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<br />was specified as one-half that required tor dumped stone (4, p. 857). <br />The supposed superiority of hand-placed riprap was refuted-by a perform- <br />ance survey of the majority of the large earth dams in the United States <br />conducted by the Corps of Engineers in 1946. The survey showed that hand- <br />placed riprap was not as satistactory as an equivalent thickness of dumped <br />riprap. The percentage of failures in hand-placed riprap slope protection <br />was six times that of dumped riprap, and the percentage of failures of con- <br />crete pavement used for slope protection was slightly over seven times that <br />of dumped riprap (1, p. 204). <br /> <br />The Corps of Engineers opinion, as stated by Middlebrooks (j, p. 713), <br />is: "Hand-placed riprap is not as satisfactory as an equivalent thickness <br />of dumped riprap, and a filter layer underneath all riprap is essentia1. <br /> <br />Hand-placed riprap requires a much firmer support tram the bank being <br />protected tban does dumped riprap because it does not have the strength to <br />resist nor the capability to adjust to movement of the supporting material. <br />Hand-placed riprap is particularly susceptible to damage from ice floating <br />in the stream. <br /> <br />Except for method of placing and greater emphasis on firm support and <br />protection of blanket edges, the discussion under dumped riprap applies to <br />hand-placed riprap. <br /> <br />2.3 - Design at Wire-Enclosed Riprap <br /> <br />The use of wire-enclosed riprap is generally restricted to locations <br />where the only rock economically available is too small for dumped riprap. <br />The design of wire-enclosed riprap is somewhat arbitrary, being dependent <br />upon the size of rock available. The mesh size of the wire is also depen- <br />dent upon the size of rock used for riprap. Wire-enclosed riprap has been <br />used in some instances as toe protection for other types ot riprap. This <br />type of protection is flexible to an extent, but the protection is limited <br />to the lite of the wire used tor enclosing the stone. California (6, <br />p. 144) has found that wire-enclosed riprap does not work well on cUrves <br />where displacement might require a lengthening or shortening of the <br />protection. <br /> <br />The wire baskets are first formed and then filled with stone. The <br />baskets are tied together to torm a mattress and anchored to the slope. <br />For light exposure, a continuous blanket of small stones retained <br />between top and bottom spreads of wire tencing might suffice. On all <br />designs the blanket should be divided into compartments so that one <br />compartment can fail without losing all of the blanket. <br /> <br />Baskets 4 or 5 feet square are a convenient size, although larger <br />sizes might be used. The dimensions of cOllllBrcial fencing available might <br />govern the dimensions of the baskets in order to minimize cutting of the <br />wire in tabricating the baskets. Mattresses lJlB.y be placed with the long <br />dimension either traverse to the slope or parallel to the slope. The <br />practice of the California Division of Highways is given on pages 143 to <br />151 at reference 6. <br /> <br />11-10 <br /> <br />e <br /> <br />. <br /> <br />. <br /> <br />e <br /> <br />. I <br />~ I <br />I <br />I <br />el <br />I <br />