The current 1977 CE Guide Specification (U.S. Army, Corps of Engineers, 

 1977) introduced the gradient ratio (GR) based on Calhoun's original work. 

 Determination of gradient ratio is set forth in Appendix C. 



(b) Forest Service Criteria . The U.S. Department of Agriculture 

 Forest Service (USDAFS) has conducted numerous laboratory and field tests, 

 including an evaluation of geotextile filter performance in various types of 

 structures. Their criteria for piping resistance (soil retention) and 

 clogging are similar to Calhoun's, as discussed above and stated in Appendix 

 C, the EOS-POA combined criteria. The service does have some disagreement 

 with the current 1977 CE Guide Specification (U.S. Army, Corps of Engineers, 

 1977) as discussed by Steward, Williamson, and Mohney (1977). They feel that 

 the currently recommended GR test should be modified to represent the range 

 of varying seepage rates and fabric strains accompanying the enlargement of 

 openings in nonwoven fabrics due to stretching anticipated in the field, and 

 that intermittent flow should be added. They suggest that the GR test has 

 not been confirmed by monitoring field performance. Concern is also ex- 

 pressed that due to higher elongation, EOS of nonwoven fabrics will be more 

 variable and more subject to change under load than that of woven fabrics. 



For all critical and severe filter applications the U.S. Forest Service 

 indicates that only woven geotextiles should be used (Steward, Williamson, 

 and Mohney, 1977). The USDAFS definitions of these terms are quoted below: 



"Critical : Projects where failure of the filter could result in 

 failure of an expensive or environmentally sensitive part of a 

 project, such as: 



(1) rock blankets greater than or equal to a 3-foot-horizontal 

 thickness, 



(.2) retaining structures, 



(3) road fills greater than 10 feet in height, 



(4) underdrain trenches greater than 5 feet in depth, and 



(.5) bridge repair. 



Severe : Conditions of moderate to high seepage out of erodible 

 soils with a hydraulic gradient evident moving from soil toward 

 the filter, such as: 



(1) spring areas, 



(2) soils with flowing ground water, and 



(3) soils with high internal hydrostatic pressure." 



Both the above definitions seem applicable to most coastal applications. 

 Calhoun (1972) also concluded that only woven fabrics be used in coastal 

 projects. The Forest Service indicates a preference for the woven filters 

 and that the sometimes "lower material cost of the lightweight non-woven 

 fabrics for critical or severe seepage conditions appear to be outweighed by 



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