Geonets

A Geonet drainage layer can be chosen if the HELP model shows it to be more effective than sand in controlling leachate production and is comparable in cost. It is assumed that the HELP model predicts a 75 to 80% reduction in leachate production. A Geotextile layer would be laid on either side of the Geonet drain to prevent clogging. A minimum slope of 3% would be provided to meet state requirements. 

An example of the effects of waste settlement can be illustrated by a recent incident at a hazardous waste landfill facility in California.5 At this facility, waste settlement led to sliding of the waste, causing the standpipes (used to monitor secondary leachate collection sumps) to move 60-90 ft downslope in 1 day. Because there was a very low coefficient of friction between the primary liner and the geonet, the waste (which was deposited in a canyon) slid down the canyon. There was also a failure zone between the secondary liner and the clay. A two-dimensional slope stability analysis at the site indicated a factor of safety (FS) greater than 1. A three-dimensional slope stability analysis, however, showed that the safety factor had dropped below one. Three-dimensional slope stability analyses should therefore be considered with canyon and trench landfills. 

The drainage materials for the liquid management system must allow for unimpeded flow of liquids for the intended lifetime of the facility. In a leachate collection system, the drains may consist of pipes, soil (gravel), geonets, or geocomposites. 

The synthetic materials that best meet in-plane flow rate regulations are called geonets. Geonets require less space than perforated pipe or granular material, promote rapid transmission of liquids, and, because of their relatively open apertures, are less likely to clog. They do, however, require geotextile filters above them and can experience problems with creep and intrusion. 

Table 26.6 presents a compilation of various geonets. The structure and properties of each are also identified. Geonets used in drainage design must be chemically resistant to the leachate, support the... 

Geonet must be equivalent to MTG regulations for natural materials ... 

Generally, geonets result in high factors of safety or DRs, unless creep becomes a problem or if adjacent materials intrude into the apertures. 

The LDCR system is located between the primary and secondary liners in landfills, surface impoundments, and waste piles. It can consist of either granular soils (i.e., gravels) or geonets. 

For a geonet LDCR system, the flow rate for the geonet is determined in the laboratory by using the ASTM D4716 test method, and the value is modified to meet site-specific situations. The geonet flow rate DR is then determined in the same way as for the granular system. No pipe network is needed. 

U.S. EPA specifies that the minimum detection time for the leachate entering the LDS of an LDCR system is <24 h. Response time calculations are based on velocity in the geonet and/or granular soil drainage layer. Darcy s law is used to calculate flow velocity in the geonet, and a true velocity must be used for granular soil. 

Surface water drainage systems can be composed of granular soils, geonets, or geocomposites, but the majority of drainage systems use granular soil. This is particularly true in frost regions where it is necessary to have 3-6 ft of soil above the FML to satisfy the requirements for frost... 

The design for the surface water-collector system is determined by an allowable flow rate divided by a required flow rate. Allowable rates for geocomposites are determined experimentally by exactly the same method as for geonets. The specific cross section used in the test procedure should replicate the intended design as closely as possible. For the required flow rate, Darcy s law or HELP36 37 can be used. Then the design-by-function concept is used to determine the DR, or FS. 

This section discusses chemical compatibility (resistance) of geosynthetic and natural liner materials with wastes and leachates. Even in a relatively inert environment, certain materials deteriorate over time when exposed to chemicals contained in both hazardous and nonhazardous leachate. It is important to anticipate the kind and quality of the leachate a site will generate and select liner materials accordingly. The chemical resistance of any FML materials, geonets, geotextiles, and pipe should be evaluated before installation.39... 

Physical property tests on geotextiles and geonets must be designed to assess different uses, weights, and thicknesses of these materials, as well as construction methods used in the field. Some tests for geonets and geotextiles recommended by U.S. EPA are listed in Table 26.8. 

Evaluation of the Chemical Resistance of Geotextiles, Geonets, and Pipe... 

Drainage Sand or gravel Geonet or geocomposite Recycled or reused waste (e.g., tire chips)... 

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