Prediction of hollows in abandoned underground workings at shallow depth

Composite statistical analysis of the lithological composition of the rock mass above underground workings in coal seams and experimental work (laboratory mechanical tests and boring for the determination of hollows) in Donetsk city (Ukraine) have been used to develop a method for predicting the presence of hollows. The empirical criteria K₁ - K₂ > 0 and K₃ ≤ 2.5 which relate to the physical characteristics of the overlying strata were found to predict the existence of such hollows in abandoned underground workings at shallow depth. The values of K₁ - K₂ and K₃ depend on the thicknesses of the different rock layers and the uniaxial compressive strength of the immediate roof over underground openings. The different layers e.g. sandstones, argillites, aleurolites and alluvium in the rock mass are shown to influence the existence of hollows in abandoned workings. Large thicknesses of sandstones in the rock mass or relatively high uniaxial compressive strength rock in the immediate roof contribute to the existence of hollows in abandoned workings. On the other hand, sandstones of small thickness, weak alluvium, argillites and aleurolites only give additional weight on immediate roof of the opening and allow collapse of the rock mass. The method of prediction for hollows was developed from the back-analysis of data from 41 boreholes which were drilled from the ground surface above underground openings.