Document Type

Conference Paper

Publication Date


Publication Details

Kenneth William Mills, A review of the mechanics of pillar behaviour, in Naj Aziz and Bob Kininmonth (eds.), Proceedings of the 2019 Coal Operators Conference, Mining Engineering, University of Wollongong, 18-20 February 2019, 42-57.


In recent years, the drive to reduce the impacts of surface subsidence has led to mine layout designs that rely for their effectiveness on the long-term stability of pillar systems. This paper reviews the mechanics of coal strength behaviour inferred from laboratory testing of coal specimens as a context to better understand the appropriateness of different pillar design strategies. Laboratory testing of coal specimens to very high confining pressures (163 MPa) illustrates the independence of the two fundamental components of coal strength: cohesive strength and frictional strength. Testing of numerous coal samples from the same coal seam and coal samples from different coal seams illustrate the variability of cohesive strength. The significant influence of frictional strength when confining pressure is available is also apparent. These two fundamental components of coal strength combine to influence the range of pillar behaviours observed in practice. This paper explores the characteristics of these two components and their implications for the application of various pillar design approaches.