SAFETY PERFORMANCE OF DAMS IN CHILE’S HIGHLY SEISMIC ENVIRONMENT


Most tailings dams constructed before 1965 were built with the same tailings, in various cases combined with sand, when some tailings classification process was included. The usual construction system was called the upstream method in which the dam grows in the upstream direction, resulting in the dam being formed by tailings or sand tailings (with some compaction generally) founded on already deposited tailings, as shown in Figure 36.


Figure 36. Methods of tailings dam construction[28]


The upstream construction relies on the strength capacity of the tailings for the stability of the deposit. Because of that condition this method requires intense ongoing supervision and control during its entire construction/operation period (longer tailings beaches/small ponds/ phreatic line control, foundation conditions control and others). This construction method has been successfully used for decades in several countries keeping adequate control and generally using flat downstream slopes and well-specified and controlled compaction as the construction proceeds.


This was not the case for most upstream tailings dams in Chile where much steeper


downstream slopes were used, such as 2.0 to 2.5:1 (H:V), with no or little compaction, poor or inefficient cycloning effort, if any, and many times too high dam raising velocities. This type of upstream construction, without adequate design and poor control, is highly vulnerable to failure by liquefaction during strong earthquakes or even by static liquefaction as in the case of Feijao dam 1 in Brazil[32]. Table 3 shows the best-known tailings dams that have failed during earthquakes in Chile[28], all of which correspond to upstream tailings dams. Of these dams only Barahona, El Cobre and Cerro Negro are considered to be large dams, based on their maximum heights. The remaining two are rather small tailings deposit and not proper dams. Figure 37 shows the eight tailings dams that failed during strong earthquakes – independent of their dimensions or condition – according to earthquake magnitude and distance of the dams from the earthquake epicentre. As can be observed in this figure all the failures occurred for earthquakes where M ≥ 7.0, but it is quite probable that other smaller and older dams could have failed in the past from earthquakes with a lower magnitude and have not been registered because of different circumstances, e.g. too distant from populated areas, too small or relatively low impact on environment.


Vol XXXI Issue 3


DAM ENGINEERING


219


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