Geonets
A geonet is a geosynthetic material consisting of integrally connected parallel sets of ribs overlying similar sets at various angles for in-plane drainage of liquids or gases. Geonets are often laminated with geotextiles on one or both surfaces and are then referred to as drainage geocomposites. They are competitive with other drainage geocomposites having different core configurations.
Geonets are formed by a continuous extrusion process into a netlike configuration of parallel sets of homogeneously interconnected ribs. There are three categories of geonets. The following are illustrated:
Each of the above categories have variations within themselves (mainly thickness) and new product development by various manufacturers is quite active.
All geonets that are currently available are made from polyethylene resin. The density varies from 0.94 to 0.96 mg/l, with the higher values forming the more rigid products. In this regard, the resin is true high-density polyethylene (HDPE) unlike the density used in HDPE geomembranes that is really medium density. The resin is formulated with 2.0 to 2.5% carbon black (usually in a concentrated form mixed with a polyethylene carrier resin), and 0.25 to 0.75% additives that serve as processing aids and anti-oxidants.
While quite different in the manufacture or configuration than geonets are competitive geosynthetic products called "geospacers". Their drainage cores consists of nubs, columns, cuspations, or 3-D networks of stiff polymer strands. They are generally used for drainage behind retaining walls, plaza decks or green roofs.
Since the primary function of a geonet is to convey liquid within the plane of its structure, the in-plane hydraulic flow rate, or transmissivity, is of paramount importance. However, other features, which may influence this value over the service lifetime of the geonet, are also of importance. Thus, a number of physical, mechanical, endurance, and environmental properties will also be mentioned.
The tests for physical properties are either covered in ASTM, ISO or GRI Standards.
A series of environmental related issues can have impact on the flow-rate performance of geonets.
Design-by-function requires the formulation of a factor of safety as follows:
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