Chapter 20 - Pullout Resistance of Grouted Soil Nails

Abstract

Soil nailing is a technique accomplished by installing closely spaced, passive, structural inclusions, known as nails, into the soil to increase its overall shear strength. Soil nails have been widely used for stabilization of slopes and earth-retaining structures in many countries and regions. In Hong Kong, grouted soil nails have been used in most slope stabilization works since the late 1980s. Although the design guidelines are available in many countries, some design issues remain ambiguous among some engineers and researchers. The soil nail ultimate pullout resistance is a key parameter for the design of soil nails. Many of the issues under debate are related to the determination of this key parameter. Some influencing factors are not well qualified in the current design methods. For example, the effective stress method, adopted in Hong Kong, does not account for factors including soil arching, soil dilatancy, soil saturation condition, roughness of drill-hole surface, and grouting pressure. Better design can be achieved if the pullout resistance of grouted soil nails is more accurately predicted. In this chapter, the behavior and design methods of the soil-nailing system are first reviewed. Different methods for determination of soil nail pullout resistance are discussed. Then, the technique and test procedures for laboratory pullout testing of grouted soil nails under controlled overburden pressure, degrees of saturation, and grouting pressure are recounted. Next, based on the series of laboratory pullout tests, the effects of some key factors on the pullout resistance of grouted soil nails are systematically discussed. These factors include overburden pressure, degree of saturation, and grouting pressure in both unsaturated and saturated conditions. Finally, a probabilistic estimation of the pullout resistance of grouted soil nail is reviewed. By using the Bayesian model class selection approach and all the pullout resistance data from laboratory tests, an optimal formula is proposed and it possesses rational balance between the data-fitting capability and robustness to measurement noise and modeling error. It is found that the most important factors in the estimation of soil nail pullout resistance are the degree of saturation and the product of grouting pressure and overburden pressure.