Laboratory investigation of pullout behavior of hollow and solid shaft helical nail in frictional soil

Helical soil nails are passive elements installed in the soil which attains its bond strength through skin friction and bearing from helices. The present study examines the behavior of helical soil nail installed in cohesionless soil subjected to pullout force under varying parameters such as helical nail configuration (shaft diameter, helical diameter, helical pitch, number of helices), nail shaft types (roughness and stiffness), installation torque, and overburden pressure. The installation torque and corresponding nail pullout capacity can be established using a torque correlation factor (Kt). Kt value decreases with increasing embedded nail area and is inversely proportional to the nail shaft diameter. From pullout tests result, it is found that pitch in the range of 24.5–35.5 mm shows better pullout capacity. Also, results show that additional helices will only contribute to pullout capacity if located outside the region of soil mobilized in the failure mechanism of lower helix. Moreover, higher axial strains are found for hollow shaft nail, which alters with the increase in number of helices. Test results also indicate that various hollow shaft helical nails have nearly equal interaction friction angle to solid shaft helical soil nails with lesser shaft diameter. Therefore, it is concluded that solid shaft helical nails can be replaced by hollow nails without compromising on pullout capacity adding to reduction in construction cost. Tests results show linear correlation between maximum pullout shear stress and overburden pressure following a Mohr–Coulomb failure for different helical nail types.