Design of plate and screw anchors in dense sand: failure mechanism, capacity and deformation

Abstract

Plate and screw anchors provide a significant uplift capacity and have multiple applications in both onshore and offshore geotechnical engineering. Uplift design methods are mostly based on semi-empirical approaches assuming a failure mechanism, a normal and a shear stress distribution at failure and empirical factors back-calculated against experimental data. However, these design methods are shown to under- or overpredict most of the existing larger scale experimental tests. Numerical FE simulations are undertaken to provide new insight into the failure mechanism and stress distribution which should be considered in anchor design in dense sand. Results show that a conical shallow wedge whose inclination to the vertical direction is equal to the dilation angle is a good approximation of the failure mechanism in sand. This shallow mechanism has been observed in each case for relative embedment ratios (depth/diameter) ranging from 1 to 9. However, the stress distribution varies non-linearly with depth, due to the soil deformability and progressive failure. A sharp peak of normal and shear stress can be identified close to the anchor edge, before a gradual decrease with increasing distance along the shear plane. The peak stress magnitude increases almost linearly with embedment depth at larger relative embedment ratios. Although further research is necessary, these results lay the basis for the development of a new generation of design criteria for determining anchor capacity at the ultimate limiting state.

Physical modelling to demonstrate the feasibility of screw piles for offshore jacket-supported wind energy structures

“…This over-prediction by a factor of two is also Cerfontaine et al (2019a) investigated the failure mechanism generated by the model screw piles 539 reported herein. The results which showed an angle of the failure plane of 17° and 10.25° to the 540 vertical for the very-dense and medium-dense conditions respectively (Cerfontaine et al, 2019a). 541…”

“…The helix tensile capacity (Qht) generated by the uppermost helix can have either a shallow or deep 123 failure mechanism depending on the embedment depth of the helix. A shallow failure mechanism 124 results in a conical failure surface, emanating from the shallowest helix, reaching the soil surface 125 ( Figure 1) (Cerfontaine et al, 2019a), whereas a flow-around mechanism occurs for deeply embedded 126 helices and the failure plane terminates below the surface instead. The initial approach adopted was to design the screw piles to operate with a shallow failure mechanism in uplift.…”