelerate the problem.

Updated: 15 August 2017