The siting of new waste containment systems is becoming increasing difficult as the public and stakeholders want more confidence that contaminant barrier systems will perform effectively for very long durations and owners want to store more wastes in the least area while knowing and limiting their long-term liabilities. These developments motivate reexamination of long-term performance issues and their implications for barrier designs. Accordingly, design principles are herein considered from the standpoint of long-term performance and management, including the ability to monitor and repair barriers. Then, some design concepts that may be implemented on the basis of these principles, especially evapotranspiration (ET) caps are discussed. Five design principles are recommended on the basis of considerations of infrastructure implementation experience and facility management experiences in other fields. The principles are: establishment of a clear and defendable design basis; design for ease of monitoring and repair; analysis of barriers as dynamic systems; working with nature and not against it; and recognition that increased complexity can reduce, not enhance, net performance. The ET caps are an excellent embodiment of these design principles. The principles are applied to ET caps, as well as design variants such as erosion armor, capillary breaks, biointrusion layers, and low permeability material layers.

The U.S. Department of Energy is scheduled to design and install hundreds of landfill caps/barriers over the next several decades and these caps will have a design life expectancy of up to 1,000 years. Other landfill caps with 30 year design lifetimes are reaching the end of their original design life; the changes to these caps need to be understood to provide a basis for lifetime extension. Defining the attributes that make a successful cap (one that isolates the waste from the environment) is crucial to these efforts. Because cap systems such as landfill caps are dynamic in nature, it is impossible to understand, monitor, and update lifetime predictions without understanding the dynamics of cap degradation, which is most often due to multiple interdependent factors rather than isolated independent events. In an attempt to understand the dynamics of cap degradation, a computer model using system dynamics is being developed to capture the complex behavior of an evapotranspiration cap. The specific objectives of this project are to capture the dynamic, nonlinear feedback loop structures underlying an evapotranspiration cap and, through computer simulation, gain a better understanding of long-term behavior, influencing factors, and, ultimately, long-term cap performance.

As part of an evaluation of closure alternatives for a radioactively contaminated facility, the Hydrologic Evaluation of Landfill Performance (HELP) Model was used iteratively with published leakage equations to predict the long-term hydraulic performance of a (degraded) multilayer soil/geosynthetic closure cap. The evaluation included an analysis of the possible effects of various degradation processes on the various components of the cap. Because the HELP Model does not account for the long-term effects of aging on the integrity of a geomembrane (a component of the proposed closure cap), a review of environmental and geomechanical degradation processes that could detrimentally affect hydraulic performance of the geomembrane was conducted. Possible effects of these processes on geomembrane hydraulic properties were estimated, and calculations performed to estimate long-term leakage rates through defects predicted to (eventually) occur in the geomembrane as a consequence of these degradation processes. Additional HELP Model simulations were performed using a range of geomembrane quality parameters until the model-predicted flux through the composite barrier layer of the cap duplicated the leakage rates computed from the leakage calculations. Results indicate that, for the cap investigated in this study, the HELP Model underpredicted long-term percolation rates through the cover barrier components, compared to rates projected using the leakage equations.

Experimental work was conducted to investigate the influence of biofilm on the hydraulic conductivity and desiccation characteristics of two barrier soils. Bacteria were used to prepare solutions of varying concentration of exopolymeric substances (EPS). These solutions were then used as the molding moisture for compacted specimens of locally available clay and sand-bentonite mixtures. Both soils/mixes exhibited notably low values of hydraulic conductivity, generally less than ${10}^{-8}\mathrm{cm}∕\mathrm{s}$ and often in the range of ${10}^{-9}\mathrm{cm}∕\mathrm{s}$ for three levels of effective stress. The hydraulic conductivity for the clay soil was decreased by a factor of $\sim 3$ , whereas a factor of 2 was more common for the sand–bentonite mixes, depending on the level of confining stress. The reductions in hydraulic conductivity are attributed to the presence of EPS and bacteria which serve to reduce the cross-sectional pore space available for flow. Depending on the initial solution concentration, the addition of EPS to the materials tested resulted in both increases and decreases in moisture retention and drying rate, although a solution containing $112\mathrm{mg}∕\mathrm{L}$ of EPS increased moisture retention. The evaporation process occurred at three different phases. The observed reductions in hydraulic conductivity and phased evaporation process can be useful in the study of naturally occurring and engineered EPS modification of barrier materials.

Containment systems for surface soils that are contaminated with radioactive substances must provide emissions control for both radionuclide migration and ionizing radiation. Herein, a design methodology for georadiological (georad) barriers is developed on the basis of the principles of gamma $\left(\gamma \right)$ radiation shielding and the practical ranges of barrier material characteristics. The intensities of gamma photons generated by parent and daughter radionuclides at the same energy level and time are summed up to derive total entry intensities for specific times ranging from 0 to 50 years, for various georad barrier thicknesses, dry densities, and volumetric moisture contents. A gamma reduction index (GRI) which ranges in magnitude from 0 to 1 in direct proportionality to $\gamma$ attenuation, is developed and used to scale the effectiveness of barriers. The methodology is applied to 1.25 MeV energy gamma emissions from a site that is initially contaminated with Co-60 and Eu-154 at concentrations of $4.64\times {10}^{-11}$ and $9.03\times {10}^{-11}\mathrm{g}∕{\mathrm{m}}^3$ , respectively. The results for a 15 cm thick georad barrier having a dry density ${\rho }^{\prime }=1.75\mathrm{g}∕{\mathrm{cm}}^3$ , and mass attenuation coefficient ${\mu }_s=5.69\times {10}^{-2}{\mathrm{cm}}^2∕\mathrm{g}$ indicate that a reduction index (GRI) of 0.79 can be achieved. For this soil contamination scenario and barrier characteristics, GRI varies exponentially with barrier thickness and reaches the maximum value of 1 at about 62 cm thickness. Exit intensities of gamma photons decrease negative exponentially with time and are much more reduced by increases in barrier thickness than by increases in barrier dry density and moisture content.

The protective cap/biobarrier experiment was constructed to compare the performance of three alternative evapotranspiration (ET) caps with that of a U.S. Resource Conservation and Recovery Act recommended cap. One alternative cap configuration was a homogenous soil cap, while two additional alternative cap configurations contained biointrusion barriers placed at different depths within a soil profile. All cap configurations were planted with two vegetation types and were tested under three climate-change precipitation scenarios. Under ambient precipitation, the three alternative cap configurations performed similarly and returned all of the water received from precipitation to the atmosphere during each growing season, regardless of vegetation type. Growing season ET on ambient-precipitation caps ranged from 133 to 338 mm. Native vegetation extracted more water from caps under augmented precipitation regimes than did a grass monoculture. End-of-season soil volumetric water content was 1.7–5.9% lower on caps planted with a native species mix. When planted with native vegetation, the alternative cap designs tested here should easily meet United States Environmental Protection Agency equivalency criteria.

The combined effects of various levels of hydraulic flow regimes and applied loads on the stability of the interface between the coarse and fine layers of a capillary barrier cover system were investigated by performing series of laboratory experiments in specially designed flow columns. The fine layer (silt loam soil) was placed at dry densities of 1.54 and $1.70\mathrm{g}∕{\mathrm{cm}}^3$ corresponding to 89 and 98% of the maximum dry density, respectively, over a coarse layer (gravel, at a dry density of $1.55\mathrm{g}∕{\mathrm{cm}}^3$ ). Water was introduced at flow rates ranging from 420 to $8,600\mathrm{mL}∕\text{day}$ corresponding to flux rates ranging from 1.57 to $55.5\mathrm{cm}∕\text{day}$ . The colloids mobilized from the silt loam soil (fine layer) used for this study did not significantly affect the stability of the interface and the structure of the fine layer. Thus, the fine layer material is self-filtering. The critical fluxes required to cause significant migration of particles from silt loam placed at 1.54 and $1.70\mathrm{g}∕{\mathrm{cm}}^3$ are 10 and $35\mathrm{cm}∕\text{day}$ , respectively. The upward intrusion of gravel materials into the overlying fine is dependent only on the placement/compaction density of the silt loam soil.

Current design of municipal solid waste landfill liner systems utilizes granular and normally consolidated soil with geosynthetics materials to provide the required environmental and public health protection. Liner systems are required to isolate the waste from the natural subsoil and groundwater. The effect of land subsidence caused by sinkholes is an important design factor for municipal solid waste landfill liner systems. A high potential for sinkhole effects is evident in the central Florida area due to the geological conditions. Sinkhole occurrence under the landfill can cause ultimate failure of the liner system, thus allowing contamination of the natural subsoil and groundwater. The liner system becomes the sole base to bridge the cavity and withstand the overburden pressure. Failure of the liner system becomes a function of the geosynthetic yield strength and its maximum allowable deformation. In this study, the design of landfill liner system for implementation above sinkholes is based on soil arching the tension member theories. It is assumed that the natural subsurface soil deforms across a developing void thereby forming an arch. The design procedure accounts for the cumulative effects of soil types as single or multiple layers with geosynthetic materials. A series of charts are developed for various sizes of sinkhole cavity versus strength parameters of typical soils used in landfill liner systems design. Design examples of single layer and multiple layer liner systems are presented. Recommendations for future landfill design projects by government, industry, and geosynthetic manufactures are also made.

Interfaces are generally the weak links in most structural systems because they are locations of stress concentration and physicochemical/biological attack from both external and internal causes. For waste containment systems in seismically active zones, stress concentration and possible damage initiation at interfaces among different layers of materials depend on input seismic parameters, site characteristics, containment system design configuration, and both material and interface characteristics of components of the system. The approaches and quantitative methods that have been used to analyze the strength and stability of waste containment structural components and systems, including adaptations from earthdam design techniques, are discussed. Using U.S. regionalization data from HAZUS (Natural Hazard Loss Estimation Methodology), containment system liner displacements in lateral and vertical directions are predicted for a liner base at 5 m depth shaken by waves with periods 0.1–0.5 at amplified ground accelerations of $0.4g,0.8g$ , and $1.5g$ respectively. The use of zonation data on seismic and soil parameters, such as those provided by HAZUS in waste containment damage models, has utility in the assessment of potential losses that can be induced by seismic events at larger spatio-temporal scales.