New tests have shown that asphalt barriers may be an alternative to traditional Subtitle D landfill liners, according to a study by the University of Missouri, Columbia, Mo.
By the late 1960s and early 1970s, asphalt was on the way to becoming the state-of-the-practice landfill liner. Of course, Subtitle D changed all that, but several U.S. facilities were constructed using asphalt concrete (hot-mix asphalt) liners, which in some cases, were combined with a sprayed-on fluid-applied asphalt layer. Asphalt liner use also declined during the oil shortage of the 1970s as new rules for hazardous and solid waste landfill designs focused on composite liners consisting of geomembranes and compacted soil.
However, by 1994, the U.S. Department of Energy (DOE), Hanford, Washington, began using asphalt to contain radioactive and mixed waste. DOE studies showed that asphalt could have a lifetime of at least 1,000 years. Initially, the agency's performance criteria for a cover system included an asphalt barrier for mixed waste sites. A test cover, including a fluid-applied asphalt (FAA) layer above the asphalt concrete, was evaluated.
The initial results of the DOE studies indicated conductivity of asphalt concrete layer's cores to range from 1.3×10-9 centimeters/second (cm/s) to 1.2×10-10 cm/s and field measured conductivities to range from 1.1×10-7 cm/s to 1.9×10-9 cm/s. The higher values in the field likely are attributed to measuring techniques and may not be representative of the asphalt conductivity. The conductivity of the FAA was reported to be 1.8×10-11 cm/s. The asphalt barrier looked very promising.
Results of other laboratory and field tests with asphalt concrete and fluid-applied asphalt show that low hydraulic conductivities can be achieved with these barriers given proper design and high level construction quality control. Several lessons learned from the data include:
The percentage of air voids must be below 4 percent in volume to achieve low hydraulic conductivity;
Asphalt cement content must be above 6 percent to achieve low hydraulic conductivity;
Fines content (less than 0.02 millimeter) must be increased to between 8 percent and 15 percent to ensure a dense graded mixture;
At least two layers of asphalt concrete should be used with a minimum thickness of a 5 centimeter layer to minimize continuity of potential defects and lateral spreading;
An asphalt cement tack coat should be applied between layers, the joints should be staggered and sloped for good compaction;
The fluid asphalt applied layer should be between 1 millimeter and 3 millimeters thick; and
The subgrade must be stable and adequately drained.
Applying these lessons, University of Missouri researchers then developed an asphalt mix design and quality control-assurance measures to test the asphalt barrier's field performance.
Hydraulic conductivity tests were performed separately on laboratory prepared fluid-applied asphalt/geotextile (FAA/GT) and asphalt concrete specimens. The FAA/GT specimens had measured hydraulic conductivities of less than 1×10-11cm/s. Conductivity tests on asphalt concrete specimens showed that specimens having 7 percent or more asphalt cement and unit weights of 22 kiloNewton per cubic meter (kN/m3) or more have conductivity of less than 1×10-9 cm/s.
“Given the longevity of buried asphalt and the high level of barrier performance shown by lab and field testing, asphalt-based liner materials are equivalent and in some ways superior to a Subtitle D liner.”
A full-scale test pad (60 m × 18 m) was tested for barrier performance. The alternative barrier system incorporated 100 mm to 150 mm of asphalt concrete overlain by a 2-mm to 3-mm thick FAA/GT. The asphalt concrete's specification was 7 percent to 7.5 percent asphalt cement with in-situ density greater than 22 kN/m3 (140 pounds per cubic foot). The asphalt concrete's top surface was sprayed with hot fluid-applied asphalt and a paving geotextile was applied followed by a surface coating of hot fluid applied asphalt cement.
FAA/GT and asphalt concrete samples were retrieved from the test pad to measure the hydraulic conductivity. Measurements on field-installed FAA/GT and asphalt concrete specimens revealed conductivities comparable to that of the lab prepared specimens.
Hydraulic conductivity of the asphalt concrete cores was 10-10 cm/s to 10-11 cm/s. The conductivity of the FAA/GT specimens was 10-11 cm/s to 10-12 cm/s. These values represent the lower limit for accurately measuring conductivity using standard American Society of Testing and Materials procedures.
In-situ hydraulic conductivity measurements also were performed. Specially designed and constructed sealed double-ring infiltrometers measured the infiltration in the field. Hydraulic conductivity was calculated from the infiltration rates. The average in-situ measured conductivity was 1×10-10 cm/s. The in-situ value represents the lower limit for accurately measuring field infiltrations. The conductivities measured on the cores from the test pad are thought to be more representative of the conductivity of the asphalt liner.
The asphalt barrier presents an alternative to existing waste containment liner technology. Given the longevity of buried asphalt and the high level of barrier performance shown by lab and field testing, asphalt-based liner materials are equivalent and in some ways, superior, to a Subtitle D liner. In addition, the asphalt liner is thinner than the conventional Subtitle D composite liner, which can save valuable airspace.