A major concern in underground infrastructures is to sufficiently seal the area from water-ingress. To achieve that, adequate spread of grout in the surrounding fractures is crucial. Cement-grouting is probably the best method for that due to the lower costs and environmental impacts. But in this method, the grout spread is disrupted by filtration and plug-building of cement that can lead to flow stop and insufficient sealing. Dynamic grouting is a technique that has been primarily developed to solve the issue by reducing the grout viscosity using high-frequency oscillation. Despite some promising results, the remaining issue has yet been quick dissipation of the oscillations along the fractures resulting in inadequate grout spread and thus insufficient sealing. Recent investigation of the authors on the method using a short slot showed significant improvement in the amount of grout passed through micro-fractures by applying low-frequency pressure impulses. The mechanism of improvement was though interpreted as successive erosion of the produced filter cakes due to the variation in flow pattern during the pressure impulses. Nevertheless, the dissipation of the pressure impulses along a fracture was debatable.
This study therefore aimed to examine the dissipation of dynamic impulses and the improvement of grout spread along a much longer artificial fracture, the so-called varying aperture long slot (VALS). The investigation was conducted under two regimes of peak/rest periods using a high-pressure gas tank and a screw pump as pressure source.
Even though the study was only based on limited number of laboratory experiments, the results were promising, showing considerable improvement in the amount of grout passed through micro-fractures with acceptable dissipation length.
The study finally showed the potential of method and suggested further development in full-scale field tests, to demonstrate the capacity of the new technique to the stakeholders in the industry.
The history of dynamic grouting probably goes back to the work of Pusch et al. (1985) in the Stripa mine, who applied a high-frequency oscillating pressure to improve the grout spread in rock fractures. Following that Borgesson and Jansson (1990) further examined a high-frequency large-amplitude oscillating pressure superimposed on an underlying pressure of 20 bar and illustrated that in such condition even a low water content grout can penetrate well through 100 μm artificial fractures. Subsequently, Wakita et al. (2003) investigated the method using oscillating amplitudes of up to 5 bar superimposed on an underlying pressure of 10 bar and similarly obtained an increase in the flow rate and the total volume of grout take. Afterwards, Mohammed et al. (2015) further elaborated on the method and once again recognized it as a potential solution to improve the grout spread. In all those investigations, the mechanism of action was interpreted as reduction in the grout viscosity due to the applied high-frequency oscillation, whereby the grout's internal structure was disrupted and reorganized to a lower viscosity suspension. Despite the promising results, use of dynamic pressure impulses has not yet been established as a common method in grouting practice in industry, due to the limited efficiency and quick dissipation of the oscillation along a fracture.