Counterfort Drain Performance in the Auckland Area

Counterfort drains are a method of lowering groundwater levels in soils and subsequently reducing pore water pressures on possible failure planes that can induce slope instability. This paper discusses the typical detail of counterfort drains employed and whether counterfort drains continue to be effective in reducing groundwater levels as has been demonstrated in Fitch (1990). Maintenance issues, monitoring and performance over time is outlined. Up to date costs of construction of counterfort drains are also given. The appropriateness of counterfort drain use in different types of Auckland Region Northland Allochthon geology is investigated. Counterfort drains are shown to generally have a demonstrable effect on groundwater levels. Their effectiveness is dependent on site specific geology rather than the overall geological unit present. Provided site specific factors are fully considered they can routinely be considered as a slope stability improvement option in the Auckland region.