The basic difference between a composite geomembrane/geosynthetic clay liner (GM/GCL) for use at the landfill base and for use within a final closure cover is that the overburden normal stress is considerably less and differential settlement is more likely to occur in a final closure cover situation.  Among the critical design considerations for this application is to evaluate the retention of the GCL permeability due to out-of-plane deformation resulting from localized settlement, and to assess the GCL strength design under hydration using slope stability analysis.

Design Objectives

1.      Estimate the GCL coefficient of permeability as a function of confining stress.

2.      Evaluate the level of strain produced in a GCL by localized subsidence and to assess the impact on retention of low permeability.

3.      Estimate the required internal reinforcement of a non-woven, needle-punched GCL composite under hydrated conditions for a specified factor of safety.

Design Equations

Hydraulic Conductivity

Compressive stress is a significant variable that controls the behavior of bentonite clay within a GCL.  Increasing the applied normal stress on a GCL decreases the void ratio (or porosity) within the bentonite clay layer, which lowers its hydraulic conductivity, analogous to compacted soils.  The relationship between hydraulic conductivity and effective stress for several types of GCLs hydrated with tap water was presented Thiel et al. (2001).  The differences in hydraulic conductivity between the various GCLs were minimal, except at lower compressive stresses. Slight differences between internally reinforced and non-internally reinforced GCLs were noted.  GCLs with internal reinforcement, such non-woven, needle-punched GCLs, were observed to have a slightly lower hydraulic conductivity under minimal confinement.  As the bentonite hydrates and swells, the reinforcing needle-punched fibers hold the encasing geotextiles together.  This mechanism provides additional confinement and compressive stress on the bentonite clay.  At high normal stresses, the differences in hydraulic conductivity between the various commercial GCLs are subtle (Thiel et al., 2001).

A preliminary design curve was recommended by Thiel et al. (2001) for a confining stress from 100 to 500 kN/m2 (14.5 to 72.5 lbf/in2).  The following equation was adapted from the graph given in their design manual:

kw = 1.78 x 10-9 {exp(-5.76 x 10-3s) Eq. 1