Soil Nailing. A Laboratory and Field Study of Pullout Capacity

Abstract: Soil Nailing is a technique for reinforcing soil in situ, increasingly utilised as an economical alternative to conventional earth reinforcing techniques. However, today the design is based on an assumed value of the pullout capacity of nails, which later is verified by pullout tests during the construction, which in turn limits the possibility of optimising the design.

The objective of this study was therefore to get a better understanding of the pullout capacity and to suggest a method for more accurate prediction. The study comprised; laboratory tests (to study the influence of overburden pressure, relative density and method of installation) and full-scale field tests (to compare the pullout capacity among different soil nails and with results of site investigation methods). The results were then compared with design methods and results from FEM analysis.

The results show that the pullout capacity in a cohesionless soil mainly depends on; the coefficient of roughness, surface area and normal stress. An increase in relative density, overburden pressure, and volume of displaced soil will result in an increase in pullout capacity. The coefficient of roughness and surface area could be estimated with quite good accuracy while the normal stress against the nail is more difficult to determine. The best prediction of the pullout capacity seems to be obtained from pullout tests on vertical nails.

The greatest pullout capacity was, as expected obtained for grouted nails, but at the test carried out about 300 days after installation a driven expansion bolt and the angle bar gave an almost equals high capacity. A significantly lower pullout capacity was obtained for the ribbed bars, and for these nails no time dependency could be found. The analysis indicates that methods based on the theory of cavity expansion could be used to calculate the normal stress after local calibration.