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City of Wollongong

Absorption disposal systems must not be used on land where the following conditions exist:

  • Slope instability
  • Loose sand
  • Clay soils that collapse in contact with water
  • Soils with a hydraulic conductivity of less than 0.36 mm/hr,
  • Rock and shale
  • Slopes greater than 15% (steep grades increase the chance of landslip and water seepage from the subgrade to the lower areas of the site reducing the amount of water infiltration)
  • Extensively paved areas such as driveways.

As a guide, sand has a hydraulic conductivity of >180 mm/hr, sandy clay 180–36 mm/hr, medium clay 36–3.6 mm/hr, reactive clay 3.6‑0.036 mm/hr.