Mathematical Analysis of Shadow Effect in Jet Grouting

Jet grouting is a ground improvement technique that involves the injection of binder (with or without air) in soil at high pressure. One aspect of this technique that has not been investigated until now is the risk of failure due to the shadow effect, defined as the interruption of continuity during jetting produced by obstructions, such as boulders. This can represent a very serious problem because boulders do not allow the jet to reach and erode the soil, thus reducing the diameter or preventing the complete formation of the columns. This paper presents a mathematical theoretical discussion considering boulders as spheres. Three conditions possibly causing a failure of the jet-grouted continuous system are identified. A statistical risk analysis related to the probability of an unidentified spherical boulder causing a failure of the watertightness requirement of a jet-grouting wall is assessed. The goal of this research is to provide a tool by which engineers can estimate the risk related to the shadow effect. (C) 2018 American Society of Civil Engineers.