sand - EOS equals No. 70 sieve), on some occasions the discrepancy 

 is reversed. (B.R. Christopher, P.E., Corporate Laboratory 

 Director, STS Consultants Ltd. (formerly Soil Testing Services, 

 Inc.), Northbrook, Illinois, personal communication, 1979-82). 



Other sources have stated similar experiences and indicate inconsistent 

 results are obtained when sand is replaced by beads. 



Geotextile filter selection criteria for piping resistance are the 

 same for all applications in coastal structures. The criteria are based on 

 the work of Calhoun (1972); the U.S. Forest Service criteria; experience 

 with laboratory testing, field experiments, installations and monitoring as 

 presented by Steward, Williamson and Mahoney (1977); and the authors' (of 

 this section) personal experience, performance records, and communication 

 with users and researchers. Geotextile filters meeting the Calhoun EOS 

 determination criteria have had field verification (service records) for 

 more than 20 years. 



(2) Clogging Criteria . In shore protection structures (such as 

 revetments), geotextile filters may be exposed to severe static and dynamic 

 loading, turbulent flows, rapid fluctuations, high-pressure differentials, 

 and sudden or regular drawdowns. Designers primarily concerned with sub- 

 surface drainage must recognize the necessity for more stringent property 

 and performance criteria for fabrics being considered for this environment. 

 It is especially true with respect to the filtration and clogging performance, 

 because, if the filter clogs, it could cause a more severe problem than if 

 it had been omitted. Usually underdrains have low rates of flow and 

 relatively low hydraulic gradients. Due to the large-grained sand present 

 in many coastal areas, filters adjacent to French and trench drains and 

 surrounding collector pipes are often exposed to higher flow rates than 

 would normally be expected in these applications. It is the responsibility 

 of the designer to specify a geotextile filter that retains the soil being 

 protected, yet will have openings large enough to permit drainage and 

 prevent clogging. 



Many fabric suppliers provide fabric permeability and waterflow rate at 

 a specified head as fabric performance criteria; however, these data are of 

 little use in establishing filter-clogging criteria. 



(a) Corps of Engineers Criteria . Calhoun (1972) developed 

 the most widely used filtration-clogging geotextile filter criteria in 1972 

 after an extensive 3-year research effort. While the criteria were rather 

 simple, they were based on numerous hydraulic-soil-fabric filtration and 

 clogging tests. Using his method described in Appendix C, Calhoun de- 

 termined the EOS of the fabric and in addition determined the POA. 



As mentioned in the discussion of piping resistance, with certain 

 restrictions for fine-grained soils, Calhoun's criteria allowed the ratio 

 of the soil's Dss to the fabric's EOS to be equal to or greater than 1. 

 The criteria also had an added limitation that no woven fabric should have 

 a POA less than 4 percent nor EOS with openings smaller than U.S. No. 100 

 Standard Sieve. Calhoun established that the larger the POA, the less the 

 fabric was susceptible to clogging. 



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