Pullout performance of inclined helical screw anchors in sand

Experimental and theoretical studies on the pullout performance of inclined helical screw anchors are presented. Previous contributions are reviewed and compared. The experimental program was conducted on anchors inclined at 15, 30, and 45° with respect to the vertical and installed into dense, medium, and loose sands. Special tests on anchors inclined at 45 and 60° installed into colored-layered sands were conducted to define the failure mechanism and the associated rupture surface. The results of these tests confirmed that the rupture surface is unsymmetrical and of a very complex nature; however, it can be represented by segments of logarithmic spiral curves. These segments differ in shape and size along the anchor perimeter. The concept of a conjugate anchor and auxiliary rupture surface was developed and introduced to correlate the pullout capacities of inclined and vertical anchors installed to the same depth in identical sand conditions. A coefficient of embedment depth was established and expressed theoretically in terms of the relative depth and the angle of inclination to facilitate the calculation of the pullout capacity of inclined anchors. A comparison of theoretical and experimental as well as field results showed reasonable agreement and confirmed the validity of the proposed theory.