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Two different clamping mechanisms are commonly used in pullout and confined extension tests of geosynthetics. The first method consists of extending the clamps inside the soil to a sufficient length that ensures the confinement of the whole specimen length during testing. The frictional resistance of the part of clamping plates inside the soil is subtracted from the results to obtain the resistance of the geosynthetic specimen. The second method consists of clamping the geosynthetic specimen outside the soil. In this method, displacement measurements are taken in the confined part of the specimen in the soil and, hence, the readings are not influenced by the possible slippage of the specimen between the clamps.
A comparison of test results using both installation techniques is presented and the boundary effects associated with both mechanisms are evaluated. In the tests where the clamps were extended inside the soil, earth pressure near the front facing was measured in order to evaluate the frictional resistance of the clamping plates. Soil pressure measurements were taken after applying the confining pressure and during the test to monitor the development of vertical stresses at the vicinity of the clamping plates. The measurements showed an apparent increase of vertical pressure above the clamps. The results were corrected for the increase of the vertical pressure due to frictional resistance of the clamping plates.
When the clamping plates were connected to the specimens outside the box, displacements were measured along the specimen length. These measurements were extrapolated to determine the front displacement of the specimen at the pullout load application point. The use of the extrapolated front displacement resulted in a more reasonable load-displacement relationship for the geosynthetic specimen.
geosynthetics, pullout test, confined extension test, clamping plates, earth pressure
Research Coordinator and Geophysical Research Manager, Louisiana Transportation Research Center, Baton Rouge, LA