24 - Design of Coal Pillar Arrays and Chain Pillars

Abstract

This chapter focuses on design of coal pillar arrays and chain pillars. Pillar formation in underground coal mining is either done as a requirement of the method of extraction itself, that is, in bord and pillar mining, or for fulfilling various functions such as panel isolation, protection of roadways, shafts or surface features, protection of current mine workings from dangers of water inundation, and as a guard against roof collapses in the face area while depillaring. Pillars for panel isolation, commonly termed as chain pillars, and those for protection against the goaf in depillaring, called ribs, are generally required for temporary stability and can have lower safety factors than the remaining pillar types, which must be permanent in nature. Design of pillar arrays is commonly required under shallow covers when subsidence at the surface or at an upper coal seam cannot be induced within the permissible limits specified. As in any design of structures, the pillar design procedure essentially consists of estimating the pillar strength and the load on pillars and linking the two through a proper safety factor. The following factors influence pillar strength: (1) uniaxial and triaxial coal strength, (2) width to height ratio of pillar, (3) pillar size or volume, (4) shape in plan, (5) pre-excavation horizontal stresses, (6) end conditions or conditions at the roof-pillar and/or floor-pillar contact and also presence of bands in the seam, (7) water and weathering underground, and (8) method of road drivage such as with road headers or with blasting.