Calculating and Limiting Local Strains in Geomembranes From Gravel Indentations
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Short-term tensile strains that arise from local gravel indentations in municipal solid waste landfill base geomembrane liners are investigated with an emphasis on quantifying how strains are obtained and providing methods for limiting long-term local geomembrane strains below allowable levels. This involved the development of new experimental techniques and apparatuses to study geomembrane strains under controlled conditions. An experimental technique to measure the radial and vertical components of displacement from local gravel indentations in a geomembrane is presented. Geomembrane strains were calculated using large strain-displacement theory, thin shell theory, and the arc elongation method. While radial displacements were found to be small relative to vertical displacements, they were shown to provide the dominant contribution to the maximum strain beneath the centre of the gravel indentation. Both the thin shell and arc elongation methods produced incorrect distributions of strain and also provided magnitudes that significantly underestimated the maximum geomembrane strain. Small-scale screening tests were conducted to gain insight into how key factors including the compressibility of the underlying clay layer, particle size of the overlying gravel, and mass of the geotextile protection layer influence local geomembrane strains. Results are provided that can be used to rule out protection layers that are unable to meet a long-term strain limit, and conversely, identify those that may be able to limit strains below a target strain for further larger scale evaluation. Compaction water contents towards the lower range of water content to achieve an acceptably low hydraulic conductivity of field compacted clay liners were found to be more successful in limiting local strains in the geomembrane. Protection from geotextiles with masses up to 2440 g/m2 were unable to limit geomembrane strains below 1.5% with nominal 50 mm gravel at a temperature of 55°C. Tests with nominal 25 mm gravel revealed that a geotextile with a mass of 1500 g/m2 successfully limited the maximum geomembrane strain below 1.5% at an applied stress of 250 kPa while another geotextile (2440 g/m2) met the strain limit up to applied stresses of 500 kPa at a temperature of 55°C.