30 Sep 01
(qreq'd) can be developed from the following realationship:
FS = (qallow)/(qreq'd)
factor of safety
allowable flow rate obtained from laboratory testing
required flow rate for design.
The planar flow rate, or transmissivity, of geonet and geocomposite drainage layers is determined using
ASTM D 4716. Laboratory tests should reflect the worst-case normal load and hydraulic gradient
expected in the field. The type of material in contact with the geonet or geocomposite drainage layer
can affect the flow properties of the drainage layer and must be considered when evaluating
transmissivity test data. The transmissivity determined in the laboratory should reflect expected field
conditions as much as possible. Other adjustments, in the form of factors of safety, may account for:
Elastic deformation or intrusion of the adjacent geosynthetics into the geonet's core space.
Creep deformation of the geonet and adjacent geosynthetics into the geonet's core space.
Physical clogging of the geonet's core space with fine soil particles.
Chemical clogging or precipitation of chemicals in the geonet's core space, or both.
Biological clogging and root intrusion of the geonet's core space.
A typical total factor of safety may range from 2.0 to 8.0. Details of the design and guidelines for the
various adjustments and reduction factors are given in Koerner (1998) and Richardson and Zhao
(1999). Guide Specification CEGS-02374 should be used in the contract documents when specifying
geonet or geocomposite drainage layers.
(b) Compatibility. Chemical compatibility between infiltrating surface water and the geonet is
generally not critical and does not require compatibility testing. However, if a geonet or geocomposite
drainage layer is used to intercept leachate exiting the landfill side slopes, the chemical resistance or
susceptibility to biological clogging of the drainage layer to the leachate may need to be evaluated.
(c) Compressive Strength. The flow capability of a geonet or geocomposite drainage layer is
reduced as it deforms under compressive loads. For landfill covers, the normal stresses imposed from
overlying fill material and live loads from construction equipment should be checked, but are usually
small and do not appreciably reduce flow capacity.