Today, the normal design of Concrete Face Rockfill Dams (CFRD), over deep alluvium,is to provide an upstream concrete cut-off wall down to rock foundation or to a sufficientdepth so that leakage under the cut-off wall is not economically important, and providingthat the resulting vertical hydraulic gradient is minor.In Chile, in 1995 construction was completed on the Santa Juana CFRD, 105 metershigh over 35 meters of alluvium with a concrete cut-off wall to the rock, in 1999,construction was completed on the Puclaro CFRD, 85 meters high, and design wascompleted for the Illapel CFRD, 55 meters high. Puclaro has a partial cut-off, 60 metersdeep, and Illapel needs a partial cut-off 40 meters deep.For these three dams, the cost of the cutoff walls as a percentage of the total cost of thedams is 4%, 37% and 39 %, respectively. This leads to the conclusion that, for small andmedium height dams, cut-off wall costs may be excessively expensive.For earthfill dams on pervious foundations, upstream impervious blanket are often usedto advantage and usually result in major project economies relative to the alternative ofproviding a vertical cut-off. It has been found that the cost of one lineal meter of aconcrete cut-off wall is 30 to 40 times the cost of a lineal meter of blanket. However, theefficiency of one meter of cut-off is only 6 to 10 times that of one meter of an upstreamblanket, so with an equivalent upstream blanket it is possible to get similar results at 15to 33 percent of the cost of a cut-off wall.In this paper are presented leakage characteristics and a cost comparison of a partialcut-off, versus two alternatives of an upstream blanket solution proposed for the Illapeldam.
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