Review of Barrier Pillar, Bleeder, Chain Pillars in Weak Strata and Thick Coal

Prior to the commencement of this project, a number of mine sites had experienced adverse conditions in roadways in the vicinity of goafs.  These adverse conditions could not be easily explained with the understanding of strata behaviour at that time.

The strata deformation at these sites appeared to be a result of strata failure typical of elevated horizontal stresses but was occurring in areas where conventional understanding of stress distributions would not indicate elevated horizontal stresses but rather a reduction. This tended to occur in thick coal seams having coal roof and in sections having weak and relatively soft roof strata. In these sections, the lateral stress associated with vertical loading in the abutment zone overcomes that associated with stress relief into the goaf.

As a result of the investigations carried out there is a range of conditions where traditional pillar design and empirical roadway serviceability criteria can be used to determine the appropriate position of roadways that will come under the influence of an adjacent goaf.  However, it has been found that there are a range of lithological and stress conditions that will initiate failure mechanisms in the strata that may severely impact on the stability of adjacent roadways.  The conditions required to initiate the failure mechanism are site specific and make empirical techniques inappropriate.

It is common for vertical loading associated with the abutment close to the goaf edge to fracture the coal ribs (and roof above the ribside yield zone). This causes a dilation of the material and a resultant displacement (pillar squeeze) toward the roadway on the pillar ribside. This causes a compression across the roadway roof.  This situation is exacerbated by a failure mechanism found to commonly occur whereby weak bedding planes in the roof will allow large scale slip of the strata above toward the goaf.

When these two situations occur together, the regional slip toward the goaf occurs on the “solid side of the roadway” and the pillar squeeze acts in an opposed direction on the pillar side of the roadway.  In this situation the roof is squeezed and typically deforms significantly.  This is common about tailgates and also travelling roadways where a combination of high abutment load and weak geological conditions are combined.  The lateral slip movements often shear cable bolts and it is usually more secure to support the roadways with standing support.