The Escondida Mine is owned and operated by Minera Escondida Limitada which has as its main shareholders BHP-Mining and RTZ. High grade copper ore is mined at 65,000 tonnes per day from an open pit, milled and floated to produce a copper concentrate. The mine is being expanded (1995) to increase production to 115,000 tpd. It is located at an elevation of 3,000 m in the Andes, 150 km southeast of Antofagasta, Chile. Water is a scarce commodity in this high desert area and recovered from a wellfield of limited capacity 45 km from the mine. Tailings disposal was initially discharged into a “salor” (closed drainage area) without need for embankments or containment structures. This ultimate tailings deposit, which will be the largest in the world (3 billion tons), is encroaching on the ultimate pit limits.
Re-development of the tailings impoundment was required to:
- implement a system of water management and conservation which would reduce water losses and therefore water consumption from 1.2 tonnes of water per tonne or ore to 0.7 tons/ton or less;
- develop a stable embankment structure between the tailings deposit and the open pit.
- Dr. Robertson, working for SRK, was appointed to perform the investigations, designs, technical specifications and construction guidance.
- The necessary water management and water savings were achieved by dividing the tailings area into 13 long thin paddies. Sixty meter wide dykes were constructed, using equipment from the mine. Laboratory and field testing were performed to establish the best methods of advancing waste rock over the soft tailings, enabling the dykes to be maintained at low cost. A computer paddy construction was developed to allow the paddy construction sequence to be modelled ahead of the construction, with determination of rates of rise, dyke building volume requirement, water returns and water losses. This model enabled the paddy system to be optimized.
- Detailed designs were prepared for all structural elements including a 120 meter high embankment around the return water pon, and 12 km underdrain to drain the tailings and reduce water pressures in the open pit slopes. Computer analyses were performed for the dynamic stability of the dykes and 120 meter high embankment, seepage analyses to the underdrain and detailed water and construction mass balance through the life of the impoundment.
- A three year operating plan and operating manual was prepared. Site investigations were performed using test piles and diamond drilling to investigate foundation condition in the salor soils, construction material identification and engineering properties determination and specialized field testing to demonstrate the constructability of this impoundment system. Construction specifications were developed and construction supervision provided for the main structural element. On-going construction and optimization inspections are provided regularly.
Minera Escondida Limitada
Tailings Dam Design