Dangerously defective Chinese steel, incompetent engineering and poor procurement have been cited by multiple former workers on Rio Tinto’s troubled Mongolian copper project as contributing to cost and schedule blowouts that workers say were understood by Rio management months before investors were told.

The explosive testimony from 12 individuals who worked for Rio or its contractors on the $US6.75 billion Oyu Tolgoi project comes as Rio’s new copper boss Bold Baatar holds crucial talks with the Mongolian government over the future of Rio’s biggest and most important growth project.