Mining giant BHP Billiton says it will defend itself against a class action lawsuit in Australia over Brazil's 2015 dam disaster...More than 3,000 investors have signed up to the lawsuit, lodged in the Federal Court of Australia in May.  The claim alleges that BHP failed to disclose the risk of the dam's failure to the stock market, and misled investors over the company's safety guarantees...BHP is also facing a US shareholder class action over the disaster, as well as ongoing criminal and civil cases in Brazil...