Fluid Migration Through Geomembrane Seams and Through the Interface Between Geomembrane and Geosynthetic Clay Liner PDF Download

Author: Madalena C. P. Barroso
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Languages : en
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Book Description
Composite liners are used to limit the contamination migration from landfills. Their successful performance is closely related with the geomembrane as it provides the primary barrier to diffusive and advective transport of contaminants. Critical issues on the performance of the geomembranes are the seams between geomembrane panels and the inevitable defects resulting, for instance, from inadequate installation activities. In landfills, where high density polyethylene geomembranes are usually used, seams are typically made by the thermal-hot dual wedge method. A literature review on quality control of the seams showed that, in situ, fluid-tightness of seams is evaluated in qualitative terms (pass/failure criteria), despite their importance to ensure appropriate performance of the geomembranes as barriers. In addition, a synthesis of studies on geomembrane defects indicated that defects varying in density from 0.7 to 15.3 per hectare can be found in landfills. Defects represent preferential flow paths for leachate. Various authors have developed analytical solutions and empirical equations for predicting the flow rate through composite liners due to defects in the geomembrane. The validity of these methods for composite liners comprising a geomembrane over a geosynthetic clay liner (GCL) over a compacted clay liner (CCL) has never been studied from an experimental point of view. To address the problem of fluid migration through the geomembrane seams, an attempt is made to provide a test method, herein termed as "gas permeation pouch test", for assessing the quality of the thermal-hot dual wedge seams. This test consists of pressurising the air channel formed by the double seam with a gas to a specific pressure and, then, measuring the decrease in pressure over time. From the pressure decrease, both the gas permeation coefficients, in steady state conditions, and the time constant, in unsteady state conditions, can be estimated. Experiments were carried out both in laboratory and in field conditions to study the suitability of this test to assess the quality of the seams in situ. The results obtained suggest that it is possible to assess the quality of the geomembrane seams from a non-destructive test conducted in situ by determining the time constant. To address the problem of fluid migration through geomembrane defects, composite liners comprising a geomembrane with a circular hole over a GCL over a CCL were simulated in tests at three scales. Flow rates at the interface between the geomembrane and the GCL were measured. Correspondent interface transmissivity was estimated based on final flow rates and observation of the wetted area. A parametric study was performed to evaluate the influence of the prehydration of the GCL, the hydraulic head on top of the liner and the confining stress over the liner system, on the flow rate through composite liners due to defects in the geomembrane, as well as to check the feasibility of an extrapolation of the results obtained on small-scale tests to field conditions. It was found that the transmissivity does not seem to be affected by the prehydration of the GCLs when low confining stresses were used. It also does not seem to be influenced by the increase in confining stress when non-prehydrated GCLs are used. Finally, the transmissivity does not seem to be significantly affected by the increase in hydraulic head. The results also suggest that predictions on flow rates though composite liners due to defects in the geomembrane, which are based on transmissivity values obtained in small scale tests, are conservative. Lastly, based on the transmissivities obtained in this study, empirical equations for predicting the flow rate through composite liners consisting of a geomembrane over a GCL over a CCL are proposed. Flow rates calculated using these equations are in better agreement with the flow rates measured experimentally than the empirical equations reported in literature. The new empirical equations provide design engineers with simple and accurate tools for calculating the flow rates through the above mentioned type of composite liners.