LARGE STRAIN MEASUREMENTS IN GEOGRID REINFORCEMENT

Strain gauges are usually used in measuring deformations of geosynthetics in reinforced soil walls, where maximum strains around 2% are usually monitored. For the higher strain levels encountered in confined extension and pullout tests, extensometers and linear variable differential transformers (LVDTs) are usually used. The use of strain gauges in monitoring larger strains (5% and higher) requires a special procedure for the attachment of the gauges and a correlation between strain gauges and LVDT readings in confined soils for the proper interpretation of the measurements. A standardized procedure for attaching strain gauges to geogrid reinforcement was developed to monitor large strains (in excess of 8%) in confined conditions. The procedure was examined for different types of geogrids, adhesives, and protective coatings. It was first evaluated in unconfined extension tests. The correlation between strain gauge measurements and strains measured across the specimen length (cross-head strains) was investigated. Strain gauges monitored up to 16% cross-head strain with a linear relationship up to 10% strain. Strain measurements under confined conditions were evaluated in confined extension tests and pullout tests. The strains between the geogrid transversal elements (element-strains) were obtained from LVDT measurements. Strain gauge readings were correlated to element strains at different locations along the specimen. Strain gauge measurements were less than those calculated from LVDT measurements. The relationship between both was linear up to 8% strain. A correlation factor to correct strain gauge measurements to element strains is used and a numerical procedure is used to estimate the tension forces at various locations along the geogrid reinforcement from strain measurements.