Compensation Grouting in Sand

Khalid Eisa, Cambridge University
Geotechnical Engineering Group


Compensation grouting is being widely used for the mitigation of ground settlements during different geotechnical activities; particularly in tunnelling. The processes involved in the implementation of compensation grouting include fracture (or hydrofracture) grouting, compaction grouting as well as intrusion grouting. Yet the current practice of compensation grouting is mainly dependent on experience rather than the scientific understanding of soil-grout interaction. As a result, the application of compensation grouting might be hindered in some cases by the uncertainty about the grout behaviour in soil.

This research thus aims to develop a fundamental understanding of the soil-grout interaction when compensation grouting is carried out in sand. The importance of such knowledge is enhanced by the fact that compensation grouting is mostly used in relation to underground construction in highly-congested cities and in close proximity to valuable structures. To this regard, large-scale experimental work was carried out using two different, but comparable, experimental setups at the University of Cambridge, UK, and GeoDelft (currently Deltares), the Netherlands.

The study concentrated on cement-based grouts; as they are the most common type of grouts used. In order to relate the grout behaviour in soil to grout characteristics, injection parameters and soil properties, a parametric study was conducted. The grout properties considered included the water-cement ratio, content of fine particles (bentonite percentage) and the type of materials employed. The injection characteristics evaluated were injection rate, injection pressure, injected volume as well as the number of injections. The influence of confining pressure and relative density of the soil were also studied. In order to gain a better control on the grout movement into soil, the method of injection with an oscillating rate, termed dynamic injection, was applied to compensation grouting. Cavity expansion analyses were performed in order to get a better interpretation of the outcome of the experimental work.

The results of the research provided a basic understanding of the different processes involved in sand fracturing. The phenomenon was found to be mainly controlled by the grout characteristics in terms of water-cement ratio and the content of fine particles. Injection rate appeared to have a minor influence on sand fracturing under the tested conditions. The influence of the water loss by pressure filtration on the outcome of grout injection was highlighted. The parametric study performed made it possible to predict the outcome of the implementation of a certain set of grouting parameters and various scenarios were offered to cover different parametric combinations.

This research was, therefore, successful in meeting the set objectives and the obtained results constitute a step towards gaining a full understanding of the grout behaviour in soil when compensation grouting is performed in sand.