Hidden Valley Mine

The Hidden Valley mine is a gold/silver mine in Papua New Guinea (PNG) with the first large engineered tailings storage facility (TSF) since large-scale mining began in the country over 80 years ago. The TSF is comprised of two large earth and rock fill dams, raised by the downstream method. The dams are constructed of pit waste and therefore have the dual function of storing tailings and waste rock. The TSF has been in operation since August 2009.

KCB staff involved with this project combined their previous experiences working in the PNG environment and dam building from other locations. This process led to close interaction between the mine operators, designers and construction teams.

 

“I like KCB because they provide an independent opinion and keep Harmony honest down at the TSF.”

-Charles de Villiers, EGM Operations, Harmony SE Asia

Challenges

  • In PNG, mine tailings were typically discharged into local river systems or directly into the ocean, rather than contained in impoundments, as high rainfall and active seismic conditions were considered unsuitable for tailings dam construction
  • High rainfall, seismic activity, weak erosive soils, material availability, dense vegetation and remoteness of the site provide constant challenges to construction and operation
  • Changing conditions exposed during embankment construction requires ongoing modifications to TSF design
  • Different material types for the various internal embankment zones can only be placed under certain weather conditions, which can be impacted by resource availability (equipment and personnel) and the requirement to mine ore from the borrow area

Solutions

  • With improvements in tailings dam design techniques and site investigation methods, the tailings dam at Hidden Valley was designed to withstand weak tropical soils and weathered rock in seismically active zone
  • KCB also designed large waste rock dumps to manage sediment loads and acid-generation from mine waste during routine mining activities
  • Observational approach to construction was recommended by the designers and adopted by the mine operator, allowing changes to the design during construction
  • Different construction techniques used to manage high rainfall, including:
    • placing fine-grained materials in dry conditions only, and rock in dry or wet conditions
    • lower placement rates during wet periods
    • parts of dam closed to vehicular traffic during wet periods to avoid damage
    • exaggerated cross-fall on dam crest to manage surface water

Successes

  • Design and construction of Hidden Valley tailings storage facility is culmination of over 30 years of work by KCB in PNG, and marks historic achievement in socially and environmentally sustainable mining practices in PNG
  • Local villagers continue to be employed and trained by the mine owner, Harmony Gold, with assistance from KCB
  • Regular design updates are provided to Harmony and subjected to annual external review by third party consultants to meet current best practice
  • The practical challenges are managed to the best of the earthworks crew’s capabilities and requires experience personnel to manage the ever-changing conditions on a daily basis. Short and medium-term planning needs to be flexible and construction under these challenging conditions continues to enjoy moderate success.

Paper:

Rynhoud, M.S., D. Johns and L. Murray. 2017. “The Hamata Tailings Dam Design and Construction at the Hidden Valley Mine in Papua New Guinea,” in ANCOLD 2017 Conference, October 26-27, 2017. Hobart, Tasmania.