Cemented hydraulic fill is made by using a large volume of water to transport the binding agent and aggregate material (sand concentrator tailings, or crushed rock) down into the space to be filled. The method is based on gravity, but pumping can also be used, if necessary. The excess water is filtered through the filling material, filter pipes and dams and is pumped up out of the mine. Cemented, dry waste rock or ore concentrator tailings with optimum water content are used when suitable for this fill method. The compressive strength requirements of the cemented fill depend on the type and the amount of fill loads. A compressive force results from one or several of the following factors: the weight of the fill material; the weight of water on the fill; pore pressure and solid material on top of the fill (e.g. pieces of broken rock). Also, rock above the mine can be supported partly or totally by filling the mine.