Dynamic pullout resistance of anchors buried in dry sand

The vertical pullout resistance of buried anchors subjected to dynamic loading was investigated by means of a laboratory model study. A dry uniformly graded sand, compacted by vibration to 92% of its relative density, was used as the test medium. Tests were performed in a 24-inch square tank using 3.0 and 5.0-inch diameter half and full-anchors. The anchors were tested at depths ranging from 5.0 to 21.0 inches. It was assumed that the dynamic failure mechanism of earth anchors was the same as the static failure mode, and not distinct behavior. Thus static tests, i.e., tests using a constant strain rate, were performed to confirm this assumption. Static test results were also used to proportionately preload anchors prior to the application of dynamic loading. Emphasis was placed on determining the geometry of the breaking-out mass of sand under both static and dynamic loading. This was accomplished by stressing half-anchors which were embedded in sand against the plexiglass window of the test box. Still pictures and high-speed movies were used to record failure surface development. Comparison of the failure surface profiles for static and dynamic loading revealed no discernible difference. A difference was noted however, between shallow and deep anchor profiles. The shape of the failure surface for shallow anchors could be approximately by a truncated cone with an apex angle equal to the friction angle of the sand. Deep anchor failure surfaces were generally indistinct. Consequently further analysis was limited to shallow anchor behavior. A method for predicting static uplift resistance was derived based on the weight of the anchor and sand within the failure surface, and the shear resistance along the surface of the uplifted sand mass. Dynamic pullout resistance was found to be greater than static resistance. It was theorized that this additional resistance was due to the inertial forces and increased shear resistance developed under accelerated strain rates. An expression for dynamic capacity was derived by modifying the static expression to include these factors. Comparison of the measured results from a number of model and full-scale studies with the results predicted by the expressions derived in this study, generally showed agreement within 10% --Abstract, pages ii-iii