III. 



REVETMENT FILTERS 



A filter (Fig. 1) is a transitional layer of gravel, small stone, or 

 fabric between the fine material of an embankment and the revetment armor. 

 The purposes of the filter are to: ( a ) Prevent fine embankment material 

 from being pulled through the armor, (b) distribute the concentrated 

 armor unit weight to prevent settlement, and (c) to provide relief of 

 hydrostatic pressures inside the embankment. An adequate design for most 

 permanent revetments will require a filter. Some common filters and their 

 application are shown in Figure 3. 



TYPE SECTION REMARKS 



Grovel 



Gravel or Quorry 

 Spoils 



Grovel 12 in. to 24 In. thick. Quarry Spoils 



18 in. to 36 in. thick. 



Quorry Spoils 



and 

 Filter Cloth 



-Armor 

 -Quorry Spoils 

 -Filler Cloth 



Filter Cloth is open -weave Monofilament, 

 Polyvinylide Chloride, or Polypropylene. 

 (Borreft, 1966) 



Filter Cloth 



It .^^^Arm 



<^>-<»-- Filter Cl( 



Armor 

 Iter Cloth 



Armor Rock ploced directly on Filter 

 Cloth can tear Clothjhowever.this 

 method has occasionally been successful. 

 (Borreft, 1966) 



Filter Cloth 



""Grovel 

 •Filter Cloth 



Cloth for Filter, Gravel for Hydrostatic 

 Uplift Pressure Relief. 

 (Barrett, 1966) 



Filter Cloth 

 Anchor 



"Blocks 

 -Filter Cloth 

 Sond-Filled Filter 

 Cloth Anchor 



Anchor could be filled with Sand, Gravel, 

 or Rocks. (Cox, 1971) 



Figure 3. Revetment filters. 



The general requirement for a rock or gravel filter for a riprap 

 revetment is: D15 riprap/Dss filter <4, i.e., the diameter of the rip- 

 rap 15-percent size fraction must be equal to or greater than four times 

 the 85-percent size fraction of the filter layer (Ahrens, 1975). D15 

 identifies the grain diameter in a sample-size gradation where 15 percent 

 of the sample is smaller. A standard riprap gradation range is from a 

 maximum weight of 4W 50 to a minimum weight of W 50 /8 (U.S. Army, Corps of 

 Engineers, 1971). The approximate W15 weight of O.4W50 and Wss weight is 

 2W50 • All underlayers of the riprap armor should meet the D15/D85 <4 

 criteria. Additional information on rock filters is found in the SPM, 

 chapter 7. 



