Mining, preparation and consumption of coal produce large amounts of waste, of which the coal washery rejects account for a major part. Currently emplacement of coal washery waste not only requires large land, but also pollutes the air, soils and underground water. A number of methods have been developed to make use of coal wastes to fill the voids and strata gaps formed from coal extraction, including dry material backfill, paste backfill and overburden slurry injection. Without the need of underground transport system and interference with coal production, the overburden injection technology is considered a cost-effective method in which the coal washery slurry is injected from the surface down to the caved zone of the longwall goaf and fill the voids. In order to understand the mechanism and behaviour of the grout flowing in the caved zone, laboratory experimental and numerical studies were conducted. The laboratory experiment visually simulated the process of coal washery flowing in the caved zone. The process was also numerically simulated by developing a Computational Fluid Dynamics (CFD) model. These studies provide better understanding of the injection and flow mechanism of the grout in the broken medium. The agreement between the experimental and numerical models indicates that the CFD model is able to simulate the complicated flow and can be used to optimise the injection system design and operation parameters.