LUIS VALENZUELA P, RAMON VERDUGO A, JOSE CAMPANA Z & CARMEN G OPAZO A


control of fines content in the sand, and hydraulic transport of the sand with positive displacement pumps. In 1978 this design concept was applied at the Pérez Caldera N°2 tailings dam (135m high)[28] which was fully instrumented, allowing for verification of the high positive behaviour this type of dam could represent. This concept was applied and improved in many ways to the Las Tórtolas dam, built in 1992 (170m estimated maximum height), which has become internationally known as the representative example of what has been recognized as the Chilean sand tailings dam practice[28,41]. Other dams of this type followed: El Chinche (110m), Torito (78m), Quillayes (198m), Ovejería (120m) and El Mauro (237m). Of these dams Las Tórtolas, Ovejería and El Mauro dams are still in operation. None of these dams which have followed the general concept applied at Las Tórtolas have suffered any significant damage during earthquakes.


Basically, the contrast in permeability between the tailings or slimes and the compacted sand is the key factor associated to the Chilean tailings dams, as shown in Figure 42.


Figure 42. Example of flow net in a typical cross-section of a downstream sand tailings dam[38]


The following are the characteristics that explain the good performance of these sand tailings dams, esspecially in such a highly seismic country as Chile:


• Centralized cyclone station to guarantee quality of specified sand, mainly in terms of maximum content of fines (particles below mesh #200 ASTM). • Specification of maximum fines content between 15% and 20% to guarantee ample contrast of permeability with that of the tailings or slimes deposited. • Distribution of sand from the crest of the dam as it grows forming a gently downstream slope to facilitate sand compaction with bulldozers (typically between 3.5 to 4.0:1 (H:V). • Sand compaction to a minimum of 95% Standard Proctor or equivalent to guarantee dilatant behaviour of sand at least for a certain threshold of confining pressure.


226 DAM ENGINEERING Vol XXXI Issue 3


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