Another way of economizing on the amount of concrete used is to build the dam as a series of connected buttresses, like the `right-wing dam’ at ltaipu. To build solid gravity dams at Itaipu, instead of the hollow and buttress dams, half again as much concrete would have been needed.
Existing dams can also be modified to possess qualities of the buttress dams. An example of this modification would be the Butt Valley dam in Plumas county, California. The dam is located within 0.5 km (0.31 mi) of the Butt Valley Fault zone and within 7 km (4.3 mi) of the Lake Almanor Fault. The location of the dam makes it highly susceptible to seismic failure leading engineers to reinforce it with rock fill buttresses.
Another example of applying buttresses would be East River from Astoria to the Bronx, where it was necessary to construct a concrete blanket to resist the upward thrust of water under considerable pressure. There was insufficient room for a blanket that would provide the required resistance due to its weight, so the concrete was laid in the form of an inverted arch.
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