Geosynthetics April/May 2020 - 8

UPDATE
A guide for specifying
drainage geocomposites
By Lisa L. Damiano and Eric S. Steinhauser

n September 2017, ASTM International released its Standard Guide for Specifying
Drainage Geocomposites (ASTM D7931). The standard provides a guideline for
calculating engineering properties related to drainage geocomposites, specifically
allowable flow rate, associated reduction factors and shear strength properties. In
basic terms, it codified design procedures so that the engineering community would
have a uniform design approach. ASTM D7931 addresses a variety of drainage geocomposites, including biaxial and triaxial geonet geocomposites, and multilinear
drainage geocomposites.
Drainage geocomposites are used to meet the regulatory requirements for liquid or
gas removal in a variety of applications including foundations, mechanically stabilized
walls, landfills and other subsurface drainage system applications. Commonly used
drainage geocomposites are comprised of a high-density polyethylene (HDPE) geonet
sandwiched between two needlepunched nonwoven geotextiles; however, innovative
products like multilinear drainage geocomposites use perforated mini-pipes in place
of the geonet.

Basic design formula:

qallow = allowable flow rate for a
drainage geocomposite
q100 = initial flow rate determined
under simulated conditions for
100-h duration
RFCR = reduction factor to account
for long-term behavior
RFCC = reduction factor for chemical
clogging
RFBC = reduction factor for
biological clogging
RFGI = reduction factor for
geotextile intrusion past the
initial 100-h seating time

Lisa L. Damiano, P.E., is project manager for
Sanborn Head & Associates Inc.
in Concord, N.H.
Eric S. Steinhauser, P.E., CPESC, CPSWQ, is
senior vice president for Sanborn Head &
Associates Inc. in Concord, N.H.
All figures courtesy of the authors

What's in ASTM D7931?
The heart of ASTM D7931 is to provide a standard equation that can be used to design
and specify a drainage geocomposite for most applications (Equation 1). The equation should be familiar to many in the industry and is included in the Geosynthetic
Institute's GRI Standard GC8, Standard Guide for Determination of the Allowable Flow
Rate of a Drainage Geocomposite.
The performance of drainage geocomposites is limited by several factors that should
be considered when specifying/designing (1) geotextile intrusion into the geonet,
RFGI; (2) geonet crushing, RFCR (i.e., creep); and (3) biological and chemical impacts,
RFCC and RFBC.
A summary of typical ranges for each of the reduction factors for some common
applications of drainage geocomposites is provided in Table 1 along with the reduction
factors that can be used for multilinear drainage geocomposites.
As seen in Table 1, one of the benefits of using multilinear drainage geocomposites is that there is no creep or geotextile intrusion over time and under load.
How is this possible?

Multilinear drainage geocomposites
Unlike what happens with geonet drainage geocomposites, normal load has little effect
on the transmissivity of multilinear drainage geocomposites. Transmissivity is not
affected because the stresses on the mini-pipes are reduced due to soil arching and

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Geosynthetics April/May 2020

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