Pullout Resistance Increase of Soil Nailing Induced by Pressurized Grouting

Pressurized grouting has been frequently adopted in soil-nailing systems that are widely used to improve slope stability. In most geotechnical applications, soil nailing using pressurized grouting has been empirically performed without theoretical validation because the interaction between the pressurized grout and adjacent soil mass is very complicated. The present paper deals with a series of pilot-scale chamber tests performed on four different granite residual soils to evaluate the effect of pressurized grouting on the soil-nailing system. When grout is injected into a cylindrical cavity in the soil mass, the pressure exerted around the cavity perimeter initially increases with time up to a peak value and then gradually decreases to a residual stress. The pressure reduction may result from the seepage of water originally retained in the grout paste into the adjacent soil formation. With the application of pressurized grouting, in situ stresses can be increased by approximately 20% of the injecting pressures during the experiments. To develop a desirable residual stress in a soil-nailing system, it is necessary to select an appropriate minimum injection time for which the grout pressure should be maintained. The required minimum injection time increases with an increase in either the fine-grain content or the injection pressure. Moreover, a series of in situ pullout experiments has been performed on soil-nailing systems, using both pressurized grouting and common gravitational grouting to compare the pullout loads of both cases and to verify the effectiveness of the pressurized grouting on the soil-nailing system. The pullout load of soil nailing using pressurized grouting is approximately 36% higher than that of soil nailing using gravitational grouting. This is attributed to the additional compaction of soil by cavity expansion and to an increase in the residual stress and in the dilatancy angle by pressurized grouting. The field experimental results have been verified with analytical solutions by estimating the dilatancy angle from the pressurized grouting tests.