Earth anchors are becoming a very useful technique for securing temporary and permanent foundation systems subjected to uplift loads. Laboratory tests were performed with one-quarter scale single-helix model anchors in dry sand at a constant relative density and embedment depth. The two parameters investigated were the displacement amplitude and prestress load. It was concluded that the screw-in anchor installation technique and the application of a prestress load both produced an increase in horizontal soil stresses and soil densification in the vicinity of the single-helix anchor. In the prestress range investigated, a critical ratio of dynamic load to effective static capacity exists. Above this critical ratio, a failure of a prestressed anchor will occur earlier than a dead anchor failed by cyclic loading. Prestressed anchors below this critical ratio will experience an increase in anchor life.