Granular soil densification using expansive resins

Specialist expansive resins have been developed over time for the purposes of ground improvement and soil stabilisation. They are injected into the soil in two phase liquid form (resin and hardener) and on reaction expand into a hardened material. The expansive forces generated cause granular soils to be compacted. This technology has been developing since the 1970's and it is installed mainly by employing the observational approach to grouting, which is based on the measurement of lift and the execution of pre and post in-situ testing to verify performance. This is an iterative process, which may require secondary or even tertiary injections to meet specified requirements. The objective of this paper is to propose an innovative design approach to the densification of coarse granular soils via the injection of expansive resins. This will enable designers to more accurately calculate the required amount of material to meet the engineering objectives of ground improvement and soil stabilisation projects. The design approach is based on a combination of the cavity expansion theory and a cubic compaction model which equates the volumetric expansion of the resin to an increase in the angle of friction of granular soils. This approach has been tested in the laboratory to quantify the cavity expansion and by finite element methods to confirm the strain generated is proportional to the expansion of the resin. Two ground improvement case studies are described, which were undertaken in loose to very loose coarse granular soils. Expansive resin treatments were designed using the new method described and predictive results compared to the current state of the art. Following the treatment an increase in soil densification of between 49.5% and 51.6% was measured on the two sites. The mean difference between the actual and predicted performance of the new design approach on these two cases was - 0.3% compared to a difference of -19.2% for the current state of the art. The surface boundary conditions, depth of injection and in-situ density are particularly important factors to define when undertaking expansive resins design. The case studies and PLAXIS modelling undertaken indicate that the new model will slightly underestimate actual compaction when the surface movement is constrained but overestimate it, if the soils are very loose, when not due to soil heave. The innovative design approach is the first to be verified by actual field case studies and provides a method to optimise the mass of expansive resin to be used for practical application cases, which should lead to a more sustainable and accurate approach to design.