Volume 6, Issue 2 (June 1983)
Equivalent Opening Size of Geotextiles
A rapid increase in the use of geotextiles in civil engineering has highlighted the necessity to identify the geotextile's properties associated with different applications. One property of importance in filtration and separation is the nominal equivalent opening size (EOS). The EOS is generally defined as equivalent to the smallest spherical particles of which 5% by weight fails to pass through the geotextile after 20 min or other specified times of sieving. Unfortunately, data show that for thick nonwoven geotextiles in particular and to a lesser extent for other geotextiles experimental variations can be substantial unless careful procedural techniques are followed including correct positioning of specimen, suspension of specimen, rigorous production and maintenance of silica sand fractions, and minimizing of humidity effects. In certain applications, one of which is presented in some detail, the use of a 95% retention definition is shown to be a more useful practical definition and even this may be less than in situ performance EOS values (that is, the particle sizes retained in situ by the geotextile). In such cases a clear understanding of geotextile behavior and a reporting of test results over a complete testing range of 95% particle retention to 95% particle passing may be of considerable value in the proper selection of a geotextile. Field performance data are presented on the importance of EOS to railway track rehabilitation where lower than normal manufactured values of EOS are considered preferable.