Laboratory investigation of multiple grout injections into clay

Compensation grouting is a multiple injection process in which large numbers of grout injections are carried out in a designated grouting zone. Each injection alters the in-situ stress state at the site of neighbouring injections, influencing the grout behaviour as well as the deformation of the surrounding soil. In this study, multiple grout injection tests were performed in the laboratory on clay specimens prepared at different overconsolidation ratios ranging from 1 to 10. Two types of grouting mode for compensation grouting were examined: (a) facture grouting by injecting epoxy resin, and (b) compaction grouting by expanding a latex balloon placed inside the soil specimen. The sequence of multiple injections was also varied by injecting either simultaneously or sequentially with different waiting periods. For highly overconsolidated clay the grout efficiency, defined as the ratio of the volume of heave achieved to the injected grout volume, was close to I irrespective of grout spacing and injection sequence. For normally consolidated and lightly overconsolidated clays the grout efficiency increased when the separation in space and time between the injections was reduced. A better grout efficiency was obtained in compaction grouting than in fracture grouting. The results from finite element analyses of the laboratory tests show that the magnitude and extent of excess pore pressure reduce when many closely spaced simultaneous injections are performed. A few non-simultaneous injections create large stress concentrations around the injection points, leading to larger soil consolidation.