Direct measurement of geomembrane strain from aggregate indentations

High localized strain in geomembranes caused by indentations from coarse, uniform drainage aggregate is associated with long-term stress cracking and has been widely studied. A number of large-scale lab tests were carried out with direct measurement of the distribution of the major and minor principal (engineering) strain across the area of a large laboratory sample of geomembrane. This 'grid-point' method of strain estimation is compared to the method developed by Tognon et al. which has been recommended as the best available approach at this time. It has been found that the method by Tognon et al. tends to over- and under-predict geomembrane strain depending on the aggregate and protection layers used. Indentations with higher smaller ratios of membrane to bending strains tend to be underestimated. A correction factor is proposed based on the ratio of membrane to bending strains and fits the measured grid strain method used in this study well.