Investigation on stress variation in soil surrounding a soil nail during installation and pull-out

Pull-out strength of a soil nail is an important parameter in the design of a soil nailing system. The pull-out strength is determined by the normal stress acting on the soil nail surface during pull-out. The variation of normal stress in soil surrounding a drill-and-grout soil nail during drilling, grouting and pull-out is studied through both laboratory pull-out tests and numerical simulation. It is found that for a drill-and-grout soil nail, most of the normal stress in the soil surrounding the hole is released after drilling. For a soil nail grouted under low pressure, the normal stress does not substantially recover after the nail is installed. During pull-out, the normal stress acting on the nail increases with the increase in pull-out distance because of the constrained dilatancy of the soil surrounding the nail, which eventually increases the pull-out resistance. It is indicated that the dilatancy of the soil is very important to a drill-and-grout soil nail. For soils with low dilatancy, further measures, such as high pressure grouting should be employed to compensate the released stress during drilling.