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corrected to a viscosity at 20 o C (Suits, 1986). Standard Test Method D 4491 may be
conducted with either a constant head or a falling head (ASTM, 1987c).
ASTM D 4751, Standard Test Method for Determining Apparent Opening Size of
a Geotextile, is accepted by most regulatory agencies. Glass beads are sieved through a
fabric specimen by means of lateral shaking. The test is repeated for different bead sizes
until 5% of the beads, maximum by weight, pass through the specimen. The average
AOS of five tests are reported as bead size [mm] or as the number of the U.S. Standard
Sieve with nominal openings equal to or just larger than the bead size (ASTM, 1987a).
The U.S. Standard Sieve Sizes are given in Table 2.1 (Das, 1985).
This test is not a good indicator for non-woven fabrics because of the tortuosity of
the pore passages of the fabric. The test is limited to particles with a diameter greater
than 75 µm (0.075 mm), lacks reproducibility (Rollins, 1986), and does not simulate
actual field conditions where graded particles may act to form a natural filtration face
(Suits, 1986).
Table 2.1 U.S. Standard Sieve Sizes (Das, 1985)
Sieve
Number:
Opening
(mm):
Sieve
Number:
Opening
(mm):
4 4.750 50 0.300
6 3.350 60 0.250
8 2.360 80 0.180
10 2.000 100 0.150
16 1.180 140 0.106
20 0.850 170 0.088
30 0.600 200 0.075
40 0.425 270 0.053
2.4.3 Previous Silt Fence Research
An alternative test which may reproduce field conditions more accurately is the
Virginia test method or VTM-51, (Wyant, 1993). The filtration efficiency and slurry flow
rate of a silt fence fabric are determined with a suspension created using a site-specific
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