Laboratory Investigation of Hollow and Solid Shaft Helical Soil Nail by Displacement Controlled Pullout Testing

A sequence of laboratory pullout tests was conducted to examine the installation and pullout behavior of hollow and solid shaft helical soil nails. The methodology involves installation and corresponding pullout testing of 12 mm, 14 mm, 16 mm and 18 mm hollow and solid shaft helical soil nails with varying number of helical plates between single to triple and shaft surface roughness from smooth to rough using a displacement-controlled apparatus. A total of 18 different helical soil nail combinations were investigated under surcharge pressures 5 kPa, 12.5 kPa, 25 kPa, and 50 kPa to examine the influence of shaft dimeter, shaft roughness, number of helices, pitch, interaction friction angle, embedment depth ratio on both installation and pullout capacity. Using the pullout capacities and corresponding installation torque, capacity-to-torque ratio (Kt) relationship was also established which yielded ranges of 19 m−1 to 61 m−1 and 23 m−1 to 58 m−1 for hollow and solid shafts, respectively. Moreover, interaction friction angles depicted that with similar number of helices, an increase of 25% in diameter of hollow shaft helical soil nail renders equivalent reinforcing action to a corresponding solid shaft. Also, for both hollow and solid shafts, double helical soil nails were observed to show the maximum efficiency, while the effect of number of helices was found to diminish beyond the embedment ratio of 11. Within the pitch range of 30–35.5 mm, pullout response for both hollow and solid shafts were found to be linearly dependent on its installation.