For years, the Polish hard coal mining has been struggling with the problem of fire hazards in
areas with coal residue, mainly in goafs. Currently, a common method of limiting this hazard is the
fire prevention involving use of fine-grained hydromixtures based on power generation waste, mainly
fly ashes. The hydromixture is introduced into the caving zone created by the advancement of exploitation
face and its task is to fill in voids, limiting the possibility of access to the mine air oxygen to
a minimum. The first part of the article presents theoretical fundamentals of determining the parameters
of gravitational hydraulic transport of water and ash hydromixtures used in the mining pipeline
systems. Each hydromixture produced based on fine-grained wastes is characterized by specified
rheological parameters that have a direct impact on the future flow parameters of a given pipeline
system. Additionally, the gravitational character of the hydraulic transport generates certain limitations
concerning the so-called correct hydraulic profile of the system in relation to the applied hydromixture
characterized by required rheological parameters that should ensure safe flow at a correct
efficiency. This paper shows an example of optimisation of the composition of a selected fly ash-water
hydromixture in relation to its capacity for hydrotransport in gravity pipeline installations, as well as
the amount of excess water that will always drain from the location of feeding the hydromixture to
the underground workings.