biological attack, with the possible exception of polyamide (nylonl. 

 Research has shown, however, that bacterial activity in the fabric inter- 

 stices can clog a fabric, reducing its permeability. B.C. Beville, U.S. 

 Department of Agriculture, Soil Conservation Service, Orlando, Florida, in 

 1968 performed the following test (Calhoun, 1972). Two slotted collector 

 pipes were installed in separate trenches. Each was wrapped with a geo- 

 textile filter of different physical type and chemical composition. In a 

 matter of weeks, the nonwoven glass fiber fabric on one pipe became clogged 

 with an iron sludge. The sludge was formed by iron bacteria that oxidized 

 and precipitated iron into the water. There was no sludge buildup on the 

 woven polyvinylidene chloride cloth on the other pipe. 



Other factors discussed in detail below can also have an adverse effect 

 on the performance or physical properties of geotextiles. 



(1) Ultraviolet (UV) Radiation . All synthetics discussed pre- 

 viously, without UV stabilizers, are subject to degradation when exposed to 

 UV radiation. The fabric will be exposed to UV rays during construction. 

 The length of time of exposure will vary with the size of the project and 

 the construction sequence. In a drainage installation, when completed, 

 there is no concern for the effects of UV radiation. However, for certain 

 types of coastal structures, such as revetments, UV resistance must be 

 considered. Continuous or intermitent UV exposure may result from any one 

 or a combination of the following: 



(a) The stone armoring may be relatively thin and rays 

 penetrate to the fabric through voids in the armor; 



(b) the armor may be precast cellular block, cast with a hole 

 through the concrete from the top to the bottom of the block, per- 

 mitting daily exposure to UV rays; 



(c) armoring materials may be rearranged or removed by storm 

 or other occurrence exposing the previously shielded fabric; and 



(d) a construction oversight may have permitted the geo- 

 textile filter to be exposed after the structure's completion. 



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

 requires that the fabric "contain stabilizers and/or inhibitors added to 

 the base plastic if necessary to make the filaments resistant to deteriora- 

 tion due to ultraviolet and/or heat exposure." For most coastal installa- 

 tions the phrase "if necessary" should be eliminated. 



Of the synthetics discussed, in an untreated state (no stabilizers or 

 inhibibitors added to the polymer) polyester has the greatest resistance 

 and polypropylene and polyethylene the least resistance to UV degradation. 

 Steward, Williamson, and Mohney (1977) report that untreated nonwoven 

 polypropylene and polyester fabric samples completely disintegrated within 

 18 months when left exposed in the field. Bell and Hicks (1980) indicate 

 that, when constructed of the same fiber and having an equal amount of UV 

 stabilizers, woven monofilament fabrics would be the most resistant to UV 

 radiation, multifilament woven and the nonwoven fabrics would have inter- 

 mediate resistance, and slit-film woven would be the least resistant. 



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