Leakage through defects in geomembrane liners under high hydraulic heads
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Dams are among the most critical of civil engineering structures and are susceptible to damage due to seepage. Geomembranes have been used to minimize seepage problems in dams since 1959. However, geomembranes are vulnerable to damage, in the form of tears and punctures, during installation and throughout the service life. These defects impact the effectiveness of a geomembrane as an infiltration barrier resulting in leakage through the geomembrane liner and into the body of the dam. The majority of previous studies conducted to investigate leakage through defects in geomembranes considered heads below 0.3 m, which corresponds to flow for conditions experienced by landfill liners. The main objectives of this study were to quantify the leakage rate through defects in geomembrane lining systems for dam and to evaluate the implications of the leakage on the performance of dams. Experimental, analytical and numerical studies were conducted as part of this investigation. The experimental testing program included permeameter cell tests for the following lining systems: i) geomembranes over clay layers, ii) geomembranes over sand layers and iii) geomembranes over geosynthetic clay liners. The permeameter tests were conducted to investigate the effects of soil type, initial water content, dry unit weight and hydraulic head on leakage through a defect in a geomembrane liner. The analytical component was conducted to evaluate the transmissivity at the interface and radius of wetted area for the tests conducted as part of the experimental component. The numerical component of this study was performed using the finite element method to simulate the experimental tests and the performance of actual dams. Ultimately, the laboratory tests and subsequent analyses were used to develop a set of tools (e.g., empirical models, design recommendations) that are expected to assist engineers in the design of geomembrane lining systems for dams. Also, the study was used to identify and address potential concerns (e.g., global stability) that may result in failure of systems in which geomembrane liners are used. This study is expected to contribute to a better understanding of the performance of geomembranes as dam liners. The presence of a geomembrane, even with a defect, was found to reduce the amount of leakage that occurs through a soil layer. New predictive equations were developed to estimate leakage through defects in composite lining systems. The finite element analyses indicated that a dam with a geomembrane with very large defects would not develop pore pressures that were similar to those observed for an unlined dam. The factor of safety for the unlined dam was improved by the placement of a geomembrane on the upstream face. Consequently, a geomembrane may also eliminate the need for a blanket drain at the downstream toe, which could decrease the cost of construction.