Transmissivity of geonets in secondary leachate collection systems of waste containment facilities

• Main Author: Choy, Harvey Wayne
• Language: English
• Published: 2009
• Online Access: http://hdl.handle.net/2429/4894

The leak detection system of a waste containment facility may comprise of a geonet between two geomembranes. One of the main design parameters for such a system is the inplane flow capacity or transmissivity. Test devices with stress-controlled boundaries were specially designed and constructed to determine the flow capacity of such geosynthetic leak detection systems under different operating conditions. Two separate testing programs were performed. The Series W tests were performed over a period of 5 days at hydraulic gradients of 0.02, 0.04, 0.06, 0.08, and 0.10, and confining stresses of 25, 100, 200, 300, and 400 kPa; in addition, sedimentation tests were performed. This series of tests were performed to determine the influence that material properties, hydraulic gradient, and confining stress had on the transmissivity. Series WLC tests were performed at a constant hydraulic gradient of 0.02 and confining stress of 300 kPa for a period of 5 or 10 days and were intended to examine the long-term flow capacity. The series W test results indicate that increasing the hydraulic gradient increased the flow capacity, but not in a direct relationship because the flow regime in the geonet was semiturbulent to turbulent. Increasing the confining stress decreased the flow capacity as a result of geomembrane intrusion into the apertures of the geonet and compression of the geonet. Stiffer geomembranes showed less tendency to intrude into the pore space of a geonet, and geonets with narrower flow channels were less susceptible to intrusion from geomembanes. Geonets made of high density polyethylene, have high compressive strengths and tend to compress less. Extrapolation of the series WLC test results indicate that the flow capacity will decrease by 30% to 67% after a period of 28.5 years; a factor of safety for creep and intrusion of 1.75-3.0 may be appropriate, depending on material properties, for a performance period of about 30 years. Comparison of the data to granular soils indicate the geonets are approximately equivalent to 1/2 inch to 3/4 inch gravel and have a flow capacity equivalent to a 0.75 m thick layer of gravel possessing a hydraulic conductivity of 0.05-0.10 m/s. The results have some implications for materials testing; it is recommended that seating periods and orientation of test specimens be standardized. Boundaries of the test device should simulate field conditions, and be flexible to allow intrusion of adjacent construction materials into the geonet. A hydraulic gradient of 0.02 should be the minimum used in testing.