Dynamic grouting - Water hammer

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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. In this case also some cement particle aggregates may be dispersed. An inexpensive way of producing pressure variations in the grout is to make use of the water hammer effect. This occurs when a flow of liquid is abruptly shut off with a valve. The liquid up-streams the valve will want to continue flowing, but is stopped, producing a strong positive pressure-wave propagating in the up-stream direction. Down-stream, similarly, the liquid will want to continue flowing, but no new liquid is added from up-streams. Thus a negative pressure pulse is created down-stream the valve and it is propagating in the down-stream direction. The negative down-stream pressure pulse will agitate the thixotropic grout and keep its low viscosity although a plug-flow may be present. It will also create a backward gradient when it reaches a place in the fractures where a filter is building up. Down-stream of the filter a high pressure will be present when the low pressure impulse reaches the filter. Thus the high pressure gradient will destroy the filter. The low pressures obtainable with this method can be very low. Close to the valve we have measured negative pressures in water. Negative pressure cannot exist in a gas, but in a liquid they may do that a short time before cavitation develops. There are two problems with this method and it is that the higher the valve-closure rate is, the lower will be the average flow. The other problem is that the efficiency of the valve closing depends on the rate of flow, thus the method is only useful when the flow is high.

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