Dynamic grouting - Feedback resonance

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The idea behind dynamic grouting is to counter increasing viscosity due to plug-flow and build-up of filters in narrow cross sections of the fracture system that would obstruct grout flow. The viscosity in a thixotropic fluid can be kept low by oscillating the pressure in the grout-flow. This will cause a back-and forth movement of the cement particles, which of course must be combined with a positive static pressure which produces a net flow of grout. The same oscillations can have the effect to keep the build-up of filters down. The idea behind this is that if the pressure up-streams a filter is reduced, the pressure down-streams of the filter will remain high and cause a filter breakdown from the downstream side. When working properly the filter build-up process will be kept very low and no real filters will develop. A system of grout pump, oscillator, rubber hose and injection pipes and fractures have a combined resonance frequency. If the oscillator is made to work at that frequency very high pressure variations can be obtained with a small force. This is because new energy is continually added to the oscillating system. The resonance is most obvious in the steel-reinforced grouting tube but the counter pressure in the rock could perhaps also influence the resonance frequency and amplitude. These assumptions have been verified by experiments in which the oscillations are produced by an electrically controllable hydraulic cylinder, which in turn operates on a grouting pump cylinder. The system allows any type of pressure or flow function to be established. It is e.g. possible to create transient pressure pulses as well as sinusoidal pressure variations.

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