Horizontal Pullout Behavior of Novel Open-Ended Pipe Helical Soil Nail in Frictional Soil

The reinforcing action of soil nails is governed by its interaction with the surrounding soil generally investigated in terms of interface friction. The reported literature depicts that increase in interface friction enhances the reinforcing action of a soil nail. Thus, with the aim of utilizing additional interface friction from internal surface of a hollow pipe and bearing resistance from helical plates attached to it, the present work investigates the pullout behavior of newly developed open-ended pipe helical soil nails. The novel open-ended pipe helical soil nails are installed and subjected to pullout force in a test tank filled with frictional soil using installation/pullout machine. The present study also examined the effect of varying soil nail diameter, number of helices and phenomenon of soil plugging on pullout capacity. Additionally, torsional and axial strains developed during pullout are also observed. The installation torque and pullout capacity are correlated using a dimensionless factor called as ‘Torque factor (Kt)’ with values ranging from 27 to 55 m−1 in compression and 28.12–53.3 m−1 in tension. A theoretical torque model for open-ended helical soil nails is also developed and is verified by the laboratory test results. The test results indicate that soil plug contributes about 11.5% of the total mobilized skin friction during pullout. The pullout capacity increases with increase in the number of helices and nail shaft diameter. However, soil plug length is independent of number of helices. Besides, higher torsional and axial strains are found for helical nails with smaller diameter.