Behavior of Helical Soil Nails An Experimental and Theoretical Study

Abstract

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 behaviour 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 for a model helical soil nail the pitch in the range of 24.5 to 35.5 mm shows better pullout capacity. Also, results shows 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 a nail, which alters with 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. 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 or hollow shaft hel