Prediction of Pullout Resistance of Helical Soil Nails: A Limit Equilibrium Approach

Soil nailing is a widely used technique for stabilizing slopes and retaining walls, specifically in challenging terrains. Helical soil nails are a relatively new advancement that has gained popularity due to their ease of installation. Several experimental studies have investigated the pullout response of helical soil nails; however, no analytical method has been reported yet for predicting their pullout resistance. This research proposes a limit-equilibrium-based prediction method to estimate the bearing resistance of the helix. Numerical modelling of the installation and pullout of helical nails was carried out using a coupled Eulerian-Lagrangian finite-element approach to identify the propagation of the failure surface. Based on the results, a logarithmic spiral slip surface model was adopted to derive the bearing resistance by considering the equilibrium of external forces acting on the soil volume enclosed by the slip surface. The pullout resistance factors obtained for the helical nails were found to depend on both the soil properties and the nail's geometric parameters. Additionally, a critical penetration depth of five times the helix diameter was observed, indicating the transition from a shallow failure to a deep failure mode. Experimental studies were also conducted to validate the numerical approach and the proposed bearing resistance equation.