filters adjacent to coarse-grained soils containing 50 percent or less 

 particles by weight passing U.S. No. 200 sieve, the piping resistance is 

 calculated using: 



85 of protected soil £ 1 

 EOS 



where Dgs is the effective grain size in millimeters for which 85 percent 

 of the sample by weight has smaller grains. For geotextile filters adjacent 

 to fine-grained soils containing more than 50 percent particles by weight 

 passing U.S. No. 200 sieve, the EOS should be no larger than a U.S. No. 70 

 sieve. Fabric with the largest possible EOS should be specified to promote 

 drainage and reduce the likelihood of clogging. Geotextiles with an EOS 

 smaller than the U.S. No. 100 sieve should not be used as filters. 



When the protected soil contains particles ranging from a 2.54-centimeter 

 (1 inch) size to those passing the U.S. No. 200 sieve, only the gradation of 

 soil passing a U.S. No. 4 sieve should be used in selecting the fabric. 

 Whenever the protected soil is so sized or graded that a fabric cannot 

 satisfy the above requirements and the soil is to be protected with a multi- 

 layered granular filter, a geotextile filter will often satisfy the require- 

 ments of all but the filter layer immediately adjacent to the protected soil 

 (primary filter layer) . 



There are additional restrictions regarding the percent of open area 

 (POA) of the geotextile which must be considered in applying the piping 

 resistance formula. The POA determination method is described in Appendix 

 G. These criteria for determining piping resistance have been widely and 

 successfully used by the engineering profession. 



The original EOS determination method developed by Calhoun (1972) was 

 based on the sieving of rounded to subrounded sands. Geotextiles rated by 

 this method were used in the filtration and clogging tests which resulted 

 in the piping criteria formula stated above. The 1977 CE Civil Works 

 Construction Guide Specification CW-02215 (U.S. Army, Corps of Engineers, 

 1977) modified the original EOS determination method by substituting glass 

 beads for sand. The 1977 Guide Specification EOS determination method is 

 described in Appendix C. Many geotechnical engineers and soil testing 

 laboratories who had experience with the sand determination method prior to 

 1977 indicate the sand behavior is more typical of the material to be 

 protected. Most often stated objections to beads are: 



(a) they develop static electricity; 



(b) there is a size problem: many are not "true" when received 

 from the manufacturer; 



(c) a continuous breakdown of the beads occurs during sieving; 



(d) different results are obtained for the same geotextile 

 when beads are used compared to the results when sand is employed: 

 generally beads yield a larger apparent opening value (smaller 

 sieve number) than sand (i.e., beads - EOS equals No. 50 sieve vs. 



294 



