30 Sep 01
geomembranes. Geomembranes can be manufactured with textured surfaces, which improve the
slopes greater than about 10. In general, textured geomembranes are more expensive, have
diminished physical properties, have potentially more manufacturing defects, and are more difficult to
inspect than non-textured geomembranes. Guide Specification CEGS-02372 should be used in
contract documents when specifying geomembranes. Designing with Geosynthetics (Koerner 1998)
provides a detailed summary of geomembrane design considerations.
i. Barrier Layer--Clay.
(1) General. The purpose of the clay barrier layer in a composite cover is to inhibit the
movement of water that may pass through any holes in the geomembrane. Soils used for the clay barrier
layer are selected to meet a specific hydraulic conductivity requirement (typically 1 107 cm/s). The
designer needs to ensure that a sufficient amount of suitable soil is available for the clay barrier layer and
specifications are written so that the soil is properly placed, compacted, and protected. If soils found in
the vicinity of the project do not contain a sufficient amount of suitable clay, a common practice is to
blend available natural soils with bentonite (typically 2 to 10% by weight) to achieve required
properties. Clay barrier layers are normally a minimum of 600 mm (24 in.) in thickness. Clay barriers
are vulnerable to increases in permeablility from dessication cracking because of drying, especially
during construction. Procedures outlined in CEGS-02377 should be followed to ensure that the clay
barrier does not dry out. Freezethaw cycles can also increase the permeability of a clay layer.
Therefore, the cover should be designed so that the clay layer is located below frost depth.
(2) Design Criteria. Guide Specification CEGS-02377 should be used in contract documents
when specifying clay barrier layers. A clay borrow source can be located during the design process, or
it can be left up to the contractor. If locating and testing a borrow source is left up to the contractor, the
designer should verify that acceptable borrow sources exist in the area during the design phase. In
general, the testing and quality control steps required for construction of a clay barrier layer are as
A potential borrow source is located and explored to determine its vertical and lateral extent and to
obtain representative samples for testing.
Once construction begins, quality control (QC) and quality assurance (QA) tests are performed to
confirm the suitability of materials being used.
After a lift of soil has been placed, additional tests are performed to verify the soils are being placed
and compacted properly.
(3) Borrow Assessment Testing. EPA/600/R-93/182 describes the required properties of a
clay barrier borrow soil. Tests should be conducted on each principal type or combination of proposed
materials to assure compliance with specified physical properties and to develop compaction