Document Type

Conference Paper

Publication Date


Publication Details

This conference paper was originally published as Frith, R, Recovering from Major Roof Cavities on the Longwall Face – A Current Perspective, in Aziz, N (ed), Coal 2006: Coal Operators' Conference, University of Wollongong & the Australasian Institute of Mining and Metallurgy, 2006, 52-63.


Recovering from roof cavities on the longwall face is an endemic aspect of longwall mining and this is especially the case today as longwall faces are wider and higher than they have ever been, yet powered support ratings are effectively technology constrained. There is little doubt that the ever-increasing dimensions of longwall faces are consequently increasing the likelihood of major roof falls occurring, especially in those more challenging geotechnical environments containing poor immediate roof conditions at depths of cover greater than 250 m. As a result, the efficiency and safety of longwall roof cavity recoveries is being given increased attention and is more relevant than ever to the success of the coal industry. This has led to the now almost universal use of cavity fills during the recovery of major roof falls on the longwall face, albeit that there is industry discussion regarding what constitutes ideal properties for cavity fill material. The paper discusses the geotechnical reasons why it is believed that major roof falls on longwall faces are becoming more likely with time and defines a conceptual geotechnical model for the cavity recovery process and the inherent ground control problems involved. Furthermore the paper considers the needs of mine operators during the cavity recovery process and how these can be best achieved. Specifically the paper contrasts foaming cements and phenolic foams as the two main generic types of cavity fill and ranks them according to such parameters as strength, foaming properties, rate of cavity fill, material cost, effectiveness, safety and overall cost effectiveness. The paper concludes that on a holistic basis, phenolic foams are the more suitable means of cavity filling on the longwall face, accepting that foaming cements are also an effective medium. However in the final analysis neither type of cavity fill is cost effective when compared to the benefits of preventing such cavities occurring in the first place through layout design, equipment maintenance and good operating practices and this is the major point of the paper.