13. Determination of Percent of Qpen Area (PQA) 



Each of five unaged samples should be placed separately in a 2 by 2- 

 inch glass slide holder and the image projected with a slide projector on 

 screen. Select a block of 25 openings near the center of the image and 

 measure to the nearest 25.4 micrometers (0.001 inch) the length and width 

 of each of the 25 openings and the widths of two fibers adjacent to each 

 opening. The percent open area is determined by dividing the sum of the 

 open areas of the 25 openings by the sum of the total area of the 25 

 openings and their adjacent fibers. 



14. Determination of Gradient Ratio (GR) 



A constant head permeability test shall be performed in a permeameter 

 cylinder on soil specimens representative in classification and density of 

 those materials to be protected, and in accordance with EM 1110-2-1906, 

 Appendix VII, with the following modifications: 



(1) A piece of hardware cloth with 0.64-centimeter (0.25 inch) openings 

 shall be placed beneath the filter fabric specimen to support it. The 

 fabric and the hardware cloth shall be clamped between flanges so that no 

 soil or water can pass around the edges of the cloth. 



(2) The soil specimen shall have a length of 10.16 centimeters (4 

 inches). Piezometer taps shall be placed 2.54 centimeters below the fabric, 

 and 2.54, 5.08, and 7.62 centimeters above the fabric. 



(3) Tapwater shall be permeated through the specimen under a constant 

 head loss for a continuous period of 24 hours. The tailwater level shall be 

 above the top of the soil specimen. The gradient ratio shall be determined 

 from the readings taken at the end of the 24-hour period. 



(4) The gradient ratio is the ratio of the hydraulic gradient over the 

 fabric and the 1 inch of soil immediately next to the fabric (ii), to the 

 hydraulic gradient over the 2 inches of soil between 1 and 3 inches above 

 the fabric {±2) • 



i-1 



GR = t- 

 12 



415 



