The world’s biggest miner, BHP Billiton, is declaring force majeure at its coal mines in Australia’s Bowen Basin due to prolonged union strikes and heavy rain, the company said on Monday, in a move that could spur prices of metallurgical coal. About 3,500 unionised workers have been staging rolling work stoppages at the mines since mid-2011 and Australia’s eastern state of Queensland, where the mines are located, was lashed by heavy flooding last year and heavy rain this year. BHP Billiton operates seven coal mines in the Bowen Basin that supply about a fifth of the world’s metallurgical coal, or coking coal used in steelmaking.