Subsoil drainange of the road pavement and subgrade is important to the performance and life expectancy of road pavements. With the development of geosynthetic materials, composite drains comprising geosynthetic core wrapped with geotextile have been increasingly used overseas over the past 20 years, and more recently in New Zealand. Research that was carried out in 1998–99 considered the benefits and constraints associated with the predominant types of geosynthstic composite drains available in New Zealand. Guidance is provided for designers on the factors that need to be considered when designing, specifying, and installing geosynthetic composite drains. The geosynthetic drainage products may have rigid, semi-rigid or flexible cuspate cores, generally wrapped with a commonly used geotextile.
A flowchart is provided for considering design and construction issues, and for selection and detailing of appropriate systems. Recommendations are made for updating the present specifications for subsoil drain construction, and for construction trials with different types of backfill around New Zealand.