TABLE 4.1 Performance Concerns for Barrier System Components

 

Performance Time Frame

Barrier System Component

Short-Term Concerns

Medium- and Long-Term Concerns

Earthen barriers

Defective material; inadequate compaction (density and/or moisture content); desiccation; slope stability

Cracking due to shrink/swell, freeze/thaw, root penetration, differential settlement, desiccation; chemical incompatibility; waste and slope stability

Geomembranes

Defective material; physical damage due to construction activities; defective seams

Puncture; global and local stability; degradation

Geosynthetic clay liners

Defective material; seam separation

Cracking due to shrink/swell, freeze/thaw, root penetration, differential settlement, etc.; chemical incompatibility; local and global stability; reinforcement degradation (needle-punched reinforced GCLs); inadequate hydration (encapsulated GCLs)

Granular and geosynthetic drainage layers

Inadequate capacity

Clogging due to soil infiltration, biological action, and mineral precipitation; geosynthetic drainage layers are also susceptible to soil and geosynthetics penetration and creep of the geonet core

Evapotranspirative barriers

Defective material; inadequate thickness; inability to establish vegetation

Inadequate storage capacity for infiltration; inability to sustain vegetation; cracking and development of other secondary permeability features; erosion, penetration by vegetation or animals

Vertical barriers

Defective material; “windows” due to caving and trapped low-quality material; leakage at joints between panels; lack of continuity in grouted barriers and extraction well systems

Cracking; desiccation of earthen barriers above the water table; chemical incompatibility of earthen and concrete barriers; corrosion of metallic barriers and of reinforcement in concrete barriers; anti-oxidant depletion and stress cracking of polymer barriers; clogging of vertical extraction trenches and wells

Asphaltic cement barriers

Defective material

Cracking due to shrinkage or deformation, degradation of the asphalt binder or supplemental material (e.g., crumb rubber)

NOTE: GCL = geosynthetic clay liner.

Relatively good agreement was achieved between hydraulic conductivities measured in the field using lysimeters (underdrains) and sealed double-ring infiltrometers, with geometric means of 9 × 10−11 m/s (8 cases) and 5 × 10−10 m/s (77 cases), respectively. However, the field-measured hydraulic conductivities did not correlate well with laboratory values of hydraulic conductivity measured on samples recovered from the test pad using thin-walled tubes when the soils were compacted dry of the optimum moisture content. This result reaffirms the need to compact clay liners wet of the optimum moisture content. Provided that this criteria is met and that there are no obvious visible secondary features (e.g., desiccation cracks), experience has shown that the hydraulic conductivity obtained in the laboratory on samples recovered with thin-walled tubes correlates well with values obtained in the field using lysimeters (e.g., Rowe et al., 2004).

Direct measurements of field hydraulic conductivity using sealed double-ring infiltrometers, as described in Box 4.1, are generally made only on test pads to establish compaction procedures and index properties (e.g., compaction moisture content and density) for quality control of the actual liner. Although the majority of field hydraulic conductivity measurements on compacted clay liners are made during or just after the completion of liner construction, some data are available on the hydraulic conductivity of a liner in the medium term. Box 4.2 describes the performance of a compacted clay liner test section subject to a constant hydraulic head of 0.3 m over a 14-year period. The performance of a liner in a waste containment environment may be influenced by the effects of increased temperatures, accompanying waste decomposition, and overburden pressures, as illustrated by the case history in Box 4.3.

4.1.2
Secondary Features

The difference between laboratory- and field-measured saturated hydraulic conductivity values for compacted earthen barriers can be attributed to macrostructure that often occurs when the soil is compacted at or dry of the optimum moisture content (Benson et al., 1999). Secondary features



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