Influence of physical characteristics of weak rock mass on height of caved zone over abandoned subsurface coal mines

This is a study of the height of caved zones over abandoned subsurface coal mines in Donetsk, Ukraine. The heights of the caved zones in porous weak rock mass over shallow, abandoned underground workings (up to 80 m) were detected by drilling and linked with laboratory measurements of the physical characteristics of the rock mass overlying the underground openings. An empirical model is developed to determine the height of a caved zone in weak porous rock mass over shallow, underground workings. The model shows that the decrease in uniaxial compressive strength of the immediate roof and the increase in average porosity of rock layers over the immediate roof lead to an increase in the ratio between the height of caved zones and the height of underground coal workings. It is suggested that the bulking of the weak immediate roof is insufficient to arrest caving over underground workings and therefore the significance of the rock mass overlying the immediate roof in formation of caved zones is increased. It is argued that the caving height in weak porous rocks may reach 4.1-11.25 times the height of underground coal extraction.