SAFETY PERFORMANCE OF DAMS IN CHILE’S HIGHLY SEISMIC ENVIRONMENT
chimney, drains and filter and transition zones on the downstream side of the core. The downstream shoulder is built of compacted earthfill, borrowed from alluvial fans. Near the toe a secondary drainage system collects seepage from the underlying aquifer and paleo stream in the dam foundation. The dam responded elastically to the Valparaiso earthquake, as post-earthquake surveys indicated no measurable crest settlements. No cracking nor other disturbances of the dam slopes were reported[20]. A dynamic back analysis of the dam as it was in 1985, considering the dam loaded with the seismic registers of the accelerometers installed in the dam, allowed for calibration of the numerical model used to estimate the dynamic behaviour of the final dam for the design earthquake, which provided relatively small deformations for the final conditions. In February 2010 the dam, with its full height, was shocked by the Maule earthquake but did not present any measurables settlements or deformations[40].
6.2.2 Rockfill dams retaining tailings
There are two main rockfill tailings dams in Chile, both using mining waste rockfill, i.e. Pampa Pabellón dam and Candelaria dam.
Pampa Pabellón dam was initially planned for a maximum height of 90m and is still in operation. The compaction of rockfill has occurred naturally from the passing of high tonnage mining trucks and bulldozers that work on the distribution of rock dumped by the trucks. A transition (filter) between the rockfill and the tailings is obtained from borrows near the dam (gravelly soil).
Candelaria dam is also a rockfill dam built with waste rock from the nearby open pit mine. Construction started on the dam in 1995 and was completed in 2017 with a height of 171m∗. It was designed as a dam without a berm supporting it, but it was known that a large waste rock dump would be formed immediately downstream of the dam. The rockfill was compacted by the passing of mining trucks and D-8 bulldozers with a zone of about 10m width near the upstream slope in layers of 2m thickness, and in the downstream direction a zone with layers of 4m thickness and towards downstream zones of 8m thickness and 16m thickness with no compaction. The dam was founded partially over a wide alluvial channel with a generous drain built inside and no cut-off. About 1km downstream of the dam the alluvial channel was much narrower, and a deep clay cut-off approximately 27m deep was built. The seepage water interrupted by the cut-off was then deviated to a tunnel towards a shaft where the water (over 200l/sec) was pumped back to the processing plant. For many years the Candelaria mine held the record for the mine with the lowest freshwater consumption.
6.2.3 Sand tailings dams
After the collapse of El Cobre dam in 1965 a new dam was built in the mine in 1969. El Cobre N°4 dam (68m high) was constructed using the downstream method with compacted sand obtained for the first time from tailings in a centralized cyclone station with strict
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