Following a number of “severe convergence events” after employing the top coal caving method at the Broadmeadow coal mine in Queensland, Australia, in 2010, BHP and SCT Operations Pty have put in place a geotechnical monitoring programme to mitigate these issues during the operation of the latest longwall mine.

In a paper titled, Management of Initial Convergence Events at Broadmeadow Mine, R Coutts, K Mills, D Lynch and M Martin; BHP, Moranbah, Queensland, Australia and SCT Operations Pty Ltd, Wollongong, New South Wales, Australia, will discuss these practices at the SME Annual Conference & Expo on Monday.

“The mine introduced a top coal caving (TCC) longwall (LW) face in 2010, but since then experienced a series of severe convergence events soon after starting each panel,” the authors said in a presentation abstract. “During LW8, LW9 and LW10, weighting events occurred at 60-70 m retreat resulting in equipment damage and the longwall almost becoming iron bound.”

The authors said considerable effort was required in LW10 to regain sufficient clearance for operations to resume, with LW11 characterised to have a similar risk of a weighting event to LW10.

The authors said: “This presentation documents the learnings from a geotechnical monitoring programme undertaken during LW11 and details the operational methods used to mitigate these initial convergence events. It also comments on the significance of the top coal caving technique to the convergence events.”

BHP commented in its recently released half-year results to end-December, that coal volumes were up compared with the same period a year ago; a period when Broadmeadow’s output was affected by roof conditions.

Broadmeadow is owned by the BHP Mitsubishi Alliance joint venture, which owned 50:50 by BHP and Mitsubishi.

When the top coal caving method was initially employed at Broadmeadow it was the company’s first mine in Queensland to use it. Employing this method was expected to lift recoveries from around 60% to 90% in the Goonyella Middle Seam and increase production rates, it said in a 2013 briefing.