Laboratory experiments to investigate siltation of pipe drainage systems in New Zealand soils

Abstract

Drainage lysimeters were used to ascertain the pipe siltation risk associated with draining several wet New Zealand soil types. A simple laboratory column study, which aimed to simulate the soil conditions immediately around the apertures in a pipe drain, was also carried out. Results indicate that many wet soil types throughout New Zealand are likely to require a pipe filter if subsurface-drained. Soil types of greatest risk are mainly coarser textured, cohesionless soils under rising water table conditions. Although particle size analysis is a useful indicator of possible pipe siltation, the effects of soil structure, particle shape, and particle density should be included as an index of soil stability. The hydraulic gradient and hydraulic conductivity determine the velocity of water flow, also affecting the degree of soil stability. Results from the laboratory column study indicate that this technique could be a quick and useful method to gauge the relative siltation potential of different soil types. Filtersok (a synthetic filter cloth fitted around the corrugated plastic drain tube) proved to be effective at preventing sediment entry. There is no evidence to suggest that Filtersok will become clogged with fine particles and eventually fail. It functions by physically supporting and stabilising the soil surrounding the pipe. This in turn minimises movement of soil particles as water flows to the drain.

Keywords:

• drainage
• pipe siltation
• soils
• sedimentation