An investigation of compaction grouting for liquefaction mitigation

The compaction grouting technique is examined from the perspective of liquefaction mitigation. Compaction grouting is a ground improvement technique that involves the sequential injection of stiff grout into weak soil with the intention of forming an essentially columnar mass of hardened material. The injected grout does not mix with the surrounding soil and improvement is produced through displacement and straining of the surrounding soil. Volumes of weak soil can be improved by performing injections over a regularly spaced grid. As the name suggests, the technique is presumed to increase the density of the treated soil through displacement and thereby reduce its liquefaction potential. The technique has attractive benefits for application in retrofit projects and has witnessed increased application in this area.An analytic study is performed of the process to determine if the assumptions made about the mechanism of improvement are warranted. Initially, a single grout column is analyzed as an expanding cylindrical cavity. A critical state soil model is adopted to correctly model the volume change characteristics of sands under the large magnitude shear strains that the technique induces. The results obtained from this one-dimensional study indicate that very little volumetric strain occurs and that the improvement measured by in-situ tests can largely be explained by increases in mean stress. That stress changes rather than densification is responsible for the improvement has implications for liquefaction resistance.The two-dimensional nature of the treatment is assessed through the use of an appropriate two-dimensional finite difference grid. It is found that for initially loose sand states that the interaction that occurs is largely elastic in nature.A set of design charts is provided for assessing the effects of compaction grouting in terms of cone penetration resistance under given initial conditions using the one-dimensional approach.A possible modification for the technique in which the grout column is expanded cyclically to exploit the volume change characteristics of sand under cyclic shear strains is also examined. It is found that the small magnitude of cavity strain that occurs during cavity unloading severely limits the volume of soil affected by shear strain reversals.