SAFETY PERFORMANCE OF DAMS IN CHILE’S HIGHLY SEISMIC ENVIRONMENT


• Generous drains, basal and lateral fingers, capable of instantaneously getting rid of water during construction (operation) stage thus obtaining a low phreatic line at the level of drains. Once operation is complete the drains will have an extraordinarily high capacity of guaranteeing adequate conditions at closure.


• Discharge of tailings and/or slimes over the upstream slope (typically 2:1 (H:V). An impervious geomembrane is placed over the upstream slope in order to avoid the possibility of water reaching the sandy slope although such membrane would be partially destroyed due to the drag downward forces produced by the consolidation of tailings, but at that moment the tailings and not the water will be in contact with the sand body of the dam.


Las Tórtolas Tailings Storage Facility (TSF) is located at an elevation of 700masl, 45km north of Santiago, in Chile’s central valley. In this dam, shown in cross-section in Figure 43, conservative design criteria initially included a double cyclone station to guarantee 10% maximum fines content (FC) in the sand. The design considered the implementation of a network of instruments, including piezometers and accelerometers. This is the first TSF subjected to a dynamic stability and deformational analysis using the finite differences method (DSAG at the time, which later gave birth to FLAC).


Figure 43. Cross-section of Las Tórtolas dam[28]


Satisfactory performance of this TSF, which began operating in 1992, made it possible to increase FC to 15% a few years later and to define a new maximum height of 170m. The current height of this dam is near 120m. Design of this deposit initially set a maximum capacity of 1000 million tons of tailings, but recent estimates increased that capacity to 2000 million tons. The deposit has two other smaller cyclone sand dams, all constructed using the downstream method. In the main dam the starter dam consists of compacted earth 17m high. Approximately 5m of loose alluvial soil was excavated under the starter dam and part of the sand dam, after which dynamic compaction was applied to the foundation. The dam is equipped with generous basal drains that were built in stages. The dam is constructed with cyclone sands compacted to 95% Proctor Standard. The sand was initially deposited forming a slope of 1:4 (V:H). After a few years of operation, and after verification of very satisfactory dam performance confirmed by density controls


Vol XXXI Issue 3 DAM ENGINEERING 227


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