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

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

Effect of soil deformability on the failure mechanism of shallow plate or screw anchors in sand

Most analytical approaches for the design of shallow plate and screw anchors in tension are based on the limit equilibrium of a rigid soil wedge for which a horizontal stress distribution acting on the failure plane is assumed. Finite element analysis for a wide range of soil properties was carried out to identify the shape of the failure mechanism and to study the stress distribution at failure. Results show that soil deformation modifies the stress field around the anchor and increases the uplift capacity. A semi-analytical approach is proposed to describe this stress distribution, based on peak friction angle