Re-analysis of Abutment Angle Method for Moderate and Deep Cover Retreat Room and Pillar Mines and Investigation of Loading Mechanics Using Finite Volume Modeling

Mining-induced stresses in underground coal mines play a significant role in pillar and ground support design, hence in the safety of mining operations. In the US, Analysis of Longwall Pillar Stability (ALPS) and in Australia, Analysis of Longwall Tailgate Serviceability (ALTS) software are used for designing Longwall coal mine layouts; and in the US, Analysis of Retreat Mining Pillar Stability (ARMPS) software is used to design retreat room-and-pillar mine layouts. All these software determine the adequacy of the design by comparing the estimated loads to the load-bearing capacity of the pillars and they use the “abutment angle” concept and a square decay stress distribution function to calculate the magnitude and distribution of the mining-induced loads. The abutment angle concept has been successfully applied to US longwall coal mines with the use of ALPS and ALTS in Australia. ARMPS uses the same concept for retreat room and pillar coal mine design in the US. The suggested abutment angle for coal mines in the US was derived as 21° by the back analysis of underground stress measurements from the 1990s and implemented in ALPS and ARMPS. The ALPS methodology was re-examined and calibrated for Australian conditions with additional Australian stress measurements and resulted in the original ALTS methodology which has been continually improved and expanded with additional cases. In this paper, some recent stress measurements are back-analyzed, and the abutment angles are investigated to verify the applicability of using 21° in retreat room and pillar mines with different depths and mining dimensions. For shallow mines, the derivation of the 21° abutment angle is supported by the new case histories. However, at depths greater than 200 m, the abutment angle was found to be decreasing with increasing depth. In this study, a new equation for the calculation of abutment angle for moderate and deep cover cases was constructed and tested for its applicability in retreat room and pillar mines. The differences in the mechanism of complete side abutment loads in shallow and deep cover mines are further analyzed by applying the finite volume modeling (FVM) approach to two case study mines, one shallow, and one deep cover. A 2D model of each mine is created and one-side and two-side abutment loads of consecutive panels are analyzed. Analysis of the deep cover mine indicated that the prior panel gobs provide a considerable amount of support to the overburden strata. These higher gob loads prevent a higher percentage of overburden loads from being transferred to the active panel workings, and this is in agreement with the lower abutment angles observed for deep cover mines. The findings of this study should only be used for retreat room-and-pillar mines’ production pillar loads since these are calculated geometrically using the abutment angle concept.