Transition Depths for Deep Circular Anchors in Saturated Sand

Helical anchors have the potential to become a cost-effective anchoring system for a wider range of offshore structures than those currently in use. However, there is still uncertainty in evaluating capacity based on analytical expressions or numerical models, particularly for deep anchor response. This paper presents results from 74 uplift tests on 50mm, 152mm, and 254mm diameter screw-in anchors in a saturated sand prepared in two states, one with a moderate and the other with a high friction angle. Tests are performed on anchors embedded to depths between 1.8 and 15.6 diameters. Sand characterization included cone penetration tests and a suite of triaxial tests. This study focused on evaluating the transition between shallow and deep anchor response in uplift for sand samples with relatively well-known state and strength characteristics. Deep anchor transitions occur at depths that are greater than previously suggested recommendations based on field tests. A functional form for an empirical equation is presented to assess uplift capacity across the transition from shallow to deep response. Input parameters for the tests performed are provided.