A proper understanding of the soil-geosynthetic interaction is essential to the design of soil-geosynthetic systems under both ultimate (or large displacement) and service (or small displacement) conditions. Pullout and interface direct shear tests have been among the most widely used experimental techniques to evaluate this interaction. However, correlation between the results obtained using each test-ing method and the suitable testing scale have remained unclear. A pilot experimental program was con-ducted using pullout and direct shear devices of various scales. Experimental data obtained in the tests was analyzed to gain insight into the effect of the scale and testing method on the soil-geosynthetic inter-action behavior. Specifically, load and displacement data, average interface shear values, and normalized displacements were compared for a baseline geosynthetic and backfill material tested in three testing sce-narios: (1) large- and small-scale pullout tests, (2) large- and small-scale direct shear tests, and (3) same-scale pullout and direct shear tests. Overall, the ultimate interface shear was found to be similar in various testing scales and devices. However, the slope of the relationship between the interface shear and relative displacement between soil and geosynthetic was found to be sensitive to both test method and testing scale. Implication of the experimental observations in modeling soil-geosynthetic interaction was evaluat-ed by simulation of a small-scale pullout test using a t-z analysis approach. Comparison of the simulation results with the experimental data further underlined significance of adopting suitable test method and testing scale for calibration and validation of analytical and numerical soil-geosynthetic interaction mod-els.
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