Fundamental Study of Compensation Grouting

Alfred Au, Cambridge University
Geotechnical Engineering Group


A successful and effective application of compensation grouting does not only depend on a good monitoring system, but also the understanding of grout behaviour in soils. The aim of this thesis is to investigate the fundamental behaviour of compensation grouting (compaction and fracture grouting). The research programme is designed to examine the behaviour of both single and multiple injections (simultaneous, sequential and re-grouting) during the injection and consolidation stages.

Laboratory single and simultaneous injection tests were performed on kaolin clay to investigate the effect of overconsolidation ratio, grout material properties (grout viscosity and water cement ratio), grout injection volume, radial boundary and injection rate on the effectiveness of compensation grouting, grout deformation pattern, fracture orientation and initiation mechanism. The effect of the duration between each injection for sequential injection and re-grouting was examined by another series of laboratory tests.

The experiments involved the injections of various grouts into clay specimens consolidated in modified consolidometers, which have one or four injection needle(s) installed from the base into the samples. Modified consolidometers with different diameters were used to examine the effect of the boundary size (i.e. distance between the injection point and the outer rigid boundary) on grout behaviour. During the single and simultaneous injections, the injection pressure and the changes in sample volume were recorded. For the sequential injection and re-grouting, the time between each injection was varied. After the injection, time dependent change in the sample volume was monitored while the excess pore pressures developed during the injection stage dissipated as the samples consolidated. It was found that the long-term behaviour of compensation grouting is largely affected by the overconsolidation ratio of the clay, grout types and the boundary conditions. Post-test examination of the samples revealed that the fracture pattern and orientation depended not only on the initial stress condition but also on the boundary conditions, injection volume, grout material properties and injection rate.

In addition, finite element analysis was adopted for the simulation of the cavity expansion tests (ideal compaction grouting tests) and the results demonstrated the effect of multiple injections by examining the variation of stress path, pore pressure generation and the effect of overlapping excess pore pressure zone. The results were also compared with the laboratory tests data. Moreover, a field scale finite element analysis was performed to examine the applicability of the general trend observed in the laboratory scale model.

Based on the research findings, new design criteria are proposed to improve the effectiveness of compensation grouting.