Floor Stability – A comprehensive investigation into failure mechanisms and controlling factors

Several underground coal mines in Australia have been experiencing excessive floor deformation in roadways, better known as floor heave. Although the phenomenon does not generally have safety implications, unexpected floor heave can cause financial losses due to production delays. The modes of floor failures observed from Australian underground coal mines indicated buckling type failures, however, studies on the coal mine floor, especially the buckling failure mechanism in the floor and relevant analysis techniques, are still insufficient.

Numerical analyses using the Universal Distinct Element Code (UDEC) were conducted to understand the failure mechanisms. In general, a strong floor unit overlying weak floor units was present immediately below the coal seam at the locations of floor heave. The modelling results indicated that the failure of the underlying weak floor unit induced the upward movement, and subsequent deformation and failure of the overlying strong floor unit. The floor displacement decreased as the thickness of the overlying strong unit increased, in areas not affected by geological structural defects such as faulting. This was attributed to the confinement generated by the strong unit. The failure modes simulated by the UDEC Trigon approach were generally in agreement with the field observations.

Based on the underlying failure mechanisms, a new floor classification system, the Coal Mine Floor Rating, is proposed that addresses the buckling type failure. The new floor rating system quantifies the stability of floor strata by considering two main factors: uniaxial compressive strength and discontinuity spacing of the floor units. The Coal Mine Floor Rating is incorporated into an empirical method, the Floor Heave Index, developed by statistical analysis of a database obtained from actual floor failure cases. The floor heave and non-floor heave cases can be determined depending on the Coal Mine Floor Rating and the Horizontal Stress Rating, a proxy that represents the magnitude of horizontal stress.

This project gives detailed insight into the failure mechanisms, with the possible involvement of the underlying floor strata below the strong floor unit. The new floor classification system and the empirical method help assess the potential of significant buckling type floor heave in underground coal mine roadways for new mining projects or future workings.