Behaviour of grouting material used for floor reinforcement in underground mines

In underground coal mines, floor heave is a major problem that is caused by groundwater, high strata stress and weak floors. Controlling the stability of the roadways is the key to maintaining the safety and efficiency of underground mining. Appropriate cement grouting is an effective and attractive floor reinforcement technique that can control excessive floor heave and closure. Cement grouting also reduces the permeability of the rock mass and consequently controls the movement of groundwater into the roadway. Injecting cement grout into roadway floors provides a more uniform reinforcement throughout the floor. It may even be possible to reinforce an extremely fractured floor. In this respect, this technique is superior to floor bolting/dowelling. This technique may be performed either as a pre- or post- development reinforcement. It is very difficult, however, to reinforce a floor that is not heavily fractured by the injection technique. Therefore, before floor injection, floor blasting is performed in order to create an adequate number of fractures in the floor. Fracturing by blasting also relieves and improves stress conditions in the floor. From this point of view, the behaviour of grouting injection as floor reinforcement material is discussed. That is to say that the results of an injection analysis, an injection experiment and a viscosity measurement to understand the injection process of cement grout into soil voids were studied. The injection analysis was performed by combining two analyses, the analysis of the clogging process, and the analysis of the transportation process. As to the analysis of the transportation process, since a cement-based grouting material can be described as a Bingham fluid, the equation of plug flow related to a Bingham fluid inside a circular pipe, instead of Darcy’s law, was expanded and analysed. The injection experiment was performed under low-pressure on a decomposed granite sample. The injection s experiment was performed using three mixtures with cement/water ratios (C/W) of 1/10, 1/5, and 1/3.5, respectively, to examine the influence of variable cement/water ratios. The viscosity model is proposed from the result measured with the aid of Brookfield viscometer. To calculate yield stress, viscosity data obtained with the Brookfield viscometer was used. However, for the determination of plastic viscosity, because some cement particles will be in the state of aggregation and so on, the viscosity measurements obtained by the use of the Brookfield viscometer are overestimated in comparison with a theoretical equation that assumed that cement particles are dispersed in the solution completely. However, it is thought that cement particles cannot form aggregates easily in the soil voids or cracks. Therefore, the plastic viscosity model of grouting material is expected to follow the proposed theoretical equation. Finally, the assumption that the grouting material was injected in the state of laminar flow was verified by data obtained from the Lugeon test. The field data which are satisfied with this condition is simulated using the plastic viscosity and the yield stress model of grouting material.