ABSTRACT
Compaction of weak strata and coal pillars, recompaction of gob rock over adjacent previously mined out panels, and separation of beds in overburden strata are some typical void sources. In order to quantify a void source, the concept of void source intensity is introduced. It means the void volume created or absorbed in an unit volume during the considered period. With the increase in height above void source, the void passing range in the horizontal plane gradually expands outward, which basically determines the distribution of surface subsidence, this process is referred to as “void diffusion”. Density of void diffusion is defined as the void volume passing through any unit area on a vertical plane during a considered period. The term convection is borrowed from general diffusion problem such as heat transfer. In void diffusion problem, it actually means the shift of diffusion center with the increase in height above void source.
