mineral precipitation. While unpublished laboratory studies show that geonet drainage layers are quite susceptible to clogging in a manner similar to that of granular drainage layers, little research has been done on these systems and few field data exist on their performance.
Evapotranspirative barriers are used in capacitive cover systems (monolithic covers and capillary break covers). This type of cover has only recently been considered as an alternative for more traditional types of covers, such as those required under RCRA Subtitles C and D for hazardous and MSW disposal facilities. Thus, most available data cover only a few years. The results of field-scale evaluations suggest that evapotranspirative covers can be an effective alternative to compacted clay or composite covers for infiltration control in arid and semiarid climates where evapotranspiration is an important component of the water balance. Evapotranspirative covers may also be effective alternatives in wetter climates where infiltration control is not a primary concern. However, significantly more data over much longer time frames are required to make a reliable prediction of the long-term performance of evapotranspirative barriers.
Short-term performance concerns for vertical barriers include gaps in the wall as a result of poor mixing, defective material, and high-permeability zones caused by caving or trapped low-quality material and leakage at joints between panels. Medium- and long-term performance concerns include chemical incompatibility, desiccation above the water table, cracking caused by ground deformation, and deterioration of the barrier material. The addition of supplemental cementitious materials (e.g., fly ash, blast furnace slag) can significantly reduce the saturated hydraulic conductivity of cement-bentonite barriers, at least over the short term. Although few monitoring data on field-installed cutoff walls exist, available evidence suggests that the primary short-term issue affecting vertical barrier wall performance is poor construction. An important medium- and long-term issue for soilbentonite barriers includes chemical incompatibility. Overall, it is not yet possible to determine whether vertical barrier walls are effective containment barriers for the long term.
Short- and medium-term performance of cementitious barriers appears, in general, to be good. Long-term performance concerns for cementitious barriers are associated primarily with degradation and cracking of the cementitious barrier material. Cracking caused by restrained shrinkage during curing, settlement during placement, flexure, thermal changes, drying shrinkage, and chemical shrinkage can all create microstructural features that increase the bulk permeability of a PCC barrier. Sulfate-induced degradation of concrete and expansion due to chloride-induced corrosion of reinforcement within the concrete are also important degradation mechanisms for PCC barriers. A variety of mitigation techniques have been developed to address these problems, including the use of ASTM Type V cement; the addition of supplemental cementitious materials, superabsorbent polymers, and saturated lightweight aggregate to the concrete mix; and the use of nonmetallic reinforcement. However, little is known about the long-term behavior of concrete barriers that incorporate these mitigation measures. Although 5,000-year-old asphalt artifacts have been recovered, this does not demonstrate the ability of asphalt concrete to serve as effective hydraulic intrusion barriers for thousands of years. ACC is subject to many of the same cracking mechanisms as PCC. Nonmetallic reinforcement (e.g., synthetic fibers, geogrids, geotextiles) has been used to mitigate cracking in ACC, but little is known about its long-term performance. Asphalt rubber mixtures have been shown to have reduced cracking potential compared to conventional ACC for pavements, but no work has been done on the use of rubberized asphalt in engineered barriers.
The performance of many of these systems could be improved by the use of better component materials. Areas that are fruitful for future research on the properties and behavior of different materials include
HDPE and other geomembrane formulations that have greater resistance to degradation by thermal oxidation.
Drainage materials that are more resistant to clogging.
GCLs that are more effective acting as semipermeable membranes.
Geomembranes that have lower coefficients of thermal expansion and contraction.
Admixtures that reduce shrinkage and cracking of cement-bentonite, soil-cement-bentonite, and concrete barriers.