Geotechnical Report from June 23, 2001 Peruvian Earthquake
5.0 GEOTECHNICAL ASPECTS OF MINE FACILITY PERFORMANCE
The Quebrada Honda Tailings Dam includes a starter dam constructed of borrow material and is being raised by the downstream method using compacted sand (underflow) from cycloned tailings. The dam is currently approximately 85 meters high and is owned and operated by Southern Peru Copper and located near Toquepala. The dam is designed for an ultimate height of 130 meters and a crest length of 3.9 km. At its ultimate height, the dam will retain approximately 530,000 cubic meters of tailings from the Cuajone and Toquepala mines. Liquefaction in the impounded tailings during the earthquake was reported by mine personnel. Earthquake effects observed at the tailings dam were limited to minor cracks along the upstream dam crest.
Figure 5.1: Crest of dam and cyclones at the Quebrada Honda tailings disposal facility.
Figure 5.2: Crest of dam (left) and tailings impoundment (right) at the Quebrada Honda tailings disposal facility.
Figure 5.3: Aerial view of downstream face of Quebrada Honda dam. Note hydraulic cells used for dewatering of sands during raising near the middle of the picture, and tailings impoundment at the top.
Torata dam is a 130-meter high concrete face rockfill dam (CFRD) that is owned and operated by Southern Peru Copper and located near Cuajone. The dam was completed in 2001 and creates a reservoir with a capacity of approximately 16 million cubic meters. The Torata Dam is part of the Cuajone Mine expansion project. Because no emergency spillway exists, the dam was designed with an extra crest width to allow overtopping across the crest and over the downstream face. This extra width was not compacted in the downstream part of the dam. Earthquake effects at the dam site included minor cracking and joint separation in the concrete face near the left abutment, and densification cracking in the uncompacted portion of the downstream rockfill.
Figure 5.4: Aerial view of Torata Dam.
Figure 5.:5 Crest of dam (left), upstream concrete face (center), reservoir (right), and outlet works (extreme right) at the 130 meter high Torata Dam.
Figure 5.6: Separation of joint in concrete facing along Torata Dam.
Figure 5.7: Cracking along the downstream crest of Torata Dam within uncompacted rockfill.
The fifth lift of a heap leach pad liquefied at Cuajone. The pad is lined with a 100-mil HDPE geomembrane bottom liner. The pad consists of 2-meter thick lifts, each of them underlined by a thin (20-mil) geomembrane liner. The ore consists of *-inch minus material. The fifth lift liquefied during the earthquake and underwent considerable displacement. Another nearby heap leach pad, as well as lined solution ponds, were undamaged.
Figure 5.8: Top of fifth lift at Cuajone heap leach pad.
Figure 5.9: Flow of liquefied ore at Cuajone Mine.
Pad #4 is built on a valley fill and lined with a 60-mil LLDPE geomembrane bottom liner. It is one of the largest (or the largest) copper pads in the world. The pad is approximately 1.8 km long and 1 km wide. The ore is 3/8-inch-minus material. It is being filled in 5-m thick lifts, without interlift liners. At the time of the earthquake the sixth lift had been completed. No damage was observed to the pad, the perimeter liner anchor trench, or the adjacent solution collection ponds. Shortly before the earthquake a stability berm had been completed to enhance stability of the front slope.
Figure 5.10: Aerial view of Cerro Verde Leach Pad #4.
Figure 5.11: Front slope of Cerro Verde Leach Pad #4, approximately 30m high, and collection pipes discharging to collection pond.