Effect of Installation Disturbance on Pullout Capacity of Helical Soil Nails in Cohesionless Soils: Experimental and Numerical Studies

Soil nailing is a widely accepted method for stabilizing slopes, excavations, and so forth. However, conventional gravity or pressure-grouted soil nails are difficult to install in some unfavorable soil conditions. The drilling operations can cause soil stress relaxation and collapse of the drill hole. Helical nails have been developed recently to overcome such difficulties associated with conventional nailing techniques. A major advantage of this technique is its ease of installation by applying torque and crowd. However, this rotary movement of helical nails can disturb the surrounding soil and reduce the pullout resistance. The present research investigated the influence of installation disturbance on the pullout response of helical nails through experimental and numerical studies. The experimental results showed that the driving process causes a significant soil disturbance and reduces the pullout capacity. To capture the extent of the disturbed zone and failure mechanism, numerical simulation of the exact field installation process and pullout loading was carried out using the coupled Eulerian–Lagrangian approach. The diameter of the disturbed zone is observed to be in the range of two to three times the helix diameter depending on the number of helices. The failure pattern is governed by penetration depth-to-diameter ratio and helix spacing.