Geosynthetics April/May 2021 - 32

Comparative analysis of the flow and filtration capabilities of sediment retention devices-Part 2

FIGURE 5 ASTM D7351 toe-of-slope
filter sock testing (upstream)

Geosynthetics | April May 2021

SRD tested and the lack of a burial trench.
Larger nominal diameter allows for greater
ponding area upstream of the SRD and
means a greater volume of product fill
per unit length, which typically leads to a
higher unit weight per length of the product. Also, in contrast to two-dimensional
SRDs, which are typically installed with
a 6-inch (152-mm) burial trench sealing the fabric to the soil surface, threedimensional SRDs are often installed in a
shallow installation trench or laid directly
on the soil surface. This lack of a sealing
burial trench allows water to more freely
seep under the installed three-dimensional
SRD, further relying on the unit weight
per length of the product to seal against
the soil surface and slow seepage. The
three-dimensional SRDs tested in this
research ranged in nominal diameter,
and thus nominal height, from 8 to 24
inches (203 to 610 mm). Unsurprisingly,
the products with the highest water retention effectiveness were all of larger nominal diameter. The two 24-inch (610-mm)
nominal diameter products produced
29.8% and 23.3% water retention effectiveness, and the 18-inch (457-mm) nominal

diameter product produced 22.0% water
retention effectiveness.
There are two caveats relating to
the performance of three-dimensional
products that should be discussed. The
first caveat to the data presented here is
that the nominal diameter/height and
functional height are often different.
Installation of three-dimensional SRD
products often includes trenching, staking, stapling or strapping and can affect
the functional height of the installed SRD.
Thus, a three-dimensional product with
a nominal height/diameter of 24 inches
(610 mm) and a deep installation trench
could have similar functional height to a
three-dimensional product with a nominal
height/diameter of 18 inches (457 mm)
and a shallow installation trench. The second caveat is that unit weight of the threedimensional SRD should be considered.
A lightweight three-dimensional SRD
of a given nominal height/diameter may
not provide the performance of a heavyweight three-dimensional SRD of a similar
nominal height/diameter. Unsurprisingly,
this phenomenon is represented in the
data above. A 9-inch (229-mm) nominal
height/diameter straw wattle had lower
performance in both sediment retention
effectiveness and water retention effectiveness (88.5% and 15.7%) than an 8-inch
(203-mm) nominal height/diameter
compost filter sock (89.7% and 19.7%).
The straw wattle has a lower unit weight
(kg/m) than the compost filter sock. This
lighter weight of the straw wattle can
potentially lead to floating or movement
under hydrostatic load, which could be the
cause for the differences in performance.
Based on the data presented above
and the discussion of variables in threedimensional SRD performance, designers must balance factors such as nominal
height/diameter, functional height, unit
weight and installation details when determining the appropriate three-dimensional
SRD for a sediment control application.

Geosynthetics April/May 2021

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