Calculating Strain and Stress in Geomembranes at Gravel Indentations
New methods were developed, validated and applied to calculate strain and stress in geomembrane liners from the local deformed shape imposed by overlying coarse gravel under vertical pressure. Previous strain calculation methods consider only vertical displacements of the geomembrane or neglect bending effects and, consequently, underestimate the maximum strain. Here, the geomembrane was assumed to behave as a thin plate that undergoes vertical and lateral displacements during deformation. The vertical displacement is the only measurement required while horizontal displacements are related to the deformed shape by strain compatibility equations and obtained by solution to Airy’s stress function. A viscoplastic model was developed and verified against data from physical experiments to get new insight of stress and strain developed in the wide-strip tensile test, verify the new strain calculation method, and to convert the calculated strain to stress. An axisymmetric single gravel indentation was created by pushing a machined probe into a geomembrane disc clamped around its perimeter. The new strain calculation method yielded strain in very good agreement (within 4% of the maximum strain) with the numerical simulation of the single gravel indentation test. The calculated strains were converted to stress using a direct stress-relaxation method. The method utilizes the fading memory concept where the deformation path is taken to have a minor effect on stress if strains remain constant for some time before calculating stress at the time of interest. The method was verified against, and found to be in good agreement with, stresses from finite element models of gravel indentation problems. The new strain and stress methods were applied to analyse the deformed shape of a geomembrane from a physical test with coarse gravel above and compressible clay beneath the geomembrane to examine the new insights gained from protection layer testing. Comparison with previous strain calculation methods showed that past simplifying assumptions lead to inaccurate values of maximum strains, location and direction. The stress analysis showed that high tensile strain can be obtained under compressive stresses, and therefore, protection layer design should also consider stresses developed in conjunction with strain.
URI for this recordhttp://hdl.handle.net/1974/25915
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