the risk and consequence of possible failure at this time". They also state 

 that in similar installations, graded aggregate filters have a 50 percent 

 chance of functioning properly, while woven geotextile filters have a rate 

 near 100 percent. 



(c) Combined Criteria . In order to develop a laboratory and 

 field performance verified geotextile selection criteria for filtration- 

 clogging properties, the authors of this section relied on the experience of 

 knowledgeable users, their own personal experience, and a combination of 

 parts of reports citing criteria relevant to coastal applications by Calhoun 

 (1972), Steward, Williamson, and Mohney (1977), and CE 1977 Guide Specifi- 

 cation (U.S. Army, Corps of Engineers, 1977). 



To achieve desired clogging resistance, woven geotextile filters adjacent 

 to soils containing 50 percent or less particles by weight passing through a 

 U.S. No. 200 sieve, should have an effective POA equal to or greater than 4.0 

 percent. (When overlayed with stone, the POA of the geotextile is the effec- 

 tive percent open area. If half of the geotextile is covered by flat based 

 concrete blocks without a gravel layer between the fabric and the blocks, a 

 POA equal to or greater than 8.0 percent is required to yield an effective 

 POA equal to or greater than 4.0 percent.) 



Nonwoven geotextiles in the same application should have a gradient ratio 

 equal to or less than 3.0. This same gradient ratio is used as the criteria 

 for selection of all geotextiles adjacent to soils with more than 50 percent 

 particles by weight passing the U.S. No. 200 sieve, or soils with a very 

 slight gradation curve or those that are skip-graded (gap- graded) . Geotex- 

 tiles with the largest possible POA available in the required EOS sieve 

 number should be specified. 



As stated previously, ASTM Subcommittee D-18.19/D-13.61 is developing 

 test methods for geotextiles. When these, or other, test determination 

 methods and formulas are submitted to the specifier, they should be 

 evaluated to determine if their results meet the requirements discussed 

 above. It is recommended that both sets of tests be conducted and the 

 results correlated to the requirements stated in this report. 



d. Placement . The geotextile filter must be laid loosely, not in a 

 stretched condition but free of wrinkles, creases, and folds for all 

 applications on slopes and beneath jetties. When the slope continues above 

 and beyond the structure, the filter should be keyed-in by being placed in 

 a trench at the upper terminus of the structure. When a gravel layer is 

 placed on the geotextile, it must have sufficient permeability so that it 

 does not reduce the flow from the filter. The largest size sheets available 

 should be used to reduce the number of overlaps required. Overlaps of 

 adjoining sheets should be a minimum of 46 centimeters (18 inches) and 

 staggered for installations in the dry. For underwater applications, the 

 overlaps should be 1 meter (3 feet) . Strict inspection and enforcement is 

 required with respect to drop height limitations and overlaps. 



On slopes, construction begins at the toe and then proceeds up the 

 slope. Horizontal underwater placement (such as groins, jetties, and scour 



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