Geosynthetics October/November 2019 - 43

of the hillside (slope stability issues), and
the risk of TRM entangling with wildlife,
maintenance equipment and hikers, this
option was immediately eliminated.
Riprap
Riprap is typically used as an energy dissipater to slow water down. The size of
riprap determined for this application
was 6 to 12 inches (15.2 to 30.5 cm).
Using riprap would affect the hillside and
hiking trail because the drainage ditch
would require over-excavating to produce
the volume capacity to install the riprap
and the anticipated flow rate of runoff
water. The other main issues with riprap were the long-term maintenance for
weed control and allowing water from the
drainage ditch to saturate the subsurface
that could potentially cause instability for
the slope and endanger hikers and their
pets if they accidentally walked onto the
riprap. Therefore, riprap was eliminated
as an option.
Tied or articulate concrete block mat
Tied or articulate concrete block mats are
used similarly to riprap as an energy dissipater. They are manufactured concrete
blocks formed to a certain shape that
are secured together by their capability
of interlocking or mechanically tying
together via a grid or cable system. The
thickness ranges from 2 to 12 inches (5.1
to 30.5 cm). It was determined that the
block mat has essentially the same concerns as riprap mentioned above and so
was eliminated as an option.
Shotcrete/poured concrete
Shotcrete or poured concrete was
reviewed, but access to the site is limited
and concrete trucks could not enter the
trail. The trail has a single access point
with limited width (a tight 10 feet [3.0
m]), steep slopes and no place to turn
around. So, any concrete or shotcrete
work was deemed unfeasible.

Fabric-formed concrete
Revetment mats are special woven fabrics
that are sewn together to create pockets
or envelopes to be filled with a pumpable
fluid concrete mix. It met the criteria, eliminating water peculation and vegetation
issues and was easy to install. However, the
difficulty for the fabric-formed concrete
was it required over-excavation and access
for the pump truck at the site, so it was
eliminated from the options.
Geosynthetic concrete
composite mat (GCCM)
A geosynthetic concrete composite mat
(GCCM) is a flexible concrete-impregnated
geotextile on a roll. When deployed, it
hardens on hydration to form a durable, fiber-reinforced concrete layer. The
material is 0.2 to 0.5 inch (0.5 to 1.3 cm)
thick and weighs from 1.9 to 5.3 psf. The
material met all the criteria and exceeded
expectations, including no special contractor (in-house labor could install it), flame
resistance (fire protection), no weather
constraints during installation, easy-tofollow ground surface contours, and minimum construction crew and equipment,
which would allow the hiking trail to be
open during the installation. This product
was selected for the project.

3b
FIGURES 3a and 3b Existing storm inlet
drains with silt protection

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Geosynthetics October/November 2019

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